Self-Focusing of a Pulsed Electron Beam in Gases and Their Ionization. A Novel Approach to Discharge Chemistry

摘要

A relativistic high-intensity pulsed electron beam generated from a Febetron 706 was strongly self-focused in two pressure regions, i.e., below and above 5 Torr. The dependence of the electron energy spectrum on pressure and path length in He was at first studied by measuring depth-dose distributions in an aluminum-blue cellophane stack. Then, maximum doses of the depth-dose curves in the dosimeter placed on the beam axis at 10.4 cm from the cell window were measured as functions of pressure in He, Ne, Ar, Kr, Xe, H sub 2 , D sub 2 , N sub 2 , O sub 2 , N sub 2 O, CO sub 2 , SF sub 6 , CH sub 4 , C sub 2 H sub 2 , C sub 2 H sub 4 , C sub 2 H sub 6 , C sub 3 H sub 8 , CH sub 3 F, CHClF sub 2 , CCl sub 2 F sub 2 , He + X, Ar + X, and O sub 2 + X (X: additive gas). The strong self-focusing at pressure lower than 5 Torr is attributed to space-charge neutralization by positive ions due to escaping of secondary electrons. Therefore, relative total ionization cross sections for beam electrons could be obtained in this region. When the space-charge neutralization time becomes shorter than a rise time of the pulsed beam, secondary electrons are accelerated by a backward electric field E sub(z) induced by the pulsed beam so that the self-focusing declines abruptly due to electron avalanching. The beam is self-focused again gradually with further increasing pressure because of suppression of this avalanching. The avalanching was analyzed self-consistently for He, Ar, H sub 2 , N sub 2 , and CH sub 4 by a computer simulation in the pressure region between 5 and 300 Torr. The present computational results indicate that the larger cose is given by the longer mean ionization time t sub(i) which depends on E sub(z)/p. The value of t sub(i) increases with increasing pressure in the pressure region of gradually-increasing self-focusing. (Atomindex citation 14:771098)