Helicon waves are right-hand polarized (RHP) waves which propagate in radially confined magnetized plasmas for frequencies ! ci ! ! ce where ! ci is the ion cyclotron frequency and ! ce is the electron cyclotron frequency. They are part of a much larger family of waves which can propagate down to zero frequency and constitute a very rich field for studying complex propagation characteristics and wave-particle interactions. This paper gives a historical perspective of the waves and their relationship to plasma source development up to the mid 1980's, presents a simple description of their propagation characteristics in free and bounded plasmas, and finishes with their first reported use in plasma processing experiments.

Helicon sources have been proposed by at least two groups for generating ions for space propulsion: the HDLT concept at the Australian National University (ANU), and the VASIMR concept at the Johnson Space Center in Houston. These sources normally require a large electromagnet and power supply to produce the magnetic field. At this stage of research, emphasis has been on the plasma density and ion current that can be produced, but not much on the weight, size, impulse, and gas efficiency of the thruster. This paper concerns the source itself and shows that great savings in size and weight can be obtained by using specially designed permanent magnets (PMs). This PM helicon design, originally developed for plasma processing of large substrates, is extended here for ion thrusters of both the HDLT and VASIMR types. Measured downstream densities are of order 10 12 cm -3 , which should yield much higher ion currents than reported so far. The design principles have been checked experimentally, showing that the predictions of the theory and computations are reliable. The details of two new designs are given here to serve as examples to stimulate further research on the use of such sources as thrusters.

Abstract-Self-consistent computations of electric fields (E-fields) and ion orbits inside trenches were done in order to verify Hashimoto's hypothesis of damage that is induced during plasma processing. In his well-accepted theory, Hashimoto proposed a mechanism for electron-shading damage, whereby the photoresist at the tops of trenches and vias collects a negative charge from the thermal electrons, creating an E-field, which prevents electrons from reaching the trench bottom, where collector is located. The sheath E-field accelerates the ions and drives them straight into the trench where they impinge on the collector, and charge it positive if it is isolated. In the computations presented in this paper, it is shown that ion orbits depend only on the E-fields at the entrance and are sensitive to changes in the shape of the photoresist layer there. In addition to the electronshading mechanism, there is an "ion-shading" effect that protects part of the trench walls, and the number of ions that strike the wall is too small to cause any deformation of the walls.

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