Plasma control by modification of helicon wave propagation in low magnetic fields

Abstract

By making use of nonuniform magnetic fields, it is shown experimentally that control of helicon wave propagation can be achieved in a low pressure ͑0.08 Pa͒ expanding plasma. The m = 1 helicon waves are formed during a direct capacitive to wave mode transition that occurs in a low diverging magnetic field ͑B 0 Ͻ 3 mT͒. In this initial configuration, waves are prevented from reaching the downstream region, but slight modifications to the magnetic field allows the axial distance over which waves can propagate to be controlled. By changing the effective propagation distance in this way, significant modification of the density and plasma potential profiles can be achieved, showing that the rf power deposition can be spatially controlled as well. Critical to the modification of the wave propagation behavior is the magnetic field strength ͑and geometry͒ near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56 MHz.

Electron–cyclotron damping of helicon waves in low diverging magnetic fields

“…9,10 Cyclotron damping is a process where electrons can collisionlessly absorb power from an electromagnetic wave and occurs when the electron cyclotron frequency is close to the wave frequency (due to Doppler-shifting effects though, this region can be quite broad [9][10][11] ), so that electrons effectively "see" a static wave electric field. The simulation results provide strong evidence to support electron-cyclotron damping as the mechanism responsible for the previously observed wave trapping 8 and demonstrates an additional electron heating process in low magnetic fields that is usually absent in typical helicon discharges. In this paper, a 1D electromagnetic particle-incell PIC simulation that we have developed is used to simulate the original experimental reactor, 8 and together with experimentally measured density and magnetic field profiles, helicon wave propagation is investigated and compared with the results of previous 8 measurements made with a B-dot probe.…”

“…8 In this previous investigation, 8 wave propagation in a low-field helicon mode was investigated with a number of external magnetic field configurations, produced using a combination of the magnetic field coils shown in Fig. 8 In this previous investigation, 8 wave propagation in a low-field helicon mode was investigated with a number of external magnetic field configurations, produced using a combination of the magnetic field coils shown in Fig.…”

“…Because of the diverging magnetic field, the magnitude of the field at this location gives electron cyclotron frequencies close to the wave frequency. A simple analytical model based on the warm plasma dispersion relation was developed, 8 suggesting cyclotron damping could be a feasible mechanism to explain the wave trapping observations, but this relation assumes a uniform magnetic field and plasma density, which are not true in the experiment. 9,10 Cyclotron damping is a process where electrons can collisionlessly absorb power from an electromagnetic wave and occurs when the electron cyclotron frequency is close to the wave frequency (due to Doppler-shifting effects though, this region can be quite broad [9][10][11] ), so that electrons effectively "see" a static wave electric field.…”

“…Helicon waves cannot propagate past such a cyclotron resonance point, 1 and it thus appears as if waves are either being reflected or rapidly attenuated due to electron-cyclotron damping. In this paper, a 1D electromagnetic particle-incell PIC simulation that we have developed is used to simulate the original experimental reactor, 8 and together with experimentally measured density and magnetic field profiles, helicon wave propagation is investigated and compared with the results of previous 8 measurements made with a B-dot probe. A simple analytical model based on the warm plasma dispersion relation was developed, 8 suggesting cyclotron damping could be a feasible mechanism to explain the wave trapping observations, but this relation assumes a uniform magnetic field and plasma density, which are not true in the experiment.…”

Effect of inhomogeneous magnetic field on plasma generation in a low magnetic field helicon discharge

“…It is also noted that with increasing rf power the density peak shifted to larger magnetic field values. The similar observations can also be found in the previous experiments 15,24,25 . It has already been established that the density peaking at the low magnetic field is the results of wave coupling rather than capacitive or inductive coupling 17,19 .…”