7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Characterization of a linear helicon plasma device in convergent magnetic fields
Y. Teranishi and K. Takahashi
Department of Electrical Engineering, Tohoku University
e-mail (speaker): yume.teranishi.q7@dc.tohoku.ac.jp

Helicon plasma sources [1], which utilize radiofrequency (rf) waves propagating in magnetized plasmas,
can efficiently produce high-density plasmas above ne ~1019 m-3 for an rf power of a few kW. A typical helicon
plasma reactor has a simple structure composed of justtwo distinct regions: a plasma source and a diffusion
chamber. Due to the potential performance as a highdensity plasma source, the helicon sources have attracted
much interest in various practical applications such as anelectric propulsion device called a helicon thruster [2, 3].
Researches on the helicon plasmas have been actively
conducted over the last few decades. One of the mostinteresting issues is “the density limit” of the helicondischarges, as it is remarkably hard to produce highdensity plasmas with a density greater than 1020m-3. The
density limit has been considered to result from a neutral
depletion phenomenon leading to a lack of fuel gas
densities in the plasma core [4, 5]. Sustaining an
extremely dense plasma of ~1020 m-3 is an indeed
challenging issue, which contributes to performance
improvement of the helicon thruster because the thrust canbe given by the electron pressure term and the electron
diamagnetic effect [2].
In the previous studies, two helical antennas wereemployed to increase the deposited power and a maximum
plasma density of 8 × 1019 m-3 has been obtained [5]. In
this study, the effect of a magnetic field on helicon plasma
densities is focused on and experimentally investigated.
The experiments are performed in the Heliex device
shown in Figure 1(a) [ADD Ref. submitted]. The device
mainly consists of a source tube, an about 1-m-longcylindrical vacuum chamber, and seven solenoid coils
around the source tube and the chamber. Each solenoid
has an independent DC power supply; hence the magnetic
field configuration and strength can be controlled byadjustments of the solenoid currents. To avoid high heatload on the solenoids by large continuous currents, pulsed
currents are applied to the solenoids. Figure 1(b) shows acircuit diagram of the pulsed solenoid current [6]. After
capacitor is charged by a DC power supply, an IGBT gate
signal is input and the IGBT is turned on; then the pulsed
current flows to the solenoid. In this work, the capacitanceis chosen as 37.6 mF and the peak value of the current
applied to the solenoid is 60 A for the charging voltage of300 V, giving the magnetic field strength of about 0.2 T.
The densities are measured by a Langmuir Probe (LP)inserted from the downstream vacuum port as shown inFig.1(a).
The results show the plasma densities nearly saturate forthe high rf power due to the neutral depletion whenincreasing the rf power, while the maximum density canbe increased by applying the strong magnetic field. The
detailed results will be shown in the presentation.

Figure 1. Schematic diagram of (a) the linear plasmadevice, (b) the switching circuit of the solenoid current [6].
References[1] R.W. Boswell, Phys. Lett. A, 33, 457 (1970).
[2] K. Takahashi, Rev. Mod. Plasma Phys. 3, 3 (2019).
[3] K. Takahashi, Sci. Rep. 12, 18618 (2022).
[4] R. M. Magee et al., Phys. Plasmas. 20, 123511 (2013).
[5] D. G. Miljak et al., Plasma Sources Sci. Technol. 7,537 (1998).
[6] K. Takahashi et al., Plasma Sources Sci. Technol. 22,055002 (2013).