With nonperturbative laser-induced fluorescence measurements of ion flow, we confirm numerical simulations of spontaneous electric double-layer (DL) formation in a current-free expanding plasma. Measurements in two different experiments confirm that the DL is localized to the region of rapidly diverging magnetic field. The measurements indicate that the trapped ion population is a single Maxwellian, that the spatial gradient of the energy of ions accelerated through the DL matches the magnetic field gradient, and that DL formation is triggered when the ion-neutral collisional mean-free path exceeds the magnetic field gradient scale length.

A one-dimensional particle-in-cell code using Monte Carlo collision techniques ͑MCC/PIC͒ for both ions and electrons is used to simulate our earlier experimental results which showed that a current-free electric double layer ͑DL͒ can form in a plasma expanding along a diverging magnetic field. These results differ from previous experimental or simulation systems where the double layers are driven by a current or by imposed potential differences. Both experiment and simulation show accelerated ions with energies up to about 60 eV on the low potential side of the plasma. A new numerical method is added to the conventional PIC scheme to simulate inductive electron heating, as distinct from the more common capacitively driven simulations. A loss process is introduced along the axis of the simulation to mimic the density decrease along the axis of an expanding plasma in a diverging magnetic field. The results from the MCC/PIC presented here suggest that the expansion rate compared to the ionization frequency is a critical parameter for the existence of the DL. For the DL to be absolutely current free, the source wall has to be allowed to charge: having both ends of the simulation at the same potential always resulted in a current flow. Also, the effect of the neutral pressure and of the size of the diffusion chamber are investigated. Finally we show that this particular type of DL has electrons in Boltzmann equilibrium and that it creates a supersonic ion beam.

No abstract

A portable, low-power, diode laser-based laser-induced fluorescence ͑LIF͒ diagnostic incorporating a heated iodine cell for absolute wavelength reference was installed on the Chi-Kung helicon source ͓K. K. Chi, T. E. Sheridan, and R. W. Boswell, Plasma Sources Sci. Technol. 8, 421 ͑1999͔͒ to measure the ion velocity distribution function of argon ions as they transited a current-free double layer ͑DL͒ created where the solenoidal magnetic field diverges at the junction of the plasma source and the diffusion chamber. Based on LIF measurements of the transiting ion beam energy, the strength of the potential drop across the DL increases with decreasing neutral pressure and increasing magnetic field strength in the source. The location of the double layer also moves further downstream of the helicon source with increasing pressure. LIF measurements of the ion beam energy were found to be in good agreement with measurements obtained with a retarding field energy analyzer and also with numerical predictions.