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Les Sources d’ É lectrons de Forte Intensit é et de Faible Emittance. T. Garvey, Laboratoire de l’Acc é l é rateur Lin é aire – Orsay. Journ é es Acc é l é rateurs de la SFP – Roscoff, 10 octobre, 2005. Motivation for new high brightness injectors – new synchrotron radiation sources. - PowerPoint PPT Presentation
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Les Sources d’Électrons de Forte Intensité et de Faible Emittance
T. Garvey,
Laboratoire de l’Accélérateur Linéaire – Orsay.
Journées Accélérateurs de la SFP – Roscoff, 10 octobre, 2005.
Motivation for new high brightness injectors – new synchrotron
radiation sources• SASE Free Electron Lasers at VUV and
X-ray wavelengths – High peak brightness
• Energy Recovery Linacs– High average brightness synchrotron radiation source.
Not an exhaustive review of all existing or planned projects!Will evoke major issues in the design of injectors for FEL and ERLProjects.
Single-pass Free Electron Lasers
Energy:
Energy width:
Narrow resonance E/E ≤
Gain Length:3/1
2
23
0ˆ
2
3
1
IKe
mcL ur
g
21
2
2
2
Kuem
Beam size:
r small high electron density for
maximum interaction with radiation fieldEmittance ≤
Peak current inside bunch:Î > 1 kA feasible only at ultrarelativistic energies, or may dilute emittance bunch compressor
SASE FEL Projects
• TTF VUV-FEL (6 – 30 nm)
• European X-FEL (0.1 – 6 nm)
• SPARC (500 nm)
• LCLS (~ 0.15 nm)
• SCSS (~ 3.6 nm)
• BESSY (50 – 1 nm)
• DUV-FEL…….
Gun types DC
(or pulsed HV)
RF
Thermionic emission Thermionic emission
Photo-emission Photo-emission
Cathodes – metallic,semi-conductor (high Q.E.)
CeB6, LaB6….
GaAs (polarised beams)
Also – Field emission guns for intense e-beams.
RF gun – electrons are “born” in high field (~ 100 MV/m) region (c.f. DC guns ~ few MV/m) ⇒quickly accelerated to high energy ⇒ reduces detrimental effect of space-charge forces on the beam emittance.
Laser-driven allows generation of beams with ⇒same temporal structure as laser ⇒ rapid ( ~ pico-seconds, c.f. gridded DC guns ~ 1ns).
Advantages of laser triggered RF Gun
CTF-3 gun under designed at LAL/Orsay
RF Photo-injectors use many technologies
• Photo-cathodes– Quantum efficiency, thermal emittance, dark current,
aging……
• Cathode lasers– Long pulse train, intensity stability, synchronisation,
temporal and spatial homogeneity…..
• Gun cavity– Geometric form, symmetric coupling, surface fields….
• Analytic theory and numerical simulations– Longitudinal bunching, transverse emittance
compensation, wake-fields….
• Diagnostics– Ultra-short pulses, low emittances, E.O. techniques, RF
deflectors……
Gun geometry design
Waveguide coupler in exit cell –Coupling from both sides – reduces dipole “kick”
Form of irises – reduce peak surface field for given accelerating field
Bunch compressionNeed high peak currents (~ 1 - 2 kA) for SASE FEL.RF gun limited to < 100 A (1nC, 4 ps for TTF) to mitigate space charge effects ⇨ need to perform longitudinal compression.
• Magnetic compressionnot necessarily part of the injector
• Ballistic compression → “classical” injector approach
• “Velocity” (or RF) bunching → incorporates bunching into the injector.
Magnetic Chicane CompresserMagnetic Chicane Compresser
Powerful radiation generates energy spread in bendsPowerful radiation generates energy spread in bends Powerful radiation generates energy spread in bendsPowerful radiation generates energy spread in bends
Causes bend-plane emittance growth (DESY experience)Causes bend-plane emittance growth (DESY experience) Causes bend-plane emittance growth (DESY experience)Causes bend-plane emittance growth (DESY experience)
x = Rx = R1616((ss))E/EE/E
bend-plane emittance growthbend-plane emittance growth
ee––RR
zz
coherent radiation coherent radiation forforzz
overtaking length:overtaking length: L L00 (24 (24zzRR22))1/31/3
ssxx
LL00
Ballistic bunching
• Usually at low energy, typical in injectors with DC guns.• Buncher cavity imparts energy chirp to give compression in downstream drift space.
Velocity BunchingFor non-rigid bunch, relative movements takeplace within bunch to reduce phase spread.
Technique considered for CTF probe beam linac
Observed on DUV-FEL linac (BNL)
2
w
ˆ 3I0
8
3
ˆ I
2Ioth
' 0
Emittance Compensation: Emittance Compensation:
Controlled Damping of Plasma OscillationControlled Damping of Plasma Oscillation
100 A => 150 MeV
L. Serafini and J.B. Rosenzweig, Phy Rev. E, Vol. 55, 1997.
Space charge effects can dominate to high energies for these intense beams and low emittances. “Injector” can be > 100 MeV
Space-charge effects and non-linear RF fields play a role in the evolution of the RMS emittance. Must determine at what point to accelerate and ‘freeze’ the emittance oscillations.
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10Z_[m]
GunLinac
rms beam size [mm]
rms norm. emittance [um]
-0.04
-0.02
0
0.02
0.04
0 0.001 0.002 0.003 0.004 0.005 0.006
z=0.23891
Pr
R [m]
-0.05
0
0.05
0 0.0008 0.0016 0.0024 0.0032 0.004
z=1.5
Pr
R [m]
-0.04
-0.02
0
0.02
0.04
0 0.0008 0.0016 0.0024 0.0032 0.004
z=10
pr_
[rad]
R_[m]
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
-0.003 -0.002 -0.001 0 0.001 0.002 0.003
z=0.23891
Rs [m
]
Zs-Zb [m]
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
-0.003 -0.002 -0.001 0 0.001 0.002 0.003
Z=10
Rs [m
]
Zs-Zb [m]
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
-0.003 -0.002 -0.001 0 0.001 0.002 0.003
z=1.5
Rs [m
]
Zs-Zb [m]
Final emittance = 0.4 m
Matching onto the Local Emittance Max.,
Example of an optimized matchingExample of an optimized matching
M. Ferrario et al., “HOMDYN Study For The LCLS RF Photo-Injector”, Proc. of the 2nd ICFA Adv. Acc. Workshop on “The Physics of High Brightness Beams”, UCLA, Nov., 1999, also in SLAC-PUB-8400
Movable Emittance-Movable Emittance-MeterMeter
for the SPARC projectfor the SPARC project
0
1
2
3
4
5
6
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 0.5 1 1.5 2 2.5 3
HBUNCH.OUT
sigma_x_[mm]enx_[um]
Bz_[T]
sig
ma
_x
_[m
m]
Bz
_[T
]
z_[m]
emittance envelope
Free-Electron Laser at the TESLA Test Facility
cath
od
e la
ser
magnetundulator
superconducting cavities
bunch compressor
rf-gunbooster
FELbeam
electron dump
Photon DiagnosticsArea
The TESLA Test Facility at DESY: TTF-FEL I
60 mm-keVLong. emittance σE σl
1.2 - 1.5 kAPeak current
~ 1 psBunch length (rms)
~ 8 mm-mradTransverse projected emittance (rms)
~ 3 nCBunch charge
220-270 MeVBeam energy (during FEL operation)
value electron beam parameter
S-band Photo-injectors – high peak currents
• Waveguide coupling to cavity• Tuners for frequency regulation
CTF-2 Gun - CERN
ELYSE Gun – LAL/Orsay
Waveguide and tuners break circular Symmetry of gun bad for emittance.⇒
1800 pulses
Free-Electron Laser at the TESLA Test Facility
The TTF-1 photo-emission electron source
•UV laser impinging a Cesium Telluride cathode provide electrons via photo-emission•The cathode is located in a 1-1/2 cell rf-gun (f=1.3 GHz) with peak E-field of 40 MV/m•The laser is a 4 ND:YLF laser (262 nm)
TTF injector II
typical parameters for TTF 1-FEL:repetition rate: 1 Hz
bunch frequency: 1 - 2.25 MHz
bunch charge: 1- 3 nC
bunch length (rms): ~3 mm (1 nC, after booster )
norm. emit., x,y: ~ 4 µm ( @ 1nC)
p/p: 0.13 % rms ( @ 16 MeV )
injection energy: 16 MeV
Free-Electron Laser at the TESLA Test Facility
•Coaxial input coupler prevents transverse rf- kicks (and the associated emittance growth)•Laser will be upgraded to generate plateau-like distributions•In its final upgrade the source will generate 7200 bunches stack on a 800 sec rf-pulse
The new electron source
• RF-gun commissioned at the DESY-Zeuthen PITZ facility
•Requirements : < 2 mm-mrad at the undulator, Q=1 nC
BESSY Berlin, CCLRC Daresbury,
DESY (HH + Z), INFN Frascati,
INFN Milano, INR Troitsk,
INRNE Sofia, LAL Orsay,
MBI Berlin, TU Darmstadt,
U Hamburg, YERPHI Yerevan
Collaborating Institutes:
(1.3 GHz)
PITZ Collaboration
VUV-FEL Gun: Longitudinal Phase Space
max. mean momentum:4.72 MeV/c
min. rms momentum spread:33 keV/c
4.4
4.5
4.5
4.6
4.6
4.7
4.7
4.8
4.8
10 20 30 40 50 60 70f / degree
pm
ea
n /
MeV
/c
0
20
40
60
80
100
120
140
160
pR
MS /
keV
/c
bunch length:
Minimum bunch length: FWHM = (21.04 ± 0.45stat ± 4.14syst) ps = (6.31 ± 0.14stat ± 1.24syst) mm
good agreement with simulations !
Q = 1 nC
beamlet size is measuredfor 3 slit positions: 1,0,1
7.0 22
n
nYy screeny
screen
n
Transverse Emittance Measurements
beam spot at screen 2
single slitpositions
beamlets at screen 3
Single Slit Scan Technique222 xxxxnx
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
0 10 20 30 40 50
Ibuck, A
no
rm. e
mit
tan
ce /
mm
mra
d
ExEySQRT(Ex*Ey)ASTRA sim.
Q = 1 nC,
= m – 5°
Imain = 305 A
Start-up requirement of TTF2 is clearly fulfilled !
requirement for VUV-FEL (30 nm) = 3
requirement for VUV-FEL (6 nm) = 2
requirement for XFEL = 0.9
1.3 GHz 1 1/2 cell RF Gun
Main SolenoidBucking Solenoid
VUV-FEL Gun: Transverse Emittance
ATF at BNL•Measurement of impact of transverse non-uniformity on emittance•Used a mask•Q=0.5 nC (kept constant)•Emittance for uniform beam is about 1.5 mm-mrad •Long. Length is 3 ps FWHM
100 %
60 % 50 %
90 %
5060
7080
90100
•As predicted by simulation, uniform beam gives the best emittance •Emittance doubles for the 50 % modulation case
(extracted from ATF News Letter 03/2002)
On 23.06.2003 longitudinal shape changed to flat top
FWHM = 7 ± 1psFWHM 18-23 ps
rise and fall time about 5-8 ps
Until 23.06.2003 - Gaussian longitudinal laser shape:
PITZ: Cathode Laser Pulse Profile
Minimum measured emittance
≥ 3 mm mrad 1.6 mm mrad
012345678
0 1 2 3 4 5 6 7 8 9 10z / m
Xrms (CDS14) / mm
EmX (CDS14) / mm mrad
Xrms (no booster) / mm
EmX (no booster) / mm mrad
Outlook PITZ2
Booster Cavity
study emittance conservation principle
booster cavity (TESLA, CDS - Cut Disc Structure)
further improvement of beam quality
work on laser, cavities, photo cathodes,developments on simulation tools
Towards to the XFEL Photo Injector:• 60 MV/m at the cathode• Cathode laser improvement
Simulations for 60MV/m at the cathode
Free-Electron Laser at the TESLA Test Facility
•Injection directly into a TESLA accelerating module makes the emittance compensation scheme more efficient (~1.3 mm-mad)
•Use of a 3rd harm. RF-section to correct the longitudinal phase space distortions
The new injector configuration
BC
ACC. MOD. ACC. MOD.
Matchingsection
Diagnosticssection
3rd harmrf-structure
dump
rf-gun
BESSY FEL2.2 GeV linac using TESLA cavities1 kHz repetition rate, 40 MV/m cathode field→ 75 kW average dissipation !!c.f. needs of Arc-en-Ciel.
Gun being developed in framework of EUROFEL
A variation of the RF-gun concept: the pulsed photodiode
M. Van der Wiel et al.,
T.U. Eindhoven,
2 MV HV 1 ns pulse on a 2 mm diode gap:100 pC @ 100 fs bunch Bn=1.2.1015
Advanced Laser-Plasma High Energy Accelerators towards X-rays
J. Rodier – LAL/Orsay
University of Strathclyde, Glasgow.
Energy Recovery Linac Injectors• ERL – combines LOW transverse emittance beam properties with HIGH average (high duty cycle / CW) current.• Beam properties at high energy are determined by injector quality and not by dynamic equilibrium properties of a Storage Ring
High brightness light source
4GLS – Daresbury Laboratory
High Current ERL Injector Requirements
• Output energy ~ 7 – 10 MeV
• CW average current ~ 100 – 500 mA
• Transverse emittance < 6 mm-mrad
• Longitudinal emittance ~ 150 keV – ps (rms)
• Bunch length ~ 2 – 7 ps
• Energy spread < 0.5 % @ 7 MeV
• High Power RF feed-through possibilities.
JLab High Brightness Gun
Extremely reliable
Delivered 5.5 kiloCoulombs from one cathode
Same cathode for 2 years
Modest improvements for 10 kW Upgrade
Jefferson Lab Gun Approach• Highest average brightness is produced by lowest charges at
highest frequency (J.B. Rosenzweig PAC95 pp. 957-960) • However FEL needs high peak current, contrary to this scaling
• Compromise: highest frequency for which charge (when later bunched to high peak current) has sufficient small signal gain and linac is stable to BBU, etc.
– Highest brightness is achieved by high gun gradient and fast acceleration
• IR Demo operated 3.8 to 4.2 MV/m limited by cesium on gun ball
• Upgrade should achieve >6 MV/m with cesium only on GaAs
• Get to >10 MeV as quickly as possible using srf high gradient cavities
• Long cathode lifetimes (measure in coulombs/cm2 not time!)
• 5,500 Coulombs from one cathode demonstrated (3 mm spot radius)
• Limited by ion back bombardment from background gas
• Vacuum pumping in DC gun 100 times rate in rf gun
JLab 10 kW Upgrade IR FEL Injector demonstrated
performance•Pulsed operation at 8 mA/pulse (110 pC/bunch) in 16 ms-long pulses at 2 Hz repetition rate
•CW operation at 9.1 mA (75 MHz) with 122 pC/bunch
•Routinely delivers 5 mA CW and pulse current at 135 pC/bunch for FEL operations
•400 A peak current at wiggler
The injector is driven by a350 kV DC GaAs Photocathode Gun
Beam
Hybrid DC gun /SRF cavity photo-injector in design engineering
• SBIR development by AES Corp. partnered with JLab
• 750 MHz operation
JLab concept of a high voltage DC gun married to a low frequency rf cavity (shown here with SNS cavities)
133 pC / RF bucket→ 100 mA @ 10 MeV100 kW of beam power!!!
Twin Coupler Attached to the CORNELL ERL Injector Cavity
Modified version of TTF-IIICoupler.Mechanical changes to Increase average power – 75 kW
Normal (Cs2Te) and superconducting (Nb) photo-cathodes under study.SC cavity prohibits use og magnetic field for Emittance Compensation.
Conclusions• Recent years (~ 10) have seen impressive progress in the
development of photo-injectors for short wave-length SASE FEL’s.– Many subjects need to be developed further – cathode life-time, dark
current levels, shaping of laser pulse (temporally and spatially), gun cavity technology for high duty cycle, longitudinal beam compression techniques which leave transverse emittance un-disturbed.
• Increasingly sophisticated BD simulations are guiding injector design for low emittance.
• DC guns, originally investigated for nuclear physics, are being up-graded for ERL requirements.– marriage of DC gun and SC RF technolgy
• The development of superconducting RF technology has had a major influence on the design of new synchrotron light sources.
Acknowledgements
I thank the following people for providing material :
Ph. Piot (FNAL)K. Flöttmann (DESY Hamburg)F. Stephan (DESY Zeuthen)M. Krasilnikov (DESY Zeuthen)F. Marhauser (BESSY)D. Jaroszynski (Univ. Strathcyde)M. Ferrario (INFN Frascati)G. Neil (JLab)