Low‐Frequency Beam‐Plasma Interactions in a Finite‐Sized Plasma

Abstract

Results are presented for an experimental and theoretical investigation of low‐frequency waves in a beam‐generated plasma. Waves are excited at frequencies immediately above the ion‐plasma frequency by sinusoidally modulating the electron beam current. The study is aimed at obtaining ion heating at the lower‐hybrid resonant frequency. An electron beam of 400‐1000 V energy and a few milliamperes of current is used to generate plasmas with densities in the range of 109 cm−3 in a magnetic field of a few hundred gauss. Hydrogen, argon, deuterium, and neon gases are used. The rf electric field is probed as a function of r, zr,z, and frequency, and energetic ions are detected by a gridded probe. Two or three resonances are found in the probe response above ωpiωpi, and are shown to be multiple half‐wavelength resonances of standing plasma waveguide axisymmetric modes. A normal‐mode analysis is used to calculate the probe responses and is found to correctly predict the effects of varying beam voltage, plasma density, ion mass, and dc axial magnetic field. The excited wave may be viewed as an electrostatic wave propagating at an angle very close to 90 deg from the magnetic field.