An experimental test of a microwave inverse Cerenkov accelerator (MICA)

We describe first tests of one module of a dielectric-lined vacuum "inverse Cerenkov" accelerator. The module was fabricated by Titan-Beta, based on a design for a smooth-bore, slow wave accelerator powered by S-band microwaves from a SLAC klystron which also produces the injected 6 MeV bunched beam from a thermionic cathode rf gun. The dielectric liner (/spl kappa/ = 9.6) has an outer diameter of 3.14 cm, an inner diameter of 8.9 mm (through which the bunches pass), and a length of 30 cm. Power up to 200 kW was launched into the accelerator in the TM/sub 01/ mode. The dimensions of the alumina dielectric were chosen so that the phase velocity of the wave is slowed to just below c. Observations are reported for acceleration and deceleration of the injected bunch, as a function of the relative phase of the bunch injection. An energy change up to 2% was found. The energy change scales in proportion to the axial electric field. Maximum acceleration was limited by microwave breakdown most likely caused by deficiencies in design of the input coupler and not by the limiting breakdown strength of alumina.