d.h. dowell/mit talk, may 31, 2002 1 david h. dowell stanford linear accelerator center photocathode...
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D.H. Dowell/MIT Talk, May 31, 2002 1
David H. DowellStanford Linear Accelerator Center
Photocathode RF Guns andBunch Compressors for High-Duty Factor FELs
IntroductionArchitecture of SASE FELs
RF Gun Technologies
Example: A Low-Frequency, High-Duty Factor RF Photoinjector at 433 MHz
Bunch Compressor Physics
Summary and Conclusion
D.H. Dowell/MIT Talk, May 31, 2002 2
Architectures of the SASE X-Ray FEL
Single Pass, Normal Conducting => Leutl, VISA(ATF), SDL, LCLS
RF Gun AcceleratorBunch
CompressorAccelerator
Long UndulatorX-Rays
High EnergyE-beamDump
3rd HarmonicLinearizer
Energy Recovered Linac (ERL), SRF => JLab, Cornell
RF Gun
SRFAccelerator
Low EnergyE-beamDump
Long Undulator
X-Rays
Bend/CompressorBend/
De-Compressor
Accelerator
SRFAccelerator
BunchCompressor
3rd HarmonicLinearizer
D.H. Dowell/MIT Talk, May 31, 2002 3
RF Gun Technologies for the Two SASE Architectures
Single Pass, Normal Conducting:
Energy Recovered Linac (ERL), SRF:
Low Duty Factor: Single microbunch at 1 to 120 Hz, 0.1 to 1 nC / bunchS-band (2856MHz) , 1.6 cell, BNL Gun, ~100 MV/m cathode fieldMetal Cathodes: Cu, MgUV Drive Laser: Freq. Quadrupled Nd:Glass, Ti-Sapphire; 2-10 ps(fwhm)
High Duty Factor: CW, microbunches at 10 to 70 MHz, 0.05 to 1 nC / bunchDC, 433 MHz, L-band (1500&1300MHz) Guns: 10 to 30 MV/m cathode fieldSemi-Conductor Cathodes: GaAs, CsTe, CsKSbVisible Wavelength Drive Laser: Frequency Doubled Nd:YLF
High Duty Factor requires low-frequency gun with semi-conductor cathodeBuild upon experience gained from high power RF gun development at Boeing & LANL
D.H. Dowell/MIT Talk, May 31, 2002 4
Example of Demonstrated Technology: A Low-Frequency, High-Duty Factor
433 MHz RF PhotoinjectorDeveloped by Boeing and Los Alamos
D.H. Dowell/MIT Talk, May 31, 2002 5
Historical Perspective
Motivation:Design, build and test an RF photocathode
gun capable of operating at high current and high duty factor.
Result:A 1992 demonstration of a two-cell, 433 MHz
photocathode gun at 32 mA of average current and25% duty factor.
D.H. Dowell/MIT Talk, May 31, 2002 6
Photoinjector Design Philosophy Use a CW low frequency photocathode gun to generatehigh charge (1-5 nC) and long (50 ps) micropulses.
Advantages:Capable of CW operationHigh chargeLong micropulses
Disadvantage:Cathode field limited to 25-30 MV/m
Accelerate in Low frequency RF cavities.Advantages:
Minimizes wakefieldsCW operation
Disadvantage:Accelerating gradient limited to 5 MV/m
Linearize and compress to high peak current at 20 MeV or higher.Advantages:
Linearizing improves compressionReduces space charge emittance growth
Disadvantage: Emittance growth due to coherence synchrotron radiation
Excellent Beam Qualityat High Beam Current
D.H. Dowell/MIT Talk, May 31, 2002 7
RF Gun433 MHz
BoosterAccelerator Section
433 MHz
LongitudinalLinearizer
1.3 GHz
BunchCompressor
Main Accelerator1.3 GHz
K2SbCsCathode
PhotoInjector
Layout of the 433 MHz PhotoInjector
D.H. Dowell/MIT Talk, May 31, 2002 8
2 MeVElectron Beam
527 nmDrive Laser Beam
CsKSbPhotocathode
RF Cavities
Defocusing and Focusing RF Lenses
Focusing Injector Coil
Electron Beam Optics of the 433 MHz Photocathode Gun
fE
Erfbeam
gain
21
sin
Cathode B-field bucking coil
D.H. Dowell/MIT Talk, May 31, 2002 9
The Boeing 433 MHz RF Photocathode Gun
D.H. Dowell/MIT Talk, May 31, 2002 10
Photocathode Performance:Photosensitive Material: K2CsSb MultialkaliQuantum Efficiency: 5% to 12%Peak Current: 45 to 132 amperesCathode Lifetime: 1 to 10 hoursAngle of Incidence: near normal incidence
Gun Parameters:Cathode Gradient: 26 MV/meterCavity Type: Water-cooled copperNumber of cells: 4RF Frequency: 433 x106 HertzFinal Energy: 5 MeV(4-cells)RF Power: 600 x103 WattsDuty Factor: 25%, 30 Hertz and 8.3 ms
Laser Parameters:Micropulse Length: 53 ps, FWHMMicropulse Frequency: 27 x106 HertzMacropulse Length: 10 msMacropulse frequency: 30 HertzWavelength: 527 nmCathode Spot Size: 3-5 mm FWHMTemporal and Transverse Distribution: gaussian, gaussianMicropulse Energy: 0.47 microjouleEnergy Stability: 1% to 5%Pulse-to-pulse separation: 37 nsMicropulse Frequency: 27 x106 Hertz
Gun Performance:Emittance (microns, RMS): 5 to 10 for 1 to 7 nCoulombCharge: 1 to 7 nCoulombEnergy: 5 MeVEnergy Spread: 100 to 150 keV
Demonstrated Performance of 433 MHz Photocathode Gun, 1992 H-D Test
D.H. Dowell/MIT Talk, May 31, 2002 11
Types of Photocathodes
Material QE Range Drive Laser Wavelength
Cat
ho
de
Fab
.
Vac
uu
m R
eq.
Dri
ve L
aser
Metal ~0.02-0.06% 260 nm, UV None 10-7 T Difficult (Cu, Mo…)
CsK2Sb 10-14% 527 nm Difficult 10-10 T Moderate
CsTe 10-14% 260 nm Easy 10-9 T Moderate to Difficult
LaB6 ~0.1% 355 nm Easy 10-7 T Difficult
Ga As (Cs) 1-5% 527 nm Moderate 10-11 T Moderate
D.H. Dowell/MIT Talk, May 31, 2002 12
Semi-Conductor Photocathode Fabrication Chamber
VacuumValve
Connectionto Gun Cavity
QE MeasurementLaser (GreNe)
RGA Head
Thin Film Monitor
Sb, K, CsSources
N2
Inlet/Outlet
2 meter Cathode Stick
D.H. Dowell/MIT Talk, May 31, 2002 13
0
2
4
6
8
10
12
14
16
6/12
/95
7/12
/95
10/2
4/95
11/1
4/95
12/1
3/95
1/3/
961/
23/9
62/
1/96
2/13
/96
2/28
/96
3/19
/96
5/2/
965/
14/9
65/
23/9
66/
3/96
Fabrication Date
Qu
antu
m E
ffic
ien
cy (
%)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4
QE x Drive Laser (% microjoules)A
ccel
erat
ed M
icro
pu
lse
Ch
arge
(n
C)
4.3 x 3.8 mm FWHM
2.8 x 2.7 mm FWHM
4.386 QE x Elaser x 0.72
QE Fabrication History and Gun Space Charge Limits
D.H. Dowell/MIT Talk, May 31, 2002 14
0.1
1
10
100
1000
10000
100000
1.00E-12 1.00E-11 1.00E-10 1.00E-09
WATER PARTIAL PRESSURE (TORR)
1/e
LIF
ET
IME
(H
OU
RS
)
Fabrication Chamber
RF Cavities (original-vacuum)
Least Squares Fit
RF Cavities (improved-vacuum)
Cathode Lifetime Vs. H2O Partial Pressure
D.H. Dowell/MIT Talk, May 31, 2002 15
Photocathode 1/e Lifetime Vs. Duty Factor
0.1
1
10
0 0.05 0.1 0.15 0.2 0.25 0.3 Duty Factor
2.3 Hour Lifetime
1/e
Lif
etim
e (H
ours
)
D.H. Dowell/MIT Talk, May 31, 2002 16
5.50%
6.00%
6.50%
7.00%
7.50%
8.00%
0 20 40 60 80 100 120 140
Time (minutes)
QE
(%
)
0
20
40
60
80
100
120
140
Cat
ho
de
Tem
per
atu
re (
deg
rees
C)
Temperature
QE
Cathode Rejuvenationand
Improving Lifetime by Operating with Hot Cathode
0
20
40
60
80
100
120
140
0 20 40 60 80 100
Time (minutes)
Ca
tho
de
Te
mp
era
ture
(d
eg
ree
s C
)
0
1
2
3
4
5
6
QE
(%
)Temperature
QE
Rejuvenating a used K2CsSb cathode by heating it to 120 degrees C.
The quantum efficiency increases at the rate of 2.5% / hour
Photocathode quantum efficiency at elevated temperature in the
RF cavity vacuum
D.H. Dowell et al., NIM A356(1995)167-176
D.H. Dowell/MIT Talk, May 31, 2002 17
Heating the Cathode With a
High Power Diode Laser
2 MeVElectron Beam
527 nmDrive Laser Beam
K2CsSbPhotocathode
RF Cavities
Cathode B-field bucking coil
800 nmHeater Laser Beam
D.H. Dowell/MIT Talk, May 31, 2002 18
Drive Laser Configuration Used in 1992 High Duty Test
108 MHz Modelocked Nd:YLF oscillator
54 MHzPockels
Cell
27 MHzPockels
Cell
FaradayIsolator
LBO Crystal
Nd:YLF Amplifier Heads
OutputPockels
Cell
ToCathode
D.H. Dowell/MIT Talk, May 31, 2002 19
Beam Emittance at 3 nCGaussian-Gaussian Distributions
0
5
10
15
20
25
30
35
40
260 280 300 320 340 360 380 400 420
Injector Coil Current (amperes)
Nor
mal
ized
RM
S E
mit
tanc
e (m
icro
ns)
High Duty Test
New Data
PARMELA
3 nC per Micropulse
Emittance = 1.0 + 3.2 * (Micropulse Charge, nC)
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5 6 7 8 9 10
MICROPULSE CHARGE (nC)
EM
ITT
AN
CE
(rm
s, m
icro
ns)
Gun Emittance Vs.
Microbunch Charge
433 MHz Gun Transverse Beam Quality Measurements1992 and 1994-1996 Test Results
D.H. Dowell/MIT Talk, May 31, 2002 20
PARMELA_B Simulations at 0.5 nC
Transverse RMS Emittance vs. Coil CurrentSP, Q=0.5(nC), a=0.1(cm), Brk012,015,016,017,018,019,020
0
0.5
1
1.5
2
2.5
3
300 310 320 330 340 350 360 370 380
Icoil (A)
xem
it (
pi
mm
mra
d) L=15(ps) (16)
L=20(ps) (12)
L=25(ps) (15)
L=30(ps) (17)
L=35(ps) (18)
L=40(ps) (19)
L=45(ps) (20)
PARMELA_B calculations provided by B. Koltenbah, Boeing
D.H. Dowell/MIT Talk, May 31, 2002 21
Bunch Compressor Physics
D.H. Dowell/MIT Talk, May 31, 2002 22
Non-Linearities in Bunch Compression
Long microbunches are distorted in longitudinal phase spacedue to wakefields and RF curvature.
433 MHz cavities introduce minimal wakes, but still causesignificant curvature.
Introduce a RF section at third harmonic (1300 MHz)to cancel curvature of 433 MHz booster.
Magnetic Pulse Compression Using a Third Harmonic RF LinearizerD.H. Dowell, T.D. Hayward and A.M. Vetter, Proceedings of the 1995 PAC, pp.992-994.
D.H. Dowell/MIT Talk, May 31, 2002 23
433 MHz Accelerator 1300 MHz LinearizerChicaneBuncher
StreakCamera
QuadTriplet
QuadTriplet
QuadTriplet
EmittanceMeasurements
BeamDump
The 20 MeV RF Photoinjector Demonstration
Photocathode GunLinearized energy
programming for buncher
To high voltageaccelerator
D.H. Dowell/MIT Talk, May 31, 2002 24
D.H. Dowell/MIT Talk, May 31, 2002 25
Cooling and RF Feed for 433 MHz 5-Cell Section
D.H. Dowell/MIT Talk, May 31, 2002 26
3-Cell and 5-Cell APLE Cavity Booster
3-Cell Accelerator Cavity 5-Cell Accelerator Cavities
D.H. Dowell/MIT Talk, May 31, 2002 27
1300 MHz (third harmonic)energy spectrum programming
for bunch compression
Three dipole magnetic buncherand diagnostics
1300 MHz Linearizer and Three-Dipole Chicane Compressor
D.H. Dowell/MIT Talk, May 31, 2002 28
Boeing Chicane Compressor
Achromatic chicane composed of three n=1/2 dipoles.
30o
60o
30o
19.5o 19.5o
384 mm 384 mm
600 mm
D.H. Dowell/MIT Talk, May 31, 2002 29
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
1 6 0 0
1 8 0 0
2 0 0 0
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0
T i m e ( p s )
M e V
4 0 p s
E n t r a n c e t oL i n e a r i z e r
E x i t o f L i n e a r i z e r
E x i t o f B u n c h e r
4 0 p s
C o m p r e s s e d ,N o n - L i n e a r i z e dE l e c t r o n P u l s e
C o m p r e s s e d ,L i n e a r i z e d
E l e c t r o n P u l s e
1 7 p s
9 p s
HT
1
2
H T
H
T
Time
Time
LinearizerRF Waveform
433 MHzRF Waveform
20 MeV
20 MeV
9= 2.2 Mev
Pulse compression occurs at two linearizer phases, but the pulse islinearized only at the decelerating phase and at 1/9 the 433 MHz RF field.
D.H. Dowell/MIT Talk, May 31, 2002 30
0
5
10
15
20
25
0 50 100 150 200 250 300
Peak Current (amperes)
RM
S T
rans
vers
e E
mit
tanc
e (
mm
*mR
)
Bend-Plane
Vertical-Plane
Bend Plane Emittance Growth During Pulse Compression
D.H. Dowell/MIT Talk, May 31, 2002 31
Coherent Synchrotron Radiation Induced Emittance Growth
Tail Radiation
Electron Microbunch Traveling in an Arc
CSR occurs when the bending of a relativistic electron beam allows the synchrotron radiation emitted by the tail of the microbunch to "catch up" with the head electrons. If the arc length of the bend is long enough, this radiation sweeps along the entire length of the microbunch and transfers
energy from the tail to the head. Therefore CSR tends to increase the energy of the head while lowering that of the tail.
Ref: Y.S. Derbenev et al., DESY TESLA-FEL Technical Note 95-05(1995)
D.H. Dowell/MIT Talk, May 31, 2002 32
-5 0 5 10 15 20
-50
0
50
Energy LossGradient(keV/m)
Charge Distribution
z/z
sb/ z=.25
sb/ z=1 sb/ z=4sb/ z=8 sb/ z=15
Transient CSR Radiation at the Magnetic Field Boundary
F sb xdx
x xx sb
x
1 1 3( , )
'
( ' ) '/
(x' )
x
( ) /s ez
s z 1
2
2 22
Ref: D.H. Dowell and P.G. O’Shea, “Coherent Synchrotron Radiation Induced Emittance Growth in a Chicane Buncher”, contribution to PAC’97.
D.H. Dowell/MIT Talk, May 31, 2002 33
0
5
10
15
20
25
0 50 100 150 200 250 300Peak Current (amperes)
No
rmal
ized
rm
s E
mit
tan
ce
CSR Emittance
ParmelaSC Emittance
SC+CSR Emittance
Comparison of Experiment with PARMELA and CSR Emittance Calculations.
See also recent work by P. Emma and M. Borland
D.H. Dowell/MIT Talk, May 31, 2002 34
Summary and Conclusion
Architectures of Single-Pass and Energy Recovery SASE FELs
RF Photocathode Injector Design Philosophy/Approach: Gun, Booster, Linearizer and Compressor
Review of high duty gun technology at 433 MHzCathode Lifetime
Cathode FabricationDrive LaserRF Design
Bunch compressor physicsCoherent synchrotron radiation
Technology developed in 1990’s is directly applicable to the new generation of Energy Recovered Linac SASE FELs