• J.B. Rosenzweig, A.M. Cook, M.C. Thompson, R.B. Yoder
  UCLA, Los Angeles, California

Recent proposals for using plasma wakefield accelerators in the blowout regime as a component of a linear collider have included very intense driver and accelerating beams, which have densities many times in excess of the ambient plasma density. The electric fields of these beams are widely known to be large enough to completely expel plasma electrons from the beam path; the expelled electrons often attain relativistic velocities in the process. We examine here another aspect of this high-beam density scenario: the motion of ions. In the lowest order analysis, for both cylindrically symmetric and "flat" beams, it is seen that for the recently discussed "after-burner" scenario the ions completely collapse inside of the electron beam. In this case the ion density is significantly increased, with a large increase in the beam emittance expected as a result. Particle-in-cell simulations of ion-collapse in the nonlinear regime are discussed.

• M.C. Thompson, H. Badakov, J.B. Rosenzweig, G. Travish
  UCLA, Los Angeles, California
• H. Edwards, R.P. Fliller, G.M. Kazakevich, P. Piot, J.K. Santucci
  Fermilab, Batavia, Illinois
• J.L. Li, R. Tikhoplav
  Rochester University, Rochester, New York

An underdense plasma lens experiment is planned as a collaboration between UCLA and the Fermilab NICADD Photoinjector Laboratory (FNPL). The experiment will focus on measuring the variation of the plasma focusing along the longitudinal beam axis and comparing these results with theory and simulation. The experiment will utilize a thin gaussian underdense plasma lens with peak density 6 x 10¹² cm-3 and a FWHM length of 1.6 cm. This plasma lens will have a focusing strength equivalent to a quadrupole magnet with a 180 T/m field gradient. A 15 MeV, 8nC electron beam with nominal dimensions sr = 400 μm and sz = 2.1 mm will be focused by this plasma lens onto an OTR screen approximately 2 cm downstream of the lens. The light from the OTR screen will be imaged into a streak camera in order to directly measure the correlation between z and sr within the beam. Status and progress on the experiment are reported.

• M.C. Thompson, H. Badakov, J.B. Rosenzweig, G. Travish
  UCLA, Los Angeles, California
• M.J. Hogan, R. Ischebeck, R. Siemann, D.R. Walz
  SLAC, Menlo Park, California
• P. Muggli
  USC, Los Angeles, California
• A. Scott
  UCSB, Santa Barbara, California
• R.B. Yoder
  ,

An experiment is planned to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range. This new UCLA/SLAC collaboration will take advantage of the unique SLAC FFTB electron beam and its demonstrated ultra-short pulse lengths and high currents (e.g., sz = 20 μm at Q = 3 nC). The electron beam will be focused down and sent through varying lengths of fused silica capillary tubing with two different sizes: ID = 200 μm / OD = 325 μm and ID = 100 μm / OD = 325 μm. The pulse length of the electron beam will be varied in order to alter the accelerating gradient and probe the breakdown threshold of the dielectric structures. In addition to breakdown studies, we plan to collect and measure coherent Cerenkov radiation emitted from the capillary tube to gain information about the strength of the accelerating fields. Status and progress on the experiment are reported.