Online Photon Beam Diagnostics

R. Treusch, HASYLAB@DESYVUV-FEL Users Workshop

on Technical Issues of First ExperimentsOnline Photon Beam Diagnostics

Online Photon Beam Diagnostics

Rolf TreuschHASYLAB @ DESY



Contents

• Required photon beam parameters

• Results from FEL operation at TTF1

• Higher harmonics & spontaneous radiation

• FEL Photon beam diagnostics



Required photon beam parameters

Spectrum: spikes ⇒ modes, pulse lengthwavelength ⇒ electron energytuning for experimental needs

Intensity: SASE optimisation, statistics,saturation, normalisation

Position & Profile: stabilisation and focusing

Timing: time resolved experiments(pump-probe ⇒ S.Düsterer)

Coherence: focusing, imaging



Results from FEL operation at TTF1

• FEL pulse energy and shot to shot fluctuations• Gain curve• Spectra• Double slit diffraction patterns / FEL coherence• Wavelength range• Summary of saturation parameters



FEL pulse energy / shot to shot fluctuations

Thermopile data ↑

Statistics recorded with MCP →



Gain curve (at 95 nm)

gain length: 0.67 m

Intensity statistics at z = 9 m

σE=0.62M =2.6

steerer magnets in the undulatorwere used to control the interaction length between e- and radiation,i.e.to switch off SASE beyond a certain position in the undulator



Spectra of single FEL pulses

Short pulse(≤100fs)

Long pulse(200fs)

FEL pulse lengthvariation via bunchcompressor settings



Diffraction of 95nm FEL radiation observed by a CCD cameraon a Ce:YAG crystal 3m behind the slits (near zone)

parallel slits, each 2mm long, 200µm wideseparation 1mm

crossed slits, 100µm wide, 4mm long

Double slit diffraction patterns

Ph.D. thesis Rasmus Ischebeck



FEL coherence

0.5 1 2 30

0.2

0.4

0.6

0.8

1

Double slit separation [mm]

Tran

sver

se C

oher

ence

y /

mm

-3

-2

-1

0

1

2

3

0

0.2

0.4

0.6

0.8

1

9 10 11 12 13 14

Undulator length [m]

FEL saturation

Ph.D. thesisRasmus Ischebeck

• almost(!) full transverse coherence• limited long. coherence (100% via seeding)

2. Fourier transform of “spiky” spectrum Longitudinal coherence

1. Transverse coherence:



VUV FEL wavelength range

45 46 47 48 49 50 51 520.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Inte

nsity

[Cou

nts]

Wavelength [nm]

2nd Harmonic

Intensity of 2nd harmonic is about 0.1 % of the fundamental

Average of 3100 pulses

Shortest wavelength ever generated by an FEL !

1st Harmonic



Summary of FEL saturation parameters at TTF1

Wavelength 82-125 nmPulse energy 30-100 µJPulse duration 30-100 fsPeak power 1 GWAverage power up to 5 mWBandwidth (FWHM) 1 %Spot size (FWHM) 250 µmAngular divergence (FWHM) 260 µmPeak brilliance up to 1028 *Averaged brilliance up to 1017 *

* = phot./sec/mrad2/mm2/(0.1% BW)



Higher Harmonics & Spontaneous Radiation

1. SASE (FEL) higher harmonics:

• 2nd and 3rd harmonic at 1% to 1 per thousand

level (see before)

• higher (if needed) with special radiators (future!!)



2. Spontaneous undulator radiation:

0 100 200 300 400 50010-1

100

101

102

103

104

105

106

107

108

109

1010

200 µrad aperture 500 µrad aperture

Phot

ons/

(s ∗

0.1

%bw

)

Energy [eV]

1. 2. 3. 4. … harmonic



Spontaneous undulator radiation power in circular aperture(assuming 1nC bunch charge, 1ps duration / bunch length, 1st harm. @ 30 nm)

a) 200µrad in 50m distance (typical detector size): 6 x 103 W 6nJ

b) 500µrad in 50m distance (typical beamline acceptance): 3.6 x 104 W 36 nJ

c) No aperture (fully integrated flux):7 x 105 W 700 nJ

FEL in saturation:3GW for 30fs 100 µJ

even without energy discrimination 3-4 orders of magnitude higher



FEL Photon beam diagnostics

• cover full dynamic range: ~ 7 orders of magnitudefrom spontaneous emission to SASE in saturation

• on-line detectors for single-pulse measurements(response < 100ns)

• low degradation under radiant exposure in the VUV

• ultra-high vacuum compatibility

• assembling under clean room conditions

Detector challenges:



• Ce:YAG and PbWO4 fluorescent crystals• PtSi photodiodes (calibrated by PTB = German

institute of standards)• YBa2Cu3O7-δ based thermopiles (= fast bolometer)• Au wire (50µm diam.) + calibrated MCP• gas ionisation detector (calibrated by PTB)• grating monochromator + ICCD (gate down to 5ns)• fast streak camera (timing)• autocorrelators / cross-correlators

Detectors

} S.Düsterer



15

10

5

3

1

0.5

Detectors

PtSi photodiode(1nJ < Epulse< 1µJ)

Apertures

Pulse energy, beam position + profile,degree of transverse coherence

Detector unit F1 for VUV FEL at TTF2

FELThermopile (Epulse > few µJ)

Ce:YAG crystal

Double slitsin thin foil(diffraction)

rear front



+ radiation hard+ good for low intensity:

spontaneous emission to ~1 µJ pulse energy(depending on wavelength)

+ calibrated by PTB- sensitive to higher harmonics- cannot be used for high intensities (saturates)

200 400 600 800 1000 1200 1400

-0,3

-0,2

-0,1

0,0

Time [ns]

Sign

al [V

]

PtSi photodiodes



TTF1 data Sept. 2001

+ good for high intensity: ~10 µJ to saturation+ independent of wavelength_ cannot be used for long pulse trains_ laser ablation at high power density

Thermopile detector



MCP Diagnostics Unit

+ large dynamic range (~7 orders of magnitude)+ can be scanned to measure beam position and profile_ depends on beam position_ will not survive long pulse trains_ the wire produces unwanted diffraction

O.Brovko, A. Fateev, M.Yurkov et al., JINR Dubna

MCP 1MCP 2



hν

Ions

e-

Differentialpumping

Interactionvolume

Faraday cup

Drift tube

10-9 hPa 10-5 hPa

Single photoionisation:

N = Nph x n x σ x l

N = number of electrons or ionsNph = number of photonsn = target densityσ = photoionisation cross sectionl = length of interaction volume

Single photoionisation:

N = Nph x n x σ x l

N = number of electrons or ionsNph = number of photonsn = target densityσ = photoionisation cross sectionl = length of interaction volume

Online monitor of single-pulse FEL intensity:+ transparent+ wide dynamic range (spont. to saturation)+ independent of beam position+ no saturation effects+ 6 nm (FEL limit) < λ < 93 nm (Xe abs.edge)+ absolute calibration (~10%)

Collaboration with PTB, Berlin, and Ioffe Institute, St. Petersburg

Gas ionisation detector



Split electrodes

hν

Gas ionisation detector as beam position monitor



Double slit diffraction patterns

⇩

setup for FELdiagnostics at TTF1



Overview: Photon diagnostics at VUV FEL

Gas ionisation cell(intensity and position)Gas absorber

(beam attenuation)

VLS Spectrometer(planned)

Streak camera(timing)

intensity and position *(TTF1 units + MCP)

Spectrum *,intensity and position

* = NOT “online”



Diagnostics in the Experimental Hall

Klimaschrank

Opticallaser

BL310 µm

BL220 µm

BL1100 µm

PG2PG1

High resol. PGMmonochromator

VLS GratingSpectrometer

(planned)

Intensity and positionmonitor (gas ionization)

Dipole radiationbeamline

~42m to undulator

Streak Camera

Gas absorber



Availability of VUV FEL diagnostics

? (experimentalstage)

YES Auto-/Cross-CorrelatorsTiming

April 2005YESStreak cameraTimingmid/end 2006YESVLS Grating Spectrom.Spectrum

now(Commissioning)

noGrazing incidencespectrometer (tunnel)

Spectrum

April 2005YESGas Ionisation DetectorIntensity &Position

nowno no

limited

thermopiles,PtSi photodiodes,MCPs

Intensity &Profile

availableOnline diagn.

Detector(s)Parameter



Summary

• Many detectors were chosen based on experience and successful FEL operation at TTF1

• New developments were needed with respect to timing, online beam position andspectrum (“online” = in parallel with experiment)

• Most experiments need online photon diagnostics incl. all beam parameters on a shot to shot basis!!

• We can provide most of these online diagnostics and also help regarding detectors in user chambers



Questions

• Which beam parameters would you need to know?

• Which detectors do you supply yourself?

• Might your detector concepts be useful for others?

• Could you profit from detectors such as photodiodes,

thermopiles or MCPs in your chamber?

Online Photon Beam Diagnostics

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