Presheath and boundary effects on helicon discharge equilibria

Abstract

The location and existence of a double layer observed in the core of an argon helicon discharge (3 to 4 mTorr, 500 W rf power) with uniform magnetic fields (900 G), previously reported by Siddiqui (2014 Phys. Plasmas 21 020707), is shown to be modulated by the distance of the downstream conducting boundary from the helicon antenna and the length of its presheath. A region of locally hot electrons (T e ≈ 10 eV) is observed in all cases. When the downstream grounded boundary is 33 cm away from the antenna no double layer is observed and the electrons cool due to heat conduction, requiring the electron density to increase concurrently in order to maintain axial electron pressure balance. When the boundary is held 27 cm or closer to the antenna, the boundary's presheath, which requires the density to decrease monotonically, does not allow enough room for the electrons to cool via conduction. The double layer only exists in these cases, and is shown to trap the hot electrons upstream from the boundary. As such, the interaction of the boundary's presheath with the upstream rf electron heating is shown to determine the overall discharge equilibrium in the system.

Spectroscopic characterization of H 2 and D 2 helicon plasmas generated by a resonant antenna for neutral beam applications in fusion

“…T e must, however, be kept below 2 eV to reduce − H detachment from electronic collisions, which is a very effective process due to the low binding energy (0.75 eV) of the extra captured electron [9]. To fully understand the mechanism behind negative ion production complete characterization of the electron energy distribution function (EEDF) may therefore be required, especially for helicon sources, which have revealed, under certain conditions, the presence of non-Maxwellian features [10,11]. An extension of the present investigation to measure the detailed EEDF in resonant antenna ion device (RAID) plasmas will be the subject of future work.…”

Laser induced fluorescence diagnostic for velocity distribution functions: applications, physics, methods and developments

“…In various bounded plasma systems from plasma processing to tokamak region of scrape off layer, the assumption that that ions enters the sheath at their own Bohm speed remains common in applications of multi-ion species plasmas [134], despite being challenged by the results discussed above. The LIF diagnostics also enabled the first test of Chodura's theory on magnetized presheath formation [135], and contested its prediction in collisional environments [73,136]. There were also a significant amount of studies on fireball and electron sheath using LIF to investigate ion transport in these structures [137,138], as well as plasma perturbation measurements measuring the perturbed IVDF [45,68].…”

“…On the other hand, Siddiqui et al also performed in depth investigation of ion dynamics in another kind of helicon double layer formed as a result of pressure balance when the helicon antenna is close to a boundary [136,149].…”

“…In most plasma devices, electron temperature is inadequate to produce such a significant amount of these electrons to populate the plasma with He II metastable ions. Plasma test bed to demonstrate He II LIF via applying of ECR heating in to a helicon source has been attempted, but laser excitation of He II fluorescence has yet been demonstrated [169].…”

Power law fitting of the ion saturation current and the three-temperature Maxwellian EEDF in a multi-dipole confined hot cathode discharge: an experimental revisitation

“…Langmuir probes has been one of the most common plasma diagnostic widely employed in studies of basic plasma physics [1][2][3][4], nuclear fusion [5,6], plasma processing [7], and electric propulsion [8], amongst other fields of study. It is by far the most cost effective diagnostic that offers direct access to the electron energy distribution function (EEDF) with the alternative being Thomson scattering [9], which can be extremely is the mechanical area of the Langmuir probe, n e is the electron density, c s is the Bohm speed (T e /m i ) 1/2 , and κ ∼ 0.6 is the constant reflecting the presheath potential drop.…”