Time: Thursday, Nov. 25th, 4:00pm
Venue: Room 413, Buil. 11th, YuQuan Campus
Speaker: Shizhao Wei
Title: Nonlinear reversed shear Alfvén eigenmode saturation due to spontaneous zonal current generation
Abstract: Energetic particles (EPs), especially alpha particles, can excite collective shear Alfvén wave (SAW) instability in tokamak plasmas,and in turn affects the behavior of EPs, resulting in EPs transport loss. Notably, reversed shear Alfvén eigenmodes (RSAE) can be preferentially excited by core localized EPs, with their frequency and radial localization directly determined by local safety factor minimum . Understanding the excitation, evolution and saturation of RSAE is important to the future study of magnetic confined controllable nuclear fusion.
The nonlinear zero frequency zonal structure (ZFZS) excitation by RSAE is an important channel of the RSAE saturation. The zonal structure (ZS), including the zonal flow (ZF) and the zonal current (ZC), are known to play important self-regulatory roles on microscale drift wave type instabilities by scattering drift waves into short radial wavelength stable domains.
RSAE frequency may sweep between those of toroidal Alfvén eigenmode (TAE) and beta-induced Alfvén eigenmode (BAE). Based on the work of the TAE and BAE nonlinearly exciting the ZFZS, in this work, we use the nonlinear gyrokinetic theory to study the nonlinear RSAE self-modulation due to ZFZS excitation. Different from TAE confined in the middle of two rational surfaces and BAE confined on the rational surface, the frequency and radial location of the RSAE is determined by , and we obtain a more general dispersion relation describing the modulational instability dispersion relation of ZFZS excitation by AEs. At the same time, we propose a unique channel of RSAE saturation. Due to the generation of ZC, the SAW continuum and q-profile may be directly modulated, which further modifies the coupling between RSAE and SAW continuum, resulting to RSAE nonlinear saturation.
Speaker: 尉国栋
Title: Four-coils Stellarator Optimization
Abstract: The stellarator concept was proposed by Spitzer in 1950s and experienced a relatively difficult period of development from 1970s to 1980s as the poor restraint performance compared to tokamaks. It was not until the concept of advanced stellarator raised that the research of stellarators was reborn. The key to get the advanced stellarator is stellarator optimization. Conventional optimization includes two stages: 1. Optimizing physics properties of stellarators from the shape of the last closed flux surface. The physics properties include neoclassical transport, MHD stabilities, bootstrap current and other desirable properties. 2. Designing 3D modular coils to fit the optimized plasma boundary in the first stage. The two-stage method was successful in design of advanced stellarators such as HSX , W7-X , NCSX . However complex 3D geometry, high engineering precision requirements and expensive construction price of stellarator coils is a new obstacle to stellarator development. It is thus necessary to develop a new way to explore the possibilities of optimizing stellarators with only controlling coils shape directly and make the complexity of coils controllable.
The Columbia Non-neutral Torus (CNT) is a nonneutral plasmas device and build with only four circular coils. Although CNT is not designed as a fusion reaction vessel originally, it still provides a possibility for stellarator constructed with simple coils. Our aim is to design a new kind of CNT-like stellarator without changing the interlocking topology of the two center coils. The optimization targets include 1/ν neoclassical transport, ideal MHD stabilities, rotational transform as well as plasma volume. In this seminar, I will show a lot of new kinds of four-coils stellarator configurations with excellent neoclassical confinement and stable ideal MHD at finite β.