Experimental Study on the Density Peaking of an RF Plasma at a Low Magnetic Field

Abstract

Density peaking of a plasma at a low magnetic field is investigated. In this experiment, a helicon plasma source with a 15-cm plasma column diameter is used. An rf power of 200 ∼ 800 W with a frequency of 6 ∼ 14 MHz is fed to a half-turn helical antenna located outside a Pyrex tube plasma enclosure. Experimental results show a loading resistance and an ion density peaking at several tens of gauss, and the peaking magnetic fields of these two parameters are reasonably the same. In addition, the dependency of the peaking magnetic field on the ion density seems to be matched with the dependence of the mode coupling magnetic field on the ion density. Mode coupling of a short-wavelength electrostatic wave called the TG mode and the helicon mode is formed at the corresponding magnetic field when the effect of electron inertia is included in the rf plasma with an axial magnetic field. The experimental result supports that density peaking at a low magnetic field is possibly caused by eigenmode formation around a mode coupling magnetic field. The loading resistance is usually found to be high for the eigenmodes, and a higher loading resistance yields an efficient rf power coupling to the plasma, thus, a higher plasma density. A plasma with a low magnetic field may have advantages, such as a relatively uniform plasma profile and a simple system for magnets and the power supply. This kind of density enhancement at a low magnetic field is, therefore, useful for applications requiring an efficient, uniform plasma.