Session TO8: MF: Heating, Current Drive, and Energetic Particles

Initial experimental results of fast wave propagation study on LAPD with a phased-array RF antenna

High harmonic fast wave (HHFW) electron heating is explored as a possible heating method for advanced beam-driven field reversed configuration (FRC) plasmas, such as TAE Technologies' current experimental device, C-2W (also called ``Norman'') [1]. To investigate the HHFW physics and gain operating experience in relevant plasma conditions, a prototype RF experiment on the LAPD machine at UCLA has been initiated. A phased-array RF antenna has been designed, built, and installed on LAPD through an international, multi-institutional, public-private collaboration. The fast wave propagation at low power was studied while varying the phasing between antenna straps, the antenna radial position, and the plasma edge density profile. The RF magnetic fields of HHFW at both the near and far fields were measured by B-dot probes and the measurements were used to validate the results of full wave simulations with PETRA-M code. The preliminary experiments on LAPD show promising results that fast wave can couple and propagate into the plasma at all antenna phases, even when the antenna is retracted close to the wall. [1] H. Gota et al., Nucl. Fusion 59, 112009 (2019).   

Progress Toward Off-Axis Current Drive Experiments in the Lower Hybrid Range of Frequencies on DIII-D

DIII-D is implementing systems to drive current non-inductively in the mid-radius region of the plasma. In addition to off-axis neutral beam injection and nearly-vertically launched electron cyclotron current drive, two systems each at the $\sim$1 MW level with toroidally-directive waves in the Lower Hybrid Range of Frequencies are being prepared. In the first experiment, fast waves at 0.48 GHz ('helicons') will be launched with a 30-element traveling wave antenna of the comb-line type from the low-field side of the torus. The comb-line modules have been fabricated and are being tested with antenna installation scheduled for Fall 2019 and experiments to commence in 2020. In the second experiment, slow waves at 4.6 GHz ('lower hybrid waves') will be launched from the centerpost (high-field side) from a location near the midplane, using an active multijunction grill with 192 powered apertures arranged in 4 rows, powered by 8 x 0.25 MW klystrons. Installation is scheduled for 2020 with first experiments in 2021. Each system will drive 0.05-0.15 MA/MW at $0.5<\rho<0.85$ in advanced tokamak discharges, with the helicon system being more useful in the high density, low field part of the AT operating space and the HFS LH system being most effective in the high field, low density region.   

Implementation of the real-time NBI code RABBIT in the discharge control system of ASDEX Upgrade

Knowledge of the fast-ion distribution arising from neutral beam injection (NBI) is important for transport analysis and magnetic equilibrium reconstruction. For sophisticated plasma control, which will be essential for the success of future fusion devices, it is very beneficial to know this distribution function already in real-time during the discharge. For this purpose, the RABBIT code has been developed. Despite its fast calculation time ($\approx$20 ms per time-step), it still shows good agreement with computationally much heavier codes like NUBEAM. In this contribution, we report on the first real-time application of the code. It has been implemented in the Discharge Control System (DCS) of ASDEX Upgrade as a DCS satellite. The input equilibrium is provided by the real-time Grad-Shafranov solver JANET and kinetic profiles come from the RAPTOR transport code, which reconstructs the profiles based on available real-time measurements. Future applications, such as for control of neutral beam current-drive, will be discussed and outlined.   

Collisional resonance function in discrete-resonance quasilinear plasma systems

A method is developed to analytically determine the resonance broadening function in quasilinear theory, due to either Krook or Fokker-Planck scattering collisions of marginally unstable plasma systems where discrete resonance instabilities are excited without any mode overlap. It is demonstrated that a quasilinear system that employs the calculated broadening functions reported here systematically recovers the nonlinear growth rate and mode saturation levels for near-threshold plasmas previously calculated from nonlinear kinetic theory. The distribution function is also calculated, which enables precise determination of the characteristic collisional resonance width.   

Global gyrokinetic investigation of the nonlinear interaction of energetic particles and turbulence in tokamak plasmas

Understanding turbulent transport is crucial for achieving a comprehensive theoretical knowledge of present day tokamaks and a predictive capability of future reactors. Energetic particles (EP), due to fusion reactions and to external heating mechanisms, have been shown to modify the turbulence dynamics both experimentally and theoretically. Due to its intrinsic multi-scale character, the problem of turbulence in toroidal plasmas is challenging, and a theoretical study is numerically demanding. This has forced previous studies to assume strong approximations on the physical models. Therefore, at present day, a comprehensive investigation is missing. In this work, the global gyrokinetic particle-in-cell code ORB5 is used to investigate in a selfconsistent way the nonlinear interaction of microturbulence modes and EPs. The effect of EPs on the turbulence dynamics is investigated here with electrostatic and electromagnetic simulations. A global model offers the opportunity of including selfconsistently the effect of meso-scale modes like zonal structures or Alfven modes, which are naturally excited in such a nonlinear system. Emphasis will be given on the analysis of the turbulent heat and particle fluxes, in dependence on the EP concentration, temperature, and radial localisation.   

The effect of species mix and fast-ion distribution on emission of fast magnetosonic waves near the ion cyclotron frequency

In the radiation belts, energetic ions drive wave emission both above and below the ion cyclotron frequency $\omega_{\mathrm{ci}}$. In a Frontier Science experiment on the DIII-D tokamak, emission of fast magnetosonic waves near $\omega_{\mathrm{ci}}$ and its harmonics is investigated using systematic scans of species mix, magnetic field, and fast-ion distribution function. For most fast-ion populations, increasing H$^{\mathrm{+}}$ in a background D$^{\mathrm{+}}$ plasma increases emission below $\omega _{\mathrm{ci\thinspace }}$but decreases emission above $\omega _{\mathrm{ci}}$, while lower magnetic field strength gives stronger emission below $\omega_{\mathrm{ci}}$ but has relatively little effect above $\omega_{\mathrm{ci}}$. Addition of a third species ($^{\mathrm{3}}$He$^{\mathrm{++}})$ sometimes introduces an additional emission band below $\omega_{\mathrm{ci}}$ reminiscent of the three electromagnetic ion cyclotron wave bands of H$^{\mathrm{+}}$, He$^{\mathrm{+}}$, and O$^{\mathrm{+}}$ in space. For higher frequencies ($\omega $\textgreater $\omega_{\mathrm{cH}})$ fast magnetosonic waves with spectral peaks at multiples of $\omega_{\mathrm{cH\thinspace }}$have been observed by satellites in the equatorial magnetosphere. Similar spectra at harmonics of $\omega_{\mathrm{ci}}$ are observed in magnetically confined fusion plasmas. Comparisons of stability calculations used in both the space and fusion communities will be shown.   

Enhanced radial energy transport induced by radially Curved Alfv\'en Eigenmode wavefronts

A surprising observation from the Electron Cyclotron Emission Imaging (ECE-I) diagnostic is that the Alfven eigenmode (AE) wave fronts are radially curved. % We show that for non-linearly saturated AEs this wavefront curvature is consistent with a spatial mismatch between the drive and damping: the mode has to transport the power gained at the location of the drive to the damping region where the power is dissipated. This radial energy transport is set up by the wavefront curvature as is deduced from the Poynting flux induced by the mode. % The Poynting flux is calculated from ideal MHD whereby the radial displacement of the mode is modified by an additional phase factor that is consistent with experiments. Without a radial phase factor, the mode-induced power flow is mainly along the field lines but including the radial phase factor strong radial power flows are generated. % The source and sink regions as determined from Poynting's theorem coincide with the fast-ion mode drive and damping regions as calculated with the NOVA-k code. Therefore, the drive and damping regions for AEs can be deduced from the observed radial wavefront curvature.   

Drift kinetic analysis of alpha particle transport in tokamaks with Alfv\'{e}n eigenmodes and ripple

Tokamak experiments fueled with deuterium and tritium have a significant population of energetic alpha particles; loss of these particles before they slow down is detrimental to experiment performance. Two powerful mechanisms for this loss are transport by Alfv\'{e}n eigenmodes and ripple. Insight into alpha behavior in cases where both of these mechanisms are important in determining device alpha transport is an area of current interest. We develop a theory that can simultaneously treat alpha transport by Alfv\'{e}n eigenmodes and ripple. Starting from the kinetic equation, a drift kinetic formulation capable of treating arbitrary perturbation frequency and toroidal and poloidal periodicity is derived. This formulation allows resonance between parallel streaming and ripple or mode periodicity. The resulting alpha flux is a sum of resonant contributions from ripple and Alfv\'{e}n eigenmodes, showing similarity between these mechanisms. This similarity allows insight into transport in cases when ripple and mode transport simultaneously affect device alpha transport. Consequences for next-generation tokamak experiment performance are considered.   

Numerical study of alpha particle confinement in CFETR plasmas

Fusion born alpha particle confinement is a key issue in burning plasmas such as CFETR. A numerical code using particle orbit tracing method (PTC) has been developed to study energetic particle confinement in tokamak plasmas. Both full orbit and drift orbit solvers are implemented to analyze the Larmor radius effects of alpha confinement. The elastic collisions between alpha particles and thermal plasma are calculated by a Monte Carlo method. A triangle mesh in poloidal section is generated for electromagnetic fields expressing. Benchmark between PTC and orbit is accomplished for verification. For CFETR burning plasmas, PTC code is used for alpha particles slowing down process calculation in 2D equilibrium, and 3D TF ripples inducing alpha particle transport. In further, numerical results show that coupling between magnetic field islands and TF ripples has significant effects for alpha particle confinement.   

Stability of alpha particle-driven Alfvén eigenmodes in the Chinese Fusion Engineering Test Reactor

The China Fusion Engineering Test Reactor (CFETR) is the next device in the Chinese roadmap for the realization of fusion energy, and is currently in design phase. In this work, the stability of alpha particle-driven Alfvén Eigenmodes (AEs) is investigated using a gyrokinetic ion/fluid electron hybrid code. It is found that the toroidal mode number of the most unstable mode is n = 10 . The excitation threshold in central alpha particle beta is found to be about 0.2\%, which is substantially below the expected value of alpha particle beta of 1\% in CFETR. This result indicates that the high- n alpha particle-driven AEs are strongly unstable in CFETR, with many toroidal mode numbers simultaneously destabilized. Furthermore, a systematic study of parameter dependence has been carried out. It is found that the stability of AE with a single toroidal mode number is sensitive to the safety factor profile. However, the overall stability of AEs is much less sensitive to the q profile when different toroidal mode numbers are considered simultaneously. It is shown that the normalized alpha particle gyro-radius and the alpha particle speed are two important parameters determining the alpha particle drive. Detailed results will be presented.   

Nonlinear Extended MHD Simulations of Turbulent CITM in 2D Slab for a Study of Transport at Edge-plasma SOL

Nonlinear simulations in a 2D slab based on the extended MHD model with Hall and FLR terms are carried out for studying a growth of the Current Tearing Interchange Modes (CITM) interacting with turbulent drift flows at edge-plasma SOL. The current diffusivity model [Miura et al., Phys. Plasmas 2017] enforces saturation of the current at a low in the SOL region as well as a current jump across the Last Closed Flux Surface (LCFS), in order to simulate open magnetic fields terminating on the divertor plates outside the LCFS. In the CITM, interchange mode under magnetic shear transforms into tearing modes, as has been shown in our single-fluid MHD simulations. The new extended MHD simulations with the Hall and FLR effects show that the CITM can grow even through interaction with turbulence originated from diamagnetic drift flow. The growth is insensitive to arbitrary parameters included in the current diffusivity model. Effects of the growth of the CITM on the heat transport under the presence of turbulence shall be discussed. \\ \\Hideaki Miura, Linjin Zheng, and Wendell Horton,“Numerical simulations of interchange/tearing instabilities in 2D slab with a numerical model for edge plasma”, Phys. of Plasmas 24, 092111 (2017)