Challenges, Achievements, and Future Plans

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Nov 15, 2013 (3 years and 4 months ago)

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The ALICE Electron Test Accelerator
-

Challenges, Achievements, and Future Plans


Professor Jim Clarke

ASTeC, STFC Daresbury Laboratory & Cockcroft Institute

JAI Lecture, 17
th

March 2011

Contents


Introduction to ALICE


Major Subsystems


Experimental Highlights


EMMA


Free Electron Laser


Future Plans


Summary




ALICE


A
ccelerators and
L
asers
I
n
C
ombined
E
xperiments



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


E
nergy
R
ecovery
L
inac
P
rototype


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…)

Aug 06
: First Electrons

Oct 08
: First Booster Beam

Dec 08
: Full Energy Recovery

Feb 09
: Coherently Enhanced THz

Nov 09
: CBS X
-
Rays

Feb 10
: IR
-
FEL Spontaneous Em.

Mar 10
: EMMA Injection Line Beam

Apr 10
: First THz Cell Exposures

Aug 10
: EMMA Ring 1000s turns

Oct 10
: IR
-
FEL First Lasing

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
m
s

0
-

100
m
s

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)

Photoinjector

Gun
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, CO
2

and H
2
O) should be < 10
-
14

mbar.


0.00
0.02
0.04
0.06
0.08
0.10
0.0
0.2
0.4
0.6
0.8
1.0
1.2


Photocurrent (a.u.)
Gas Exposure (L)
CH
4
O
2
CO
2
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

DIAGNOST
ICS ROOM

Electron beam

Laser beam

X
-
rays

Camera:

Pixelfly QE

Camera:

DicamPro

Scintillator

Be 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
46.8mm
Ø
. 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

Scattering

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

DIAGNOST
ICS ROOM

Background:

Electron beam ON

Laser OFF

Electron beam ON

Laser 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

(
N
2
)


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
THz signal amplitude, 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 Configurations

Understanding 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


3
π

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 mm

F

D

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

Septum

Kicker

Kicker

Septum

Power 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 Muratori

CERN 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
-
Locked

Fibre 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

Beamline

RF

pickup

Single Mode

Distribution

Fibre (100m)

Faraday Rotating
Mirror (50:50)

EOM

Detector

Accelerator Area

0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-250
-150
-50
50
150
250
Normalized power

Delay (ps)

Beam Arrival Time Calibration

Zero crossing for

arrival time

measurements

Trina Ng

ALICE Electro
-
optic experiments

o

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 bunch

o

confirming compression for FEL commissioning

o

examine compression and arrival timing along train

o

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 ...

propagation

direction

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 17
th

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 23
rd

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

ALIGNMENT
MIRROR

ALIGNMENT
MIRROR

(OPTICAL
TARGET)

(OPTICAL
TARGET)

POWER
METER

MCT
DETECTOR

SPECTROMETER

ALIGNMENT

WEDGES

INFRA
-
RED

DWN
-
LAM
-
02

DWN
-
LAM
-
01

UPS
-
LAM
-
02

UPS
-
LAM
-
01

HeNe

HeNe

FEL
-
M1

FEL
-
M2

CCD VIEWER
CAMERAS

FEL
-
WIG
-
TRANS
-
01

ALICE FEL Systems
Schematic

OPTICAL TARGET

OPTICAL TARGET

UNDULATOR
ARRAYS

DOWNSTREAM
FEL MIRROR

REFERENCE AXIS

LASER
TRACKER

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

ALIGNMENT

WEDGES

OPTICAL TARGET

OPTICAL TARGET

UNDULATOR
ARRAYS

DOWNSTREAM
FEL MIRROR

2. Alignment Wedges and Downstream Mirror aligned optically using Theodolite

ALIGNMENT
MIRROR

HeNe

CCD VIEWER
CAMERAS

3. Downstream Mirror aligned using Upstream HeNe

CCD VIEWER
CAMERAS

HeNe

ALIGNMENT
MIRROR

4. Upstream Mirror aligned using Downstream HeNe

5. Electron Beam steered to Alignment Wedges

POWER

METER

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
-
DETECTOR

on Exit Port 2

DOWNSTREAM
ALIGNMENT HeNe

Spontaneous Emission as a Diagnostic

February 2010: 1
st

Observation

Spontaneous emission a useful
diagnostic

7
7.5
8
8.5
9
-2
0
2
4
6
8
10
12
x 10
5
Wavelength

(
m
m)
P(

) (a.u.)


x = -1.0 mm
x = 0.0
x = +1.0 mm
10
12
14
16
18
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
Linac Off-Crest Phase (Degrees)
MCT Signal (V)
40
50
60
70
80
1
1.5
2
2.5
3
3.5
4
Cavity Length Detuning (
m
m)
MCT Signal (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
25
0
2
4
6
8
10
12
14
Cavity Length Detuning (
m
m)
Outcoupled Average Power (mW)
-5
0
5
10
15
20
25
0
10
20
30
40
50
Cavity Length Detuning (
m
m)
Outcoupled Average Power (mW)
ALICE IR
-
FEL: First Lasing

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

20
25
30
35
4
6
8
10
12
14
Average Power (mW)
Cavity L (
m
m)
20
30
40
0.8
1
1.2
1.4
1.6
1.8
FWHM B/W (%)
Cavity L (
m
m)
20
25
30
35
0.5
1
1.5
Pulse Energy (
m
J)
Cavity L (
m
m)
20
30
40
0.8
1
1.2
1.4
1.6

T (ps)
Cavity L (
m
m)
20
30
40
0
0.5
1
1.5
2
Peak Power (MW)
Cavity L (
m
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
120
5
10
15
20
25
Q (pC)
Single Pass Gain (%)
20
40
60
80
100
120
20
40
60
80
100
Q (pC)
T
sat
(
m
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)
MCT
Results from February 2011: Gap Tuning

5
5.5
6
6.5
7
7.5
8
8.5
0
0.2
0.4
0.6
0.8
1
P(

)(a.u.)

(
m
m)


g = 16 mm
g = 15 mm
g = 14 mm
g = 13 mm
g = 12 mm
6
7
8
1
1.2
1.4
1.6
1.8
2
Bandwidth (%)
Wavelength (
m
m)
6
7
8
500
600
700
800
900
1000
FWHM

t (fs)
Wavelength (
m
m)
6
7
8
0.5
1
1.5
2
2.5
Pulse Energy (
m
J)
Wavelength (
m
m)
6
7
8
1.5
2
2.5
3
3.5
P
Pk
(MW)
Wavelength (
m
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
13
0
1
2
3
4
5
6

(
m
m)
P
peak
(MW)


27.5MeV, 0.75mm Hole radius
22.5MeV, 0.75mm Hole radius
27.5MeV, 1.5mm Hole radius
22.5MeV, 1.5mm Hole radius
27.5MeV, 2.25mm Hole radius
22.5MeV, 2.25mm Hole radius
4
5
6
7
8
9
10
11
12
13
0
1
2
3
4
5
6

(
m
m)
P
peak
(MW)


27.5MeV, 0.75mm Hole radius
22.5MeV, 0.75mm Hole radius
27.5MeV, 1.5mm Hole radius
22.5MeV, 1.5mm Hole radius
27.5MeV, 2.25mm Hole radius
22.5MeV, 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