David H. Dowell

1
Photocathode RF Guns and Bunch Compressors for
High-Duty Factor FELs
David H. Dowell Stanford Linear Accelerator Center
Introduction Architecture of SASE FELs RF Gun
Technologies Example A Low-Frequency, High-Duty
Factor RF Photoinjector at 433 MHz Bunch
Compressor Physics Summary and Conclusion
2
Architectures of the SASE X-Ray FEL
Single Pass, Normal Conducting gt Leutl,
VISA(ATF), SDL, LCLS
3rd Harmonic Linearizer
Bunch Compressor
Accelerator
Long Undulator
Accelerator
RF Gun
X-Rays
High Energy E-beam Dump
Energy Recovered Linac (ERL), SRF gt JLab, Cornell
3
RF Gun Technologies for the Two SASE Architectures
Single Pass, Normal Conducting
Low Duty Factor Single microbunch at 1 to 120
Hz, 0.1 to 1 nC / bunch S-band (2856MHz) , 1.6
cell, BNL Gun, 100 MV/m cathode field Metal
Cathodes Cu, Mg UV Drive Laser Freq.
Quadrupled NdGlass, Ti-Sapphire 2-10 ps(fwhm)
Energy Recovered Linac (ERL), SRF
High Duty Factor CW, microbunches at 10 to 70
MHz, 0.05 to 1 nC / bunch DC, 433 MHz, L-band
(15001300MHz) Guns 10 to 30 MV/m cathode
field Semi-Conductor Cathodes GaAs, CsTe,
CsKSb Visible Wavelength Drive Laser Frequency
Doubled NdYLF
High Duty Factor requires low-frequency gun with
semi-conductor cathode Build upon experience
gained from high power RF gun development at
Boeing LANL
4
Example of Demonstrated Technology A
Low-Frequency, High-Duty Factor 433 MHz RF
Photoinjector Developed by Boeing and Los Alamos
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 and 25 duty
factor.
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
Demonstrated Performance of 433 MHz Photocathode
Gun, 1992 H-D Test
Photocathode Performance Photosensitive
Material K2CsSb Multialkali Quantum
Efficiency 5 to 12 Peak Current 45 to 132
amperes Cathode Lifetime 1 to 10 hours Angle
of Incidence near normal incidence Gun
Parameters Cathode Gradient 26
MV/meter Cavity Type Water-cooled
copper Number of cells 4 RF Frequency 433
x106 Hertz Final Energy 5 MeV(4-cells) RF
Power 600 x103 Watts Duty Factor 25, 30
Hertz and 8.3 ms Laser Parameters Micropulse
Length 53 ps, FWHM Micropulse Frequency 27
x106 Hertz Macropulse Length 10 ms Macropulse
frequency 30 Hertz Wavelength 527
nm Cathode Spot Size 3-5 mm FWHM Temporal and
Transverse Distribution gaussian,
gaussian Micropulse Energy 0.47
microjoule Energy Stability 1 to
5 Pulse-to-pulse separation
37 ns Micropulse Frequency 27 x106
Hertz Gun Performance Emittance (microns,
RMS) 5 to 10 for 1 to 7 nCoulomb Charge 1
to 7 nCoulomb Energy 5 MeV Energy
Spread 100 to 150 keV
11
(No Transcript)
12
Semi-Conductor Photocathode Fabrication Chamber
2 meter Cathode Stick
13
QE Fabrication History and Gun Space Charge Limits
16
10
14
9
4.386 QE x E
x 0.72
8
12
laser
7
10
6
Quantum Efficiency ()
4.3 x 3.8 mm FWHM
8
Accelerated Micropulse Charge (nC)
5
6
4
3
4
2
2.8 x 2.7 mm FWHM
2
1
0
0
0
1
2
3
4
1/3/96
2/1/96
5/2/96
6/3/96
6/12/95
7/12/95
1/23/96
2/13/96
2/28/96
3/19/96
5/14/96
5/23/96
QE x Drive Laser ( microjoules)
10/24/95
11/14/95
12/13/95
Fabrication Date
14
Cathode Lifetime Vs. H2O Partial Pressure
Fabrication Chamber
100000
RF Cavities (original-vacuum)
Least Squares Fit
10000
RF Cavities (improved-vacuum)
1000
100
1/e LIFETIME (HOURS)
10
1
0.1
1.00E-12
1.00E-11
1.00E-10
1.00E-09
WATER PARTIAL PRESSURE (TORR)
15
Photocathode 1/e Lifetime Vs. Duty Factor
10
2.3 Hour Lifetime
1/e Lifetime (Hours)
1
0.1
0
0.05
0.1
0.15
0.2
0.25
0.3
Duty Factor
16
Cathode Rejuvenation and Improving Lifetime by
Operating with Hot Cathode
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
17
Heating the Cathode With a High Power Diode
Laser
18
Drive Laser Configuration Used in 1992 High Duty
Test
19
433 MHz Gun Transverse Beam Quality
Measurements 1992 and 1994-1996 Test Results
20
PARMELA_B Simulations at 0.5 nC
PARMELA_B calculations provided by B. Koltenbah,
Boeing
21
Bunch Compressor Physics
22
Non-Linearities in Bunch Compression
Long microbunches are distorted in longitudinal
phase space due to wakefields and RF
curvature. 433 MHz cavities introduce minimal
wakes, but still cause significant
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 Linearizer D.H. Dowell, T.D. Hayward and A.M.
Vetter, Proceedings of the 1995 PAC, pp.992-994.
23
(No Transcript)
24
(No Transcript)
25
Cooling and RF Feed for 433 MHz 5-Cell Section
26
3-Cell and 5-Cell APLE Cavity Booster
3-Cell Accelerator Cavity
5-Cell Accelerator Cavities
27
1300 MHz Linearizer and Three-Dipole Chicane
Compressor
1300 MHz (third harmonic) energy spectrum
programming for bunch compression
Three dipole magnetic buncher and diagnostics
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
See also recent work by P. Emma and M. Borland
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 MHz Cathode Lifetime Cathode
Fabrication Drive Laser RF Design Bunch
compressor physics Coherent synchrotron
radiation Technology developed in 1990s is
directly applicable to the new generation of
Energy Recovered Linac SASE FELs