Development of MAGPIE2: A High Power Helicon Plasma Discharge

Abstract

The development of high-density plasmas is important to a range of application such as rapid material processing for plasma-material interaction studies, negative ion sources, neutral beam injections, fundamental plasma studies and space thruster propulsion research. In this thesis, a new large volume high power helicon plasma device, MAGPIE2, is developed and studied. Building on the program established on its smaller predecessor MAGPIE1 device (Magnetised Plasma Interaction Experiment), the high power plasma capability of MAGPIE2 will provide extreme conditions that wall materials will be exposed to in the next-generation nuclear fusion devices such as ITER.

During high power continuous steady state operation, high heat load will be generated not only from the plasma, but also from the antenna and the magnetic coils. The heat should be removed as excessive heating will damage the hardware components of the device and hinder our experiments. Improving on the design of MAGPIE1, the 20 cm diameter plasma source tube of MAGPIE2 is manufactured from the highly thermally conductive material aluminium nitride (AlN). The plasma is created by supplying radio frequency power up to 40 kW to specially designed water-cooled antenna that surrounds the AlN source tube, which at the same time helps remove heat from the source. Besides that, the magnetic coils for MAGPIE2 are also water-cooled to sustain the long period operation.

There are three key aspects to this research: (1) Optimisation of the helicon source under different conditions; (2) Investigate helicon wave excitations effects on the plasma production; (3) Investigate the characteristics of plasma generation with heavy and light ions.

In this project, the single loop antenna and the right-handed half helical antenna were studied and compared under diverging field configuration. The plasma production and electromagnetic wavefields due to helicon wave excitation were studied for both antennas on light ion and heavy ion plasmas using a range of diagnostic systems including Langmuir probes, Bdot probes and optical emission imaging. A global model was developed and used to compare the power dependence on the density measurements while the measured wavefields of the plasma were compared with an electromagnetic wavefield model.

It was found that high densities of ~1e19 m-3 in argon and ~1e18 m-3 in hydrogen can be obtained in the source region of MAGPIE2. In the source region, the densities saturate at high RF powers and at high divergent magnetic fields. However, the half helical antenna showed better plasma production in the downstream compared to the loop antenna. The density in the target region for the half helical antenna continues to increase with increasing power even though the magnetic field strength is low in the downstream region, which was found to be due to enhanced helicon wave activity. Besides that, the discharges also displayed mode transitions while varying RF power, pressure and the external magnetic field, which can be characterised by a significant increase in the on-axis electron density and the plasma emission, as well as the formation of helicon waves. Excellent agreement can also be found between experiments and the model. The underlying physics of plasma production is then discussed for both light and heavy ion plasmas.

Although the plasma density achieved is comparable to MAGPIE1's, the plasma in MAGPIE2 can be easily generated without serious overheating issue in the source chamber and the magnetic coils, unlike its predecessor MAGPIE1 which can only run at low duty cycle under similar high power range. In future experiments, more magnetic coils will be added to the target region of MAGPIE2 to create a strong converging field downstream, which is expected to further enhance the plasma density and temperature that significantly surpasses MAGPIE1's capability and towards the extreme conditions of a nuclear fusion reactor. Show more