First Lasing of the ALICE Free Electron Laser

David DunningOn behalf of the ALICE FEL team (Jim Clarke, Neil Thompson,

Mark Surman and Andy Smith), and all the ALICE team

IntroductionOn October 23rd 2010, the ALICE infra-red free electron laser (FEL)

at Daresbury Laboratory was successfully operated for the first time. This is the first FEL of its type to operate in the UK, and the first FEL operating with an energy recovery linac accelerator in

Europe.

• What it is• Motivation • How it was done • Results so far • Future prospects

Why this particular free electron laser?

• A step towards developing a cutting-edge FEL facility for the UK.

• The UK doesn’t have an FEL facility, but has aspirations to develop one:

• The ALICE FEL has allowed the team proposing advanced FELs for these facilities to develop the skills to design, build and operate such a machine.

• The ALICE FEL is an infra-red oscillator FEL - it’s not the first of its kind, but it puts us in a position to seriously consider advanced concepts in a future machine.

• In addition, a free electron laser is kind of the ultimate test for our prototype accelerator facility.

Why free electron lasers?• Light sources have served as a powerful

tool for scientific experiments for decades.

• A free electron laser is an accelerator-based light source with an exceptional combination of properties :– Applicable over a very wide

wavelength range (THz to x-ray demonstrated so far)

– Easily tuneable– High repetition rate– Short pulse– High brightness/peak power– …

• FEL facilities increasingly significant:– FLASH, LCLS

• Enabling new science

Motivation for the ALICE FEL

How an Oscillator FEL works

ELECTRONS

ELECTRONS

Electron bunches arrive with a fixed

repetition rate

The undulator is a periodic magnetic field which causes the electrons to oscillate transversely

and emit synchrotron radiation. The transverse component of the electron velocity allows the exchange of energy between the electrons and

the stored radiation field

An optical cavity stores the emitted radiation. The cavity length is set

such that the cavity roundtrip time matches the repetition time of the

electron bunches

A hole in one cavity mirror outcouples a small fraction of the

stored radiation

The interaction causes the electrons to bunch

at the radiation wavelength, and so

emit coherently

UNDULATORUPSTREAM MIRROR

DOWNSTREAM MIRROR

Key components: electron beam, undulator and optical cavity

ALICE FEL layout

UNDULATORUPSTREAM MIRROR

DOWNSTREAM MIRROR

ELECTRON PATH

Key components: electron beam, undulator, optical cavity + diagnostics…

Electron Beam

UPSTREAM MIRROR

UNDULATOR

DOWNSTREAM MIRROR

ELECTRON BEAM AT FELEnergy 27.5MeV

Bunch Charge 60-80pC

FWHM Bunch Length ~1ps

Normalised Emittance ~12 mm-mrad

Energy Spread ~0.5% rms

Repetition Rate 16.25MHz

Macropulse Duration ≤100µs

Macropulse Rep. Rate 10Hz

BUNCH COMPRESSOR

ELECTRON PATH

UNDULATOROn loan from JLAB where previously

used on IR-DEMO FEL

Now converted to variable gap

PARAMETERS

Type Hybrid planar

Period 27mm

No of Periods 40

Minimum gap 12mm

Maximum K (rms) 1.0

Undulator

Optical CavityOPTICAL CAVITY

Mirror cavities on kind loan from CLIO.

Previously used on Super-ACO FEL

PARAMETERS

Type Near Concentric

Resonator Length 9.2234m

Mirror ROC 4.85m

Mirror Diameter 38mm

Mirror Type Cu/Au

Outcoupling Hole

Rayleigh Length 1.05m

Upstream Mirror Motion Pitch, Yaw

Downstream Mirror Motion Pitch, Yaw, Trans.

UPSTREAM MIRROR

DOWNSTREAM MIRROR

Diagnostics for IR radiation

LASER POWER METER

FEL BEAMLINE TO DIAGNOSTICS ROOM

SPACE FOR DIRECT MCT DETECTOR

MCT (Mercury Cadmium Telluride) DETECTOR on Exit

Port 1

SPECTROMETER Based upon a Czerny Turner

monochromator

PYRO-DETECTORon Exit Port 2

DOWNSTREAM ALIGNMENT HeNe

FEL Commissioning Strategy

In order for the FEL to operate we needed to co-align:• Undulator axis• Optical cavity axis • Electron beam axis

FEL Commissioning Strategy

OPTICAL

TARGET

OPTICAL

TARGET UNDULATOR

ARRAYSDOWNSTREA

M FEL MIRROR

REFERENCE AXIS

LASER TRACKER

1. Undulator Arrays and two Optical Targets surveyed onto Reference Axis with Laser Tracker

UNDULATORUPSTREAM MIRROR

DOWNSTREAM MIRROR

ALIGNMENT WEDGES

OPTICAL

TARGET

OPTICAL

TARGET

DOWNSTREAM FEL

MIRROR

2. Alignment Wedges and Downstream Mirror aligned optically using Theodolite

CCD VIEWER CAMERAS

3. Downstream Mirror aligned using Upstream alignment laser (HeNe)

ALIG

NM

ENT

MIR

ROR

HeNe

CCD VIEWER CAMERAS

4. Upstream Mirror aligned using Downstream alignment laser (HeNe)

ALIG

NM

ENT

MIR

ROR

HeNe

CCD VIEWER CAMERAS

ELECTRONS

5. Electron Beam steered to Alignment Wedges

CCD VIEWER CAMERAS

POWERMETER

MCT DETECTOR

SPECTROMETER

6. Cavity length scanned looking for enhancement of spontaneous emission, then LASING.

CCD VIEWER CAMERAS

ALICE IR-FEL Commissioning

7 7.5 8 8.5 9-2

0

2

4

6

8

10

12 x 105

Wavelength (m)P(

) (

a.u.

)

x = -1.0 mmx = 0.0x = +1.0 mm

• 2009– November: FEL Undulator installation begins.

• 2010– January: FEL Undulator and Cavity Mirrors installed and

aligned: all hardware in place to start commissioning.– Throughout 2010: FEL programme proceeded in parallel with

installation & commissioning of EMMA, plus THz programme. One shift per day for commissioning. Of available beam-time, FEL programme got 12%.

– February: First observations of undulator spontaneous emission.

• Stored in cavity immediately, indicating transverse pre-alignment reasonable.

• Scanned cavity length but no enhancement (limited to 40pC bunch charge, design was for 80pC).

– May/June: Spectrometer installed and tested. First spectra of undulator spontaneous emission.

• Analysis of spontaneous emission used to optimise electron beam steering and focussing

• Also indicated pre-alignment was acceptable.

17th November 2009

4th February 2010

13th June 2010

27th July 2010

ALICE IR-FEL Commissioning• 2010 (continued)

– June: Strong coherent emission with dependence on cavity length

• Indication of correct cavity length.• BUT NO LASING! Gain not exceeding losses.

– Identified need to:• Reduce losses• Increase gain• Get reliable measure of cavity length

– July: Changed outcoupling mirror from 1.5mm radius to 0.75mm to reduce losses

• Re-gained cavity enhancement but still no lasing

• Installed encoder to get cavity length measurement• Mirror radius of curvature tested, and matched spec.• EO measurements indicated correct bunch

compression.– 17th October: installed a Burst Generator to reduce

laser repetition rate by a factor of 5, from 81.25MHz to 16.25MHz. This enabled us to increase the bunch charge to >60pC and hence increase the gain…

17th October 2010

28th June 2010

23rd October 2010: ALICE FEL First Lasing

First Lasing Data: 23/10/10 Simulation (FELO code)

-5 0 5 10 15 20 250

2

4

6

8

10

12

14

Cavity Length Detuning (m)

Out

coup

led

Ave

rage

Pow

er (

mW

)

-5 0 5 10 15 20 250

10

20

30

40

50

Cavity Length Detuning (m)

Out

coup

led

Ave

rage

Pow

er (

mW

)

• Highest recorded average power = 32 mW– Macropulse length = 100 μs, repetition rate = 0.1 s.– Micropulse repetition rate = 16 MHz, micropulse spacing =

0.0615 μs, i.e. 1626 pulses per macropulse, 16260 pulses per second.

– Energy per pulse = 32mW/16260 = 2 μJ per pulse.– Average power within a macropulse = 32 W– Transmission efficiency of vacuum window=0.7 – so power

from FEL ~1.4x higher.• The FEL pulse duration has been inferred from the

spectral width to be ~1 ps The peak power is therefore ~3 MW.

Results

Further Progress

5 5.5 6 6.5 7 7.5 8 8.50

0.2

0.4

0.6

0.8

1

P(

)(a.u

.)

(m)

g = 16 mmg = 15 mmg = 14 mmg = 13 mmg = 12 mm

6 7 81

1.2

1.4

1.6

1.8

2

Ban

dwid

th (%

)

Wavelength (m)6 7 8

500

600

700

800

900

1000

FWH

M

t (fs

)

Wavelength (m)6 7 8

0.5

1

1.5

2

2.5Pu

lse

Ener

gy (

J)

Wavelength (m)6 7 8

1.5

2

2.5

3

3.5

P Pk (M

W)

Wavelength (m)

FEL Spectra at Different Gaps

The first demonstration of wavelength tuning of the ALICE FEL has been carried out. By increasing the undulator gap the wavelength was continuously tuned from 8.0 μm to 5.7 μm, with lasing maintained throughout the range.

Further ProgressThe FEL radiation has been transported to a diagnostics room, and the first measurements of the transverse profile have been made.

• Improved electron beam set-ups with reduced energy spread and jitter.

• Improve transport of FEL beam to diagnostics room, then full output characterisation.

• Reduced Mirror ROC to improve gain, plus selection of outcoupling hole sizes to optimise output power.

• Plan to run and characterise at two different energies– 27.5MeV (5-8µm) – 22.5MeV (7-12µm)

4 5 6 7 8 9 10 11 12 130

1

2

3

4

5

6

(m)

P peak

(MW

)

27.5MeV, 0.75mm Hole radius22.5MeV, 0.75mm Hole radius27.5MeV, 1.5mm Hole radius22.5MeV, 1.5mm Hole radius27.5MeV, 2.25mm Hole radius22.5MeV, 2.25mm Hole radius

4 5 6 7 8 9 10 11 12 130

1

2

3

4

5

6

(m)

P peak

(MW

)

27.5MeV, 0.75mm Hole radius22.5MeV, 0.75mm Hole radius27.5MeV, 1.5mm Hole radius22.5MeV, 1.5mm Hole radius27.5MeV, 2.25mm Hole radius22.5MeV, 2.25mm Hole radius

Simulation results

ALICE FEL Future Plans

SummaryFurther work on characterising the FEL performance and output is continuing, with lasing now achieved routinely. The successful operation of the free electron laser is a significant achievement for the team, and provides invaluable experience for future FEL facility proposals.

Thanks to everyone who contributed to the project, and thanks to you all for listening!

ALICE FEL Systems Schematic

(OPTICAL

TARGET)

(OPTICAL

TARGET)

POWER METER

MCT DETECTOR

SPECTROMETER

INFRA-RED

ELECTRONS

ALIG

NM

ENT

MIR

ROR

HeNe

ALIGNMENT WEDGES

CCD VIEWER CAMERAS

ALIG

NM

ENT

MIR

ROR

HeNe

ELECTRONS

ALICE FEL Layout

UNDULATORUPSTREAM MIRROR

DOWNSTREAM MIRROR