RESEARCH ON BEAM SIZE MONITORS S. Csorna, Vanderbilt U. LCCOM2, June 30, ’02, Santa Cruz

1. 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.)

2. W. M. Keck-Vanderbilt Free Electron Laser Center Facilities W. Gabella, B. Feng, J. Kozub and D. Piston BiOS 2002 – Biomedical Optics and Application, 4633B-31 • College of Arts and Sciences • College of Engineering • School of Medicine

3. VANDERBILT UNIVERSITY FEL pulse structure (Mark-III) • FEL macropulse: • repetition rate: 1-30 Hz • electron duration 8 ms • IR pulse duration ~ 3-5 ms • FEL micropulse: • pulse duration ~1 ps • pulse separation ~ 350 ps (pulse-to-pulse thermal confinement)

4. Proposal for non-intercepting beam size diagnosis using coherent diffraction radiation from a slit

5. 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 DRTR • Bunch length/bunch shape measurement: • I()= I1()[N+N(N-1)F()] • F() = Bunch Form Factor = S(z)exp[i  /c z]dz2 • Density Distribution:

6. Asymmetric electron bunch In case of asymmetric electron bunch: where s is the wave number of radiation, Y(s) the phase calculated from the observed form factor by the Kramers-Kronig relation:

7. Angular distribution from DR Parallel polarization Normal polarization

8. Martin-Puplett Interferometer Electron Beam Beam Dump Coherent Light Parabolic mirror Polarization Splitter Detector 2 mirror S1 S2 Movable mirror Detector 1

9. Proposal Studies • Radiator • Longitudinal Bunch Length Experiments • Transverse Beam Dimension Experiments • Studies of Diffraction Radiation

10. BUDGET 1. Design, construction and deployment of target/slit 2000.00 2. Wire or graphite polarizer/beam splitter for interferometer 5000.00 3. Parabolic reflector (mirror) 1000.00 4. Stage for moving mirror in the interferometer 5000.00 5. 2 Golay Cell Detectors (@5000$ each) 10000.00 TOTAL 23,000.00

11. Laser Interferometer/Laser Wire Studies Transverse beam spot size is determined by using the laser interference fringe as a physical scale. Scan the electron beam through the fringes and measure the Compton scattered g rays from the photon target.

12. The Modulation Depth M = (Signal Max – Signal Avg)/Signal Max of the g ray flux is related to the transverse beam size: MUncorrected=½cos q½exp (-(1/2) (2psy /p)2) q= angle between laser beams p=fringe pitch= l/(2 sin(q/2)= p/ky (adjust by changing q or using higher harmonics) Corrections : 1/ Gaussian laser profile 2/ Power imbalance of laser beams can affect contrast 3/ 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: 70±6 nm

13. 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 by laser 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.

16. Scan through relevant beat frequencies, at each frequency measure the difference M. You get: M=(Ngmax - Ngmin)/( Ngmax + Ngmin) = (correction factor for laser radius and transverse bunch size) ´ F(wbeat)/F(0) Here F(wbeat) is the Fourier spectrum of the bunch and would look like:

17. 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. -------------------------------------------------------------------------------------------------------------------------- 

18. Electron-Beam Brightness is More Important than Current by Charles A. Brau Department of Physics Vanderbilt University

20. The quantum efficiency for 266-nm pulses increases sharply at low fields and approaches unity for high fields

21. We have achieved several orders of magnitude improvement in normalized brightness Observed currents ~ 100 mA from 0.5-mm tips produce current densities ~ 1011 A/m2 with an estimated normalized brightness ~ 1016 A/m2-steradian • Applications • Far Infrared (100-500 mm) FEL • UV/Soft X-ray (10-400 nm) FEL • Linear Collider

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