Ion acceleration is achieved in a low-pressure solenoid-free plasma expanded by permanent magnet arrays. Although a permanent magnet normally forms cusp magnetic fields which prevents plasma diffusion and double layer formation, by employing double concentric arrays of permanent magnets, a constant field area, and a diverging magnetic field can be generated near the outlet of the plasma source. In the source, a rapid potential drop with 4 cm thickness from 50 V to 20 V is generated at the diverging field area for 0.35 mTorr and a supersonic ion beam accelerated through the potential drop is observed in the diffusion chamber. The beam energy can be increased up to over 40 eV with a decrease in gas pressure. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2965497͔ Plasma expansion has attracted a great deal of attention because it self-consistently forms nonlinear plasma-potential structures causing electrostatic ion acceleration. The process has been investigated for a long time in connection with space plasmas 1 and electric propulsion devices. 2 The plasmapotential structures are divided broadly into two types: One is an electric double layer ͑DL͒ due to charge separation phenomena, another is a potential gradient determined by a pressure gradient, i.e., following the Boltzmann's relation. Recent experiments and theories have shown the DL formation and/or the ion acceleration in expanding plasmas. [3][4][5][6][7][8][9][10] Charles and Boswell have suggested that the new type of electric thruster, named the helicon double layer thruster, can yield a long-lifetime because it does not require ion extraction electrodes. Recent measurements of the electron energy distribution function upstream of the helicon double layer showed that only a single upstream plasma source is required to maintain the formation of the double layer, 10,11 hence the above-mentioned ion accelerator is considered to be applicable to the thruster.Helicon wave discharges, i.e., rf-driven plasmas under steady-state magnetic field, are well known to be the efficient plasma sources for high-density plasma productions. In conventional helicon sources, electromagnets are used for generating the steady-state magnetic fields, which consume much electricity, and make the devices large and costly. From the viewpoint of the industrial applications, compact helicon sources using permanent magnets were developed.12-14 More recently, Shamrai et al. developed the compact helicon source introducing the permanent magnets for the electric propulsion. 15 The magnetic fields produced by the permanent magnets are normally strongly nonuniform with reverse-fields, i.e., cusps. They demonstrated that the cusps prevent the plasma diffusion and the ion beam formation. In order to eliminate the cusps near the outlet of the source tube, the magnetic fields produced by the electromagnets ͑solenoidal coils͒ are superimposed in their experiments, where the ion beam of energy above 100 eV is detected as a result of optimizing the magnetic field and the spatial profile...

Abstract-Plasma potential structures and ion-energy distribution functions are measured in a magnetically expanding radiofrequency plasma using permanent magnets (PMs) and in a geometrically expanding one, where the radio-frequency power and the argon-gas pressure are maintained at 250 W and 1 mtorr, respectively. In the magnetically expanding plasma, a rapid potential drop like a double-layer structure and a subsequent supersonic ion beam are detected by retarding field energy analyzers. On the other hand, a plasma potential structure following Boltzmann relation accelerates the ions in the geometrically expanding plasma. The comparison between the results in both cases indicates that the existence of the PMs is effective for the generation of the high-speed ion beam, where the mach number of the ion beam is increased to about 3.5 by using the PMs compared with the mach number of 2.3 in the operation mode of the geometrically expanding plasma.Index Terms-Double layer (DL), expanding plasma, ion acceleration, permanent magnets (PMs), plasma potential structures.