RESEARCH ON BEAM SIZE MONITORS

S. Csorna, Vanderbilt U.

LCCOM2, June 30, ’02, Santa Cruz

• Proposal for Non-Intercepting Beam Size Diagnosis Using Diffraction Radiation. Feng/Gabella/Csorna (Vanderbilt U.)

• Laser Interferometry/Laser Wire Studies. Csorna/Ernst/Hartill(Vanderbilt,Albany,Cornell)

• Electro-Optic Technique for beam size measurements. Gabella/Feng (Vanderbilt U.)

• Bright Needle Electron Sources. Brau (Vanderbilt U.)

W. M. Keck-Vanderbilt Free Electron Laser

Center Facilities

College of Arts and Sciences

• College of Engineering• School of Medicine

W. Gabella, B. Feng, J. Kozub and D. Piston

BiOS 2002 – Biomedical Optics and Application, 4633B-31

• FEL macropulse:– repetition rate: 1-30 Hz– electron duration 8 s– IR pulse duration ~ 3-5 s

• FEL micropulse:– pulse duration ~1 ps– pulse separation ~ 350 ps

(pulse-to-pulse thermal confinement)

FEL pulse structure (Mark-III)VANDERBILT UNIVERSITYVANDERBILT UNIVERSITY

Proposal for non-intercepting beam sizediagnosis using coherent diffraction radiation from a slit

Advantages:

• Non-invasive (beam goes through a slit).

• Easy Extraction of signal since Diffraction Radiation (DR) directed perpendicular to beam if slit inclined at 45 degrees.

• Responds rapidly to changes in beam, inherently compact, could be installed in many locations.

• Intensity proportional to 2

• In the limit of zero slit width DRTR

Bunch length/bunch shape measurement:

I()= I1()[N+N(N-1)F()]

F() = Bunch Form Factor = S(z)exp[i /c z]dz2

Density Distribution:

In case of symmetric electron bunch:

S z d Fz

c

1

0

cos

6

Asymmetric electron bunch

where is the wave number of radiation, () the phase calculated from the observed form factor by the Kramers-Kronig relation:

( ) ( / ) ln ( ) / ( ) / f t f t dt2 2

In case of asymmetric electron bunch:

S z f z d( ) cos/

21 2

2

7

Angular distribution from DR

Parallel polarization Normal polarization

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Martin-Puplett Interferometer Electron Beam

Coherent Light

Beam Dump

Detector 2

Detector 1

mirror

Parabolic mirror

Movablemirror

Polarization Splitter

S1 S2

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Proposal Studies

• Radiator

• Longitudinal Bunch Length Experiments

• Transverse Beam Dimension Experiments

• Studies of Diffraction Radiation

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BUDGET

1. Design, construction and deployment of target/slit 2000.002. Wire or graphite polarizer/beam splitter for interferometer 5000.003. Parabolic reflector (mirror) 1000.004. Stage for moving mirror in the interferometer 5000.005. 2 Golay Cell Detectors (@5000$ each) 10000.00 TOTAL 23,000.00 

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Laser Interferometer/Laser Wire Studies 

Transverse beam spot size is determined by using the laserinterference fringe as a physical scale. Scan the electronbeam through the fringes and measure the Comptonscattered rays from the photon target.

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The Modulation Depth M =(Signal Max – Signal Avg)/Signal Maxof the ray flux is related to the transverse beam size:

MUncorrected=cos exp (-(1/2) (2y p)2)

= angle between laser beamsp=fringe pitch= /(2 sin(/2)= /ky

(adjust by changing or using higher harmonics)Corrections :1/ Gaussian laser profile2/ Power imbalance of laser beams can affect contrast3/ Beta Function of e beam + finite width of laser beam along beam direction, etc.Ref: See KEK preprint 96-81 by Tsumoru Shintake (tutorial) 

(PRL 74,2479(95)) SLAC FFTB: 706 nm

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WHAT ABOUT BUNCH LENGTH/BUNCH SHAPE ?

1/Don Hartill is studying the possibility of using timing information to determine bunch length/bunch shape.

2/ Longitudinal beam size can, in principle, be measured bylaser heterodyne techniques. This has not been realized experimentally…. Principle of Operation: Mix two lasers of different frequency to create an intensity modulation at the beat frequency. If the pitch of the beat wave is longer than the bunch length, large fluctuations will be seen in signal due to each pulse. If the pitch is shorter than the bunch length, the signal will be approximately constant. So you can determine a threshold, which is related to the bunch length.

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Scan through relevant beat frequencies, at each frequency measure the difference M. You get: M=(Nmax - Nmin)/( Nmax + Nmin) = (correction factor for laser radius and transverse bunch size) F(beat)/F(0)

 Here F(beat) is the Fourier spectrum of the bunch and would look like:   

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Conclusion: It is fairly clear that we need to use laser based techniques to measure nm size bunches; we hope to report a more detailed plan at next UCLC meeting.-------------------------------------------------------------------------------------------------------------------------- 

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Electron-Beam Brightness is More Important than Current

by

Charles A. Brau

Department of Physics

Vanderbilt University

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Electric Field (1010 V/m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Qua

ntum

Eff

icie

ncy

The quantum efficiency for 266-nm pulses increases sharply at low fields and

approaches unity for high fields

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We have achieved several orders of magnitude improvement in normalized brightness

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

1.E-04 1.E-02 1.E+00 1.E+02 1.E+04Current (A)

Nor

mal

ize

d B

rig

htn

ess

(A/m

2-st

era

dia

n)

rf photoinjectors storage rings

field emission thermionic emission

photoelectric field emission

Applications• Far Infrared (100-500 m) FEL

• UV/Soft X-ray (10-400 nm) FEL

• Linear Collider

Observed currents ~ 100 mA from 0.5-m tips produce current densities ~ 1011 A/m2

with an estimated normalized brightness ~ 1016 A/m2-steradian

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Conclusion: Charley Brau is currently studying the possibility of getting polarized electrons from a needle source. A detailed research plan is to follow…