Energetic electrons upstream and downstream of a current-free double layer (CFDL) in an inductively coupled helicon plasma

Abstract

CFDLs are sharp potential drops that can form self-consistently without external current forcing, in helicon plasma sources with an expanding magnetic field. The potential drop typically occurs in the transition between the source and the diffusion chamber, and as a result, an ion beam can be observed in the downstream region of the CFDL. As no external current is applied, the free electrons have to play an important role in balancing the ion beam current. To better understand this role, it is important to obtain information about the electrons and their energy distribution as well as that of the ions. We report on the measurements of the ion energy distributions and high-energy tail of electrons in the inductively coupled helicon plasma of the Njord device, in both upstream and downstream regions of the CFDL. The measurements were carried out by means of a retarding field energy analyzer (RFEA) set to ion and electron collection, respectively. In the electron collection mode, electrons with energies large enough to overcome the potential barrier between the grounded aperture of the probe and the plasma potential Vp can be detected. In a former experiment [1] a high-energy population of electrons in the downstream region of the CFDL was found, most pronounced at the magnetic field lines mapping from the outer radial region of the source onto the downstream radial position where they intersect the radially moving probe. Here a high-density and high-energy population at temperatures about 10 eV was found. This electron population originates in the source region where electrons are directly heated in the RF field, and they are energetic enough to cross the potential drop and ionize the downstream atoms, giving rise to a small increase in the plasma density. However, also a tenuous plateau of even higher energy electrons was observed within the region where the ion beam. On the other hand, the tail of the bulk electrons that reached the RFEA collector had temperatures of 5–6 eV, in agreement with Langmuir probe results. In more recent experiments, comprehensive 2D measurements in the plasma source (upstream region) of the ion and electron distributions were obtained. In the source, Vp is typically about 70 V, which presents a taller potential barrier for electrons than in the downstream region, where the plasma potential is around 50 V. However, electrons could still be detected, but with a temperature of about 10 eV. The density of these electrons appears highest in the regions of lowest plasma potential > 3 cm outside the center of the column, as is to be expected from the lower potential barrier imposed on the electrons.