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

Abstract

A small helicon source is used to create dense plasma and inject it into a large chamber. A permanent magnet is used for the dc magnetic field (B-field), making the system very simple and compact. Though theory predicts that better antenna coupling will occur at 27.12 MHz, it was found that 13.56 MHz surprisingly gives even higher density due to practical effects not included in theory. Complete density n and electron temperature Te profiles are measured at three distances below the source. The plasma inside the source is also measured with a special probe, even under the antenna. The density there is lower than expected because the plasma created is immediately ejected, filling the experimental chamber. The advantage of helicons over inductively coupled plasmas (with no B-field) increases with RF power. At high B-fields, edge ionization by the Trivelpiece-Gould mode can be seen. These results are useful for design of multiple-tube, large-area helicon sources for plasma etching and deposition because problems are encountered which cannot be foreseen by theory alone.

Central peaking of magnetized gas discharges

“…If φ(r) did not peak there, the associated E-field would drive ions inwards, 8 where they would have nowhere to go, since diffusion along B at the ion temperature is very slow. There are situations where our assumptions are not valid and n(r) is not peaked on axis, and we have observed these 7 .…”

A Compact Permanent-Magnet Helicon Thruster

“…It is seen that the ion energy peaks at ∼12-14 eV. A normal sheath drop at the wall of an argon discharge is ∼5 KT e , or ≈10 eV for KT e ≈ 2 eV [3]. Thus, the ion acceleration has the approximate expected magnitude.…”

“…2, the B-field below the magnet reaches a stagnation point not far from the magnet; the discharge is located below this, where the field is quite uniform and nearly vertical. In previous experiments, specially designed neodymium (NdFeB) magnets were used, but that work showed that B-fields greater than ∼60 G (6 mT) yielded negligible improvement in plasma density [3]. As a result, a smaller system using a commercially available magnet was designed and tested.…”

Large diameter permanent-magnets-expanded plasma source for spontaneous generation of low-energy ion beam

“…[6][7][8] The previously developed PMexpanded plasma source had a 6.5 cm diameter source cavity and the generation of the supersonic ion beam has been observed. 7 The recent direct measurement of the thrust imparted from the PM-expanded plasma thruster have suggested that the electron pressure inside the source is converted into an axial ion dynamic momentum by the electrostatic ion acceleration by the electric field and can be given as 9 where n p , k B , T e , S are the plasma density in the source, Boltzmann constant, electron temperature, and cross section of the thruster, respectively.…”