Development and optimisation of advanced auxiliary ion heating schemes for 3D fusion plasma devices

The magnetic confinement fusion devices known as the tokamak and the stellarator are progressing towards becoming viable commercial nuclear fusion reactor designs. Both ap- proaches require improvements in the applied heating sources and in the particle and energy confinement in order to become power efficient and cost effective. A better understanding of the physics associated to the different heating techniques is required to optimise perfor- mance. Ion Cyclotron Range-of-Frequency (ICRF) waves and Neutral Beam Injection (NBI) are two auxiliary fast-ion heating methods that are commonly used in tokamak and stellarator fusion devices. The largest tokamak in the world, ITER, is currently being built in Cadarache, France, and the largest stellarator project, Wendelstein 7-X (W7-X), started operation in 2015. For the latter, NBI operation will start in 2018, and ICRF experiments are foreseen in 2020. The heating scenarios to be applied experimentally in both of these machines are currently being developed. This requires the development of an understanding of how the heating methods work, proposals to be made for optimisation, and theoretical and numerical predictions to be made in advance. A known issue for stellarator devices is the relatively poor confinement of energetic particles. This is an issue for the auxiliary power efficiency of the device. Fusion reactions produce alpha particles of large energies (∌ 3.6MeV) that should be well confined and collide with the background plasma such that the plasma becomes self-heating. This reduces the requirements on the external heating sources to main- tain ideal fusion conditions, assuming these fusion alphas are themselves confined. The work presented in this thesis uses the SCENIC code package to calculate the heating performance and energetic particle production and confinement of a range of basic and advanced heating scenarios involving ICRF and NBI. The SCENIC code self-consistently calculates the magnetic equilibrium, the RF-wave propagation and absorption, the neutral beam injection ionisation and deposition, and evaluates the energetic particle distribution function evolution in the presence of the applied heating scheme. The main results of this thesis indicate that for the Wendelstein 7-X stellarator, both the 3-ion species and the synergetic RF-NBI Doppler shifted resonance heating schemes, developed in this thesis, generate highly energetic ion populations. However, the 3-ion species scheme is shown to not be ideal for energetic particle experiments for multiple reasons. The results for this heating scheme are very sensitive to the magnetic equilibrium. In particular, it is found that the standard mirror equilibrium produces and confines only up to 0.15MeV ions. Moreover, the densities of such fast ion populations are low, such that experimental detection from probes such as Fast Ion Loss Detectors (FILD) is not feasible. The 3-ion species scheme is only capable of producing particles that are deeply trapped with a strong peaking in the pitch angle. Typically, such particles have worse confinement in the 3D equilibrium. With respect to the heating transferred to the core background plasma and the generation of highly energetic particles, the most successful heating scheme applied to W7-X is the synergetic RF-NBI doppler shifted resonance heating scheme, developed for the first time in this thesis, predicting large densities of MeV range ions isotropic in pitch angle space.


Advisor(s):
Graves, Jonathan
Year:
2019
Publisher:
Lausanne, EPFL
Keywords:
Laboratories:
SPC-TH




 Record created 2019-01-31, last modified 2019-05-09

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