V VII- S D C 6 L D S-AD-M-010 Revision A 2nd Draftopto-mech.lle.rochester.edu/Training...

S-AD-M-010 Rev A Chapter 6: Beamline Laser Diagnostics

University of Rochester of 41 S-AD-M-010 Rev A Laboratory for Laser Energetics May 5, 2008

OMEGA EP

SYSTEM OPERATIONS MANUAL

VOLUME VII- SYSTEM DESCRIPTION

CHAPTER 6

BEAMLINE LASER DIAGNOSTICS

S-AD-M-010 Revision A

2nd Draft



Table of Contents Introduction..................................................................................................................................... 3

6.1 Injection and IR Alignment System................................................................................ 5 6.1.1 Injection and IR Alignment Tables............................................................................. 5

6.2 IR Diagnostic Package.................................................................................................... 7 6.2.1 IR Output Chain.......................................................................................................... 7 6.2.2 IR Diagnostics Table................................................................................................... 8

6.3 UV Diagnostic Package ................................................................................................ 10 6.3.1 UV Output Chain ...................................................................................................... 10 6.3.2 UV Alignment Table................................................................................................. 12 6.3.3 UV Diagnostics Table............................................................................................... 13 6.3.4 UV Alignment Sensor Package ................................................................................ 14

6.4 Short-pulse Diagnostics Package.................................................................................. 15 6.4.1 Short-pulse Input and Output Chains........................................................................ 16 6.4.2 Short-pulse Diagnostics ............................................................................................ 17

6.5 Alignment Diagnostics.................................................................................................. 18 6.5.1 Alignment Sensor Package (ASP) ............................................................................ 18 6.5.2 Pinhole Viewing System (PVS)................................................................................ 19 6.5.3 Parabola Alignment Diagnostic ................................................................................ 20

6.6 Energy Diagnostics ....................................................................................................... 23 6.6.1 Calorimetry ............................................................................................................... 23 6.6.2 Energy Diagnostic..................................................................................................... 24 6.6.3 Harmonic Energy Detector ....................................................................................... 26 6.6.4 Near- and Far-Field Scientific Cameras ................................................................... 28

6.7 Wavefront Diagnostics.................................................................................................. 29 6.8 Spectral Measurements ................................................................................................. 31 6.9 Temporal Diagnostics ................................................................................................... 32

6.9.1 ROSS Streak Cameras .............................................................................................. 32 6.9.2 Scanning Autocorrelator ........................................................................................... 34 6.9.3 TESSA ...................................................................................................................... 35

6.10 Focal Spot Diagnostics ................................................................................................. 37 6.11 Acronyms...................................................................................................................... 40



Introduction

A full compliment of laser diagnostic instrumentation is dedicated to each of the four beamlines of the OMEGA EP Laser System. These diagnostic packages provide comprehensive information of system performance, both in preparation for and during a target shot. Measurements are made of the beam energy, spectral content, pulse shape, pulse contrast, wavefront, spatial intensity profile and focal spot profile. Alignment diagnostics point and center the beam through the system using a series of cross-hair references.

Each of the four beamlines has a suite of laser diagnostics that includes a injection diagnostic, infrared alignment tables (IRAT), infrared diagnostic packages (IRDP), short-pulse diagnostic package (SPDP’s), ultraviolet diagnostic packages (UVDP), and a parabola alignment diagnostic (PAD). The injection diagnostics characterize the beam received from Laser Sources and is the hand off between sources and beamlines. The IRDP is used to characterize and align the main laser cavity. The SPDP characterizes and aligns the short-pulse beams of Beamlines 1 and 2. The UVDP’s characterize and align the four frequency-converted beams. The PAD supports the off-axis parabola (OAP) alignment, focal spot diagnostic (FSD), and the short-pulse wavefront control system (WCS) providing for the final beam targeting alignment. Table 6.1 describes the measurements made by each diagnostic. Table 6.1 Measured parameters for each type of diagnostic Measurement Injection IRDP SPDP UVDP Alignment � � � � Energy � � � � Spectrum � � � -- Pulse shape -- � � � Pulse contrast -- -- � � Wavefront � � � -- Near-field � � � � Far-field -- � � � Optic damage -- � � --

The laser diagnostics characterize laser performance, assess the condition of optics, and

have the capability to

• accurately and precisely align beams in pointing (±0.5 µ-rad) and centering (±1 mm); • measure the IR and/or UV energy of each beam, • measure the pulse shape, spectrum and pulse contrast, • measure the near-field wavefront quality at key image planes, • measure the far-field spot profile at and near the focal plane of the focusing optics.

The diagnostic instruments are computer controlled, and all relevant experimental on-shot data, as well as test data are stored in the OMEGA EP Experimental Database for analysis.



Section 6.1 describes the diagnostics that characterize the pulse injected from Laser Sources prior to injection into the Transport Spatial Filter (TSF). After the pulse is injected and amplified, a 0.1% portion is reflected by the Diagnostic Beam Splitter (DBS) back into the TSF where an output chain diverts this light to the IRDP. Diagnostics used to co-align the injected pulse with the Beamline optics are located on the IRAT. These two systems are described in Section 6.2. The amplified long-pulses (1 to 10 ns) from each Beamline are frequency converted and a second DBS placed just after the frequency conversion crystals (FCC’s) divert 4% of the pulse to the UV on-shot and the UV alignment tables (UVAT) for characterization. These systems are described in Section 6.3. Beamlines 1 and 2 are capable of producing short-pulse beams (1 to 100 ps) that are diverted into the Grating Compression Chamber (GCC). A 0.5% portion of each compressed pulse is down-collimated and diverted to the SPDP table for characterization as described in Section 6.4.

The diagnostics are located in three main areas in the Laser Bay as shown in Fig. 6. The

Injection, IRAT and IRDP are incorporated as adjacent tables in each beamline structure. Full aperture IR calorimeters are located on the switchyard structures south of the Target Area Structure (TAS) and inside the GCC. The UVDP for each beamline is located on the southern portions of the second and third decks of the TAS. A single UVAT is located on the Laser Bay floor south of the TAS. The SPDP table is positioned south of the GCC. While some components of the SPDP (calorimeters and supporting optics) are located inside the GCC. PAD is Ten-Inch Manipulator (TIM) based instruments. It can be located on the OMEGA EP Target Chamber in TIMs 12 and 14, and in OMEGA Target Chamber in TIM 5.

Figure 6. Physical locations of the laser diagnostic packages. (G6922aJ1)



6.1 Injection and IR Alignment Tables

The Injection and IR Alignment Tables are located under TSF vacuum vessel and rest on a polymer concrete structure that provides mechanical stability (see Chapter 5, Optomechanical System, S-AD-M-009). The input beam from Laser Sources is projected through a periscope on the IRAT table and transported to the Injection table where it passes through a vacuum window onto the injection mirror that directs it through the TSF Pass-1 pinhole (PH) towards the Booster Amplifier. The telescope from Laser Sources preceding the injection system relays the image of the apodizer and Laser Sources alignment cross-hair (relay plane -1, RP-1), to the system crosshair plane (RP-0) located on the IRAT (Fig. 6.1), that serves as the system alignment reference. The injection lens, TSF and CSF then project RP-0 to other planes in the system as described in Chapter 5, Optomechanical System, S-AD-M-009.

The two tables share a suite of diagnostics:

• Alignment Sensor Package (ASP) • Insertable calorimeter • Near-field (NF) camera • Energy diagnostic • Spectrometer (Beamline 1 & 2 only) • Retro wave front sensor The IRAT beam and Laser Sources beam co-propagate starting at a polarizer placed at just

after the periscope. The IRAT laser is up-collimated and apodized to a square beam ~15% larger then the sources beam. The IRAT laser aligned to the Alignment Sensor Package (ASP) on the injection table, this is the hand off between the sources beam and IRAT laser beam (which is use for the full system alignment).



Fig. 6.1.1 One version of the Infrared Alignment Table (B-DV-A-036) is shown above. The alignment beam and the Sources input beam are co-aligned at the beam combiner located just after the upper periscope fold mirror. A pair of waveplates are used as system throttles. The System Crosshair (relay plane 0) is also located on this table. Fiber pickoffs are provided for the IR Spectrometer and the Energy Diagnostic (ED).

In addition to the IRAT laser uses as the main alignment laser, the output of the laser can propagate to a fiber launcher. After the laser is launched into the single mode fiber, the energy is split. One fiber goes into the TSF pinhole vessel, where it is used to provide the reference wavefront beam for the IR wavefront sensor (WFS) on the IRDP table. A second fiber-optic path provides a reference beam for tiled grating compressor interferometry.

IRAT Laser

To the Injection Table

Energy Diagnostic

Upper Periscope

Fold Mirror

5x Upcollimator

Power AmpBeam

Combiner

System Cross-hair

Wave Plate



ASP - Centering Arm

ASP - Pointing Arm

NF-

CCD

Calorimeter

Injection

Lens

CCD

CCD

IR-DBS

Return

Injection

Mirror

To IRDP

From

InjectionDown Collimator

Figure 6.1.2 The optomechanical drawing of the Injection Table layout of Beamline 1 (B-DV-A-037). The lens set in front of the calorimeter provides an intentional alignment offset to correct for the 2o tilted TSF lenses. The calorimeter can be inserted to measure the energy prior to amplification or can act as a beam block.

6.2 IR Diagnostic Package

An IR diagnostic package3 containing a suite of diagnostic instrumentation is dedicated to each of the beamlines. This group of instruments provides comprehensive information about system performance in preparation for and during a target shot. During a shot, measurements are made of the beam energy, the near-field and far-field spot profiles, and the full-aperture beam wavefront. A spectrometer and a streak camera measure the spectral and temporal pulse shape. Prior to taking a shot, alignment diagnostics are used to point and center the beam from the source injection point to the beam emerging from the end of the transport spatial filter. 6.2.1 IR Output Chain

The source beam for the IRDP comes from the first-surface reflection of the IR-DBS4.

The IR-DBS is a flat, wedged plate oriented at 0.10 degrees relative to the beam normal and is located at the output end of the TSF. Approximately 0.2% of the incident light is reflected from the front surface of the IR-DBS back through the TSF output lens where the converging beam is folded to fit within the TSF pinhole area vacuum vessel. Within the TSF center vessel, the beam is collimated to form the beam that is then delivered to the IRDP table where it acts as the source beam for the various alignment and shot diagnostics. Figure 6.2.1 shows a functional layout of

3 IR Diagnostics Package Technical Requirements, D-AD-R-001 4 IR-DBS Assembly, D-AD-A-017



the IRDP optical output chain. This beam exits the TSF pinhole vessel vacuum window and is reflected into the IRDP for characterization.

Fig. 6.2.1 A depiction of the IRDP output chain optical path is shown. The source beam for the IRDP comes from the first-surface reflection of the IR-DBS. Approximately 0.2% of the incident light is reflected back through the TSF output lens where the converging beam is folded to fit within the TSF pinhole-area vacuum vessel. The beam is collimated within the TSF’s center vessel to form the beam that is then delivered to the IRDP table. (G7207J2) 6.2.2 IR Diagnostics Table

An optical layout of the IRDP table5 is shown in Fig. 6.2.2. The IRDP has two functional modes, one for alignment and a second for on-shot diagnostics. The wavefront sensor is an exception; it must be operational with the IR alignment laser and on-shot beam energies. Selection of the IRDP operational mode is achieved with a flip-in shot pickoff mirror. A design requirement to achieve maximum stability of the alignment arms dictates no moving parts in the narrow fields of view for both pointing and centering. With the flip-in shot pickoff out of the beam path, the cw alignment laser propagates to the alignment, or pre-shot diagnostics.

5 IRDP Table Layout, D-AD-A-030



Fig. 6.2.2 The input energy to the IRDP table (D-AD-A-030) enters from the lower right as a reflection from the IR-DBS. A full suite of diagnostics is used to characterize the on-shot beam. The flip-in ‘on-shot’ mirror (at the right edge of the table) is controlled by the Beamlines Operator and separates the on-shot diagnostics from the alignment ASP’s.

Pre-shot diagnostics include:

• ASP coarse and fine pointing, • ASP coarse and fine centering, • Wavefront measurement, and • Optics inspection.

Insertion of the on-shot pickoff directs the beam towards the shot diagnostics and blocks any high energy light from reaching the alignment diagnostics. On-shot diagnostics include:

• Near-field camera • Far-field camera • Wavefront Sensor • ROSS6 streak camera (via fiber pickoff) • Energy diagnostic (via fiber pickoff) • Spectrometer (via fiber pickoff)

6 Rochester Optical Streak System

FF-CCD

NF-CCD

Streak Camera

Fiber PO

Spectrometer Fiber PO

Energy Diagnostic

Fiber

Optics Inspection Arm

Input

Wavefront Sensor

CCD

CCD

ASP - Centering Arm

ASP - Pointing Arm

On-shot Mirror



6.3 UV Diagnostic Package 6.3.1 UV Output Chain

Four UV diagnostic packages7 provide comprehensive information about the system performance, both in preparation for and during a target shot. Measurements are made of the beam energies at all three harmonics, the near-field (IR and UV) and far-field spot profiles, and the pulse contrast. The IR beam energies before the FCC’s are measured in the IR diagnostic package (Fig. 6.3.4). A diagram of the UV transport and focusing system is shown in Fig. 6.3.1.

Fig. 6.3.1 The optical layout of the UV Transport and focusing system is shown. Approximately 4% of the main beam is reflected from the UV DBS to an OAP and into the diagnostic or alignment packages. The front and rear surfaces of the on-shot diagnostic pickoff provide energy to the beam characterization diagnostics. When the calorimeter pickoff is not inserted, the beam propagates to the alignment table. An insertable beam block located behind the UV-DBS (not shown) prevents propagation into the target chamber.

7 UVDP Technical Requirements, D-TM-R-001



Each of the four beamlines has its own UV diagnostic and alignment table, located near the target chamber on the target-area structure (Fig. 6.4.2). The UV alignment source is located on its own table on the Laser Bay floor in front of the TAS, and its output beam is introduced just before the FCC’s with a periscope mirror assembly (PMA) similar to that on OMEGA. The UV alignment beam is sequentially propagated through each of the four beam paths. The placement of the FCC’s before the target chamber permits more convenient beam diagnostics and allows for the rejection of unconverted light by the transport mirrors. The frequency-conversion performance is diagnosed with a 4% diagnostic pickoff located after the FCC’s in an arrangement similar to that used on OMEGA. The pickoff diagnostics include alignment sensors for co-aligning the UV alignment source to the IR alignment source using a cross-hair located after the FCC’s.

Fig. 6.3.2 An illustration of the installation of one of four UV diagnostic (on-shot) and alignment sensor tables located on the TAS.

Co-alignment of the IR and UV alignment beams is achieved by steering the periscope mirrors to point the UV alignment beam to the pointing and centering alignment sensors on the UV Diagnostic table. These sensors use achromatic optics to function at both wavelengths. The portion of the UV alignment beam that passes through the UV DBS is steered to the target by moving the transport mirrors (Fig. 6.3.1). The beam is confirmed to be aligned by retro-reflection back to sensors adjacent to the laser source. This method of UV system alignment is the same as used on OMEGA. A minor difference is that the UV beam is injected prior to the FCC’s on OMEGA EP.



6.3.2 UV Alignment Table

The UVAT8 serves as the primary laser source for alignment of the UV transport system. The UVAT also senses centering of the beam on the target mirrors, focus lenses, and detects pointing and focusing on target. The sensors on the UVAT can be used to look at light reflected back from a target. These sensors are also used for the FCC Alignment Diagnostic (FCCAD) that images light reflected from FCC’s. A functional optical layout of the UVAT is shown in Fig. 6.3.3.

Alignment with the UVAT makes use of an approximately 9-mm-diameter pencil beam

that is propagated through the UVAT sub-system. Generation of this beam utilizes a subset of the UVAT beam shaping optics, i.e. 1:1 image relay, microscope objective and collimating lens. The UVAT output is 37-cm x 37-cm collimated square beam. Fine alignment of the beamline involves pointing the UVAT source through the UV transport system while maintaining centering of the beam within the optical component clear apertures. Output beam pointing and centering is measured using the alignment sensor packages located inside of the UVDP and on the UVAT. The UVAT alignment beam is injected by the PMA9 into one of the four long-pulse beams. The PMA is a sub-system that positions a set of mirrors before the FCC’s.

Fig. 6.3.3 Optical layout of the UV Alignment Table (B-DU-A-005). A full aperture UV focus lens (not shown) collimates the expanding beam from the table prior to injection into the PMA. The PMA selectively inserts the UV alignment beam into each of the four beamlines for co-alignment with the IR alignment beam.

8 UV Alignment Table Requirements, B-DU-R-001 9 Periscope Mirror Assembly, B-EB-R-001



6.3.3 UV Diagnostics Table

The UV diagnostic table (Fig. 6.3.4) supports the instruments required to characterize the on-shot beam. Insertable beam splitters provide the incident beam energy to the NF- and far-field (FF) -scientific cameras, a 4-in calorimeter as well as fiber pickoffs to a harmonic energy detector (HED) (1ω, 2ω and 3ω), ROSS streak camera and a pulse contrast diagnostic (roving oscilloscope).

The NF-camera can be translated to image both the FCC’s and the beams image on the

final switchyard mirror (IRHR6). The FF-camera receives an image from an optical zoom system providing different fields-of-view (FOV) and resolution meeting the needs of both System Science and Experimental Systems. Table 6.2 describes the different requirements.

Table 6.2 Scientific camera specification for System Science and the Experimental Group

Specification Parameter System Science Experimental Angular FOV µ-rad +/- 119 +/- 500 Angular Resolution µ-rad/pixel 0.25 1.1

Fig. 6.3.4 The optical layout of the UVDP Diagnostics Table (D-TM-B-026) is shown above. The on-shot diagnostic pickoff (UV-BS2) front and back surface reflections provides energy to the diagnostics. A second insertable pickoff (UV-FMS1) provides a path to the calorimeter. The



dual diffuser fiber launcher provides an optical fiber to the ROSS streak camera located in Diagnostic Bay 1. The HED’s integrating sphere provides a fiber pickoff to the HED instrument located in Diagnostic Bay 1.

Insertion of the on-shot pickoff mirror directs the beam towards the shot diagnostics and

blocks any high energy light from reaching the alignment diagnostics. On-shot diagnostics include:

• Near-field camera • Far-field camera • Harmonic Energy Detector (via fiber pickoff) • UV-ROSS camera (via fiber pickoff) • Contrast Diagnostic (Diode and scope) • Calorimeter

6.3.4 UV Alignment Sensor Package

Adjacent to the diagnostics table, is the alignment table whose optical arrangement is shown in Fig. 6.3.5. The fixed alignment mirror, UV-FMA1, is aligned to the UV OAP (see Fig. 6.3.1). This optical table is bi-level by design. The lower level supports an on-shot pickoff beam splitter (UV-BS2) an insertable fold mirror (UV-FMS1) and the fixed alignment mirror (UV-FMA1). Both the front and back surfaces of the beam splitter direct energy to different instruments on the diagnostic table. The insertable mirror directs the on-shot beam to the calorimeter on the diagnostic table. The optics on the upper level table direct the alignment beam to the ASP pointing and centering cameras. Both coarse and fine centering and pointing alignment modes are provided for using flip-in optics for the coarse mode.



Fig. 6.3.5 The optical layout of the UVDP ASP table (D-TM-B-025) is shown above. In shot-mode, the on-shot pickoff mirror (UV-BS2) and the calorimeter pickoff mirror (UV-FMS1) are inserted to provide energy to the diagnostic table. 6.4 Short-pulse Diagnostics Package

The instruments in the short-pulse diagnostic package10 diagnose the properties of the

two compressed beams before they are co-aligned in the GCC. These diagnostics measure the beam quality, energy, alignment, spectrum, optical component damage, output wavefront, pulse width, and pulse contrast. A functional diagram of the diagnostics on the SPDP table is shown in Fig. 6.4.1 below. Two identical sets of diagnostics are used for the upper and lower compressors in the GCC.

10 Short-pulse Diagnostics Requirements, D-AA-R-001



Fig. 6.4.1 Functional layout of the short-pulse diagnostic package. There is one of these systems for each of the two compressed beams.

Subsystems specified in other documents include:

• Alignment sensor package • Full aperture calorimetry • Energy diagnostic • Ultrafast temporal diagnostic • Near/Far field diagnostic • Spectrometer • Wavefront sensor • Optics inspection diagnostic • Focal spot diagnostic

6.4.1 Short-pulse Input and Output Chains

The input system provides for a dual-wavelength IR alignment laser (1053 nm and 1047

nm) located on the SPDP table to illuminate the short-pulse transport paths to the target, Fizeau interferometers and alignment mirrors within the GCC. This laser is used for alignment and setup of the pulse compressors independent of the main beamline. Two wavelengths are used in the compressor alignment procedure to ensure that the grating assemblies are aligned for broadband compression. The optical input chain sends these beams from the SPDP table into the



compressors counter-propagating to the pulsed beams. Three insertable compressor alignment mirrors (CAM’s) allow for fine positioning of each of the four tiled grating assemblies (TGA’s).

The diagnostic output chain11 provides energy from the beam in the upper pulse

compressor using light transmitted through the diagnostic mirror (SPHR2) that is directed to an optic having a slight wedge to the SPDP. The first surface of the wedge is uncoated, providing a 4% reflection, while the rear surface is highly reflective. This small pointing difference between the beams reflecting from the front and rear surfaces allows selection of either low- or high-energy transmission paths to the SPDP. This flexibility enables greater attenuation for the highest-intensity short-pulse beams, providing a lower-energy diagnostic beam and minimizing the B-integral for on-shot measurements. The full aperture reflected beam is down-collimated through a pair of lenses to 65 mm before it exits the GCC vacuum window and enters the SPDP table through a beam tube. A second set of down-collimation lenses further reduces the beam size to 22 mm. After the SPDP’s ASP the beam is further reduced in size to 12 mm to accommodate the remaining diagnostics.

The lower pulse compressor has a similar, but separate output chain path to a separate

diagnostics set on the SPDP table. Since the lower compressor operates at lower energies (due to B-integral accumulation in the beam combiner optic) an attenuation wedge path is not required. A single, 45° fold mirror, in the lower compressor beam replaces the wedge and fold mirror pair.

6.4.2 Short-pulse Diagnostics

The diagnostics and alignment laser sources are located on a 5-ft x 32-ft optical table adjacent to the compressor vessel (Fig. 6.4.2). The instruments in the SPDP are nearly the same as those on the IRDP. Alignment sensors, near- and far-field cameras, energy sensors, wavefront sensors, and inspection systems are also available. Unique to this area are the focal-spot diagnostic12 and the ultrafast temporal diagnostic package13 (UTD). The FSD uses pre- and on-shot wavefront sensors and near-field spatial-profile instrumentation to characterize the spatial irradiance pattern of the beam(s) focused on target. A far-field camera with two fields of view is used PRESHOT (in narrow field) to confirm grating tiling alignment and on-shot (in wide field) to characterize any noise passing through the pinholes. The temporal instruments consist of a fast streak camera (UROSS14) and an autocorrelator used in combination to measure the pulse duration and shape.

11 Short-pulse Output Chain Requirements, D-AA-R-017 12 Focal Spot Diagnostic Requirements, D-AA-R-015 13 Ultrafast Temporal Diagnostic Requirements, D-AA-R-002 14 Ultrafast Rochester Optical Streak System



Fig. 6.4.2 The north end of the SPDP table is shown with its protective side barriers removed. The beam tubes (telescopes) provide inputs from the diagnostic pickoffs of the upper and lower pulse compressors inside the GCC and reduce the square beam edge length from 65 mm to 21 mm. 6.5 Alignment Diagnostics 6.5.1 Alignment Sensor Package (ASP)

The ASP’s are stable optical telescopes used to view an alignment beam propagating in a beamline. They diagnose the alignment of the system by sensing the pointing of the alignment lasers and the position of the laser beam on optical components by sensing the position of alignment fiducials (e.g., cross hairs). These fiducials can be remotely inserted into the beamline or manually attached to optomechanical assemblies. The ASP’s are also used to view the positions of the spatial-filter (SF) pinholes and are the primary sensors for the beamline alignment system typically residing on each of the diagnostic packages described above.

Each ASP contains a pair of CCD cameras (Sony XC-ST70 or Cohu XXXX) to view

either the focus of the telescope objective (beam pointing) to measure the directionality of the beam or the output image of the telescope to view the position of alignment fiducials (beam centering). The availability of 2 cameras facilitates simultaneous viewing of pointing and centering. Two-state devices move optics within the ASP’s to allow coarse or fine viewing. The fine ASP’s contain no remotely actuated components between the entrance pupil and point where



the beam focuses on an alignment reticle. This feature provides long-term stability of the pointing reference.

6.5.2 Pinhole Viewing System (PVS)

The pinhole viewing system is a long working distance microscope used to align the beamline in the spatially multiplex beamlines. The scan mirror inside the pinhole vessel is remotely actuated to view all of the pinholes. The alignment pinholes are smaller than the shot pinholes and are made of frosted glass. This allows a diffraction pattern formed by an upstream cross-hair to be visible at the pinhole plane.

The operator aligns the scan mirror to the pinhole, using a software reticle to locate the

center of the pinhole. Inserts a cross-hair into the main beam path, create and moves a mirror upstream of the pinhole to align that pass. The entire main beamline is aligned by viewing each pinhole pass and aligning diffraction patterns to software reticles.

Fig. 6.5.2.1 An illustration of the TSF pinhole vessel and the pinhole viewing system

Pinhole Viewing System

TSF Pinhole Assembly

Injection Mirror

PVS Scan Mirror



Fig. 6.5.2.2 An image from the pinhole viewing system 6.5.3 Parabola Alignment Diagnostic

The final optic in the imaging chain is the off-axis parabola located in the target chamber. This mirror requires precise alignment with the optical axis of the system that is accomplished using the vacuum-compatible PAD15 (Fig. 6.5.1). This diagnostic is TIM16 based and located opposing the OAP. The PAD is comprised of three main items: a 1053-nm cw source, a pointing diagnostic and a wavefront sensor. The PAD supports the alignment of the OAP detects the incoming wavefront and provides a cw laser source for the FSD17.

15 Parabola Alignment Diagnostic Requirements, D-AA-R-032 16 See Vol. VII, Chapter 7, Experimental Systems, page 23, S-AD-M-011 17 Focal Spot Diagnostic Requirements, D-AA-R-015



Fig. 6.5.3.1 An illustration of the PAD (in blue) installed in a TIM (door open). The entire PAD package will rotate manually around its optical axis to accommodate the

different angles necessary for the three different PAD/OAP locations specified in Table 6.3 below. The PAD pointing diagnostic is used to image the focal spot as well as aid in alignment of the PAD and the OAP to Target Chamber Center (TCC). The nature of the size of PAD caused the design to require a short focal length microscope objective to re-collimate the light off of the OAP. If uncorrected the OAP and microscope objective would have distorted the beam enough to render the wave front sensor unusable. To correct this error a second microscope objective and small OAP were added to pre-correct the outgoing beam and correct the incoming beam. Table 6.3 {Possible PAD locations)

Designation OAP Port Beam Port PAD Port PAD TIM Backlighter OMEGA H7 OMEGA H9 OMEGA H14 TIM5 Backlighter EP 33 EP 51 EP 45 TIM12 Sidelighter EP 54 EP 69 EP 48 TIM14

To ensure that the PAD does not drift linearly during OAP alignment, calibration, or

wavefront correction, the PAD has alignment fiducials that are monitored by the target viewing system, TVS. To ensure that the TIM does not drift linearly during OAP alignment, an autocollimator18 attached to the TAS or Target Mirror Structure (TMS) monitors the TIM’s pointing. The angular changes are recorded and the PAD motion controls make the necessary adjustments in angular position. Wedges compensate for keystone distortion caused by the OAP.

18 OMEGA EP Parabola Alignment Diagnostic Auto Collimator, D-AA-W-117



Fig. 6.5.3.2 The PAD is a self-contained TIM-based diagnostic, portable between target chambers. A Shack–Hartmann sensor uses a lenslet array to measure the wavefront reflected off the off-axis parabola, the autocollimator monitors angular displacement of the PAD optics, and a pointing diagnostic determines the point of focus of the OAP.

The PAD laser (1053 nm) is linearly polarized, fiber-coupled and apodized to the OMEGA EP spatial profile with its polarization rotated by a waveplate to match the beam reflected through TCC from the OAP. The beam can be counter-propagated through the pulse compressors and directed by insertable compressor alignment mirrors into the SPDP.

The wavefront of the compressed pulse can be determined using a Shack-Hartmann

lenslet array and CCD camera. The wavefront sensor is used to correct wavefront errors created by the short-pulse transport and focusing optics and is used in combination with the PAD source and the compressor Deformable Mirror. The wavefront sensor is located in a non-vacuum, temperature and pressure monitored, bubble inside the PAD.

OAP

1053 nm Fiber Laser Sun Ultra 45

Au

toco

llimato

r

SP Beam Entry Port

To SP Transport Mirrors

TIM Pointing Camera

Wave Front Sensor



6.6 Energy Diagnostics 6.6.1 Calorimetry

The OMEGA EP system uses six dedicated full-aperture19,20,21 (40-cm) calorimeters, four, 4-in and eight, 2-in calorimeters. Each calorimeter measures the total energy in the beam. These measurements are used to assess the beam-to-beam energy balance and to calibrate the ED’s and the HED (see Sect. 6.6.3) that serve as the primary beam energy and crystal-tuning diagnostic for target shots. The full-aperture calorimeters are located in the IR Switchyard just after the IR-DBS’s and also at the exit of each pulse compressor inside the GCC. The 4-in calorimeters are used on the UVDP tables. The 2-in calorimeters are used in Laser Sources and on each Injection table.

The calorimeters in Laser Sources, Injection table and in the GCC serve the dual role of acting as beam blocks, preventing laser light from entering the Laser Bay, propagating into the amplifier chain or into the target chamber. The calorimeters located on the UVDP tables measure all three wavelengths, ultraviolet (351 nm), green (527 nm), and remaining infrared (1053 nm), that would otherwise propagate toward the target. Auxiliary full-aperture calorimeters may also be mounted on the Short-pulse Polarizer enclosure of Beamlines 1 and 2 to measure the energy of retro-reflected light from the target.

The calorimeters are a volume absorbing design, meaning that they measure the temperature rise of an absorbing glass (NG-9) heated by the incident laser beam. As the heat from the absorbed pulse is quenched by a heat sink, Peltier devices22 located between the two convert the temperature difference to a voltage (via the Seebeck23 effect) that changes with time. Output signals from the calorimeters are acquired by a dedicated data-acquisition system24 and stored in a temporary database. The data-acquisition system uses a 1-Hz sampling rate scheme that digitizes each calorimeters output signal.

During a laser shot, the calorimeters begin acquiring data when the T-10_Enable OIP25 message is propagated (to acquire a baseline energy) by an analog-to-digital converter from the Sampling Head. A generic analog-to-digital module (GADM) records the signal. The laser pulse arrives at T-0 creating a rise in temperature on the absorber glass. The temperature change is converted to a voltage and the Sampling Head reads this voltage as a function of time. When sufficient data has been acquired the calorimeter software, syscal, halts further data acquisition. The software26 then retrieves the raw data from the GADM(s) and stores it in a temporary database as a raw HDF27 file. 19 Full-Aperture Calorimeter Technical Requirements, D-AB-R-031 20 Infrared Full-Aperture Calorimeter System, D-AB-R-035 21 Gentec E Calorimeter SK-8442C, D-AB-C-094 22 Thermocouples made of high performance crystalline semiconductor material 23 See for example: http://en.wikipedia.org/wiki/Peltier-Seebeck_effect 24 For details on the calorimeter control system, see C-AM-M-028 25 OMEGA Intercommunication Protocol 26 syscal software resides on the Beamlines Executive computer (Pluto) in the EP Control Room 27 Hierarchical Data Format



On operator request, the syscal software retrieves the raw data and calculates the total

energy by integrating the area under response curve to the 1/e time of the signal decay. This technique integrates the calorimeter signal during the rise time, peak, and initial decay. The remaining energy is estimated by fitting the heat loss of the calorimeter signal as a single exponential function. The remaining energy is predicted from the exponential time constant and added to the numerical integration to determine the total energy in pulse. Establishing a zero-signal base line by sampling prior to the laser shot enables the subtraction of calorimeter instrument drift errors. The software reduction detects, corrects (by bypassing or linear averaging) and reports minor errors in the raw data. If too many errors are detected, a critical failure is reported to the operators and the trace is not reduced. The resultant energies are stored in the ep_experimental database for future retrieval.

The full-aperture calorimeters are the primary standard used to determine the energy in the laser system. As such, there is a high demand placed on the accuracy and precision of the measurement. The calorimeters incorporate heaters as a means of full-scale signal calibration via a precision electrical stimulus. The data-acquisition system provides for in-situ calibration of each calorimeter. Electrical calibrations are performed regularly. The electrical calibration process occurs with the calorimeters in their “shot position” and shuttered from all propagating laser sources. A series of electrical energies delivered from a calibrator card are applied to the electrical heaters mounted to the back of the absorber glass. Each calorimetry stage has a different set of calibration energies based upon the calorimeter operating range and nominal optical energy received at that stage. All electrical calibrations are performed starting with the lowest energy in the set with an hours ‘rest’ between each calibration run to allow the absorber glass to return to ambient room temperature. Heater temperature is not expected to rise more than 10 degrees during the highest calibration energy. Once the electrical calibration series is complete, the raw data is reduced and compared against reference data. If necessary, the calibration factors stored in the database can be updated to recalibrate a calorimeter. Each calorimeter's calibration is thought to be highly stable, and thus the expectation is that it is rarely necessary to change a calibration factor.

6.6.2 Energy Diagnostic

The Energy Diagnostic28, measures on-shot Laser Sources and Beamlines IR energies. Integrating spheres are positioned at beam pickoff points throughout the laser system and optical fibers connect them to the ED instrument located in the Laser Sources Bay. An overview of the system is shown in Fig. 6.6.1. One ED instrument supports a network of up to 100 locations. The general specifications for the ED are listed in Table 6.4.

The integrating sphere design accepts a 5-mm input beam into a 1-in diameter integrating

sphere. Commercially available multi-mode, 300-µm-core, step index optical fiber couples the sphere to a Dalsa 1M30 CCD camera in the ED via a fiber stub.

28 Energy Diagnostic Requirement, T-AD-R-002



Figure 6.6.1 Overview of ED System. Light sampled at a pickoff point in the system is directed to an integrating sphere fiber coupled to a fiber stub bonded to the sensor chip of a Dalsa 1M30 CCD camera. The ED’s 2-dimensional 10 x 10 pixel array measures energies from a 100 different locations via the fiber network using a single instrument. (G7204J1)

Table 6.4 Omega EP Energy Diagnostic specifications Requirement Specification Input beam pulse width 1ps-10 ns Energy wavelength range 500 to 1100 nm Dynamic energy range 20 µJ to 200 µJ Number of sampling locations 100 Ability to capture ‘on-shot’ data yes Data acquisition rate 5 Hz Repeatability ‘shot-to-shot’ 1%



Signal processing is accomplished via the ED software29. The software runs on a PC-based server (tarvos) and the ED-client runs on any Unix-based networked workstation. Energy is proportional to the amount of charge produced from light incident on the 2-dimensional array of CCD pixels. The raw data is the amount of charge collected on each channel. The number of photons counted is quantified using the sum of “counts per pixel” and then combining the number of pixels within the software-defined channel. The signal resulting from photons incident upon any particular channel is calibrated to an absolute measure of the energy in the beam from the calorimetry data.

Data is acquired at 5 Hz or ‘on-shot’ as the software is an OIP client of the Laser Sources and/or Beamlines Executives. Timing signals (5 Hz and 0.1 Hz) are provided by the Hardware Timing System (HTS). Using calibration constants, the raw data is converted to absolute energy. The client gui30 displays the channel energies at an Operations console with a 1 Hz refresh rate. The raw “on-shot” data is saved to a temporary database in an HDF format file that can be further processed and retrieved using the OMEGA Data Viewer (ODV) software. An example gui showing 15 channels from the Laser Sources area is illustrated below.

Fig. 6.6.2 Operations console gui of the ED software showing the pickoff points for the Laser Sources short-pulse section of Source 1. 6.6.3 Harmonic Energy Detector

The HED31 measures the IR, green, and UV energy output after the FCC’s. To make this measurement, each beamline is configured with an uncoated pick-off located on its UV 29 EP Energy Diagnostic Software Requirements, C-AM-R-001 30 Graphical User Interface 31 Harmonic Energy Detector Upgrade FDR, D-AB-M-118



Diagnostic on-shot table (see Fig. 6.3.4). The 4% reflection from this pick-off is down collimated and reflected into a 2-in integrating sphere. The fiber optic output cable transports the optical signals to the HED instrument located in Diagnostic Bay 1. Each of the 4 beamlines provides a fiber output to the HED. A schematic of the optical spectrometer is shown in Fig. 6.6.3.

Fig. 6.6.3 is drawing of the Harmonic Energy Detector showing its internal components. The dispersion is generated by a refractive/reflective wedged meniscus element that also serves as the secondary mirror in the Öffner triplet relay system.

Each fiber input to the spectrometer is imaged onto a 2-dimensional charged coupled

device (CCD) camera32. The dispersion of the spectrometer is very low so that the three images of the fiber are displaced by slightly more than their diameters. The fibers input into the HED spectrometer are arranged in rows and columns with the appropriate spacing to fill the 2-D area of the CCD sensor with fiber images. Software segments the images into square regions centered on each fiber image. The pixel values in these image segments are summed and reduced using predetermined calibration factors to determine the energy associated with each image.

The HED’s must be calibrated using the calorimetry system. To do this a series of six shots are taken into the calorimeters, with the HED’s also acquiring. During these shots, the crystal tuning of the FCC’s is changed so that on two shots the system produces mostly UV, on two shots it produces IR and green (i.e. the tripler is detuned) and on two shots it produces IR 32 Princeton Instruments PIXIS 16-bit CCD camera with a back-illuminated, UV-enhanced, 512x512 pixel E2V chip



only (i.e. the doubler and tripler are detuned). By doing a multiple linear regression of the HED data to the calorimetry data, the HED calibration constants can be determined. An Excel spreadsheet accesses the calibration data from the database, performs the data analysis and calculates the calibration factors.

Signal processing is accomplished via the HED software. The software runs on a PC-based server (fritch) and the HED-client runs on any Unix-based networked workstation. Energy is proportional to the amount of charge produced from light incident on the 2-dimensional array of CCD pixels. The raw data is the amount of charge collected on each channel. The number of photons counted is quantified using the sum of “counts per pixel” and then combining the number of pixels within the software-defined channel. The signal resulting from photons incident upon any particular channel is calibrated to an absolute measure of the energy in the beam from the calorimetry data.

Data is acquired ‘on-shot’ and the HED software is an OIP compatible client of the Beamlines Executive. Using calibration constants, the raw data is converted to absolute energy. The client gui displays the channel energies at an Operations console. The raw “on-shot” data is saved to a temporary database in an HDF format file that can be further processed and retrieved using the OMEGA Data Viewer (ODV) software.

6.6.4 Near- and Far-Field Scientific Cameras

The near-field33 camera provides a detailed, quantitative measurement of the spatial profile and the far-field34 camera provides a detailed, quantitative measurement of the focal spot. The NF cameras are calibrated against the calorimeters to provide additional on-shot energy measurements. Dedicated cameras are found on the Injection, IRDP, SPDP and UVDP tables of each beamline.

Solid-state, CCD based, SI-800 cameras procured from Spectral Instruments™ are used extensively within the laser system as the energy detectors of choice35. These 16-bit cameras use an E2V Technologies™, 1k x 1k sensor (47-10) cooled to sub-ambient temperatures to improve dynamic range. There are two configurations: 1) with the CCD mounted face up (front lit) the camera is used for IR imaging; 2) when mounted inverted (back lit) the camera is used for UV imaging.

The cameras are interfaced to a PCI compatible frame grabber located in a PC and are triggered, on-shot, by the HTS. The commercial software is made OIP compliant and is a client of the Beamlines Executive.

33 The (SPDP) Near Field Technical Requirements Document, D-AA-R-010 provides a representative set of requirements for all the NF cameras. 34 The (SPDP) Far Field Technical Requirements Document, D-AA-R-011 provides a representative set of requirements for all the FF cameras. 35 The SI-800 camera is also the detector of choice in the streak camera applications such as that used for the (IR) ROSS, UV-ROSS and U-ROSS. See section 6.9.1.



The cameras have an evacuated ‘snout’ that has a wedged window to protect the bare CCD that prevents fringing and suppresses stray light reflections. Each camera requires a separate power supply and cooling unit. 6.7 Wavefront Diagnostics A Wavefront Control System is used with both short- and long-pulse beams to meet top-level focal-spot requirements by correcting wavefront errors. The laser system requires this adaptive optic to meet on-target intensity and repetition-rate requirements by correcting aberrations due to

• optics material and fabrication tolerance, • laser-glass, on-shot, prompt-induced distortion, • thermal distortions in the amplifiers from previous shots, • alignment errors, • gravity-induced sag in optics, • coating-induced distortion in optics, and • mount induced distortion.

Adaptive optics are used to correct wavefront errors created by the optics and the laser amplifiers. These adaptive optic systems are called Wavefront Control Systems and are comprised of a deformable mirror and a Shack–Hartmann wavefront sensor (SHS). These devices work in concert with their control hardware36 and software37 and to measure and modify the laser pulse wavefront. Each of the four beamlines, both pulse compressors in the GCC, and each parabola alignment diagnostic has a WCS. Reference (i.e. flat) wavefronts are provided by the cw IRAT, SPDP, or PAD lasers. The WFS is comprised of a lenslet array and CCD camera. The arrangement of lenslets maps to the layout of the DM actuator spacing. Based on 39-actuator design of the DM, the optimal number of lenslets is 77, shown in Fig. 6.7.1. The CCD camera (Basler 102K) is connected to a frame grabber and computer using a UNIX operating system. Each of the six DM’s is associated with a Mirror Surface Control (MSC) that controls the position of the 39 actuators upon command from the WCS Coordinator software. A WCS Coordinator manages the operation of a single WCS and is an OIP client of the Beamlines Executive Software. The beamline (cavity) DM is located at the north end of the main amplifier cavity and the SHS is located on the IRDP table. The IR DBS reflects a fraction of light exiting the TSF back through the TSF to the SHS, where the wavefront is measured. The WCS calculates the conjugate wavefront and then deforms the DM to this shape. The wavefront of the subsequent laser pulse is thus flattened or altered to a prescribed shape, resulting in improved focusability. For simple wavefront curvatures, up to +/-12 waves of correction are possible for the double-pass-reflected wavefront.

36 [WCS-Coordinator, Mirror Control System (MCS) 37 Shack–Hartmann Sensor Service (SH-SS)]



Two DM’s are located in the GCC at the outputs of the upper and lower pulse compressors to compensate for residual wavefront errors associated with the optics of the short-pulse optical transport chain. Their corresponding SHS’s are located on the SPDP table or are in the PAD in the target chamber.

Figure 6.7.1



The Shack–Hartmann wavefront sensor reconstructs the input wavefront shape by measuring the local slopes at 77 points across the beam aperture. This information is sent to the WCS coordinator that calculates the delta’s of these centroids from a reference wavefront array to determine a new voltage for the deformable mirror actuators. The reconstructed wavefront is then measured and adjusted again and, over a series of iterations, the wavefront is corrected. Each iteration can be completed within one second. The WCS can correct for static and dynamic wavefront errors if allowed to run continuously. The WCS Coordinator software is the core of the control system serving as the intermediary between the Shack Hartmann Sensor Service (SH-SS) and MCS. It coordinates wavefront-processing algorithms, calculates voltage deviations for “automated” mode of operation using the driver matrix, performs calibration algorithms, and creates driver matrices. It also monitors the wavefront in closed-loop or open-loop modes of operation and is an OIP client of the WCS sub-executive. The SH-SS provides centroids from the SHS-CCD camera to the WCS Coordinator by managing image acquisition, calculating centroids, and processing region of interest functions The MCS applies voltages to the DM actuators by accepting voltage deviations produced by the control algorithms in the WCS-Coordinator. The MCS applies new voltages to actuators via a Versa Module Eurocard (VME) driver and ensures that actuator voltages stay within specified tolerances. When a DM is actively correcting the wavefront, each WCS-Coordinator is receiving wavefront information from a SHS at ~1 Hz. After wavefront processing, the MCS is provided actuator voltages deforming the mirror to the desired shape. Prior to the start of active correction, the system needs a reference wavefront and a response matrix. The reference wavefront can be a measured wavefront (from the SHS) or a calculated wavefront (from mathematical models) and is the desired wavefront at the SHS. The response matrix is a way of relating movement of a single actuator on the DM to measured responses on the wavefront sensor. Using measurements from the WFS and the driver matrix (inverse of the response matrix), the wavefront correction system can drive the wavefront measurement at the wavefront sensor to that of the reference. The system can correct for static and dynamic wavefront errors in the laser system when it is allowed to run continuously. WCS hardware and software requirements and implementation details are described in the Technical Requirement Documents, C-AQ-R-006 and C-AQ-R-004, respectively. 6.8 Spectral Measurements The broadband spectral characteristics of the laser pulse are controlled by the pulse stretcher in Laser Sources. Spectrometer pickoffs on the Injection and SPDP tables provide comparative information that determines how the pulse spectrum has been altered during beam transport. Three Acton SpectraPro™ spectrometers, purchased from Princeton Instruments, measure the spectral characteristics of the short-pulses from pickoffs at 5 points in each of Laser Sources 1 and 2, the Injection tables and the third spectrometer, located on the SPDP table, measures the



gain narrowed and ‘compressed’ output pulse spectra from the two pulse compressors located in the GCC. The signal generated by each pixel of the spectrometers CoolSNAP™ CCD camera corresponds to the intensity of light in the wavelength band of the incident pulse. The optical design of the input for the spectrometer allows multiplexing up to six different channels onto its CCD. The spectrometers CCD readout is timed to be synchronous with the 5 Hz laser source pulse. The software38 resolves the 6 individual inputs as separate channels and displays spectral lineouts. On-shot data is also obtained by the OIP compatible software. 6.9 Temporal Diagnostics The SPDP includes an Ultrafast Temporal Diagnostic suite of instruments to measure the required range of pulse widths (0.5 to 240 ps) of the compressed pulse since no single diagnostic is capable of covering the entire range.

An ultrafast streak camera is useful for pulses between ~8 and ~240 ps. For shorter pulses (0.5 to 12 ps), a single-shot autocorrelator (SAC) or a time-expanded single-shot autocorrelator (TESSA) are employed. The advantage of this approach is that overlapping measurement windows are possible, providing additional confidence in the pulse width measurement, reducing the possibility of causing damage to the laser. 6.9.1 ROSS Streak Cameras Streak cameras measure the temporal pulse shape and provide beam timing information. The LLE designed ROSS39,40 (Rochester Optical Streak System) (fig. 6.9.1) simultaneously measures up to eight input signals and additional input for the fiducial laser calibration signals. Streak cameras utilize a conversion of photons to electrons and the passage of electrons through a sweeping electro-magnetic field to translate temporal amplitude into spatial amplitude that can be measured on a CCD camera after conversion back to electrons (via a phosphor screen). Controlling the sweep speed controls the temporal resolution. The IR ROSS camera located in Laser Sources is equipped with four sweep speeds: 30ns, 12ns, 6ns, and 2ns. An optical calibration module (OCM) generates precision optical pulse trains with repetition rate of either 500 MHz or 2.0 GHz allowing precision calibration at all of the sweep speeds.

38 Laser Sources Spectrometer Control System Software Requirements, T-AD-R-004. 39 P.A. Jaanimagi, R. Boni, D. Butler, S. Ghosh, W.R. Donaldson, and R.L. Keck, “The Streak Camera Development Program at LLE.” in 26th International Congress on High-Speed Photography and Photonics, edited by D.L. Paisley, S. Kleinfelder, D.R. Snyder, and B.J. Thompson (SPIE, Bellingham, WA, 2005), Vol. 5580, p. 408. 40 R. Boni and P. Jaanimagi, “System for Photometric Calibration of Optoelectronic Imaging Devices Especially Streak Cameras,” U.S. Patent No. 6,642,499 (19 July 1999).



Fig. 6.9.1 The Rochester Optical Streak System (ROSS) is a comprehensive diagnostic system with auto-focusing and self-calibration capability. Fiber coupled inputs from dual diffuser fiber launchers (DDFL) signals are provided from pickoffs in each IRDP in the Laser Bay. The fiber coupling achieves high coupling efficiency and relatively high tolerance of spatial profile and/or pointing variation using a 200 µm multimode fiber. A UV-ROSS camera is located in Diagnostic Bay 1 and provides pulse width measurement of the frequency converted UV-pulses from DDFL’s from each UVDP. A special UV compatible fiber conveys the signals to the camera. An ultrafast streak camera41, located on the SPDP table, provides measurement of the temporal pulse shape and timing for both short-pulse beamlines42. The two compressor outputs are spatially multiplexed onto the UROSS, but cannot be measured simultaneously because of the timing shift caused by different optical path lengths. While similar to the IR3 streak camera in OMEGA, there are some key differences as the UROSS measures pulses as short as 8-ps. To accommodate sweep durations less than 500-ps, a precision trigger is provided from the

41 See OMEGA EP Ultrafast Temporal Diagnostics Requirements, D-AA-R-002 42 A second streak camera of the same design called PDF (pretty-darn-fast) ROSS is located in the Laser Development Laboratory.



Precision Optical Trigger and Timing System43 (POTTS) that also supplies an optical ultrafast fiducial pulse train to calibrate the streak camera's time base. Key elements of the UROSS include

• SI-800 camera using of a 2k x 2k E2V CCD chip • PC with frame grabber • P820 streak tube • sweep module • two free-space coupled optical input channels • inputs for two optical fiducial channels from POTTS • input for electronic trigger signal from POTTS • optical system for coupling each beam to the streak tube photocathode

and a subset of the UROSS specifications are listed in Table 6.5.

Table 6.5 Ultrafast streak camera requirements. Feature Requirement

Pulse width (max) 300 ps Pulse width (min) 8 ps Max. tolerable error in pulse-width measurement ± 10% Trigger source(s) POTTS Number of fiducial channels 2 Source for time-base calibration POTTS fiducial pulse trains Sweep speed(s) 500 ps, 2 ns

ROSS software is provided by Sydor Instruments and runs on a local PC. The PC controls the basic function of the ROSS as well as acquisition and analysis of the CCD image. The software allows the user to observe pulses and make and store temporal measurements. The software is made OIP compliant to permit on-shot data collection. 6.9.2 Scanning Autocorrelator A Scanning Autocorrelator is used to cross-calibrate short-pulse pulse width with TESSA during PRESHOT set-up. It is a simple instrument that is easy to align and calibrate and is less sensitive than TESSA to beam and wavefront quality. In a SAC the input pulse is separated into two different pulses and then angularly mixed in a nonlinear crystal. The frequency conversion in the crystal is deterministically related to the intensity of light such that small changes in the relative timing will produce a 2nd harmonic signal proportional to the convolution of the pulses. The SAC must change the relative timing of the two pulses and map out the convolution as a function of timing shift. From this, the temporal pulse length can be determined mathematically if one assumes the pulse shape. Key elements of the SAC include

• photodiode and oscilloscope with sample-and-hold electronics

43 Precision Timing and Triggering System (POTTS), D-AA-R-031 and Precision Optical Triggering and Timing System Software Control Requirements, C-AM-R-006



• optical system for focusing two replicas of the pulse onto a nonlinear crystal • PC with National Instruments data acquisition board • motorized delay stage with Newport controller, and • trigger signal from HTS.

and a subset of the SAC specifications are listed in Table 6.6.

Table 6.6 Scanning Autocorrelator Specifications Requirement Value Achieved

On-shot acquisition No Time window tbd Pulse width (max) 30 ps Pulse width (min) 0.2 ps Max. tolerable error in pulse width ± 10% Time resolution xx ps/pixel Image plane at autocorrelator crystal Conjugate to G4 Input beam polarization s-polarized

6.9.3 TESSA The Time-Expanded Single-Shot Autocorrelator44 provides a measurement of the temporal pulse widths of short-pulse beams between pulse widths of 0.5 to 12 ps by measuring the second-order intensity autocorrelation of a single pulse. Since it does not make direct measurements of pulse shape, subsequent analysis is needed to "de-correlate" the pulse width from the measured autocorrelation. TESSA is a single channel device limiting measurements to a single beamline. The instrument is located on the SPDP table and a kinematic mirror selects either the upper or lower pulse compressor diagnostic output. A subset of the TESSA specifications is listed in Table 6.7.

Table 6.7 TESSA Specifications Requirement Value Achieved

On-shot acquisition Yes Time window 50 ps Pulse width (max) 15 ps Pulse width (min) 0.22 ps Max. tolerable error in pulse width ± 10% Time resolution 0.036 ps/pixel Image plane at autocorrelator crystal Conjugate to G4 Input beam polarization p-polarized

44 Ultrafast Temporal Diagnostics, D-AA-R-002 and Ultrafast Temporal Diagnostic Software Requirements, C-AM-R-003.



The essential elements of TESSA include; • an optical system to create ‘replicas’ of the compressed sample pulse that consists of a

grating, a cylindrical and a spherical lens • frequency doubling the replicated pulses via a KDP crystal • an optical relay to focus the 2ω pulses onto an 8-bit Basler A102K video camera • a Sun computer with VigraVision™ video frame grabber • a manually operated internal path length time delay stage for time-axis calibration • pointing camera for alignment

and the opto-mechanical design of TESSA is shown in Fig. 6.9.2.

Basler CCDKDP

crystal

PointingCameraGrating

Input pulse

Delay stage

Cylindrical lens

Spherical lens

Periscope

Fig. 6.9.2 The optical elements of TESSA are illustrated. TESSA uses a grating to provide a ‘pulse-front tilt’. A beam splitter and time delay stage create the second (delayed) pulse front that are subsequently overlapped in the second harmonic generating crystal (KDP45). An aperture blocks the collinear second harmonic generator (SHG) and a filter removes the unconverted light. The delay stage is used to calibrate the pulse width. TESSA software and controls are based upon client/server applications. The server runs on any Sun workstation and collects data at 5 Hz (and on-shot) from the VigraVision™ frame grabber. The service is linked to the ep_experimental database to read calibration data and store reduced on-shot data. The client provides the user interface gui’s and displays the ‘raw’ image and the processed autocorrelation image. The client is OIP compliant and also stores shot related, PRESHOT related and non-shot related images to the database. Example gui’s of the client software are shown in figure 6.9.3.

45 Potassium dihydrogen phosphate



TESSA Image gui TESSA Autocorrelation gui

Fig. 6.9.3 The TESSA software client is an OIP client of the Beamlines Executive. The Image gui displays the signal intensity vs. image position on the 8-bit CCD camera. The Autocorrelation gui displays the results of the algorithm that calculates the autocorrelation trace. 6.10 Focal Spot Diagnostics Each of the two short-pulse beamlines uses a focal spot diagnostic46 suite of instruments to predict on-shot target irradiance. The quantity of interest is the 80% encircled energy radius, R80. Since direct measurement of the high-energy focus of the main beam during target shots is impractical, FSD measures a low energy sample beam from the diagnostic mirror (SPHR2) in the GCC.

The data collection and analysis requires the use of a number of subsystems and is a multistep process requiring both measurements made in PRESHOT and on-shot. Supporting subsystems include the following.

1. FSD Wavefront Sensor: Makes calibrated measurements of the near-field wavefront. 2. SPDP Source: Used during the pre-shot calibration phase when quantifying the

transfer wavefront (the difference between aberrations in the main path (to TCC) and sample path to the wavefront sensor).

3. Parabola Alignment Diagnostic Source: Used during the pre-shot calibration phase when quantifying the transfer wavefront.

4. SPDP NF-CCD: Measures the on-shot near-field fluence of the sample beam. This is needed in order to calculate the on-shot focal-spot irradiance in the target plane.

5. SPDP FF-CCD: Measures the on-shot far-field fluence of the sample beam; checks the validity of the near-field wavefront measurement.

6. CAM3: This full-aperture compressor alignment mirror is inserted during the pre-shot calibration phase.

7. Focal Spot Microscope (FSM): This PAD-mounted subsystem validates the FSD measurements by directly measuring of the focal-spot irradiance at low energy.

46 OMEGA EP Focal Spot Diagnostics Technical Requirements, D-AA-R-015



FSD uses data from these diagnostics, along with PRESHOT calibration data called the ‘transfer wavefront’, to calculate47 the on-shot energy density in the target plane and R80. A schematic flow diagram for processing this data is shown in Fig. 6.10.1.

Figure 6.10.1 Overview of FSD measurement and calculation process showing relationship between PRESHOT and on-shot data collection.

The measurements are not fully automated and require the System Science group to perform the analysis. The FSD wavefront sensor48 (HASO128) measures the wavefront of the beam and calculates the resultant focal spot. Direct measurements of the focal spot are made using the Focal Spot Microscope (FSM) that is PAD based and located in a TIM. The FSD wavefront measurement in combination with the FF amplitude measured by the SPDP camera is combined to produce a calculated focal spot. The two focal spots can be compared (Fig. 6.10.2) and are seen to be similar.

47 FSD Software Requirements document, C-AM-R-010. 48 OMEGA EP Focal Spot Diagnostic Final Design Review, D-AA-M-155



Figure 6.10.2 The wavefront based focal spot measurement from FSD (left) is compared to the measured focal spot from the FSM (right). A histogram (Fig. 6.10.3) of the encircled energy is plotted vs the radius of the focal spot to determine the radius at which 80% of the encircled energy is obtained.

Figure 6.10.3 A histogram of the encircled energy vs. the focal spot radius is used to determine R80.



6.11 Acronyms Alignment Sensor Package ASP Beam Splitter BS Cavity Spatial Filter CSF Charge Coupled Device CCD Compressor Alignment Mirror CAM Continuous wave cw Deformable Mirror DM Diagnostic Beam Splitter DBS Dual-diffuser Fiber Launcher DDFL Energy Diagnostic ED Far-field FF FCC Alignment Diagnostic FCCAD Fold Mirror FM Focal Spot Diagnostic FSD Focal Spot Microscope FSM Field of View FOV Frequency Conversion Crystal FCC Generic Analog-to-Digital Module GADM Grating Compression Chamber GCC Graphical User Interface GUI Hardware Timing System HTS Harmonic Energy Detector HED Hierarchical Data Format HDF Hertz Hz Inch in Infrared Diagnostic Package IRDP Infrared Alignment Table IRAT Infrared IR Joule J Kilo k (or K) Potassium dihydrogen phosphate KDP Laboratory for Laser Energetics LLE Micro µ Mirror Control System MCS Millimeter mm Neodymium:Yttrium fluoride Nd:YLF Near-field NF Nanometer nm Nanosecond ns Optical Calibration Module OCM Off-axis Parabola OAP OMEGA Data Viewer ODV OMEGA Intercommunication Protocol OIP



Parabola Alignment Diagnostic PAD Personal Computer PC Peripheral Component Interconnect PCI Periscope Mirror Assembly PMA Pinhole PH Precision Optical Timing and Triggering System POTTS Picosecond ps Relay Plane RP Rochester Optical Streak System ROSS Region of Interest ROI Scanning Autocorrelator SAC Shack-Hartmann Sensor SHS Shack-Hartmann Sensor Service SH-SS Second Harmonic Generator SHG Spatial Filter SF Short-pulse High Reflector SPHR Short-pulse Diagnostic Package SPDP Square sq Target Area Structure TAS Target Chamber Center TCC Time Expanded Single Shot Autocorrelator TESSA Tiled Grating Assembly TGA Ten-inch Manipulator TIM Target Mirror Structure TMS Transport Spatial Filter TSF Ultraviolet UV Ultraviolet Alignment Table UVAT Ultraviolet Diagnostic Package UVDP Ultrafast Temporal Diagnostic UTD Ultrafast Streak Camera UROSS Wavefront Sensor WFS Wavefront Control System WCS

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