Induction Velometry in an Ionized Gas

A free jet (diameter 2r0) of ionized gas is considered under the influence of a uniform magnetic field B normal to the jet axis. Two appropriately placed electrodes sense the induced potential difference Δφ and the following relation: 〈u〉0 = Δφ/2r0B is taken as the basis for the experimental determination of 〈u〉0 defined as the velocity of the ionized gas averaged over the jet cross section. The experimental conditions and the analytical background are carefully examined. It is found that the necessary requirements are satisfied by the plasma properties, by the geometry of the experiment, and by the external circuitry so that the above relation is approximately valid within ±20% or better, depending on certain restrictive assumptions. Appreciable uncertainties arise from the existence of a gradient in the electrical conductivity of the ionized gas and from difficulties in the determination of r0. A plasma jet was generated by a radiation-cooled thrustor and four different gases were used at various mass flow rates and power levels so that a range of plasma velocities between 105 and 106 cm·sec−1 were available. The observed signals were induced by an ac magnetic field (60 cps) having an amplitude in the vicinity of 10 G. Concurrently with the measurements necessary for the determination of 〈u〉0, two other quantities were measured: the thrust F generated by the jet and the mass flow rate ṁ of the gas. For the experiments reported here, the values of 〈u〉0 and of F/ṁ are expected to agree within a few percent. Accordingly, the straight line: 〈u〉0 = α+β (F/ṁ) is determined by a least-squares fit to the data when 〈u〉0 is plotted versus F/ṁ. For the main group of data it is found that α = (0.38±0.24) × 105 cm/sec and β = 0.94±0.04. This is found satisfactory within experimental error and within the approximations made. Finally, several auxiliary experimental tests are carried out in order to obtain evidence supporting the approximations made in the main experiments.