2026 Theses Doctoral

Runaway-Electron Wave Interactions in DIII-D Tokamak Plasmas

Choudhury, Hari P.

Runaway Electrons (REs) are a relativistic population of electrons that can be generated in tokamak plasmas under several conditions, most notably during disruptions, which are catastrophic events where control of the plasma is lost. These REs can reach energies of several MeVs, and if they are deconfined and strike the tokamak walls, they can cause significant damage to the device. Understanding and mitigating REs is therefore a critical outstanding problem in the tokamak research community.

This thesis focuses on REs in the DIII-D tokamak. Located in San Diego, California, DIII-D is the largest operating tokamak in the United States and, with its suite of advanced diagnostics and control systems, offers a unique experimental platform to study RE physics. In particular, low-density benign Ohmic plasmas were used as they are very long-lived steady-state plasmas that can be routinely generated in a reproducible fashion.

Resonant wave-particle interactions offer a very interesting avenue to study and control REs. Because the RE population is highly anisotropic in velocity space, REs can excite waves, also known as kinetic instabilities, which are measurable using magnetic diagnostics. REs have been experimentally observed to excite several distinct kinds of plasma waves, but this thesis focuses on whistler waves primarily. Detailed measurements of the whistler wave instability in DIII-D plasmas are presented. One key finding has been that the excited whistler waves pitch-angle scatter REs, which reduces the source of the whistler waves themselves. This nonlinear cycle has been compared to a predator-prey model from which the damping rates of the whistler waves, along with other key parameters, have been inferred.

Although RE-excited waves certainly have the potential to scatter REs and limit their energies, a more suitable technique for the purposes of limiting RE energies would be to launch waves into the plasma using external antennas. To cause pitch-angle scattering, the launched electromagnetic waves must resonate with REs, and so must be right-handed and have a suitable frequency and wavelength. Helicon waves launched from DIII-D's helicon antenna match these criteria, and so experiments were conducted to study the interaction of helicon waves with REs. It is worth noting that helicon is an alternative name for whistler, and the author generally refers to the waves as helicon waves when externally launched, and whistlers when naturally excited by REs. Helicon waves targeting the normal cyclotron resonance have been found to successfully pitch-angle scatter REs. Following the application of helicon waves, pitch-angle scattering of high-energy REs causes an increase in both synchrotron and electron-cyclotron emissions. The hard x-ray (HXR) emission, a proxy for the RE population, ceases to grow. Energy-resolved HXR measurements also show a striking decrease in the number of high-energy REs (above the resonance at approximately 8 MeV) to below the noise floor and an increase in low-energy (~4 MeV) REs. This occurs despite the toroidal electric field remaining high enough to drive exponential RE growth in the absence of helicon waves.

Though the helicon antenna was designed to drive current in a specific high-beta magnetic configuration (the 'ideal' configuration), significant pitch-angle scattering of REs has been observed in the 'non-ideal' magnetic configuration, which is where the background magnetic field lines do not line up well with the antenna's Faraday screen elements. In the 'non-ideal' case, the possibility of significant power being directly launched into the slow wave, or lower-hybrid wave, cannot be ruled out. But given the effective pitch-angle scattering of REs observed, it is likely that a significant fraction of the launched power still goes into the helicon wave, even in the 'non-ideal' configuration. Significantly, it was also found that in the ideal configuration, if helicon waves were launched in the direction to target the anomalous resonance, i.e. the direction opposite to that targeting the normal resonance, the RE population was actually enhanced.

Finally, initial experimental observations on the effect of Electron Cyclotron Heating (ECH) on REs in these low-density ohmic plasmas is presented. Following the application of ECH, the HXR scintillator signal briefly peaks and then decays exponentially, indicating a rapid decrease in the RE population. These waves cannot resonate with REs in the way helicon waves can and so the cause of this RE suppression is likely non-resonant. One possible explanation proposed by Decker et al following similar experimental results on the TCV tokamak is that in addition to decreasing the toroidal electric field, ECH increases the RE loss rate. A 0D model is used to separate the effects of changes in loss rate to changes in background parameters in the DIII-D experimental data. The loss rate is found to scale with ECH power, suggesting that the decreases to the normalised electric field alone are not sufficient to explain the decay of REs in ECH plasmas.