A model for ion heating in modulated beam experiments



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In long, narrow, electron beam formed plasmas energetic ions have been observed when the electron beam was modulated at frequencies ranging from one to ten times the ion cyclotron frequency. Here a model has been developed which forms a basis for understanding the observed ion heating. The problem of finding the ion energy in a long, narrow, plasma column coaxially traversed by a strongly modulated electron beam is too difficult to handle theoretically at present. Consequently, the problem of understanding the observed ion heating is broken into two sections. Proton trajectories are calculated in ad-hoc fields - appropriate to a 60 ma 1 KeV electron beam 0.3 cm in radius, in a plasma dielectric rod - reveal a net energy transfer to the ions from the beam generated electric fields-at frequencies ranging from one to ten times the cyclotron frequency. Typically, for a modulation frequency at an ion cyclotron harmonic and near the resonant frequency of the plasma dielectric rod, 6.2 MHz, ion energies increases of [similarity]100 eV per cyclotron period are calculated, and at a non cyclotron harmonic frequency [similarity]30 eV per cyclotron period. To experimentally show the existence of radial electric fields similar to the ad-hoc electric fields used in the proton trajectory calculations, a new test ion beam diagnostic is developed and used. A heavy ion beam is passed transversly through a plasma column and the energy change of the ion beam is measured, determining the time varying electric fields of the plasma column. In the experiment to test for the electric fields, a 560 eV Argon beam is passed through a [similarity]5[dot]10[raised 9]/cm[cubed] Helium plasma column of [similarity]0.2 cm radius. Typically, a [similarity]2 ma. electron beam passing longitudinally through the plasma is modulated at [similarity]7 MHz, corresponding roughly to the Helium plasma frequency, and energy shifts in the Argon test beam of 6 eV are observed, indicating plasma dielectric enhancement of the electron beam modulation fields by a factor of 7. Consequently, it is felt that the observed heating can be understood on the basis of the model. This work was supported by the United States Atomic Energy Commission Contract AT - (liO-l) - 3777 and by National Science Foundation Grant GZ973.