He gas breakdown in the pressure range 0.5 to 10 Torr has been studied in a magnetic field near cyclotron resonance (8.76 T) using 1.22 mm laser radiation. At high focused laser intensity and low pressure the line shape of the laser absorption vs. magnetic field is dominated by laser intensity broadening rather than collisional broadening. The experimental data are in good agreement with a theory which takes into account the time dependence of the laser absorption. The theory predicts that, at cyclotron resonance, transmission through the gas breakdown volume is cut off due to the rapid growth of the electron density. The resulting transmitted pulses may be in the sub-nanosecond range.