Session C6 - Laser and Plasma Acceleration.
INVITED session, Saturday afternoon, April 29
Beacon A, LBHR
In the E-157 experiment, a 30 GeV electron beam of 2e10 electrons in a 0.65mm long bunch is propagated through a 1.4m long lithium plasma (created by UV ionization) of density \sim 2e14/cm^3. The beam density is greater than the plasma density and the head of the bunch expels the plasma electrons leaving behind an ion channel with transverse focusing fields of up to several thousand Tesla/m. There are three types of ongoing studies: (1) The zeroth order effect is the ``thick plasma lens" where the induced focusing-channel causes the beam to undergo so-called betatron oscillations where the envelope of the beam oscillates radially. (2) Transverse head-tail effects are of higher order: non-axially-symmetric longitudinal charge distributions cause different slices of the beam to oscillate radially with different periods, or may even be unstable, distorting the beam further. (3) For electron bunch lengths on the order of half the plasma wavelength, the ion column immediately behind the head of the electron bunch will cause the main bunch to lose energy. The plasma electrons expelled from the beam will rush back to the axis and produce a strong accelerating force (order GeV/m) for the particles in the tail of the same bunch. The betatron oscillations are studied by scanning the plasma density and observing time-integrated images of optical transition radiation and Cherenkov radiation from foils downstream of the plasma. Energy changes in the beam are observed from time-resolved images of the Cherenkov radiator located in a dispersive section of the downstream beamline. Head-tail effects are seen in these images and can be isolated from energy gain by acquiring time-resolved images from the Cherenkov radiator in the non-dispersive direction. Beam-position monitors and beamline feedback signals also reveal information regarding beam centroid motion induced by the plasma. Progress on the experiment will be reported.