IONIZATION AND PULSE LETHARGY EFFECTS IN INVERSE CHERENKOV ACCELERATORS

Ionization processes limit the accelerating gradient and place an uppe r limit on the pulse duration of the electromagnetic driver in the inv erse Cherenkov accelerator (ICA). Group velocity slippage, i.e., pulse lethargy, on the other hand, imposes a lower limit on the pulse durat ion. These limits are obtained for two ICA configurations in which the electromagnetic driver (e.g., laser or millimeter wave source) is pro pagated in a waveguide that is (i) lined with a dielectric material or (ii) filled with a neutral gas. In either configuration the electroma gnetic driving field is guided and has an axial electric field with ph ase velocity equal to the speed of light in vacuum, c. The intensity o f the driver in the ICA, and therefore the acceleration gradient, is l imited by tunneling and collisional ionization effects, Partial ioniza tion of the dielectric liner or gas can lead to significant modificati on of the dispersive properties of the waveguide, altering the phase v elocity of the accelerating field and causing particle slippage, thus disrupting the acceleration process. An additional limitation on the p ulse duration is imposed since the group velocity of the driving pulse is less than c and the pulse slips behind the accelerated electrons. Hence for sufficiently short pulses the electrons outrun the pulse, te rminating the acceleration. Limitations on the driver pulse duration a nd accelerating gradient, due to ionization and pulse lethargy, are es timated for the two ICA configurations. Maximum accelerating gradients and pulse durations are presented for a 10 mu m, 1 mm, and 1 cm wavel ength electromagnetic driver. The combination of ionization and pulse lethargy effects impose severe limitations on the maximum energy gain in inverse Cherenkov accelerators.