Plasma density measurements with a molecular beam

Abstract

Investigations of high-energy plasma require probing techniques that do not change the plasma characteristics. A molecular beam is such a probe for plasmas that have energetic electrons and cold ions. Attenuation of the beam is mainly by electron ionization, but other single-particle processes must be considered. A beam of hydrogen molecules is here used as the probe. That portion of the neutral beam that passes through the plasma is measured as a pressure by a specially designed differential manometer. The measured attenuation is then used to calculate plasma density using known values of the cross sections for the interactions between beam molecules and plasma electrons and ions.

Properties of a high-beta plasma produced by electron-cyclotron heating

“…Attempts to measure plasma density by the attenuation of a thermal neutral beam [6] gave useful results in that the mean free path for thermal neutrals was measured. However, the results of this measurement were not unambiguously interpretable in terms of hot electron density The two sources of error are (1) the competing effect of ionization by the cold plasma blanket whose temperature and density are not accurately known and (2) the effect of ion-neutral scattering whose cross sections have not been measured with high accuracy in this energy range.…”

“…Several experiments on electron-cyclotron plasmas have been reported [6,7,8] which are only representative of our experimental efforts to properly document the parameters of this type of plasma. The majority of the experiments that have been performed will not be mentioned in this paper, but many of those unmentioned have contributed to a body of information which confirms the results presented here.…”

Plasma density measurements with atomic beams

“…While some limited use has been made of charged particle beams for the measurement of shock wave profiles [1], plasma electromagnetic fields [2], and plasma floating potential [3], the general usage of magnetic confining field coupled with the advantage of reasonably sharp plasma boundaries requires that, for density measurements, the beam traverse the plasma perpendicular to the magnetic field and thus be electrically neutral. A thermal molecular hydrogen beam has been employed to measure plasma density [4] although prior knowledge of the electron and ion energy distribution was required since the relative motion of the beamplasma particles arises solely from the plasma constituents. For deuterium plasmas an energetic tritrium beam has been used to measure the neutron production rate and thus the deuteron density in a plasma [5].…”