The method of contact ionization is used to produce a three-component plasma (Cs+, Cl-, e-) in the Q-device BARBARA using a beam of Cesium chloride. The end-plate temperature controls the concentration of Cl-, which can be varied in a wide range. The density of negative ions is obtained from the difference between the positive ion density given by a Langmuir probe and the electron density given by a microwave resonator. A high concentration of negative ions is achieved.

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A comparison of density measurements between cylindrical Langmuir probes, resonance fluorescence scattering and microwave methods is performed in a singly ionized barium plasma of a Q-device by varying the following parameters: 1) Density n: 3 × 108 cm−3 ≤ n ≤ 3 × 1011 cm−3; 2) Plate temperature Tp: 2100°K ≤ Tp ≤ 2500°K; 3) Plate material: Re and W; 4) Magnetic field B: 2 kG ≤ B ≤ kG; 5) Mode of operation: Double and single ended operations; 6) Ion temperature Ti down to about room temperature, keeping the electron temperature Te = Tp; 7) Degree of ionization γ: 0.1% ≤ γ ≤ 100%; 8) Probe dimensions. If the density n is evaluated by equating the extrapolated ion saturation current Is at the plasma potential Vs to the random ion current, it is shown that this yields a reliable relative value of the plasma density, which is by a factor of about 2.5 higher than the most probable value.

A theoretical explanation for the experimentally observed enhanced surface recombination of the ions in a thermally ionized magnetoplasma in a Q-device is proposed. It is based on a collisionless treatment which yields a non-Maxwellian velocity distribution for the plasma particles (section 2). In section 3 the influence of rate collisions on the velocity distribution , particularly in double-ended operation, (D.E., bothendplates hot) is discussed and it is shown that even in the case of a partially thermalized plasma with a thermalization rat e of about 50% the surface recombination causes still a J0 ∼ n dependence instead of a J0 ∼ n2 dependence as found in the case of a fully thermalized plasma.