Effect of Magneic Field Configuration on Plasma Production and Ion Acceleration in a Compact Helicon Plasma Source with Permanent Magnets

Summary form only given. Permanent magnets (PMs) are used in helicon plasma sources destined for some applications that need a compact design, such as electric propulsion. The magnetic field of the PM is strongly nonuniform, with null points (cusps) near the ends, which can impede the wave propagation and plasma flow. We report the results on improving the source characteristics by finding an optimal magnetic configuration with the use of the electromagnet (EM) in addition to the PM. Such a configuration may be reproduced using the PMs only. The discharge in argon is excited in a 4.5-cm-diam quartz chamber by an m=0 antenna powered at 13.56 MHz. The basic magnetic field is created by the annular PM magnetized along the axis and situated close to the outlet. The magnetic configuration is varied by using the EM with separate current control in three sections. With the PM only, the field cusps arise between the antenna and the PM and behind the PM, near the outlet. In this case, the plasma density is maximal under the antenna and little plasma is ejected from the source. Engaging the EM permits to remove both cusps. Then the plasma density several-fold increases, up to 3times10¹² cm^-3 at 700 W input power, and peaks downstream the antenna, under the PM, which gives rise to substantial increase of plasma outflow. Electron temperature in the source amounts to 10 eV at pressures about 1 mtorr. Measurements with rotatable ion energy analyzer have shown that two groups of ions are present in the drift chamber. The first group with energies in the range of a few tens of eV relates to the plasma potential within the drift chamber. The second group comes from the source with much higher energies exceeding 100 eV at lower pressures. Wave excitation and power absorption with nonuniform profiles of the plasma and magnetic field were computed with the use of developed FEM code. Theoretical and measured RF fields are found to be in a reasonable agreement