Experimental investigation of certain beam transport issues in a pulsed transmission line linear accelerator

Abstract : The successful development of a new generation of high current, high voltage, linear induction accelerators relies on the solution of a number of beam transport problems, including radial oscillations, diocotron instabilities, transverse beam break-up (BBU), etc. Most of the instabilities appear to onset either at the injector region or at the accelerating gaps. Radial oscillations were first observed in RADLAC I, while transverse beam break-up was first observed on the SLAC accelerator, and more recently on the ETA accelerator. A low emittance, high current, high voltage injector precisely aligned with the guiding magnetic field axis and beam vacuum pipe axis is of prime importance for successful beam acceleration and transport. Similarly, an accelerating gap design which maintains radial force balance and an accelerating cavity with low Q and very small transverse shunt impedance Z perpendicular should eliminate the most dangerous radial oscillations and beam break-up instabilities. The design and experimental studies of a new 4 MeV, 40 kA electron beam injector and accelerating gap will be presented. Test bench measurements of Z perpendicular and Q on a typical radial transmission line accelerating cavity prove that BBU is not of concern unless the number of accelerating gaps become excessively large. (Author)