Microwave guiding and intense plasma generation at subcutoff dimensions for focused ion beams

The mechanism of microwave guiding and plasma generation is investigated in a circular waveguide with a subcutoff dimension using pulsed microwaves of 3GHz. During the initial phase, gaseous breakdown is induced by the exponentially decaying wave. Upon breakdown, the refractive index of the plasma medium varies radially, with the plasma density reaching close to cutoff values in the central region. At lower pressures, the waves can propagate through the peripheral plasma with a reduced wavelength, due to the collisionally broadened upper hybrid resonance region. The intense narrow cross sectional plasma bears promise for multielemental focused ion beams.