D.H. Dowell/MIT Talk, May 31, 2002 1 David H. Dowell Stanford Linear Accelerator Center Photocathode RF Guns and Bunch Compressors for High-Duty Factor

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


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


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


Example of Demonstrated Technology: A Low-Frequency, High-Duty Factor

433 MHz RF PhotoinjectorDeveloped by Boeing and Los Alamos


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.


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


RF Gun433 MHz

BoosterAccelerator Section

433 MHz

LongitudinalLinearizer

1.3 GHz

BunchCompressor

Main Accelerator1.3 GHz

K2SbCsCathode

PhotoInjector

Layout of the 433 MHz PhotoInjector


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


The Boeing 433 MHz RF Photocathode Gun


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


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


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


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


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


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

)


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


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


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


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


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


Bunch Compressor Physics


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.


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



Cooling and RF Feed for 433 MHz 5-Cell Section


3-Cell and 5-Cell APLE Cavity Booster

3-Cell Accelerator Cavity 5-Cell Accelerator Cavities


1300 MHz (third harmonic)energy spectrum programming

for bunch compression

Three dipole magnetic buncherand diagnostics

1300 MHz Linearizer and Three-Dipole Chicane Compressor


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


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.


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


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)


-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.


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


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

Documents

D.H. Dowell/MIT Talk, May 31, 2002 1 David H. Dowell Stanford Linear Accelerator Center Photocathode RF Guns and Bunch Compressors for High-Duty Factor