Transverse modulation of an electron beam generated in self-modulated laser wakefield accelerator experiments C. I. Moore, 1 K. Krushelnick, 2, * A. Ting, 1 H. R. Burris, 1 R. F. Hubbard, 1 and P. Sprangle 1 1 Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375 2 Laboratory of Plasma Studies, Cornell University, Ithaca, New York 14853 Received 9 June 1998; revised manuscript received 17 August 1999 Low energy electron beams ( E 300 keV) generated in a self-modulated laser wakefield accelerator experi- ment were observed to filament and be deflected away from the laser axis forming radial jets in the electron beam profile. At higher energies ( E 900 keV), the filamentation and jets were suppressed and smooth elec- tron beams copropagating with the laser were observed. The observed electron beam filamentation likely results from laser beam filamentation in the plasma due to relativistic self-focusing effects. The radial jets of low energy electrons are likely caused by transverse ejection of the electrons due to the radial structure of the wakefield and space charge deflection of electrons as they exit the laser focus. PACS numbers: 52.40.Nk, 41.75.Lx, 41.75.Fr, 52.35.Qz Rapid developments in high power laser technology dur- ing the past several years have allowed experimental exami- nation of new phenomena resulting from the interaction of ultrahigh intensity light with plasma 1. Ponderomotive forces associated with high intensity laser pulses can produce larger amplitude plasma waves in the ‘‘wake’’ of the laser pulse as it propagates through an underdense plasma and the large electric fields associated with such waves have been proposed as a means of accelerating electrons 2. There have been several recent experiments which have measured the temporal evolution of plasma waves in the wakefield 3,4, as well as the energy of electrons accelerated by these waves up to 100 MeV5–8. In the self-modulated laser wakefield acceleration scheme SM-LWFA9, a very high power laser pulse is focused into underdense plasma such that an instability is induced which modulates the laser pulse envelope at the plasma fre- quency   pe =(4  n e e 2 / m e ) 1/2  . The subsequent resonant in- teraction with the plasma can produce large amplitude plasma waves in the wake of the pulse which have longitu- dinal electric fields suitable for accelerating electrons. In this paper, we discuss recent measurements of acceler- ated electrons generated during SM-LWFA experiments at the Naval Research Laboratory NRL. The low energy elec- tron ‘‘beam’’ profiles showed structures characteristic of filamentation in the center of the profile and radial jets out- side the laser cone angle. High energy electrons showed rela- tively weaker jets and no filamentation in the center of the profiles. The use of circularly polarized light which may have been expected to axially ‘‘guide’’ the electrons through the generation of an axial magnetic field via the inverse Far- aday effect was not found to have a significant effect on beam propagation. The experiment was performed using the Table Top Tera- watt (T 3 ) laser facility at NRL. The laser operates at a wave- length of 1054 nm and a typical power of 2.5 TW (  laser 400 fsec). The beam was focused using an f /4 off-axis parabolic mirror into the front of a jet of helium gas. The helium was completely ionized by the front part of the pulse and the main part of the beam interacted with a plasma hav- ing an electron density of 1.410 19 cm -3 ( n e 0.01n crit ). The peak intensity was 6 10 18 W/cm 2 when focused in vacuum. The interaction of the high intensity pulse with the gas jet plasma produced a large number of energetic electrons ( N e 10 8 ) which propagated in the forward direction along with the laser pulse. This beam of high energy electrons accelerated from the background plasma was highly direc- tional, although it typically had a large energy spread, with many fewer electrons produced at the highest energies. The electron energy spectrum from 500 keV to 4 MeV is shown in Fig. 1. This spectrum was measured using an electromag- net to disperse the electrons in energy and using Kodak di- rect exposure film DEF film as the detector 7. Higher energy electrons where the electron fluence was not large enough to expose the film were measured using a higher sensitivity detector consisting of a plastic scintillator directly coupled to a PMT 7,8. *Present address: Blackett Laboratory, Imperial College, London SW7 2BZ, U.K. FIG. 1. SM-LWFA accelerated electron energy spectrum from 500 keV to 4 MeV measured using DEF film as the detector. The dark streak on the film is exposure from electrons with a spatial dependence on energy due to an applied magnetic field. PHYSICAL REVIEW E JANUARY 2000 VOLUME 61, NUMBER 1 PRE 61 1063-651X/2000/611/7885/$15.00 788 ©2000 The American Physical Society