The first phase of an
experiment to study the performance of dielectric Cerenkov wakefield
accelerating structures at extremely high gradients in the GV/m range has been
completed. This experiment takes advantage of the unique SLAC FFTB electron
beam and its demonstrated ultra-short pulse lengths and high currents (e.g.,
σz = 20 µm at Q = 3 nC). The FFTB electron beam has been
successfully 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 was varied in the
range 20 µm < σz < 100 µm which produced a range of
electric fields between 2 and 20 GV/m at the inner surface of the dielectric
tubes. We observed a sharp increase in optical emissions from the capillaries
in the middle part of this surface field range which we believe indicates the
transition between sustainable field levels and breakdown. If this initial
interpretation is correct, the surfaced fields that were sustained equate to on
axis accelerating field of several GV/m. In future experiments we plan to
collect and measure coherent Cerenkov radiation emitted from the capillary tube
to gain more information about the strength of the accelerating fields.