Localized electron heating and density peaking in downstream helicon plasma

Localized electron temperature and density peaking at different axial locations in the downstream helicon plasma have been observed in a linear helicon device with both geometrical and magnetic expansion. The discharge is produced with an $m=+1$ right helical antenna powered by a RF source operating at 13.56 MHz. Axial wave field measurement shows the presence of damped helicon waves with standing wave character folded into it even at low densities ($\sim {10}^{16}$ m${}^{-3}$). The measured helicon wavelength is just about twice the antenna length and the phase velocity $\left(v_p\right)$ is almost the speed required for electron impact ionization. These experimental observations strongly advocate the Landau damping heating and density production by the helicon waves, particularly in low density plasma such as ours. The electron temperature maximizes at 35–45 cm away from the antenna center in our experiments indicating a local source of heating at those locations. Different mechanisms responsible for this additional heating at a particular spatial location have been discussed for their possible roles. Further downstream from the location of the maximum electron temperature, a density peak located 55–65 cm away from the antenna is observed. This downstream density peaking can be explained through pressure balance in the system. (paper)