Plane and hemispherical potential structures in magnetically expanding plasmas

Abstract

Two-dimensional potential structures are measured for different gas pressure in expanding argon plasma using permanent magnets, where the magnetic field is about 100 G in the source and several gauss in the diffusion chamber. The plane potential drop is observed near the source exit for 0.35 mTorr, while the potential structure becomes hemispherical when increasing up to 1 mTorr; the hemispherical structure results in the radial divergence of the ion beam. It is found that the trajectories of the accelerated ions and the electrons overcoming the potential drop are dominated by the potential structure and magnetic-field lines, respectively. © 2010 American Institute of Physics. ͓doi:10.1063/1.3467857͔Expansion of low-pressure radiofrequency plasmas operated in inductively coupled or helicon wave mode along divergent magnetic-field lines has recently attracted a great deal of interests in communities of space and laboratory plasmas and electric propulsion because a nonlinear potential structure called an electric double layer ͑DL͒ spontaneously forms near the source exit and ions are subsequently accelerated by the potential drop.

RFEA Measurements of High-Energy Electrons in a Helicon Plasma Device with Expanding Magnetic Field

“…This behavior of the floating potential is similar to the one observed by Takahashi et al [16]. Figure 6C shows electron temperature from Langmuir probe (dots) and electron-RFEA (crosses).…”

“…Later, others [14][15][16][17] have pointed out the 2D nature of the CFDL, in that the electrons follow the magnetic field lines and escape toward the side walls, while the non-magnetized ions follow a straight path from the source into the expansion chamber [18]. A curved 2D CFDL potential structure [19,20] and the conics of enhanced plasma density along the outermost magnetic fields emerging from the source region [21] has been observed.…”

Energetic Electrons Moving Along Peripheral Magnetic Field Lines in Magnetically Expanding Plasmas

“…Measurements of electron energy distributions have previously shown that the ion beam is electrically neutralized by the electrons being energetic in the source and overcoming the potential drop of the DL [2]. For further development of the propulsion device and for understanding the physics behind the DL formation, the radial and/or 2-D measurements have been progressed recently [3]- [5]. These investigations have suggested that the energetic electrons near the source wall move along the divergent field lines and ionize the neutrals in the diffusion chamber.…”

Performance of a permanent-magnet helicon source at 27 and 13 MHz

“…It has been shown 4 that helicon sources can produce an interesting amount of thrust for that purpose, but experiments so far have used large electromagnets for the DC field, and these may be incompatible with the weight limits of spacecraft. The use of PM helicons for thrusters has already been investigated extensively by Takahashi et al 5,6,7 Their configuration of PMs is quite different from ours and does not use annular magnets. For thruster applications, that work is much more advanced than ours, since the ion energy distributions were measured in detail.…”