An Investigation of the Positive Column of a Hollow Cathode Arc in a Magnetic Field. Part I

Abstract

The positive column of the hollow cathode arc consists of a highly ionized ( > 90%) and -magnetized (ω ei τ i,i > 1) plasma. In order to obtain as much as possible information about this plasma its parameters were measured as functions of radius, r, and axial distance, z. They were varied by changing the discharge parameters - arc current I , gas feed Q, magnetic field strength B, arc length L and core diameter d. Generally the discharge was fed with Argon, but Neon, Helium, Nitrogen and Hydrogen were also used. - The radial plasma density distribution, n e (r), is approximately Gaussian for 0 ≦ r < 4 cm (n e (0)≅ 2 X 10 14 part./cm 3 ); in axial direction the plasma density does not vary much. -The ion temperature, T i , is constant with radius, contrary to the electron temperature, T e , which drops sharply near the core of the arc. In axial direction T i drops by a factor ten from the cathode to the anode (T i ≅ 1- 30 eV). T i (z = 0) increases with arc length L. - The whole plasma moves in axial direction from the cathode towards the anode ( υ z ≅ 6 X 10 4 cm/s); the angular mass velocity (Ω ≅ some 10 5 rad/s) is strongly sheared and decreases with temperature. - The axial electrical field is small (0.1 to 0.2 V/cm) corresponding to a normal electrical conductivity. The radial electrical field points inwardly and varies strongly with radius (0 ≧ E r ≳ - 10 V/cm). - Strong (nonlinear) low frequency oscillations (υ ≅ 15 kHz) are generated spontaneously in the plasma; these oscillations are due to the Kelvin-Helmholtz instability. As is shown in Part II of this investigation, the observed behaviour of the plasma column may be satisfactorily explained by the two fluid model with MHD ordering. Due to viscous effects (i, i collisions) the sheared plasma rotation has big consequences for the transport of particles and energy.