Helicon Wave Propagation in Low Diverging Magnetic Fields.

Abstract

This thesis details an experimental, theoretical, and numerical investigation into helicon wave propagation in the presence of low diverging magnetic fields (< 5 m T). Experiments are performed in the Piglet helicon reactor, which consists of a Pyrex source tube connected to a larger aluminium diffusion chamber. A double-saddle field antenna (operated at 13.56 MHz), is used to create both the plasma and launch helicon waves, while the diverging magnetic field is produced by a number of solenoids that surround both the antenna and source tube. Experiments are conducted with argon gas in the pressure range 0.04-0.4 Pa, and for rf input powers below 400 W. As the magnetic field is increased (using single solenoid), the plasma density is observed to increase rapidly over a narrow range of magnetic values (between about 1 mT < Bo < 5 mT), where a distinct density peak is formed. The density at the maximum of the peak (>1017 m-3 ) is more than an order of magnitude larger than that before or after, and is associated with a corresponding peak in the measured antenna resistance; showing that a larger percentage of the input power is deposited within the plasma. In the presence of the diverging magnetic field an ion beam is observed to form simultaneously with the low –field helicon mode. The ion beam, which is present for argon gas pressures below around 0.3 Pa, is produced by upstream ions accelerated by a decreasing plasma potential set up by the spatially decaying plasma density profile. An analytical model, based on simple flux conservation, is developed to describe the general features and behaviour of the observed ion energy distribution functions (IEDFs), which are found to be strong functions of the plasma potential profile and neutral gas pressure. During the low-field mode, m = 1 helicon waves was observed with B-dot probes in the source region of Piglet. With just a single solenoid producing the magnetic field, waves are prevented reaching the downstream region (that is, the waves appear “trapped”), but slight modifications to the magnetic field geometry allows the axial distance over which waves can propagate to be controlled. Critical to the modification of the wave propagation behaviour is the magnetic field strength ( and geometry) near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56MHz. By solving the wave equation using cold plasma approximation, and separately by making use of a 1D electromagnetic particle-in-cell (PIC) simulation, wave propagation and absorption are investigated in the presence of a low diverging magnetic field. The numerical results from both studies are in good qualitative agreement with the experimental measurements, and provide strong evidence to suggest that the observed wave “trapping” is due to electron cyclotron damping of helicon waves in the spatially decaying magnetic field; an electron heating process not usually dominant in conventional helicon discharges, thus opening up additional possibilities for the use and optimization of helicon systems in processing and propulsion applications.