Session VO07: Magnetic Confinement: Energetic Particles & RF

Live

Novel measurements of Alfven Eigenmode (AE) stability have been made via active antenna excitation in JET [Puglia 2016 NF]. For the first time in JET energetic particle experiments, we report the real-time tracking of a stable AE after its transition from being driven by RF-heated fast ions. We also report stable AE tracking at high heating power, 4MW RF and 20MW NBI in a D-3He plasma. During the 2019-2020 D campaign, 5000 MHD resonances and their frequencies (30-250kHz), damping rates (0-13kHz), and toroidal mode numbers (n$<$30) were detected. Good agreement is found between mode numbers applied by the antenna and measured of the resonances. Dedicated experiments explored plasma-antenna coupling, and resonance detection is more likely in limited as opposed to X-point plasmas and as plasma-antenna separation decreases, agreeing with simulations [Dvornova 2020 PoP]. Results are compared with MHD and gyrokinetic simulations [Aslanyan 2019 NF] to assess various drive and damping mechanisms including alpha particles in future DT experiments.   

Live

Internal kink is a common instability seen in tokamak when the axis q profile goes below unity. It impacts on the transport of bulk plasma and fast ions. In TCV ($R_0/a =0.88/0.25$) fast ions are fed to the plasma using neutral beam tangential injection at $25$ keV. TCV is equipped with magnetic coils which measure the three orthogonal components of the magnetic field at 2 MHz sampling frequency. Effect of internal kink on fast ions in TCV can be seen experimentally with Compact Neutral Particle Analyzer and Fast Ion D-$\alpha$ diagnostic. We will show comparison with interpretative modelling including enhanced fast ion diffusion caused by internal kink on shots with positive and negative triangularity. These shots are chosen to have similar q-profile and internal kink conditions. The perturbation (assumed as ideal) of the magnetic flux surfaces is modeled using KINX code. This is used to compute the transport matrices with the ORBIT code. In this step, magnetic coils measurement are used to correctly scale the normalized radial field perturbation by KINX. The last part consist in applying the \textit{kick-model} using the induced transport coefficients in TRANSP/NUBEAM. This will allow us to simulate synthetic diagnostics and evaluate the impact on integrated modeling.   

Live

Anomalous transport generated by plasma turbulence is linked to the performances of present-day and future fusion reactors. Any mechanisms able to reduce the radial propagation of particles and energy is crucial in view of experimental scenarios optimization. In this context, energetic particles, generated through external heating -- have been proved to be a surprisingly efficient way to reduce ion-temperature-gradient (ITG) driven turbulent transport. The underlying physics of the interplay between fast ions and micro-turbulence has been shown to involve different mechanisms. In strong electromagnetic regimes, energetic particles have been observed to destabilize marginally ``MHD'' modes via nonlinear coupling, reducing the bulk ion energy content and enhancing zonal flow activity. The key parameters for this nonlinear interplay to occur are identified on a particular JET discharge and the applicability to future machines discussed. By applying sophisticated energy diagnostics, we show that fast ions provide a finite amplitude for linearly stable MHD-type of modes, to which the bulk ITG turbulence can nonlinearly couple. This energy redistribution depletes the ITG drive reducing the overall transport levels. If the energy injected into these MHD-type of modes is sufficiently large, an increase in the zonal flow amplitude is observed, further decreasing the transport levels.   

Live

The full orbit following SPIRAL code [1] has been used to simulate the interaction between edge neutrals and neutral beam generated fast ions that move outside the separatrix and traverse the tokamak plasma periphery. Significant fast-ion losses were found, up to 20{\%} in a DIII-D case. It is shown that at beam energies up to \textasciitilde 100 keV fast-ion losses are considerable because of charge exchange reactions between the neutrals and the fast ions. Approximately half of the fast ions that neutralize outside the plasma get lost to the wall while the other half re-enters the plasma and ionizes again, often on better confined orbits which is visible as an enhanced beam power deposition in the plasma core (r/a \textless 0.7). As a result, the plasma performance as measured from beam-plasma fusion reactions is hardly changed despite the significant fast-ion losses toward the plasma edge. [1] G.J. Kramer et al. (2013) Plasma Phys. Control. Fusion 55 025013   

Live

The aim of the present study is to analyze the saturation of the energetic-ion-driven resistive interchange mode (EIC) in LHD plasma using FAR3d code. The non linear simulation results show similar $1/1$ EIC saturation phases with respect to the experimental observations, reproducing the enhancement of the $n/m = 1/1$ resistive interchange modes (RIC) amplitude and width as the EP $\beta$ increases, the EP $\beta$ threshold for the $1/1$ EIC excitation, the further destabilization of the $1/1$ EIC as the population of the helically trapped EP increases and the triggering of burst events.   

Live

We present a high field side (HFS) lower hybrid current drive (LHCD) multijunction launcher for DIII-D. Electric field is reduced below the 9.3 kV/cm arcing threshold by a traveling wave power divider and aperture impedance matching to the edge plasma resulting in low circulating power over a wide range of edge densities (n$_{\mathrm{e}}=$1x10$^{\mathrm{17}}$ to 1x10$^{\mathrm{18}}$ m$^{\mathrm{-3}})$. The coupler is expected to drive \textasciitilde 150 kA/MW at r/a$=$\textasciitilde 0.6-0.8 at n$_{\mathrm{\vert \vert }}=^{\mathrm{-}}$2.7 and 4.6 GHz. Additive manufacture with selective laser melting (SLM) from Glenn Research Copper 84 (GRCop-84), a Cr$_{\mathrm{2}}$Nb precipitation hardened alloy, allows configurations unachievable with conventional machining. SLM of GRCop-84 results in a fully dense, vacuum compatible RF structure. Monolithically printed LHCD launchers with self-supporting RF structures eliminate secondary machining operations. Surface finishing techniques to reduce RF losses in printed material are developed. Material failure, fracture characteristics, and precipitate morphology are examined with electron microscopy and focused ion beam milling. Work supported by the USDOE, OFES, using User facility DIII-D, under Award Number DE-FC02-04ER54698 and~DE-SC0014264. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-14-19807.   

Live

A 1 MW, 476 MHz Helicon fast wave electron current drive antenna system has been installed at the DIII-D facility. Antenna modules, transmission structures, sources and instrumentation have been tested, tuned, installed in the vessel, and evaluated. Optimization and tuning have produced antenna characteristics of 2{\%} reflection from the 30-module array and an average vacuum loss of 1.3{\%} per module. Plasma coupling experiments at low power suggest these characteristics are sufficient to drive a substantial current with a sustainable thermal load. Evaluation under power is planned during the 2020 DIII-D campaign. Infrared camera and thermocouple measurements during plasma operations with an unpowered antenna are consistent with expectations from plasma modeling. Modules for real time thermal load estimation and collision avoidance have been integrated into the DIII-D PCS to insure safe operation of the antenna. A 10-kW transmitter has been used to evaluated performance of some antenna hardware under thermally significant powers and at experimentally relevant fields. Antenna optimization, low power studies, and multiphysics simulations provide confidence that the Helicon antenna is ready for operations.   

Live

At ITER, for first operation 24 gyrotrons at 170 GHz with 1 MW power each are being prepared for ECRH and ECCD. Using the O1 heating scheme, absorption of the injected power is full. But practical limitations in relaying and launching the power will lead to a fraction of non-absorbed power. This power is incident on in-vessel component, risking excessive heating. At Wendelstein 7-X (W7-X), with ten gyrotrons at 140 GHz with up to 1 MW each, this problem was also anticipated and a microwave stray radiation test facility was built to test in-vessel components at isotropic power densities of up to 50 kW/m2 for up to 30 minutes. The facility is also operated in support of the ITER ECRH protection, for instance, exposing the Mirnov coil covers and future ITER proto type vacuum windows. The temperature increase, as well as the rate of temperature increase, of ceramics are measured as function of power density and exposure time. Various shielding concepts using metal foils, meshes and reflecting coatings are being assessed. Results are discussed in this contribution.   

Live

EAST lower hybrid current drive (LHCD) experiments at two frequencies show that lithiation extends LHCD toward high density and has a more significant impact on the scrape-off-layer (SOL) properties than changes in Greenwald fraction achieved by the varying the plasma current (300 kA to 700 kA). Density ramp experiments show that LHCD remains up to \textasciitilde 4 x 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$ with a density scaling of hard X-ray emission of n$_{\mathrm{e}}^{\mathrm{-2.5}}$ (n$_{\mathrm{e}}^{\mathrm{-3.5}})$ at 4.6 (2.45) GHz, compared to a scaling of n$_{\mathrm{e}}^{\mathrm{-3.5}}$ (n$_{\mathrm{e}}^{\mathrm{-5}})$ without lithiation. Indications of stronger RF power losses are observed at a lower source frequency (2.45 GHz). A faster rise in the density at the launcher shows a higher level of ionizations, which also coincides with the onset of parametric decay instabilities. Both phenomena agree with the expected frequency scaling. A stronger non-linear increase in the divertor D$_{\mathrm{\alpha }}$ signal is observed at 2.45 GHz, which indicates that divertor becomes denser and colder. RF power may be dissipated more there due to higher collisionality. The rise in the density at the launcher may partially be responsible for an early onset of D$_{\mathrm{\alpha }}$ signal observed at a lower RF frequency, implying that control of the launcher density may be one way to mitigate parasitic RF losses at the edge and improve CD efficiency at 2.45 GHz.   

Live

A dipole field is a basic structure in nature for plasma confinement. The Ring Trap 1 (RT-1) device is motivated by the Jovian magnetosphere, and it enables us to study both magnetospheric plasma physics and advanced fusion. The RT-1 device has demonstrated the plasma confinement in a dipole field produced by a levitated superconducting ring magnet. The produced plasma has a peaked density profile invoked by the self-organization, which is a unique feature in a dipole field configuration like a magnetosphere in nature. To evaluate the absorption of the electron cyclotron (EC) wave, the modulated EC wave is applied to the RT-1 plasmas, and the response of the diamagnetic signal is measured. The dependence of the absorption efficiency on the density is characterized experimentally. As the density increases, the absorption efficiency decreases and the upper limit of the density appears. The understanding of the propagation and the absorption profile of EC waves in a dipole field is necessary to explain the above experimental results. We applied the full wave simulation for the propagation of EC waves. The absorption is modeled by a hot dispersion relation. The results will be presented and discussed.   

Live

In the past few years, there has been an increased interest in Radiofrequency (RF) heating of low ($\sim 10 eV$) and high ($\sim 1keV$) temperature plasmas in open magnetic geometries for applications ranging from diverter simulators, electric thrusters, fusion neutron sources, fusion-fission hybrids and pure-fusion reactors. This has motivated the development for Fokker-Planck solvers to understand transport under the influence of RF heating, Coulomb collisions and kinetic effects.\\ In this work, we present progress towards Fokker-Planck transport simulations of RF heated plasmas in open magnetic geometries using a parallel, electromagnetic, hybrid-Particle-in-Cell code: Prometheus++ where the electrons are of fluid nature and the kinetic behavior of ions is retained. Furthermore, a Fokker-Planck Coulomb collision operator has been developed and added to the code to investigate the collisional transport of ions in these devices. In addition, we discuss the process of selecting initial conditions for both the plasma pressure and beta-corrected magnetic field in order to minimize the free energy at the start of simulation. Finally, we discuss plans to introduce RF heating operators.   

Live

Studying nonlinear interactions between MHD activities and energetic particles using first-principle simulation is an important and challenging task. Recently, a new kinetic module of M3D-C1 code has been developed to address this issue. The module utilizes GPU to accelerate particles pushing, which can reach up to 16 times speedup compared to CPU code. Both drift-kinetic and gyro-kinetic schemes for delta-f calculations and both pressure coupling and current coupling schemes for MHD-kinetic interaction have both been implemented. Several linear and nonlinear benchmarks have been conducted, and good agreements with other codes have been obtained. In nonlinear runs, the energy conservation of the MHD-kinetic coupling system has been tested. In addition, it is found that this module, with slight modification, can also be used to improve the runaway electron calculation used in M3D-C1, which can save computation time and provide better numerical stability.   

Abstract Withdrawn

Numerical simulations of Alfv\'{e}n Eigenmodes (AEs) and kink/fishbone modes driven by energetic particles (EPs) on EAST are performed using hybrid-kinetic MHD model implemented in the NIMROD code. Eigen-analysis of AEs on EAST is carried out using AWEAC (Alfven Wave Eigen-Analysis Code). When the safety factor at magnetic axis $q_0$ is in the vicinity of $1$, fishbone can be excited by EPs. As EP $\beta_f$ increases, fishbone is suppressed first before excited again. For sufficiently large EP $\beta_f$, fishbone can transfer to $\beta$-induced Alfv\'en eigenmode (BAE). When $q_0$ is well above $1$, BAE can be driven directly by EPs. The frequency and radial location of BAE from NIMROD calculations are consistent with results from AWEAC.   

Composite Processes in Fusion Burning Plasmas and Gained Innovative Perspectives

A new kind of tridimensional structures (ballooning in both the radial and poloidal directions) has been identified that can be maintained spontaneously in fusion burning plasmas and transfer energy from the emitted reaction products to the reacting nuclei populations. The involved resonant mode-particle interactions [1] are shown to affect the initial mode spatial profile. Minimal electron temperatures of the order of the ideal ignition temperature for DT (Deuterium-Tritium) plasmas are required in order to avoid transferring energy at significant rates to the electron population by mode-particle interactions. The observed D-D (Deuterium-Deuterium) fusion reaction rates [2] resulting from energetic neutral H-beam injection into D-plasmas are consistent with the directions emerging from the presented theory and warrant the investigation of more sophisticated and less severe ignition conditions (``cool fusion'') than those commonly considered for burning plasmas. [1] B. Coppi, Pl. Phys. Rep. \textbf{45}, 438 (2019). [2] R. M. Magee \textit{et al.}, Nature \textbf{15}, 281 (2019).