1. A Proof-of Principle Study of 2D optical streaking for ultra-short e-beam diagnostics using ionization electrons & circular polarized laser Lanfa Wang Yuantao Ding and Zhirong Huang LCLS II Physics Meeting, 5/25/2011 1

2. From RF (cm) to optical (  m) streaking eeee zzzz 2.44 m dddd ssss   90° V(t)V(t)V(t)V(t) yyyyRF‘streak’ S-band  LCLS S-band RF deflector ( λ S_RF = 10cm ) gives resolution ~ 10fs; For short e-beam, λ RF >> σ z, the streaking is not efficient;  X-band RF deflector helps( λ X_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. 2

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 lower A lot of issued to consider, and, most difficult problem is synchronization…… 3

4. synchronization problem Linear polarized Laser, the momentum kick due to the laser is 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! 4

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 electron bunch 5

6. Interaction of Laser field with ionization electron beam With ellipticity .  =0 for linear polarized laser and  =1 for circular polarized laser Polarized laser Ionization electron beam Ionization electron beam(Low energy beam, plasma electron):  It has the same profile as the high energy beam  It 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) ; 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 6

7. Low energy beam on the screen Ionization electron beam Ionization electron beam(Low energy beam):  The low energy beam is accelerated (longitudinally, Beam direction) by the DC field to the screen 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 HT  The radius of the circle depends on the laser field strength and drift time to the screen R=  t  The profile of the low energy electrons is translated to the angular distribution on the screen 7

8. Parameters used in simulation  Gas: Helium, pressure=1E-4Torr, assuming ionization length=1mm  There 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 10  m  The rms size of laser >=3 times of the beam rms size  Laser FWHM 500fs  Laser power: varies DC voltage ~ keV Required electron density ~ 3e9/mm 2 8

9. Effect of laser phase 0 90 o 180 o 270 o 9

10. On the screen: Example for 0.5/1  m bunch (Laser field only) 10

11. Other effects may spoil the distribution ……….. ………. -10kV (2) circularly- polarized laser (1) gas nozzle (3) DC field (4) screen) High energy beam field Field of Plasma electrons and ions 11

12. Effect of High energy beam field 20pC bunch 1  m bunch length Sigma_r=5  m E-field of high energy beam Energy distribution of low energy electrons without laser beam 12

13. 1  m bunch; 10pC;  r = 5  m, peak laser field 19GV/m(0.63GW), peak beam field=7GV/m head tail 13 10pc bunch 1  m bunch; 20pC;  r = 5  m, peak laser field 38GV/m (1.25GW), peak beam field=13GV/m 20pc bunch

14. Effect of laser power(  r =5  m) PL=0.9GW PL=0.45GW 14

15. Laser power effect (  r =5  m) vacuum, L=1mm, P=1e-4Torr(Helium) Neutralization factor=0.4% PL=1.2GW PL=1.5GW PL=1.8GW 15

16. beam size effect (L=0.2m, P=1e-4Torr)  r =10  m, PL=1.8GW  r =15  m, PL=5.0GW RL=7mm RL=10mm sigr15fla25w090 sigr10fla20w060 16

17. Effect of laser Power & beam size PL=1.2GW PL=0.4GW PL=0.9GW PL=1.8GW  r =5  m  r =10  m 17

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 18

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 Cons Complexity : Involved many dynamics Preliminary conclusion: This method looks promising based on the preliminary studies.  Required laser power depends on the beam: 1GW for  r =5  m  Gas pressure: 1.0e-4 Torr (mm)  Space charge of low energy particles is not included 19

20. Acknowledgment Thanks very helpful discussions with Eric Colby, Mark Hogan and Weiming An (UCLA) 20

21. Linear polarized x-ray  Need realistic model of the X-ray ionization Assuming ionized electrons are emitted only in polarization direction (NOT accurate model!) 21

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