OBSERVATIONS OF A BUNEMAN DOUBLE-LAYER AND ASSOCIATED ELECTRON HEATING IN A HIGH-VOLTAGE STRAIGHT PLASMA DISCHARGE

The spatial evolution of a potential hump (electron hole) into a Buneman double layer was for the first time observed at the instant of current limitation in the initial phase of a high-voltage straight plasma discharge with a preexisting hydrogen plasma. Potential profiles along the axis numerically calculated from the real time E-field data demonstrate the formation of a strong double layer with an inverse potential jump (e-phi-DL/kT(e) approximately 10(3)) moving toward the center of the device with a mean velocity 1.2 x 10(8) cm/sec, nearly 0.3 times the electron thermal velocity. The strong HV double layer developed near the center then quickly grew up to an extremely strong dynamic double layer propagating toward the anode. The formation of a highly anisotropic velocity distribution of energetic electrons associated with the strong double layer was evidenced by intense hard x-ray emissions that were first detected in the direction almost parallel to the magnetic field with a delay time 250-350 nsec from the generation of the double layer.