Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session YI2: Energetic Particles, Heating |
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Chair: Cami Collins, General Atomics Room: OCC Oregon Ballroom 203 |
Friday, November 9, 2018 9:30AM - 10:00AM |
YI2.00001: Three-ion ICRH scenarios: new and efficient schemes for plasma heating and fast particle generation in current and future fusion devices Invited Speaker: Jozef P ONGENA
Three-ion ICRH scenarios [1] offer new exciting possibilities for the application of ICRF heating in fusion devices. In a plasma consisting of two main ions, ICRF power absorption is strongly enhanced in the vicinity of the ion-ion hybrid (IIH) layer because the RF electric field E+ is rotating nearly completely in the direction of the plasma ions. Candidate absorber ions are those for which the resonance layer is located close to the IIH layer and thus can be a third ion species with Z/A in between that of the two main ions or fast main ions, with a Doppler shift that displaces their resonance layer in between that of the two main ions. These ions may be typical H or He isotopes or impurities intrinsic or extrinsic to the plasma such as 9Be, 22Ne, Ar, etc. Both plasma heating and fast particle acceleration are possible with this technique. We summarize recent and past experiments with various three-ion ICRH scenarios on TFTR [2], C-Mod [1], JET [1] and AUG [3], using third ions and fast accelerated beam particles [4] as resonant absorbers and with on- and off- axis heating. We will illustrate the promising potential of three-ion ICRH scenarios for future JET D-T, ITER and DEMO operations and for testing the fast particle confinement properties of the optimized stellarator W7-X at the high densities expected in this device in the coming years. Finally, applications of three-ion scenarios are not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3He-rich solar flares. [1] Ye.O.Kazakov, J.Ongena, J.Wright, S.Wukitch et al., Nat.Phys. 13,973–978(2017) [2] J.R.Wilson et al., Phys. Plasmas 5,1721(1998) [3] A.Kappatou et al., 45th EPS Plamsa Phys.Conf (Prague 2018) Paper O2.105 (2018) [4] J.Ongena, et al.,EPJ Web of Conferences, 157,02006(2017) |
Friday, November 9, 2018 10:00AM - 10:30AM |
YI2.00002: Electron Cyclotron Heating Modification of Alfvén Eigenmode Activity in the DIII-D and ASDEX-Upgrade Tokamaks Invited Speaker: Michael A Van Zeeland Joint experiments on the DIII-D and ASDEX-Upgrade tokamaks demonstrate that electron cyclotron heating (ECH) can have a dramatic effect on Alfvén eigenmode activity in fusion plasmas, with the actual outcome depending sensitively on ECH deposition location, discharge conditions and fast ion source. The most common effect in DIII-D plasmas is a shift in the observed modes from a mix of reversed shear Alfvén eigenmodes (RSAEs) and toroidicity induced Alfvén eigenmodes (TAEs) to a spectrum of only weak TAEs when ECH is deposited near the shear reversal point. Experiments in AUG have found a similarly large impact on beam driven AEs including examples with the complete stabilization of both RSAEs and TAEs by ECH. The ECH impact on RSAEs results primarily from an increase in the local electron temperature and its gradient at qmin, which causes an increase in both the geodesic acoustic frequency and RSAE frequency to a point where the RSAE has a much reduced frequency sweep range or is no longer an eigenmode of the system. A simple q-evolution model that includes these effects on the RSAE frequency sweep is in agreement with measurements and captures the relative balance of TAE or RSAE-like modes in a broad range of discharges. Additionally, MHD and gyrofluid calculations confirm this interpretation and show both modification of plasma pressure and pressure gradient play an important role in altering RSAE activity. Recent AUG experiments have extended these results to include the impact of ECH on TAEs driven by ion cyclotron heating accelerated ions. In these experiments detailed analysis showed ECH actually increased TAE amplitudes due to a locally enhanced slowing down time / fast ion pressure. The ECH modification of AE activity is of practical importance as both devices have significantly reduced fast ion transport with weaker modes. |
Friday, November 9, 2018 10:30AM - 11:00AM |
YI2.00003: Fast ion effects on gyrokinetic turbulence Invited Speaker: Alessandro Di Siena Experimental and dedicated gyrokinetic studies have reported strong fast ion effects on ion-scale plasma turbulence. Depending on the origin of the energetic tails - e.g., generated through auxiliary heating systems such as neutral beam injection or ion cyclotron resonance heating or in view of ITER from fusion reactions - these effects are found to possibly provide substantial overall confinement improvements. In particular, a wave-fast ion resonant stabilising mechanism has been acknowledged quite recently [Di Siena et al Nucl. Fusion 58 054002 2018]. Corresponding studies with the gyrokinetic code GENE and a reduced analytic model will be summarized in the first part of this contribution. The second part will deal with removing an important deficiency that is present in most of such studies, which employ equivalent Maxwellian fast ion distributions. Anisotropic velocity structures which might modify the aforementioned resonance effects and which can be particularly relevant in driving energetic particle induced modes are not captured. In the contribution at hand, the impact of fast ions is studied for the first time for realistic physics inputs taken from actual discharges with an extended version of the gyrokinetic code GENE [Di Siena et al Phys. Plasmas 5 042304 2018], which allows to relax the assumption on the energetic ion background distribution functions for flux-tube and radially-global simulations. Numerical backgrounds, computed with state-of-the-art heating codes are employed to model highly non-thermalised particles for ASDEX Upgrade and JET discharges with substantial fast ion fraction. By comparing GENE heat/particle fluxes and fast-ion driven frequency spectra with experiment, a much better agreement is observed. Based on these findings, a brief outlook on how these intriguing fast ion effects may affect ITER plasmas will be given. |
Friday, November 9, 2018 11:00AM - 11:30AM |
YI2.00004: Fast Ion Transport in the Quasi-Single Helical Reversed-Field Pinch Invited Speaker: P. J. Bonofiglo At sufficiently high Lundquist number, the reversed-field pinch (RFP) transitions to a 3D-helical equilibrium known as quasi-single helicity (QSH). A helical axis forms when the innermost resonant tearing mode grows and envelops the magnetic axis while the secondary modes maintain, or decrease, their amplitudes. This new, self-organized state has been shown to create strong thermal transport barriers, making QSH a possible scenario for a low-magnetic-field ohmic fusion reactor (Lorenzini et al. Nature 2009). The behavior of fast ions in the QSH state, however, poses a critical question for the helical RFP’s fusion relevance. Current work on MST investigates the dynamics of fast ion transport during QSH using neutral beam-born ions. Energetic particle modes (EPMs) on MST upshift in frequency with increasing core-resonant mode amplitude and disappear in high current QSH plasmas. FIR interferometry has resolved electron density perturbations associated with the EPMs and shows the EPMs moving radially outward during the QSH transition. The 3D shear-Alfven continuum solver STELLGAP describes the frequency rise as an effect of the equilibrium change on the fast ion resonance. Additionally, neutral particle analysis and neutron flux measurements suggest fast ion losses at sufficient core-resonant mode strength. The particle tracking code ORBIT corroborates rapid fast ion loss times in QSH and demonstrates weak neoclassical effects. ORBIT results indicate that the rapid fast ion transport depends heavily on the presence of the secondary tearing modes. While thermal particle orbits display reduced stochasticity and improved confinement, fast ion orbits display the opposite. While this presents an obstacle in the exploitation of the QSH regime for fusion, data in Lundquist number scaling exhibits a reduction in the secondary mode strengths, lessening their impact on fast ions. This research is supported by US DOE. |
Friday, November 9, 2018 11:30AM - 12:00PM |
YI2.00005: Quasi-linear resonance broadened model for fast ion relaxation in the presence of Alfvénic instabilities Invited Speaker: Nikolai Gorelenkov We present a realistic Quasi-linear (QL) model to find the energetic particle distribution function relaxed in the presence of Alfvénic instabilities. This approach is a numerically efficient method to capture the evolution of a beam ion distribution function. The spatial structure of the instabilities is computed by the eigenvalue solver for predictive simulations. |
Friday, November 9, 2018 12:00PM - 12:30PM |
YI2.00006: Real-time simulation of the NBI fast-ion distribution Invited Speaker: Markus Weiland Knowledge of the fast-ion distribution arising from neutral beam injection (NBI) is important for transport analysis and magnetic equilibrium reconstruction. For advanced plasma control, which will be essential for the operation of future fusion devices, it is very beneficial to know this distribution function already in real-time during the discharge. |
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