Abstract
In the inductively coupled plasma of the Njord helicon device we have, for the same parameters as for which an ion beam exists, measured a downstream population of high-energy electrons emerging from the source. Separated measurements of energetic tail electrons was carried out by Retarding Field Energy Analyzer (RFEA) with a grounded entrance grid, operated in an electron collection mode. In a radial scan with the RFEA pointed toward the source, we found a significant population of high-energy electrons just inside the magnetic field line mapping to the edge of the source. A second peak in high-energy electrons density was observed in a radial position corresponding to the radius of the source. Also, throughout the main column a small contribution of high-energy electrons was observed. In a radial scan with a RFEA biased to collect ions a localized increase in the plasma ion density near the magnetic field line emerging from the plasma near the wall of the source was observed. This is interpreted as a signature of high-energy electrons ionizing the neutral gas. Also, a dip in the floating potential of a Langmuir probe is evident in this region where high-energy electrons is observed.
Highlights
An electric double-layer forming spontaneously in a helicon plasma source with an expanding magnetic field was first measured by Charles and Boswell [1]
We report the first measurements of energetic electrons in the downstream region of the current-free double layer (CFDL) in the Njord device, applying a Retarding Field Energy Analyzer (RFEA)
Direct measurements of high-energy electrons have been performed with a RFEA set up for detection of energetic electrons
Summary
An electric double-layer forming spontaneously in a helicon plasma source with an expanding magnetic field was first measured by Charles and Boswell [1]. It was named current-free double layer (CFDL), as it formed self-consistently without external current forcing. As no external current is necessary to form this ion beam, the free electrons in the CFDL in the downstream plasma, play an important role in balancing the ion current in the beam. Takahashi et al [10] observed, in a similar experiment, high-energy electrons at a magnetic field line mapping to the edge of the source by measuring the electron energy distribution function (EEDF) with a RF compensated Langmuir probe
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