Probe diagnostics of anisotropic plasma in a hollow cathode arc

Measuring the second derivatives of electron retarded current characteristics of a plane probe at different angle positions, the coefficients f*i(ε) of the velocity distribution function developed as a Legendre polynomial are determined for the anisotropic plasma of a hollow cathode arc discharge. The integral values deduced from this for the electron concentration ne and the mean electron energy ε̄, as well as the drift velocity vd, show only a limited dependence on the number of angle positions used for measurement. Even for measurements at a single angle position (Druyvesteyn method), the values obtained for ne, ε̄, and vd differ by less than 10% from the corresponding values measured with the multiposition method, so long as vdrift ≤ vthermal. On the contrary, the shape of f*i(ε), like the angle dependence f*(θ)ε=const, is clearly affected by the number of angle positions used. A Maxwell distribution superimposed by a group of directed fast electrons proves to be a sufficient model for the plasma investigated here.