Session KI02: MFE: Energetic Particles and Fast Ions

Alpha Particle Losses within JET's 2021/2022 DT-Campaign

Alpha particle confinement is crucial for sustaining burning plasmas and designing future reactor concepts. Along with classical/prompt losses, various MHD instabilities can lead to wave-particle interactions which can transport alpha particles outward from the plasma. These processes are detrimental to plasma self-heating and require further study. Since prior experiments in the mid-90's, advances in experimentation, diagnostic capabilities, and numerical modeling tools have warranted renewed DT-operations. JET's 2021/2022 DT-campaign provided new opportunities for alpha particle experiments in ITER-like plasmas with state-of-the-art energetic particle diagnostics. This work will present alpha particle loss measurements from JET's Faraday cup fast ion loss detector array and scintillator probe with supporting measurements from neutron diagnostics. Losses from low frequency MHD activity are examined with comments on alpha transport, confinement, and heating in the bump-on tail distribution, "afterglow," and baseline scenarios. Alpha particle losses were recorded from both coherent (NTMs, kinks, fishbones, etc.) and non-coherent sources (ELMs and sawteeth). In particular, coherent losses with low frequency (<100 kHz) MHD were strongly observed. An estimate of the maximum alpha loss fraction was determined due to be ~5% from the coherent, low frequency, MHD activity, which could pose an issue for future reactors. Sensitivities to alpha energy, pitch, and spatial loss deposition were also observed. An interesting discharge exhibiting losses maximal in two regions of pitch-space is being examined for NTM induced ripple losses. Additionally, correlations with plasma shaping parameters, impurity concentrations, and fast ion orbit topology were conducted to assess shaping effects, wall interactions, and detector susceptibility. Lastly, comments on higher frequency toroidal Alfven eigenmode (TAE) induced alpha losses will be shared.   

Simulation of the AE activity in JET D-T discharges using a Landau closure model

The plasma in future nuclear fusion reactors will be heated by neutral beam injectors (NBI) and high frequency electromagnetic waves as well as fusion born alpha particles. Energetic particles (EPs), with energies up to two orders of magnitude larger than the thermal plasma, can trigger Alfven Eigenmodes (AE) and induce harmful EP losses, reducing the plasma heating efficiency and the economical viability of the reactor. The present study is dedicated to analyze the AE activity in JET D-T discharges, the closest experiment to reactor-like operation. There, AEs are driven by the combined effect of tangential NBIs and ion cyclotron resonance heating (ICRH) driven EP. Linear and nonlinear simulations are performed with the gyro-fluid FAR3d code to reproduce the AE activity observed in the discharge #99896, including the effect of multiple EP populations. The linear simulations reproduce the AE activity in the frequency range of 150-180 kHz, identifying unstable n=2 to 4 TAEs with frequencies between 152 to 164 kHz at the inner plasma region, triggered by highly energy passing Deuterium populations injected by the tangential NBIs, further accelerated by the effect of the ICRH up to ~1 MeV. In addition, low frequency modes below 40 kHz are identified as fish-bones, triggered by energetic trapped Hydrogen induced by the ICRH. On the other hand, the alpha density is too small to destabilize AEs in the experiment. Nonetheless, increasing artificially the alpha density by one order of magnitude, an n=1 BAE with a frequency around 60 – 80 kHz can be destabilized in the inner plasma region. Nonlinear simulations including single or multiple EP species are performed to study the AEs saturation phase. Important nonlinear couplings are identified between different EP populations along with the generation of zonal structures that may affect the performance of future fusion reactors.   

Spatial mode structure and propagation of ion cyclotron emission in the DIII-D tokamak

Ion cyclotron emission (ICE) spatial mode structure in DIII-D has been characterized for the first time, with the toroidal magnetic field fluctuation amplitude found to be consistently larger than its poloidal counterpart (δB_tor/δB_pol ~ δB_‖/δB_⊥ ≥ 2), thus demonstrating compressional polarization near the probe location. The modes were found to be poloidally extended through comparison of signal amplitude measured by loops on the low- and high-field sides of the machine, corroborated recently by δn measurements made by the high-frequency Doppler backscattering diagnostic¹. Finally, toroidal mode numbers of n ~ [-10, 5] were determined via phase information collected from three toroidally displaced outer wall loops. Measurements of ICE represent a possible fast ion diagnostic technique compatible with high radiation environments in future burning plasma experiments, which are prohibitive to many current fast ion diagnostics. However, we must better our physics understanding to discern fast ion characteristics from observed ICE spectra. To this end, an array of new toroidal and poloidal in-vessel loops was installed on the DIII-D ICE diagnostic, and the resultant spatial mode structure measurements will be used as crucial constraints for ongoing modeling work. ICE dependence on scrape-off layer (SOL) width was also investigated to assess mode propagation to the ICE diagnostic loops. ICE spectra were found robust to changing the SOL width from ~4–12 cm, suggesting very weak attenuation between the plasma and loop location and meaning that future diagnostics might be moved farther from areas of high radiation without adversely impacting measurement capability.

[1] N.A. Crocker et al. NF (2022) 026023.   

Simulating energetic ions and enhanced neutron rates from ion-cyclotron resonance heating with a new fast, self-consistent full-wave / Fokker-Planck model

Ion-cyclotron resonance heating (ICRH) is a key plasma actuator in experiments such as the JET, EAST, and WEST tokamaks, and use of ICRH is planned in the burning plasma experiments on SPARC and ITER. Previous experiments on Alcator C-Mod (P.T. Bonoli, Fusion Sci. Tech. 2007) and recent results from the JET DTE2 campaign (P. Jacquet, Conf. RF Power in Plasmas 2022) have demonstrated that ICRH generated fast-ions are important to understanding enhanced neutron rates and MHD instabilities.

Reproducing fast-ion effects in simulations is difficult. It requires self-consistent coupling of a full-wave solver and a Fokker Planck code which evolve multiple simultaneously resonant ion species. We introduce a new self-consistent model that iterates the TORIC or AORSA full-wave solvers with the CQL3D Fokker-Planck solver using the integrated plasma simulator (IPS). This model iteratively evolves the bounce-averaged ion distribution functions in both parallel and perpendicular velocity-space with a quasilinear RF diffusion operator (Lee, Phys. Plasmas 2017) valid in the ion finite Larmor radius (FLR) limit and the RF electric fields with the resultant non-Maxwellian FLR dielectric tensor (N. Bertelli, Nucl. Fusion 2017). This model provides non-Maxwellian ICRH simulations that are, fully self-consistent, fast, and interoperable with integrated modeling frameworks such as TRANSP/GACODE/IPS-FASTRAN. We demonstrate our model’s capabilities by performing simulations of Alcator C-Mod experiments where enhanced neutron rates resultant from fast-ions driven by harmonic damping of ICRH on deuterium were present in D(H) minority heating experiments, and we perform the first RF heating simulations of SPARC and ITER using self-consistent non-Maxwellian ion distributions to investigate the potential to enhance fusion gain using ICRF-generated fast ions.