brookhaven science associates u.s. department of energy a few comments on the photoinjector...

Brookhaven Science AssociatesU.S. Department of Energy


A Few Comments on the Photoinjector Performance

X.J. WangNational Synchrotron Light SourceBrookhaven National Laboratory

Upton, NY 11973, USA

Presented at the UCLA High-Power Brightness Beams WorkshopNov.9, 2004


AcknowledgementAcknowledgement

I would like to thank many colleague who have educated me over many years on various subjects I mentioned here,

BNL: M. Babzien, K. Batchelor, I. Ben-Zvi, X.Y. Chang, A. Doyuran, J. Fisher, W. Graves, H. Loss, J. Murphy, T. Rao, J. Rose, B. Sheehy, J. Sheen, Z. Wu, V. Yakimenko and L.H. Yu

ANL: S. Biedron, M. Conde, W. Gai, J. Lewellen, S. Milton, J. Power

SLAC: R. Miller, D.T. PalmerJapan: A. Endo, K. Kobayashi, F. Sakai, J. Urakawa, Uesaka and

M. Washio

And Many others. Thank you!


BNL NSLS PERL Gun Workshop (1/2001)BNL NSLS PERL Gun Workshop (1/2001)

• High QE Green or Red Cathode.

• 2 ½ Cell low temperature RF gun

•DC and RF gun both capable of CW operation.

Dc: 10 mm-mrad

RF: 1.0 mm-mrad


Challenges in High-Brightness Electron Source R&D

Challenges in High-Brightness Electron Source R&D

• Stability and Reliability• Timing jitter and its control• Better Theoretical Understanding• Thermal Emittance – fundamental limit and

importance of beam instrumentation• Improve performance – 6-D optimization• Next Generation Electron source:1. CW injector –DC, RF, SRF, what should be? 3H -

Heat, Heat and Heat;2. Brighter sources - Higher field gun, pulse DC

gun, laser plasma source and others.


High Brightness Electron InjectorsHigh Brightness Electron Injectors

500 kV Spring-8 DC Injector

How to create the Greenfield FEL injector?

• Optimize 6-D phase space, not just Optimize 6-D phase space, not just nn or I or Ip p

• To realize this the GFEL injector should To realize this the GFEL injector should achieve:achieve:

G > 500 MV/m , E > 50 MeV

in order to produce and preserve the beam.

Type DC Gun RF Gun GreenField

E [MeV] 0.5 5 5050G[MV/m] 10 100 500500 [ps] 500 10 <1Ip [A] 10 100 500Q [nC] 0.5 1 <0.5n [m] 1 1 0.1

BNL RF Photoinjector


XFEL vs Linear Collider

M. Brandin et al, CLIC Note 543 (2002)


ISSUESISSUES

•Introduction – What are the performance and Applications

•Vacuum and QE do Matter. •6-D Performance Optimization •Thermal Emittance•Timing Jitter – What is required?•Theoretical Challenges of RF Gun •Femto-second Kilo-Ampere Electron

Beam Generation - Longitudinal Emittance Compensation


They all driven by a photocathode RF Gun Based Linac

They all driven by a photocathode RF Gun Based Linac


IntroductionIntroduction

All the FEL reach saturation does not test the limit of the emittance performance:

1. LEUTL and TTF I < 6-10 mm-mrad2. VISA <2.0 - 2.5 mm-mrad3. DUV-FEL < 5 mm-mra

Does Any Physics Experiment Test the limit:1. Laser Compton Scattering < 2 mm-mrad2. IFEL Micro-bunching - Stella Experiment <

1-2 mm-mrad


Why Photocathode RF Gun

Why Photocathode RF Gun

• 6-D performance – smaller emittance and shorter bunch.

• Flexibility.

But it bring more issues, mainly laser and cathode:

• Stability • Reliability• Uniformity – QE, transverse and

longitudinal distribution• Jitters – position and time


Vacuum and QE Do Matter

Vacuum and QE Do Matter

(at Room Temperature)

Pressure(Torr)

Molecular Density(molec./cm3)

Molecular Incidence(molec./cm2sec)

Mean FreePath(cm)

MonolayerFormation Time

(sec)

760 2.49 x 1019 2.87 x 1023 3.9 x 10-6 1.7 x 10-9

1 3.25 x 1016 3.78 x 1020 5.1 x 10-3 2.2 x 10-6

10-3 3.25 x 1013 3.78 x 1017 5.1 2.2 x 10-3

10-6 3.25 x 1010 3.78 x 1014 5.1 x 103 2.2 x 100

10-9 3.25 x 107 3.78 x 1011 5.1 x 106 2.2 x103 (37 min)

10-12 3.25 x 104 3.78 x 108 5.1 x 109 2.2 x 106 (25.5 days)


Laser-Induced Explosive Emission(X.J. Wang et al, J. Appl. Phys. 72(3), 888-894 (1992))

Laser-Induced Explosive Emission(X.J. Wang et al, J. Appl. Phys. 72(3), 888-894 (1992))


What We would like Photoinjector doWhat We would like Photoinjector do

Programmable in both transverse and longitudinal distribution

•No timing jitter

•No energy fluctuation

•Perfect point stability

•7/24 available

•Remote controllable

•NO laser physicist.

rms peak

Timing jitter 50 - 100, fs 200 - 400, fs

energy 1,% 5,%

Point stability 0.25, 1,%

Transverse uniformity

2.5,% 10,%


Photo-injector Beam Diagnostics

•Energy

•Charge

•RF Gun Phase

150 160 170 180 190 200 210 2203.8

3.9

4

4.1

4.2

4.3

4.4

4.5

Relative RF Gun Phase (deg.)

Pho

toel

ectr

on B

eam

Ene

rgy

(MeV

)


Quantum Efficiency MeasurementsQuantum Efficiency Measurements

0 2 4 6 8 10 12 140.5

1

1.5

2

2.5

3

3.5

Laser Energy ( J)

Ph

oto

ele

ctro

n C

ha

rge

(n

C)

Ocotober, 1999March, 2002


Stability and Reliability Leads To Better Performance


Thermal EmittanceThermal Emittance

22 cm

Er

e

kth

0 10 20 30 40 50 60 70 800.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

RF phase (degrees)

N (

mm

-mra

d)

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

1

1.2

Horizontal RMS laser size (mm)

N (m

m-m

rad)

Electrons are emitted with a kinetic energy Ek

laser spot assumed uniformwith radius r

RFRFRFk EhE sin

Example of measurement for Cu-cathode

AG EE or ,

(Courtesy of W. Graves)

Linear fit gives Ek=0.43 eVNonlinear fit gives rf=3.1+/-0.5, cu=4.73+/-0.04 eV, and Ek=0.40 eV

ICFA/BD Sardinia July 2002


Mg thermal EmittanceMg thermal Emittance

., constaiswhereL res

quadresres

8.2 mlinac

SolenoidRF gunQuadrupol

e

BPM

Faraday Cup


Slice Emittance


RF Photoinjector TheoryRF Photoinjector Theory

• Are all emittance uncorrelated?

K-J.’s theory:

Emittnace growth (Rieser):

scrfther222

2/1

02

0515

~1

w

UxNr

in

c

ni

nf

)(sin

11

4 0

AI

I

k xA

scnx


• 1300 MHz• Eb = 15-20 MeV• Imacro = 100-400 mA• Q = 1-4 nC• rms = 1.6 mm-mrad• = 0.2%• Injection = 30o

• Solenoid = 300A• Bucking Sol. = 310A

The Advanced FEL Photoinjector Operates at 20 MV/m Gradient and 200 mA Average Current

(D. Nguyen’s talk at BNL PERL workshop, Jan 2001)


Emittance Optimization at 45 MeV


Timing jitter effects – photocathode RF gun

Timing jitter effects – photocathode RF gun

15 20 25 30 35 40 45 50 550

1

2

3

4

5

6

RF Gun Phase (Deg.)

Ele

ctro

n b

ea

m b

unch

leng

th (

FW

HM

.ps)

40 pC

80 pC

150 pC

200 pC

Jitter smaller < 200 fs (rms)


Longitudinal Emittance Compensation

15 20 25 30 35 40 45 50 550

1

2

3

4

5

6

RF Gun Phase (Deg.)

Ele

ctro

n b

ea

m b

unch

leng

th (

FW

HM

.ps)

40 pC

80 pC

150 pC

200 pC

25 30 35 40 45 50 55 60 650.5

1

1.5

2

2.5

3

3.5

RF Gun Phase (Deg.)

No

rma

lize

d R

MS

Em

itta

nce

(m

m-m

rad

)

¬ 100 pC

¬ 300 pC

450 pC ®

•Phys. Rev. E. 54, R3121 (1996)

• PAC 97

Energy vs. initial phase for Ecathode=25.50,100Mv/m

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 50 100 150

25Mv/m

50Mv/m

100Mv/m


Linac 2000)


BuncherBuncher

head

tail


M r e c y1 p s

1 %

20 pC at 40 MeV

Presented at the linac 2000


10 fs kilo-Ampere Electron beam generation

10 fs kilo-Ampere Electron beam generation

20pc,100Mv/m,drift=3.05,12.0degree,8ps,R=0.75mm,Bf=0.70 sigma-z

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

1.40E+03

0.00E+

00

1.00E+

02

2.00E+

02

3.00E+

02

4.00E+

02

5.00E+

02

6.00E+

02

7.00E+

02

8.00E+

02

5 1.5 0.0039616 1.529 0.0039047 1.57 0.0038468 1.623 0.0037879 1.687 0.003727

10 1.764 0.00366611 1.853 0.00360412 1.952 0.00354113 2.057 0.00347714 2.17 0.00341315 2.287 0.00334716 2.41 0.00328117 2.537 0.00321418 2.618 0.00317319

20pc,100Mv/m ,drift=3.05,12.0degree,8ps,R=0.75m m ,Bf=0.70 em ittance-z

0

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150 200 250

20pc,100Mv/m ,drift=3.05,12.0degree,8ps,R=0.75m m ,Bf=0.70 em ittance-x

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 50 100 150 200 250

20pc,100Mv/m ,drift=3.05,12.0degree,8ps,R=0.75m m ,Bf=0.70

s igm a-x

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

1.20E-01

1.40E-01

0.00E+00 2.00E+02 4.00E+02 6.00E+02 8.00E+02

20pc,100Mv/m ,drift=3.05,12.0degree,8ps,R=0.75m m ,Bf=0.70 s igm a-z

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

1.40E+03

0.00E+0

0

1.00E+0

2

2.00E+0

2

3.00E+0

2

4.00E+0

2

5.00E+0

2

6.00E+0

2

7.00E+0

2

8.00E+0

2

20pc,100Mv/m,drift=3.05,12.0degree,8ps,R=0.75mm,Bf=0.70 energy

0

5

10

15

20

25

30

35

40

45

0 100 200 300 400 500 600 700 800

20pc,100Mv/m,drift=3.05,12.0degree,8ps,R=0.75mm,Bf=0.70 de/e

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 50 100 150 200 250

10. 8 fs


0 1 2 3 4 5 6 7 80

200

400

600

800

1000

1200

1400

Distance (m)

RM

S b

un

chle

ng

th (

fs)

RMS bunch length increase from12.9 to 19.3 fs

0 1 2 3 4 5 6 7 80.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Distance (m)

No

rma

lize

d R

MS

em

itta

nce

(m

m-m

rad

) no focusingfocusing

Charge: 50 pCrms bunchlength:19 fs


Summary of Kilo-Ampere Electron Beam Summary of Kilo-Ampere Electron Beam

20 40 60 80 100 120 140 160 180 20010

20

30

40

50

60

70

80

90

Charge(pC)

RM

S B

unch

leng

th(f

s)

20 40 60 80 100 120 140 160 180 2000.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Charge (pC)

RM

S e

mitt

an

ce (

mm

-mra

d)

X.J. Wang and X.Y. Chang, NIMA 507 (2003) 310–313

brookhaven science associates u.s. department of energy a few comments on the photoinjector...

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