MEASUREMENTS OF PLASMA ROTATION IN A HOLLOW CATHODE DISCHARGE

The plasma column of a hollow cathode, hollow anode vacuum arc is found to rotate in the direction of the ion gyro motion with velocities of several times 105 cm/sec. The maximum velocity is reached when the magnetic field strength is increased to a characteristic value (ωcτ 1); it then decreases gradually at higher field strengths. The measurements were made with a small pendulum hanging above the arc and were confirmed spectroscopically. The experimental results are described by the equation of motion of the ions with the electrons being able to move practically unhindered by the magnetic field, as was suggested before for the explanation of diffusion measurements. Changing parameters - such as ion mass, neutral gas density and radial electrical field - makes the value of the characteristic magnetic field strength for maximum velocity shift in the expected direction. The radial particle flux is, however, found to be smaller than that expected from the collisional diffusion theory of Simon (1955). Thus one has to assume that the azimuthal velocity of the ions is controlled by a much higher effective collision frequency than their radial diffusion current. It is suggested that this result is related to density perturbations of high energy (eφ/kT e 1) found to be propagating in the direction of the electron gyro motion with a frequency near the drift frequency.