Progress of Novel Vacuum Laser Acceleration Experiment at ATF Xiaoping Ding, Lei Shao ATF Users’ Meeting, Apr. 4-6, 2007 Collaborators: D. Cline (PI), X. Ding, and L. Shao UCLA, USA Y.K. Ho (Co-PI), Q. Kong, and J. Xu Fudan University, China I. Pogorelsky, V. Yakimenko, and K. Kusche BNL, USA

Approval letter for novel VLA experiment at ATF

Outline Review of Theory behind Experiment. Capture and Acceleration Scenario (CAS) Scheme. Simulations Based on Possible ATF Experimental Conditions. Experimental Setup and Plans.

Review of Theory behind Experiment Lawson-Woodward theorem: in the plane wave V  >c, the electrons may experience the acceleration and deceleration phases alternately. Net energy gain is zero. Ponderomotive scattering in a non-focused laser: electrons are accelerated in the front of pulse, and decelerated in the tail of pulse. The net energy gain is nearly zero. Ponderomotive scattering in a tightly focused laser: electrons may obtain a larger energy near the focal point, and lose less energy at the deceleration phase (diffraction effects break the symmetry of acceleration and deceleration process). But the energy gain is limited. Novel VLA (CAS scheme): the diffraction not only changes the intensity distribution of the laser, but also its phase distribution, which results in V  <c in some areas. Thus, in some special regions, which overlaps features of both strong longitudinal electric field and low laser phase velocity, electrons can receive high energy gain from the laser.

Schematic electron beam trajectory Capture and Acceleration Scenario (CAS) Scheme

Contour of phase velocity in a tightly focused laser (Gaussian beam)

Quality factor and Acceleration Channel

Required Parameters for CAS to emerge

E-beam energy routinely varies from 40 MeV to 70 MeV 20 MeV electron beam at 200pC has been successfully tuned to the end of Beam Line 1 (BL1) by Feng Zhou with normalized transverse emittance below 3.5 μm and energy spread smaller than 0.15%. The availability of 10 MeV e-beam is under study μm CO 2 laser with 5J and 5ps, 1TW level is available; 3TW will come soon Higher resolution and wider energy acceptance energy spectrometers with 90 0 and 4 0 dipoles, respectively. Simulations Based on Possible ATF Experimental Conditions

Simulation of Electron Dynamics

Parameters for Simulation: Electron-beam: 10MeV Laser Intensity: 3TW, 20 waist size, 10.6 wave length. Laser description: As we can see, the simulation clearly demonstrates that electrons can get energy gain from laser beam after the entire interaction. Under the parameters we used above, the electron can be accelerated of the gradient about 5MeV/1.7mm.

Distribution of Phase Velocity on y=0 plane

Transverse and Longitudinal E&M force performance during Interaction

Basically, according to the simulation work, this new vacuum laser acceleration scheme could have a chance to be carried out in a real experiment under the promising ATF experimental conditions, which are expected in the near future. By then it would turn out a revolution of a new acceleration concept, which gives acceleration gradients as high as GeV/m in vacuum laser acceleration without any optics.

10-MeV e-beam study Parmela shows 10 MeV electron beam at 200pC with energy spread below 0.1% can be obtained at the end of Linac

Experimental Setup and Plans

Plans To demonstrate novel VLA - Continue efforts to tune e-beam at lower energy, such as 10-MeV - Adjust laser optics to produce incident laser injection to e-beam - Measure angular distribution and energy gain In addition to our leading this experiment at ATF, Prof. Ho from Fudan University is also leading a similar experiment in China by using SILEX-I Laser Facility (Ti-Sapphire, 286TW,30-fs laser ) and we will help the experiment.

SILEX-I: Super Intensity Laser for Extreme Experiments SILEX-I: Super Intensity Laser for Extreme Experiments Generation of 286-TW, 30-fs Laser Pulses SILEX-I SILEX-I Laser Facility

SILEX-I:286TW-stage SILEX-I: 286TW-stage  80mm Ti:sapphire slab with cladding  80mm Ti:sapphire slab with cladding Beam diameter 60mm Beam diameter 60mm Four-pass amplifier Four-pass amplifier Pumping: 90-J, 8-ns pulse green light Pumping: 90-J, 8-ns pulse green light (for 286TW, only 50J needed) Four-grating single-pass compressor Four-grating single-pass compressor High gain coefficient laser medium ASE-physical issue for large aperture rod High refractive index 1.76 Index-matched thermoplastic material for cladding Pump Laser: Energy:150J/1064nm, 80~90J/532nm Beam diameter: 64mm Pulse duration: 8ns

Thanks a lot!