Numerical investigation of energetic electrons in RF-wave driven spherical Tokamak plasmas
Fluid Mechanics
Final Report Abstract
Compared to conventional energy sources such as fossil fuels, nuclear fission or renewable energies, the concept of nuclear fusion is characterized by numerous advantages. Regarding security issues, fuel reserves, minimization of environmental hazards, and efficiency, nuclear fusion may significantly contribute to the future world-wide power supply. The substantial scientific and engineering challenges which are tightly coupled to the concept of nuclear fusion, first have to be solved convincingly whereas the known problems are already tackled by global research programs. To realize a stable thermo-nuclear fusion process, complex procedures such as plasma heating to rampup and to maintain the plasma current play a fundamental role. In order to reach the necessary conditions for thermo-nuclear fusion, low-aspect-ratio Tokamaks are a promising concept. However, to minimize the aspect ratio, the central solenoid which is required for heating and driving the current has to be removed and thus, other methods to replace the functionality of the solenoid are needed. The application of lower-hybrid (LH) waves represents a promising way to accomplish this task. However, the substantial influence of energetic electrons in low-density plasmas on the effectivity and stability of the ramp-up procedure of Tokamak configurations has not been investigated sufficiently yet. The goal is to investigate the role of energetic electrons on lower-hybrid-wave induced ramp-up procedures in spherical Tokamaks using novel numerical methods to develop and optimize the current ramp-up procedure. First, a solver using non-linear magneto-hydrodynamics coupled to kinetic particles was extended to deal with energetic electrons and by using experimental and reference simulation results a model distribution function is developed and implemented. Then, the mhdkinetic-particles-solver was coupled with a ray-tracing solver and a full-wave solver to further investigate realistic spherical Tokamak configuration and compare to experimental results to obtain an answer about the role of energetic electrons during the ramp-up procedure. A semi-analytical model has been developed to mimic in a simplified fashion the distribution function of the energetic electrons caused by the lower-hybrid wave injection. However, it was found that the simple model neglects important aspects of the distribution function such as loss cones or energy plateaus. It is possible to study some basic properties of the energetic electrons, such as finite orbit widths or co-and counter-rotating currents. However, for an in-depth analysis, the distribution function had to be founded on more reliable sources. To that end, a simulation environment has been developed coupling different numerical methods, and its application to the plasma configurations of the spherical tokamak TST-2 was thoroughly analysed. These plasmas are formed and driven by radio-frequency waves without the use of the central solenoid, and are characterized by low density and low magnetic field. A hybrid simulation environment which is divided into two groups, one using magneto-hydrodynamic (MHD) as well as particle-in-cell (PIC) approaches, and the second group using ray-tracing and Fokker-Planck solvers, was applied to describe the behaviour of energetic electrons, bulk plasma, wave propagation, and the wave-particle interaction. Both groups of solvers can be coupled via the energetic-particle velocity distribution function and the equilibrium conditions of magnetic field, pressure, and density profiles to obtain a self-consistent solution. First results show the impact of a self-consistent equilibrium on ray trajectories and current density profiles. Therefore, new insights in LH-wave-driven plasmas of TST-2 can be obtained using the proposed hybrid simulation environment. That is, thanks to the achieved goals of this project, these complex plasma configurations can be investigated in a fully self-consistent way.
Publications
- (2018) A simulation environment to simulate lower-hybrid-wave-driven plasmas efficiently. Computer Physics Communications 230 38–49
Roidl, Benedikt; Todo, Yasushi; Takase, Yuichi; Tsujii, Naoto; Ejiri, Akira; Yoshida, Yusuke; Yajima, Satoru; Shinya, Takahiro
(See online at https://doi.org/10.1016/j.cpc.2018.04.018) - Numerical modeling of lower hybrid current drive in fully non-inductive plasma start-up experiments on TST-2. Nuclear Fusion, 57(12), 126032
Tsujii, N.; Takase, Y.; Ejiri, A.; Takahiro, S.; Togashi, H.; Yajima, S.; Yamazaki, H.; Moeller, C.; Roidl, B.; Sonehara, M.; et al.
(See online at https://doi.org/10.1016/j.cpc.2018.04.018) - Plasma current start-up experiments using outboard-and top-launch lower hybrid wave on the TST-2 spherical tokamak. Nuclear Fusion, IOP Publishing, 2016, 57, 036006
Shinya, T.; Takase, Y.; Yajima, S.; Moeller, C.; Yamazaki, H.; Tsujii, N.; Yoshida, Y.; Ejiri, A.; Togashi, H.; Toida, K.; H. Furui, H.; Homma; Nakamura, K.; Roidl, B.; Sonehara M. et al.
(See online at https://doi.org/10.1088/1741-4326/57/3/036006) - Measurements of edge plasma parameters during internal reconnection events in the TST-2 spherical tokamak. Physics of Plasmas, 2017, 24, 062504
Furui, H.; Ejiri, A.; Nagashima Y.; Takase, Y.; Sonehara, M.; Tsujii, N.; Roidl, B.; Shinya, T.; et al.
(See online at https://doi.org/10.1063/1.4985077)
