Measurements of the sheath potential in low density plasmas

Abstract

The authors have measured the sheath potential around a probe in a range of different plasma conditions in the UMIST quadrupole GOLUX and in a related experiment in which the plasma expands freely to supersonic velocity. In the latter case, the sheath potential agrees well with an appropriately modified form of the usual expression for a field-free plasma, for both hydrogen and argon plasmas. In GOLUX, however, the sheath potential is found to be significantly less than the accepted value, even when the magnetic field is taken into account. For the slow-moving plasma in the outer part of the quadrupole confining field, they present both theoretical and experimental results showing that the reduction is due to truncation of the electron velocity distribution as the probe drains electrons from a closed flux-tube faster than they can be replaced. In the central hot plasma, however, this explanation cannot apply. Here the plasma is moving at about sonic speed and magnetic effects are weak. Nevertheless the results are significantly different from those in the field-free experiment.

Plasma potential mapping of high power impulse magnetron sputtering discharges

“…The space resolution is limited by the size of the probe including its sheath; a resolution of 1 mm has been demonstrated. 40 Emissive probes have been successfully applied to measure plasma potentials in a wide range of magnetic fields, from tens of mT for a quadrupole device 41 to 0.5 T at the edge of a Tokamak 38 or Q-machine. 42 It should be noted that the emitted electrons have energies of about 0.3 eV, corresponding to a wire temperature of 3000 K w T  .…”

The interaction of a flowing plasma with a dipole magnetic field: measurements and modelling of a diamagnetic cavity relevant to spacecraft protection

“…Using the direct measurement of the upstream floating potential and evaluating the plasma potential using the second derivative of the Langmuir probe characteristic [15], we obtain α = 1.7, considerably less than that expected for an isotropic plasma. Supersonic ion flow will tend to reduce the value of α and a more detailed analysis [10,13,16,17] suggests a full expression for α:…”

Experiments on drift wave launching: I. Dispersion curves and damping rates

“…The plasma density profile has a maximum near the separatrix and falls to a low value at the critical surface. Close to the separatrix there is a fast-moving plasma stream with drift speed about 2 × 10 4 m s −1 , bounded by a steep velocity shear layer at ψ/I Q = 0.04; the outer plasma which carries the drift waves is cool (T e ≈ 1 eV) and drifts slowly away from the source at a speed of the order of 1000 m s −1 (Bradley et al 1992). This is less than, but comparable to, the diamagnetic drift speed, so allowance for the Doppler shift is required in comparing the experimental and theoretical dispersion curves.…”