Lanfa Wang Yuantao Ding and Zhirong Huang

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

2. V(t) sy From RF (cm) to optical(m)streaking RF ‘streak’ 2.44 m e- S-band sz D 90° • 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. bd bs 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 lower A lot of issued to consider, and, most difficult problem is synchronization……

4. synchronizationproblem Linear polarized Laser, the momentum kick due to the laser is Similar as the deflecting cavity The phase jitter causes the difficulty in the measurement! Deflecting from circular (RF) mode D. Alesini, DIPAC 09. • The whole circle is just one rf period  calibration; • No Phasing problem.

5. 2D streaking with ionization electrons & circularly-polarized laser High energy bunch (1) gas nozzle ………. ……….. ……….. (4) screen) Laser beam (2) circularly-polarized laser Beam ionization (3) DC field -10kV Ionization electron bunch

6. Interaction of Laser field with ionization electron beam 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) ; Polarized laser With ellipticity . =0 for linear polarized laser and =1 for circular polarized laser For a circular polarized laser, the momentum kick due to the laser is 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

7. Low energy beam on the screen Ionization electron beam(Low energy beam): Ionization electron beam • The low energy beam is accelerated (longitudinally, Beam direction) by the DC field to the screen a On the screen, the low energy electrons form a circle (arc) because: H T • The kicker strength from the circular laser is constant (approximately); • And the angle linearly depends electron birth time(z) • 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

8. Parameters used in simulation DC voltage ~ keV Required electron density ~ 3e9/mm2 • 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

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

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

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

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

13. 1m bunch; 10pC; r= 5m, peak laser field 19GV/m(0.63GW), peak beam field=7GV/m 10pc bunch head tail 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=0.45GW PL=0.9GW

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

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

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

18. Similar idea may work for x-ray pulse measurement (1) gas nozzle ………. ……….. ……….. (4) screen) (2) circularly-polarized laser Xray- ionization (3) DC field -10kV • Laser wavelength >~ xray wavelength

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

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

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!)