Formation of the double layr in tow temperature plasma

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A current-free double layr near the cathode wall has been experimentally investigated in a two-electron-temperature plasma by using a LaB_6 hollow cathode tube. This LaB_6 hollow cathode tube allows us to have stable discharge with two-electron-temperature plasma in the glow-arc transition during which the discharge characteristics change from the glow to the arc. In the glow-arc transition, the interactions of beam electrons is very strong because of the high-voltage discharge and the thermionic electron emission effect. The electron distribution composed of two Maxwellians at different temperatures is observed in the LaB_6 hollow cathode tube. The electron population at this tube consists of a cold, high-density Maxwellian and a hot, low-density Maxwellian. The ratio of n_h to n_c is controlled by varying the argon neutral gas pressure while maintaining a constant discharge current (I_d = 1.0 A). As the gas pressure is decreased, n_h/n_c increases in the LaB_6 hollow cathode tube. At the same time, the current-free double layr are localized electrostatic potential structure created at the distance from the cathode wall of 5 mm.