Two-dimensional Cherenkov emission array for studies of relativistic electron dynamics in a laser plasma

Abstract

In laser-produced plasmas there are several effects which will scatter a longitudinally probing relativistic electron beam. In vacuum, the laser itself will ponderomotively defocus the electron beam, while in plasma the ponderomotive force can dig an ion channel which would focus the electron beam. In the cases of plasma wave excitation via the beat-wave or wake-field mechanisms, the thermalization of the electron distribution function can lead to large scale magnetic fields via the Weibel instability. One way of studying such phenomena is to time resolve the transverse current distribution of the electron beam after it exits the plasma. A wire mesh has insufficient time resolution for this purpose, so we instead use a mesh of optical fibers. When the electron beam strikes the fiber mesh, Cherenkov radiation is generated within whichever fibers have current running across them. The Cherenkov radiation from all the fibers can then be analyzed on a streak camera. This allows the reconstruction of j(x,y,t) where j is the current density. We have successfully implemented this technique in the study of beat-excited laser plasmas.