Ion cyclotron range of frequencies (ICRF) heating in fusion plasmas is significantly hampered by the phenomenon of RF sheath rectification. Addressing RF sheaths and their related effects, such as impurity generation and convective cell formation, is important to make ICRF an effective heating option for future fusion devices. Experiments were performed on the Large Plasma Device (LAPD) using a single strap ICRF antenna to better understand how to mitigate RF sheath formation and its subsequent effects.
The initial set of experiments explored the effects of electrically insulating antenna enclosures on RF-rectified sheaths. A single-strap RF antenna was powered using a high-power amplifier and matching network. Although the high-power amplifier and matching network were constructed during prior work, some improvements and changes were incorporated for this thesis work. For example, the amplifier, the matching network, and the antenna were modeled using the software LT-spice and simulations helped guide the changes made to the matching network. Additionally, the antenna design was updated to better shield against RF noise otherwise broadcasted into the lab, contaminating several data signals and electronics.
Data from three experiments were compared where the enclosure material was made of copper, MACOR (electrically insulating), and MACOR over copper, respectively. The non-conductive MACOR material was exposed to the bulk plasma in the case of the MACOR-copper side walls, but a layer of copper was placed below to let image currents flow. All three experiments were carried out in a helium plasma with a background magnetic field of 1kG. In each of these three experiments, a single-strap, high-power (100kW) RF (2.4MHz) antenna was used to launch fast waves into the dense core of the magnetized helium plasma. The core density of the plasma was $n_e \approx 5 \times 10^{12} \ \mbox{cm}^{-3}$ to $8 \times 10^{12} \ \mbox{cm}^{-3}$ during each experiment. No Faraday screens were used on the front face of the antenna enclosure for all three experiments.
In the case of the copper enclosure, RF rectified potentials, many times the local electron temperature, and associated formation of convective cells were observed and reported \cite{Martin2017}. The experiments with MACOR and MACOR-copper enclosures showed a considerable reduction in RF rectification. Furthermore, neither of these last two experiments indicated convective cell development. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with a 3-strap antenna optimized to reduce image currents on the antenna limiters \cite{bobkov2016first}, the MACOR-copper experiment seems to suggest that insulating plasma-facing materials have at least an equally strong impact on reducing potential rectification.
To further explore the DC RF sheath mitigation seen in MACOR and MACOR-copper experiments, another set of experiments were executed with different thickness MACOR enclosures. A 1D voltage divider model by Myra and others has been presented to predict mitigation behavior depending on the insulator material and plasma properties. A series of experiments were conducted to investigate the effect of insulator material qualities and plasma properties on the degree of sheath mitigation. These experiments were conducted using enclosure walls made of copper, 1mm, 2mm, and 5mm MACOR. Also, each experiment was carried out under various plasma conditions by varying the time during discharge when the experiment was performed. RF rectified potentials in the copper enclosure experiment were used as a benchmark to determine the degree of mitigation in the MACOR studies. Findings from the various MACOR thickness and plasma parameters demonstrate that, with a few exceptions, sheath mitigation often follows the indicated trend of the voltage divider model. Moreover, the model's projected mitigation quantities and the measured sheath potentials do not agree well. To more accurately predict sheath mitigation in these experiments, the voltage divider sheath model will need to consider the 2D impacts of evolving density and plasma potential.
In addition to the sheath mitigation work outlined above, additional work was done to document the parasitic lower hybrid (slow) wave in the LAPD edge. Most fusion experiments where coupling to the slow wave is a concern often have plasma densities and temperatures that are far too harsh for in-vessel diagnostics to be placed in the plasma volume. This work is unique because a fast-wave RF antenna was used to launch fast-wave, typically used for heating, in the core while simultaneously launching the unwanted slow-wave in the edge. Furthermore, this simultaneous coupling of waves was documented using electric dipole probes. Two new dipole probes were developed for this work to allow for better wave propagation mapping along the LAPD's length. One of the big challenges with this work has been achieving a range of densities in the LAPD that span the propagation region of the slow wave and fast wave. With the newly upgraded large $LaB_6$ source, several different plasma configurations were explored using annular limiters, different species plasmas, and a range of accessible frequencies. After the work done for documenting slow-wave propagation with the new $LaB_6$ source, we are better equipped to run high-power slow-wave experiments for future work.
