Gas breakdown at cyclotron resonance with a submillimeter laser

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Abstract
A pulsed 496‐μm CH3F laser is used to produce gas breakdown in He at pressures between 1 and 300 Torr in an intense longitudinal magnetic field. Breakdown is detected by the observation of visible light when the electron cyclotron frequency (eB/m) equals the laser frequency, which occurs at B=216 kG for λ=496 μm. At the lowest helium pressures and near cyclotron resonance, the focused laser intensity of 40 kW/cm2 gives rise to very large electron heating rates, well beyond the limit of validity of conventional equilibrium breakdown theory. The observed result is an intensity‐dependent resonant linewidth, much larger than predicted by equilibrium theories.