Transcript Photocathode RF Guns and Bunch Compressors for High

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
D.H. Dowell/MIT Talk, May 31, 2002
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Architectures of the SASE X-Ray FEL
Single Pass, Normal Conducting => Leutl, VISA(ATF), SDL, LCLS
RF Gun
3rd Harmonic Bunch
Accelerator Linearizer Compressor
Accelerator
Long Undulator
X-Rays
High Energy
E-beam
Dump
Energy Recovered Linac (ERL), SRF => JLab, Cornell
Accelerator
RF Gun
3rd Harmonic
Linearizer
SRF
Accelerator
Bunch
Compressor
Bend/
De-Compressor
SRF
Accelerator
Bend/
Compressor
Low Energy
E-beam
Dump
Long Undulator
X-Rays
D.H. Dowell/MIT Talk, May 31, 2002
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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 Nd:Glass, 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 (1500&1300MHz) Guns: 10 to 30 MV/m cathode field
Semi-Conductor Cathodes: GaAs, CsTe, CsKSb
Visible Wavelength Drive Laser: Frequency Doubled Nd:YLF
High Duty Factor requires low-frequency gun with semi-conductor cathode
Build upon experience gained from high power RF gun development at Boeing & LANL
D.H. Dowell/MIT Talk, May 31, 2002
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Example of Demonstrated Technology:
A Low-Frequency, High-Duty Factor
433 MHz RF Photoinjector
Developed by Boeing and Los Alamos
D.H. Dowell/MIT Talk, May 31, 2002
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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.
D.H. Dowell/MIT Talk, May 31, 2002
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Photoinjector Design Philosophy
Use a CW low frequency photocathode gun to generate
high charge (1-5 nC) and long (50 ps) micropulses.
Advantages:
Capable of CW operation
Excellent Beam Quality
High charge
at High Beam Current
Long micropulses
Disadvantage:
Cathode field limited to 25-30 MV/m
Accelerate in Low frequency RF cavities.
Advantages:
Minimizes wakefields
CW operation
Disadvantage:
Accelerating gradient limited to 5 MV/m
Linearize and compress to high peak current at 20 MeV or higher.
Advantages:
Linearizing improves compression
Reduces space charge emittance growth
Disadvantage:
Emittance growth due to coherence synchrotron radiation
D.H. Dowell/MIT Talk, May 31, 2002
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Layout of the 433 MHz PhotoInjector
PhotoInjector
Booster
RF Gun Accelerator Section
433 MHz
433 MHz
K2SbCs
Cathode
D.H. Dowell/MIT Talk, May 31, 2002
Bunch
Compressor
Main Accelerator
1.3 GHz
Longitudinal
Linearizer
1.3 GHz
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RF Cavities
Cathode B-field
bucking coil
Electron Beam Optics of the
433 MHz Photocathode Gun
527 nm
Drive Laser Beam
CsKSb
Photocathode
2 MeV
Electron Beam
f rf  2
Ebeam 1
E gain sin 
Defocusing and Focusing RF Lenses
Focusing Injector Coil
D.H. Dowell/MIT Talk, May 31, 2002
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The Boeing 433 MHz RF Photocathode Gun
D.H. Dowell/MIT Talk, May 31, 2002
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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
D.H. Dowell/MIT Talk, May 31, 2002
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Types of Photocathodes
Material
QE Range
Metal
(Cu, Mo…)
~0.02-0.06%
Drive Laser Wavelength
260 nm, UV
None
10-7 T
Difficult
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
D.H. Dowell/MIT Talk, May 31, 2002
Moderate
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Semi-Conductor Photocathode
Fabrication Chamber
Sb, K, Cs
Sources
Thin Film
Monitor
2 meter
Cathode Stick
Connection
to Gun
Cavity
N2
Inlet/
Outlet
RGA Head
Vacuum
Valve
QE Measurement
Laser (GreNe)
D.H. Dowell/MIT Talk, May 31, 2002
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QE Fabrication History and Gun Space Charge Limits
Accelerated Micropulse Charge (nC)
10
14
12
10
8
6
4
2
0
6/12/95
7/12/95
10/24/95
11/14/95
12/13/95
1/3/96
1/23/96
2/1/96
2/13/96
2/28/96
3/19/96
5/2/96
5/14/96
5/23/96
6/3/96
Quantum Efficiency (%)
16
9
4.386 QElaser
x xE0.72
8
7
6
4.3 x 3.8 mm FWHM
5
4
3
2
2.8 x 2.7 mm FWHM
1
0
0
1
2
3
4
QE x Drive Laser (% microjoules)
Fabrication Date
D.H. Dowell/MIT Talk, May 31, 2002
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Cathode Lifetime Vs. H2O Partial Pressure
Fabrication Chamber
100000
RF Cavities (original-vacuum)
Least Squares Fit
10000
1/e LIFETIME (HOURS)
RF Cavities (improved-vacuum)
1000
100
10
1
0.1
1.00E-12
1.00E-11
1.00E-10
1.00E-09
WATER PARTIAL PRESSURE (TORR)
D.H. Dowell/MIT Talk, May 31, 2002
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Photocathode 1/e Lifetime Vs. Duty Factor
1/e Lifetime (Hours)
10
2.3 Hour
Lifetime
1
0.1
0
0.05
D.H. Dowell/MIT Talk, May 31, 2002
0.1
0.15
0.2
Duty Factor
0.25
0.3
15
Cathode Rejuvenation
and
Improving Lifetime by Operating with Hot Cathode
6
120
5
100
Temperature
4
80
3
60
QE
2
40
1
20
0
0
0
20
40
60
80
100
Time (minutes)
140
120
Photocathode quantum efficiency
at elevated temperature in the
RF cavity vacuum
QE (%)
7.50%
Temperature
7.00%
80
6.50%
60
QE
40
6.00%
20
5.50%
D.H. Dowell et al., NIM A356(1995)167-176
100
0
0
20
40
60
80
100
120
140
Time (minutes)
D.H. Dowell/MIT Talk, May 31, 2002
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Cathode Temperature
(degrees C)
8.00%
QE (%)
Rejuvenating a used K2CsSb cathode
by heating it to 120 degrees C.
The quantum efficiency increases
at the rate of 2.5% / hour
Cathode Temperature
(degrees C)
140
RF Cavities
Cathode B-field
bucking coil
Heating the Cathode
With a
High Power Diode Laser
527 nm
Drive Laser Beam
K2CsSb
Photocathode
2 MeV
Electron Beam
800 nm
Heater Laser Beam
D.H. Dowell/MIT Talk, May 31, 2002
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Drive Laser Configuration
Used in 1992 High Duty Test
108 MHz Modelocked Nd:YLF oscillator
54 MHz
Pockels
Cell
To
Cathode
Nd:YLF Amplifier Heads
Faraday
Isolator
Output
Pockels
Cell
27 MHz
Pockels
Cell
LBO Crystal
D.H. Dowell/MIT Talk, May 31, 2002
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433 MHz Gun Transverse Beam Quality Measurements
1992 and 1994-1996 Test Results
Beam Emittance at 3 nC
Gaussian-Gaussian Distributions
Gun Emittance
Vs.
Microbunch Charge
EMITTANCE (rms, microns)
16
14
Emittance = 1.0 + 3.2 * (Micropulse Charge, nC)
12
10
8
6
4
2
0
0
1
2
3
4
5
6
7
MICROPULSE CHARGE (nC)
8
9
10
Normalized RMS Emittance (microns)
40
3 nC per Micropulse
35
High Duty Test
New Data
PARMELA
30
25
20
15
10
5
0
260
280
300
320
340
360
380
400
Injector Coil Current (amperes)
D.H. Dowell/MIT Talk, May 31, 2002
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420
PARMELA_B Simulations at 0.5 nC
Transverse RMS Emittance vs. Coil Current
SP, Q=0.5(nC), a=0.1(cm), Brk012,015,016,017,018,019,020
3
xemit (pi mm mrad)
2.5
L=15(ps) (16)
2
L=20(ps) (12)
L=25(ps) (15)
1.5
L=30(ps) (17)
L=35(ps) (18)
1
L=40(ps) (19)
L=45(ps) (20)
0.5
0
300
310
320
330
340
350
360
370
380
Icoil (A)
PARMELA_B calculations
provided by B. Koltenbah, Boeing
D.H. Dowell/MIT Talk, May 31, 2002
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Bunch Compressor Physics
D.H. Dowell/MIT Talk, May 31, 2002
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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.
D.H. Dowell/MIT Talk, May 31, 2002
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The 20 MeV RF Photoinjector Demonstration
433 MHz Accelerator
1300 MHz Linearizer
Quad
Triplet
Photocathode Gun
Chicane
Buncher
Quad
Triplet
Linearized energy
programming for buncher
D.H. Dowell/MIT Talk, May 31, 2002
Streak Emittance
Camera Measurements
Beam
Dump
Quad
Triplet
To high voltage
accelerator
23
D.H. Dowell/MIT Talk, May 31, 2002
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Cooling and RF Feed for 433 MHz 5-Cell Section
D.H. Dowell/MIT Talk, May 31, 2002
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3-Cell and 5-Cell APLE Cavity Booster
3-Cell Accelerator Cavity
D.H. Dowell/MIT Talk, May 31, 2002
5-Cell Accelerator Cavities
26
1300 MHz Linearizer and
Three-Dipole Chicane Compressor
1300 MHz (third harmonic)
energy spectrum programming
for bunch compression
D.H. Dowell/MIT Talk, May 31, 2002
Three dipole magnetic buncher
and diagnostics
27
Boeing Chicane Compressor
19.5o
19.5o
600 mm
30o
30o
384 mm
60o
384 mm
Achromatic chicane composed of three n=1/2 dipoles.
D.H. Dowell/MIT Talk, May 31, 2002
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Pulse compression occurs at two linearizer phases, but the pulse is
linearized only at the decelerating phase and at 1/9 the 433 MHz RF field.
40 ps
40 ps
Linearizer
RF Waveform
Entrance to
Linearizer
MeV
T
20 MeV
H
Exit of Linearizer
2
9
T
Exit of Buncher
= 2.2 Mev
Time
H
1
Compressed,
Linearized
Electron Pulse
Compressed,
Non-Linearized
Electron Pulse
2000
1800
433 MHz
RF Waveform
1600
20 MeV
H
1400
T
1200
1000
800
9 ps
600
400
Time
17 ps
200
0
0
20
40
60
80
100
120
140
160
180
200
Time (ps)
D.H. Dowell/MIT Talk, May 31, 2002
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Bend Plane Emittance Growth During Pulse Compression
25
RMS Transverse Emittance
(mm*mR)
Bend-Plane
20
Vertical-Plane
15
10
5
0
0
50
100
150
200
250
300
Peak Current (amperes)
D.H. Dowell/MIT Talk, May 31, 2002
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Coherent Synchrotron Radiation Induced
Emittance Growth
Electron Microbunch Traveling in an Arc
Tail Radiation
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
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Transient CSR Radiation at the Magnetic Field Boundary
Charge Distribution
 ( s) 
50
sb/ sz=1
Energy Loss
Gradient
(keV/m)
sb/ sz=4
sb/ sz=8
1
2 s z
es
2
/ 2s 2z
sb/ sz=15
sb/ sz=.25
0
x
dx'
x  sb
( x  x' ) 1/ 3
F1 ( sb, x )  
  (x' )
 x'
-50
-5
0
5
10
15
20
z/sz
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
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Normalized rms Emittance
Comparison of Experiment with
PARMELA and CSR Emittance Calculations.
25
20
SC+CSR Emittance
Parmela
SC Emittance
15
CSR Emittance
10
5
0
0
50
100
150
200
250
Peak Current (amperes)
300
See also recent work by P. Emma and M. Borland
D.H. Dowell/MIT Talk, May 31, 2002
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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 1990’s is directly applicable to the
new generation of Energy Recovered Linac SASE FELs
D.H. Dowell/MIT Talk, May 31, 2002
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