The ALICE Electron Test Accelerator - Challenges, Achievements, and Future

Plans

Professor Jim ClarkeASTeC, STFC Daresbury Laboratory & Cockcroft

Institute

JAI Lecture, 17th March 2011

Contents

• Introduction to ALICE• Major Subsystems• Experimental Highlights• EMMA• Free Electron Laser• Future Plans• Summary

ALICE

• Accelerators and Lasers In Combined Experiments

• An R&D facility dedicated to accelerator science and technology– Offers a unique combination of accelerator, laser and free-

electron laser sources – Enabling studies of electron and photon beam combination

techniques– Provides a range of photon sources for development of

scientific programmes and techniques

Reminder: 4GLS

• Energy Recovery Linac Prototype• To develop skills and technologies for

4GLS:– Operation of photo injector electron

gun– Operation of superconducting

electron linac– Energy recovery from a FEL-

disrupted beam– Synchronisation of gun and FEL

output

ERLP Funded in 2003

ALICE

Parameter Value

Gun Energy 350 keV

Injector Energy 8.35 MeV

Max. Energy 35 MeV

Linac RF Frequency 1.3 GHz

Max Bunch Charge 80 pC

ALICE Milestones (Champagne Moments…)

Au

g 0

6:

Firs

t Ele

ctro

ns

Oct

08

: Fi

rst

Boost

er

Beam

Dec 0

8:

Full

Energ

y R

eco

very

Feb

09

: C

ohere

ntl

y E

nhance

d T

Hz

Nov 0

9:

CB

S X

-Rays

Feb

10

: IR

-FEL

Sponta

neous

Em

.

Mar

10

: EM

MA

Inje

ctio

n L

ine

Beam

Ap

r 1

0:

Firs

t TH

z C

ell

Exposu

res

Au

g 1

0:

EM

MA

Rin

g 1

00

0s

turn

s

Oct

10

: IR

-FEL

Firs

t La

sing

ALICE parameters

Parameter Design Value

Operating Value

Injector Energy 8.35 MeV 6.5 MeV

Total beam energy 35 MeV 27.5 MeV

RF frequency 1.3 GHz 1.3 GHZ

Bunch repetition frequency

81.25 MHz 81.25 MHz or 16.25 MHz

Train Length 0 - 100 ms 0 - 100 ms

Train repetition frequency 1 - 20 Hz 1 - 20 Hz

Compressed bunch length

<1 ps rms <1 ps rms (measured)

Bunch charge (81.25 MHz)

80 pC 40 pC

Bunch charge (16.25 MHz)

80 pC 80 pC

Energy Recovery Rate >99% >99% (measured)

PhotoinjectorGun ceramic was major source of delay (~1 year)Alternative ceramic on loan from Stanford was installed to get us started – still in use today! Limits gun voltage to 230 kV (cf 350 kV)Original ceramic is on shelf waiting for opportunity to be installed

First electrons August 2006

Photoinjector Vacuum• XHV needed for good lifetime of cathode (GaAs)

– UHV is not good enough!• A new in-situ bakeout procedure was developed which

monitored the ratio of gas species in the vacuum system during the bake.

• Evidence suggests that partial pressures of any oxygen containing species (CO, CO2 and H2O) should be < 10-14 mbar.

0.00 0.02 0.04 0.06 0.08 0.100.0

0.2

0.4

0.6

0.8

1.0

1.2

Pho

tocu

rren

t (a.

u.)

Gas Exposure (L)

CH4

O2

CO2

CO

Standard Bake Optimised Bake

Photoinjector upgrade• Never need to let up gun

vacuum• Photocathode activated

offline• Reduced time for

photocathode changeover, from weeks to mins

• Higher quantum efficiency– Allows practical

experiments with photocathodes activated to different electron affinity levels

– 15% achieved in offline tests (red light)

• Allows tests of innovative photocathodes

• Installation?

Photocathode preparation facility

Loading chamber

Hydrogen rejuvenation chamber

Activation chamber

Superconducting Linacs

• Both linacs were procured from ACCEL (now Research Instruments)

• They each contain two 9-cell ILC type cavities (as used by XFEL) – 1.3 GHz

• Linacs only designed to operate in pulsed mode (20Hz)

• Would not be suitable for 4GLS or NLS type, high-rep rate, facilities

Linac Collaboration

• International initiative led by ASTeC to develop linac module suitable for CW operation as required by a high rep rate facility (eg NLS) – Higher power and adjustable input couplers– Higher beam currents, CW operation– Piezo actuators provide improved stability control– Improved thermal and magnetic shielding– Better HOM handling– 7 cell cavities so space for HOM absorbers– Same footprint as ACCEL linac so can install in ALICE easily– Validation with beam

Current Module

Linac Collaboration

New Module

Will be installed into ALICE in 2011

Linac Collaboration

7 cell cavity

Input coupler testing

HOM absorber

Outer cryomodule

assembly

DIAGNOSTICS

ROOMElectron beam

Laser beam

X-rays

Camera:Pixelfly QE

Camera:DicamPro

ScintillatorBe window

Dipole magnet

Quadrupole-04

Quadrupole-03

Correctors

Interaction region

To linac and beam dump

deflection and focussing mirrors

Vertical beam size: 39 µm RMS

Horizontal beam size: 27 µm RMS

~40pC/bunch, 29.6 MeV

800 nm pulses, ca. 70 fs duration, 500 mJ pulse power @ 10 Hz

(50. 8mm mirror) when seen in the holder in the straight on position you can only see 46.8mmØ. When rotated through 45° the vertical is 46.8mm and the horizontal is 41.14mm because of the mirror holder

200mm

135mm

Size is not known because this would depend on the lens and the camera, but this should only be

small.

Size of foil in the straight on is 47.5mm. When turned through 45° the vertical height of the foil is 47.5 but the horizontal is only 39.77mm because of the clamping ring

E Beam

OTR Camera

Compton ScatteringGeneration of short x-ray pulses by interacting a conventional laser with a low energy electron bunch

DIAGNOSTICS

ROOM

Background:Electron beam ONLaser OFF

Electron beam ONLaser beam ON

First data November 2009

Time delay

Evidence points to mis-alignment

Only 2 days of actual experimentation

Head on Collisions

Use of THz

• CSR generated in THz region because bunch length ~1 ps

• Output enhanced by many orders of magnitude (N2)

• Dedicated tissue culture lab

• Effect of THz on living cells being studied

• Source has very high peak intensities but very low power – so no thermal effects!

0

0.5

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10

TH

z si

gnal

am

plit

ude,

V

Bunch charge, pC

EMMA• Fixed Field Alternating Gradient accelerators are

an old idea (invented in 1950s)• They use DC magnets with carefully shaped

pole profiles• The beam orbit scales with energy so the magnet

apertures are large

EMMA• Non-Scaling Fixed Field Alternating Gradient

accelerators are a new idea (invented in 1990s)• They use simple DC magnets (eg quadrupoles)• The beam orbit changes shape with energy

enabling the magnet apertures to be small• EMMA is the first of this type – a proof of principle

Non-scaling FFAG• Born from considerations of very fast muon acceleration

– Breaks the scaling requirement– More compact orbits ~ X 10 reduction in magnet

aperture– Betatron tunes vary with acceleration (resonance

crossing)– Parabolic variation of time of flight with energy

• Factor of 2 acceleration with constant RF frequency• Serpentine acceleration

• Can mitigate the effects of resonance crossing by:-– Fast Acceleration ~15 turns– Linear magnets (avoids driving strong high order

resonances)• Or nonlinear magnets (avoids crossing resonances)

– Highly periodic, symmetrical machine (many identical cells)

• Tight tolerances on magnet errors dG/G <2x10-4

Novel, unproven concepts which need testing Electron Model => EMMA!

EMMA Goals

Graphs courtesy of Scott Berg BNL

Lattice ConfigurationsUnderstanding the NS-FFAG beam dynamics as function of lattice tuning & RF parameters

Graphs courtesy of Scott Berg BNL

Time of Flight vs Energy

• Example: retune lattice to vary longitudinal Time of Flight curve, range and minimum

• Example: retune lattice to vary resonances crossed during acceleration

Tune plane

EMMA

ALICE Provides the Beam

EMMA Parameters

Injection Line

Diagnostics Beamline

Frequency (nominal)

1.3 GHz

No of RF cavities

19

Repetition rate 1 - 20 Hz

Bunch charge 16-32 pC single bunch

Energy range 10 – 20 MeV

Lattice F/D Doublet

Circumference 16.57 m

No of cells 42

Normalised transverse acceptance

3π mm-rad

EMMA Ring Cell

• 42 identical doublets

• No Dipoles!

Long drift 210 mm

F Quad 58.8 mm

Short drift 50 mm

D Quad 75.7 mmFD

Cavity

210 mm

110 mm

Beam stay clear aperture

D

65 mm

55 mm

Magnet Centre-lines

Low Energy Beam

High Energy Beam

Field Clamps

Independent slides

Injection

SeptumKicker

Kicker

SeptumPower supply

Realisation of EMMA August 2010

First Data ...

First Turn Second Turn

September 2010 - beam circulates more than 1000 turns

Aug 2010 - First turns

Bruno MuratoriCERN 07/10/10

Extraction (07/03/11)• Going clockwise towards

extraction– Yellow = Inj. Kicker1– Pink = Ext. Kicker1– Green = Ext. Kicker2– Blue = beam

• Action of injection kicker too early to be seen

• Spikes = turns• Effect of extraction clearly

visible• Image seen on first YAG

screen in extraction / diagnostic line

Optical Clock Distribution Scheme

Mode-LockedFibre Ring Laser

(81.25 MHz)

Link Operation 60 fs pulses are distributed to BAM

sites around ALICE.

Half the pulse power will be reflected back at the far end to enable detection of optical path length changes.

Timing is actively stabilized with a fibre stretcher and delay line.

The other half of the timing stabilized pulses will be used to measure the arrival time of electron bunches and other diagnostics.

Feedback Loop

Circuitry

Fibre Stretcher

Fibre Stabilization Interferometer

Highly stable clock distribution across large scale facilities is important for the synchronisation of beam generation, beam manipulation components and end station experiments. Optical fibre technology can be used to combat the stability challenges in distributing clock signals over long distances with coaxial cable.

An actively stabilised optical clock distribution system based on the propagation of ultra-short optical pulses has been installed on ALICE. Femtosecond pulses emerging at the far end are currently used to implement a beam arrival monitor. However, the clock signals could also be integrated into other diagnostic systems such as electro-optical beam diagnostics.

Beam Arrival Monitor

BeamlineRF

pickup

Single Mode Distribution Fibre (100m)

Faraday Rotating Mirror (50:50)

EOM Detector

Accelerator Area

-250 -150 -50 50 150 250

0

0.5

1

1.5

2

Beam Arrival Time Cali-bration

Delay (ps)

Norm

ali

zed

pow

er

Zero crossing for arrival time measurements

Trina Ng

ALICE Electro-optic experimentso Energy recovery test-accelerator

intratrain diagnostics must be non-invasive

o low charge, high repition rate operation typically 40pC, 81MHz trains for 100us

Spectral decoding results for 40pC buncho confirming compression for FEL commissioningo examine compression and arrival timing along traino demonstrated significant reduction in charge requirements

S.P. Jamison

Laser-electron Beam Interactions

• New concepts & proof-of-principle tests• Developing technique for direct phase-space

manipulation of electrons with longitudinally laser & unipolar THz pulses.

• Aim to adjust phase-space without need for modulators/chicanes

ALICE experiment in final stages of preparation ...

propagationdirection

EM Source development and testing

Oscillator FEL Process

ALICE IR-FEL

Dec 2009/Jan 2010: FEL Undulator and Cavity Mirrors installed and aligned. Throughout 2010: FEL/THz/CBS programmes proceeded in parallel with

installation of EMMA. One shift per day of beamtime for commissioning. Of available beamtime, FEL programme gets ~15%. Progress:

Feb 2010: First observations of undulator spontaneous emission. Stored in cavity immediately.

But no lasing could be found. Problem was that we were limited to 40pC: above 40pC @ 81.25Mz beam loading prevented constant energy along 100µs train.

On 17th October 2010 we installed a Burst Generator to reduce laser repetition rate from 81.25MHz to 16.25 MHz and increased bunch charge to 60pC.

A week later, on 23rd Oct 2010 achieved first lasing @ 8µm Shutdown Nov/Dec 2010 Jan/Feb 2011: Lasing from 8.0-5.7µm Mar 2011: IR transported out of ALICE area to beyond shield wall

FEL SYSTEMS + Transverse/Longitudinal Alignment

ALIG

NM

EN

T

MIR

RO

R

ALIG

NM

EN

T

MIR

RO

R

(OP

TIC

AL

TA

RG

ET)

(OP

TIC

AL

TA

RG

ET)

POWER METER

MCT DETECTOR

SPECTROMETER

ALIGNMENT WEDGES

INFR

A-R

ED

ELECTRONS

DWN-LAM-02 DWN-LAM-01 UPS-LAM-02UPS-LAM-01

HeNeHeNe

FEL-M1

FEL-M2

CCD VIEWER CAMERAS

FEL-WIG-TRANS-01

ALICE FEL Systems Schematic

OP

TIC

AL

TA

RG

ET

OP

TIC

AL

TA

RG

ETUNDULATOR

ARRAYS

DOWNSTREAM FEL

MIRROR

REFERENCE AXIS

LASER TRACKER

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

ALIGNMENT WEDGES

OP

TIC

AL

TA

RG

ET

OP

TIC

AL

TA

RG

ETUNDULATOR

ARRAYS

DOWNSTREAM FEL

MIRROR

2. Alignment Wedges and Downstream Mirror aligned optically using Theodolite

ALIG

NM

EN

T

MIR

RO

R

HeNeCCD VIEWER

CAMERAS

3. Downstream Mirror aligned using Upstream HeNe

CCD VIEWER CAMERAS

HeNe

ALIG

NM

EN

T

MIR

RO

R

4. Upstream Mirror aligned using Downstream HeNe

ELECTRONS

5. Electron Beam steered to Alignment Wedges

POWERMETER

MCT DETECTOR

SPECTROMETER

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

FEL Overview

UPSTREAM MIRROR

UNDULATOR

DOWNSTREAM MIRROR

ELECTRON BEAM AT FEL

Energy 27.5MeV

Bunch Charge 80pC

Bunch Length ~1ps

Normalised Emittance

~12 mm-mrad

Energy Spread ~0.6% rms

Repetition Rate 16.25MHz

Macropulse Duration

100µs

Macropulse Rep. Rate

10Hz

BUNCH COMPRESSOR

FEL Undulator

UNDULATOR

On 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

FEL Resonator

RESONATOR

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

FEL Local Diagnostics

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

Spontaneous Emission as a Diagnostic

February 2010: 1st Observation

Spontaneous emission a useful diagnostic

7 7.5 8 8.5 9-2

0

2

4

6

8

10

12x 105

Wavelength (m)

P(

) (a

.u.)

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

10 12 14 16 181.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

Linac Off-Crest Phase (Degrees)

MC

T S

ign

al (

V)

40 50 60 70 801

1.5

2

2.5

3

3.5

4

Cavity Length Detuning (m)

MC

T S

ign

al (

V)

1. Spectrum used to optimise steering in undulator

2. Coherent enhancement used to set minimum bunch length

3. Interference of coherent SE used to set correct cavity length

Shortest wavelength + Narrowest Bandwidth when

beam on reference axis

Intensity enhancement at maximum bunch

compression

Intensity Oscillations at λ/2 in

cavity length indicating round trip

interference

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

Po

wer

(m

W)

-5 0 5 10 15 20 250

10

20

30

40

50

Cavity Length Detuning (m)

Out

coup

led

Ave

rage

Po

wer

(m

W)

ALICE IR-FEL: First Lasing

Results from First Lasing Period (23-31 October 2010)

20 25 30 354

6

8

10

12

14

Av

era

ge

Po

we

r (m

W)

Cavity L (m)

20 30 400.8

1

1.2

1.4

1.6

1.8

FW

HM

B/W

(%

)

Cavity L (m)

20 25 30 350.5

1

1.5

Pu

lse

En

erg

y (

J)

Cavity L (m)

20 30 40

0.8

1

1.2

1.4

1.6

T

(p

s)

Cavity L (m)

20 30 400

0.5

1

1.5

2

Pe

ak

Po

we

r (M

W)

Cavity L (m)

Implies electron bunch length ≈1ps, in agreement with

previous EO measurements of a similar ALICE setup

Results from First Lasing Period (23-31 October 2010)

20 40 60 80 100 1205

10

15

20

25

Q (pC)

Sin

gle

Pa

ss

Ga

in (

%)

20 40 60 80 100 12020

40

60

80

100

Q (pC)

Tsa

t(s

)

Gain determined from cavity rise time From one pulse train to the next (@10Hz) the

gain jitters Cause under investigation. Phase jitter in

pulsed RF? Laser jitter?.... On average the gain is lower than we want:

rms Energy spread of 0.6% is too big: degrades the gain significantly

Aim to halve energy spread and double gain

Can then change to outcoupler with larger hole

Can set up beam to achieve this (set injector to deliver shorter bunch to linac) but haven’t yet lased with this setup – still to be understood! Should work, but doesn’t!

NB: No optimisation done at higher charges (just turned up the

PI laser power (to 11))

3.54 3.56 3.58 3.6 3.62 3.64

x 10-4

10-2

10-1

T (s)

MC

T

Results from February 2011: Gap Tuning

5 5.5 6 6.5 7 7.5 8 8.50

0.2

0.4

0.6

0.8

1P

()(

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

Ba

nd

wid

th (

%)

Wavelength (m)6 7 8

500

600

700

800

900

1000

FW

HM

t

(fs

)

Wavelength (m)6 7 8

0.5

1

1.5

2

2.5

Pu

lse

En

erg

y (

J)

Wavelength (m)6 7 8

1.5

2

2.5

3

3.5

PP

k (M

W)

Wavelength (m)

ALICE FEL Future Plans

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

Transport of FEL beam to diagnostics room, then full output characterisation.

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

Plan to run at 27.5MeV (5-8µm) and 22.5MeV (7-12µm)

Beyond that depends on funding being obtained for specific exploitation programmes.

But ALICE itself will not run indefinitely.

We are now thinking beyond ALICE….

4 5 6 7 8 9 10 11 12 130

1

2

3

4

5

6

(m)

Pp

eak (

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)

Pp

eak (

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

The Future ...

Concepts for post-ALICE future hundred-MeV-scale electron test accelerators are currently under development in consultation with other stakeholders (including JAI!).

Potential topics of interest: Ultra-Cold injectors (low emittance, low charge, velocity

bunching, fs bunches…..) Novel acceleration (laser plasma….) Compact FELs (short period undulators….) Attosecond FEL pulse generation (slicing, modelocking…) Novel FEL seeding schemes (HHG, self-seeding, EEHG….) FEL pulse diagnostics

Will be a national and international collaboration taking ~12 months to develop the plans in more detail.

Summary• ALICE is an extremely versatile and flexible test accelerator• We have gained practical experience/skills of several key

accelerator technologies– Photoinjectors– SRF & 2K cryo– High power laser/electron interactions– FELs– Timing & Synchronisation– Energy Recovery– Coherent SR– .....

• EMMA is currently being commissioned (using ALICE as the injector)

• Plans are being drawn up for future test facilities – please join in the discussion!

Acknowledgements

• Thanks to the following for providing slides and other material– Neil Thompson– Bruno Muratori– Elaine Seddon– Neil Bliss– Rob Edgecock– Steve Jamison– Peter McIntosh– Susan Smith– Keith Middleman– Trina Ng