1

A Proof-of Principle Study of 2D optical streaking for ultra-short e-beam diagnostics using

ionization electrons & circular polarized laser

Lanfa WangYuantao Ding and Zhirong Huang

LCLS II Physics Meeting, 5/25/2011

2

From RF (cm) to optical (m) streaking

e-

sz

2.44 m

bd bsD 90°

V(t) sy

RF‘streak’

S-band

LCLS S-band RF deflector (λS_RF = 10cm) gives resolution ~ 10fs; For short e-beam, λRF >> σz, the streaking is not efficient; X-band RF deflector helps(λX_RF = 2.6cm), one after undulator is planned; How about going to optical wavelength(um)?

• > 10 um wavelength;• typically a wiggler is required for interaction with high-E e-beam;• the required laser power ~10s GW.• synchronization is a problem.

We are proposing a new method to overcome the disadvantages (power & synchronization) using a circularly-polarized 10 um laser.

3

THz-driven x-ray streak camera

Nature Photonics, 3, 523. Both x-ray and THz are generated from the same e-beam, phase locked; X-ray and THz co-propagate at the same direction; Photoelectrons are modulated by THz and detected by TOF detector. Very similar to the RF zero-phasing method for e-beam diagnostics.

Phil Bucksbaum suggested to us long time ago about streaking the ionized electrons from high-E electron gas interaction for high-E electron bunch diagnostics. Advantage: The required laser power is lowerA lot of issued to consider, and, most difficult problem is synchronization……

4

synchronization problemLinear polarized Laser, the momentum kick due to the laser is

))(sin()()( 0 zzeEzPL

xx

D

The whole circle is just one rf period calibration; No Phasing problem.

Deflecting from circular (RF) mode

D. Alesini, DIPAC 09.

Similar as the deflecting cavity

The phase jitter causes the difficulty in the measurement!

5

2D streaking with ionization electrons & circularly-polarized laser

……

…..

……

…..

……

….

-10kV

(2) circularly-polarized laser

(1) gas nozzle

(3) DC field

(4) screen)

Beam ionization

High energy bunch

Laser beam

Ionization electronbunch

6

Interaction of Laser field with ionization electron beam

-1.5 -1 -0.5 0 0.5 1 1.5-20

-15

-10

-5

0

5

10

15

20

t ()

EL (G

V/m

)(GV/

m/p

s) FieldField Gradient

yLLL tttEtE eex )sin()cos(1

)()(2

0

With ellipticity . =0 for linear polarized laser and =1 for circular polarized laser

Polarized laser

)(0 )()( zi

L

ezeEz

D P

Z

Zi

Ionization electron beamIonization electron beam(Low energy beam, plasma electron):

It has the same profile as the high energy beamIt doesn’t move longitudinally (very slow), so the laser beam passes the whole low energy and modulates its energy(momentum) according to the electron birth time (z) ;

0/)( czz L

If E(Z) is constant during the short period of bunch pulse, then all electrons receive the same amount transverse kicker with angle linear dependence on their position in z

For a circular polarized laser, the momentum kick due to the laser is

7

Low energy beam on the screen

)(0 )()( zi

L

ezeEz

D P

Z

Zi

Ionization electron beamIonization electron beam(Low energy beam):

The low energy beam is accelerated (longitudinally, Beam direction) by the DC field to the screen

0/)( czz L

On the screen, the low energy electrons form a circle (arc) because:

The kicker strength from the circular laser is constant (approximately);And the angle linearly depends electron birth time(z)

a

H T

(z)

DV

H

T

a

R=DV*Dt

The radius of the circle depends on the laser field strength and drift time to the screen

R=DDtThe profile of the low energy electrons is translated to the angular distribution on the screen

00 2/)(

LL

zcz

2D

LhBunchlengt

8

Parameters used in simulation

Gas: Helium, pressure=1E-4Torr, assuming ionization length=1mmThere is no field ionization;Neutralization factor=0.4%, consider ionization length,the density of low energy electrons is much lower (by a factor of 1.0e5) than the density of high energy beam

Laser wave length 10mThe rms size of laser >=3 times of the beam rms sizeLaser FWHM 500fsLaser power: varies

DC voltage ~ keVRequired electron density ~ 3e9/mm2

9

Effect of laser phase

0

90o 180o

270o

10

On the screen: Example for 0.5/1 m bunch (Laser field only)

100 200 300 400 500 6000

0.2

0.4

0.6

0.8

1

1.2

1.4

angle(degree)

Dst

ribut

ion

after shift

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree) D

strib

utio

n

after shift

11

Other effects may spoil the distribution

……

…..

……

…..

……

….

-10kV

(2) circularly-polarized laser

(1) gas nozzle

(3) DC field

(4) screen)

Z

Zi

•High energy beam field•Field of Plasma electrons and ions

12

Effect of High energy beam field

0 200 400 600 800 1000 12000

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

energy (eV) D

strib

utio

n-15 -10 -5 0 5 10 150

2

4

6

8

10

12

14x 10

9

t (fs)

E r (V/m

)

20pC bunch1 m bunch lengthSigma_r=5 m

E-field of high energy beamEnergy distribution of low energy electrons without laser beam

13

1m bunch; 10pC; sr= 5m, peak laser field 19GV/m(0.63GW), peak beam field=7GV/m

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

head

tail

10pc bunch

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

1m bunch; 20pC; sr= 5m, peak laser field 38GV/m (1.25GW), peak beam field=13GV/m

20pc bunch

14

Effect of laser power(sr=5m)

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effectPL=0.9GW

PL=0.45GW

15

Laser power effect (sr=5m)vacuum, L=1mm, P=1e-4Torr(Helium) Neutralization factor=0.4%

PL=1.2GW

PL=1.5GW

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

w.o. beam effect

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

w.o. beam effect

PL=1.8GW

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

w.o. beam effect

16

beam size effect (L=0.2m, P=1e-4Torr)

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

sr=10m, PL=1.8GW

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

sr=15m, PL=5.0GW

RL=7mm

RL=10mm

sigr15fla25w090

sigr10fla20w060

17

Effect of laser Power & beam size

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

w.o. beam effect

PL=1.2GWPL=0.4GW PL=0.9GW

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

P=0.9GW

w.o. beam effect

100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

angle(degree)

Dst

ribut

ion

after shift

w.o. beam effect

PL=0.9GWPL=1.8GW

sr=5m

sr=10m

18

Similar idea may work for x-ray pulse measurement

……

…..

……

…..

……

….

-10kV

(2) circularly-polarized laser

(1) gas nozzle

Laser wavelength >~ xray wavelength

(3) DC field

(4) screen)

Xray- ionization

19

Summary

Circularly-polarized laser, no phase synchronization problem;Interaction in vacuum, no wiggler needed; Streaking the ionized low-E beam, required laser power is lower;

Pros

ConsComplexity : Involved many dynamics

Preliminary conclusion:This method looks promising based on the preliminary studies.Required laser power depends on the beam: 1GW for sr=5mGas pressure: 1.0e-4 Torr (mm)Space charge of low energy particles is not included

20

AcknowledgmentThanks very helpful discussions with Eric Colby, Mark Hogan and Weiming An (UCLA)

21

Linear polarized x-ray

(z) DV

P0

Need realistic model of the X-ray ionization

Assuming ionized electrons are emitted only in polarization direction (NOT accurate model!)

22