Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session JI01: Invited: Magnetic Fusion: Alternates and RFLive
|
Hide Abstracts |
Chair: Paul Bonoli, MIT |
Tuesday, November 10, 2020 2:00PM - 2:30PM Live |
JI01.00001: Impurity temperature screening in stellarators close to quasisymmetry Invited Speaker: Mike Martin Impurity temperature screening is a favorable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough $\eta^{-1}\equiv d\ln n/d\ln T$. In contrast, neoclassical fluxes in generic stellarators will depend on the radial electric field, which is predicted to be inward for ion-root plasmas, potentially leading to impurity accumulation. Here we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry-breaking in the magnetic field. For the majority of this work, neoclassical fluxes have been obtained using the SFINCS drift-kinetic equation solver with electrostatic potential $\Phi=\Phi(r)$, where $r$ is a flux surface label. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. Thus, the small departures from symmetry in nominally quasisymmetric stellarators are large enough to significantly alter the neoclassical impurity transport. A further look at the magnitude of these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in configurations optimized for quasisymmetry when compared to turbulent fluxes. [Preview Abstract] |
Tuesday, November 10, 2020 2:30PM - 3:00PM Live |
JI01.00002: Evidence for thermonuclear neutron production from a sheared-flow stabilized (SFS) Z-pinch. Invited Speaker: James Mitrani Plasma confinement and heating via a Z pinch is one of the oldest and more straightforward fusion-energy concepts. However, if not adequately stabilized, the configuration is prone to virulent MHD instabilities that accelerate high-energy ion beams. Such ions can generate non-thermal, beam-target fusion and have caused significant misinterpretation of past Z pinch experiments. In this work, we present the first direct evidence of non-beam, thermonuclear fusion on the Fusion Z-pinch Experiment (FuZE). FuZE is a sheared-flow stabilized (SFS) Z-pinch that uses radially sheared, axial plasma flows to limit growth of MHD instabilities [Y Zhang \textit{et al.}, PRL 2019]. The confined, plasma column is 50 cm-long with a \textless 6~mm diameter and operates with pinch currents up to 400 kA. The device has achieved neutron yields up to 1e7 with durations up to 8 \textmu s. The axial extent of the neutron emitting region (34 cm) is comparable to the length of the plasma column (50 cm) [JM Mitrani \textit{et al.}, NIMA 2019]. Neutron signals are measured with plastic scintillator detectors operating in pulse-counting mode and digitized with a high bandwidth oscilloscope. Neutron energies are determined by constructing pulse integral spectra from measured neutron traces. Pulse integral spectra are a function of the energies of recoil protons in plastic scintillators, which in turn are a function of incident neutron energies. Analysis of pulse integral spectra from detectors placed upstream and downstream of the plasma column indicates spatially isotropic neutron energy emission within 200 keV, which precludes the presence of axial, deuteron beams with energies \textgreater 20 keV. This result rules out axial, beam-target fusion reactions as the dominant source of neutron emission and is encouraging for scaling SFS Z pinches toward reactor conditions. [Preview Abstract] |
Tuesday, November 10, 2020 3:00PM - 3:30PM Live |
JI01.00003: Explore and expand regimes of synergy with two frequencies of lower hybrid power Invited Speaker: Wilkie Choi EAST has demonstrated steady-state, fully non-inductive operation using their unique setup of two lower hybrid (LH) systems at 2.45 GHz and 4.6 GHz. Theory predicts that injecting the two LH waves simultaneously would produce higher current drive (CD) efficiency than injection of either frequency alone, but evidence for this synergy between the two LH waves has not been observed in the experiments to date. A recent experiment scanned the power fraction from the two antennas while maintaining constant total injected power, at two different density conditions. Analysis of this experiment indicate that simultaneous injection can improve CD efficiency. \par The time-dependent evolution of an EAST plasma with simultaneous injection of two frequencies of LH waves has been simulated for the first time using the TRANSP code together with the ray-tracing/Fokker-Planck codes GENRAY/CQL3D. In addition to requiring accurate density and temperature profiles for simulation to emulate experiment, it is also found that at low density the injected power spectrum needs to be modified with a tail model in order to reproduce the observed core deposition. \par The time-dependent simulations show that, when scanning the injected power ratio of the two frequencies of LH at low density ($n_{e,lin}\approx 2.0 \times 10^{\mathrm{19}}$ m$^{\mathrm{-3}})$, a shot with simultaneous injection of 0.6 MW at 2.45 GHz and 0.4 MW at 4.6 GHz achieved an LHCD efficiency higher than 1 MW of either 2.45 GHz alone (by $\sim$39{\%}) or 4.6 GHz alone (by $\sim$8{\%}) injected in similar conditions. However, at high density ($n_{e,lin}\approx 3.3 \times 10^{\mathrm{19}}$ m$^{\mathrm{-3}})$, LHCD efficiency was found to monotonically increase with fraction of LH power at 4.6 GHz. The possible operating regimes with synergy and their sensitivity to plasma density and injected wave spectrum will also be investigated, which will further optimize access to long pulse scenarios at high non-inductive fraction. [Preview Abstract] |
Tuesday, November 10, 2020 3:30PM - 4:00PM Live |
JI01.00004: Suppression of Magnetic Islands and Avoidance of Disruptions via RF Current Condensation Invited Speaker: Allan Reiman Large islands produced by off-normal events routinely cause disruptions. We identify a nonlinear effect, ``RF current condensation'', that facilitates island stabilization, allowing the stabilization of larger islands than would otherwise be possible [1]. The effect arises from the sensitivity of the power deposition and driven current to the temperature perturbation in the island. There is a nonlinear enhancement of the temperature, with the increased power deposition leading to a further increase in the temperature perturbation. In combination with the sensitivity of the current drive to the temperature, this leads to the RF current condensation effect. There is a discontinuous jump in the steady-state solution to the thermal diffusion equation at a threshold in power and island width, with the temperature above the threshold saturated by depletion of the wave power [2] or by the ITG threshold [3]. This bifurcation is associated with a hysteresis effect that can cause the islands to shrink to smaller widths than would otherwise be the case. For lower hybrid waves, the effect narrows the generally broad deposition, and it can lead to automatic, passive stabilization of islands [4]. The stabilizing effect can be further enhanced by pulsing the RF power [5]. We have developed a code for simulating the effect, and we find that current condensation and bifurcation can occur in ITER plasmas with the available EC power [6]. [1] A. Reiman and N.Fisch, Phys. Rev. Lett. \textbf{121}, 225001 (2018). [2] E. Rodriguez, A. Reiman and N. Fisch, Phys. Plasmas \textbf{26}, 092511 (2019). [3] E. Rodriguez, A. Reiman and N. Fisch, Phys. Plasmas \textbf{27}, 042306 (2020). [4] S.J. Frank, A.H. Reiman, N.J. Fisch and P.T. Bonoli, Nucl. Fusion, to appear. [5] S. Jin, N. J. Fisch, and A. H. Reiman, Phys. Plasmas 27, 062508 (2020). [6] R. Nies, A. Reiman, E. Rodriguez, N. Bertelli, N. Fisch, \underline {http://arxiv.org/abs/2005.05997}. [Preview Abstract] |
Tuesday, November 10, 2020 4:00PM - 4:30PM Live |
JI01.00005: Study of turbulence-induced refraction of Lower Hybrid waves using synthetic SOL blobs Invited Speaker: Bodhi Biswas Ray-tracing/Fokker-Planck (RT/FP) simulations with realistic 3D scrape-off layer (SOL) turbulent density profiles provide evidence of significant RF-turbulence interaction that affects Lower Hybrid current drive (LHCD) in tokamaks. Synthetic SOL turbulence profiles that account for dense field-following blob-like structures are coupled to the RT code GENRAY. Modification of ray-trajectories due to turbulence-induced refraction (TIR) is shown to increase with blob density and decrease with blob width, consistent with previous theories. However, it is found that spatially intermittent density fluctuations lead to increased angular diffusion of the perpendicular wavevector (k$_{\mathrm{\bot }})$ compared to previous models that assume drift-wave-like turbulence. In a modelled Alcator C-Mod discharge, TIR results in significant diffusion of ray-trajectories in phase-space, greatly increasing the robustness of RT/FP models in the multi-pass regime. In C-Mod, TIR also results in smoother current profiles, diminished off-axis current peaks, and increased near-axis current, all of which contribute to better match experiment. The role of k$_{\mathrm{\bot \thinspace }}$broadening, due to turbulent scattering, versus broadening of the incident parallel wavenumber (k$_{\mathrm{\vert \vert }})$, due to parametric decay instabilities (PDI), in explaining the spectral gap is also investigated. A previous study reports that \textasciitilde 50{\%} power redistribution of the launched k$_{\mathrm{\vert \vert }}$-spectrum is required to match experimental hard x-ray measurements in C-Mod [Y. Peysson, et al., PPCF 58, 044008 (2016)]. The inclusion of k$_{\mathrm{\bot }}$ broadening from TIR lowers the necessary power redistribution to a more realistic \textasciitilde 10{\%}. Lastly, simulations predict that modification of the current profile due to TIR is mitigated in a DIII-D high-field side single-pass damping scenario, where shorter propagation distances result in decreased ray stochasticity. [Preview Abstract] |
Tuesday, November 10, 2020 4:30PM - 5:00PM Live |
JI01.00006: Confinement Improvement through Impurity Induced Profile Modification on W7-X. Invited Speaker: Robert Lunsford Pulsed injection of boron carbide granules into W7-X discharges transiently increase the plasma stored energy and core ion temperature above standard W7-X operation by up to 30{\%}. In a series of 4MW ECRH heated experiments, the PPPL Probe Mounted Powder Injector provided 50 ms bursts of 100 micron granules every 350 ms at quantities ranging from \textless 1mg/pulse to 30 mg/pulse. For quantities up to 10 mg/pulse, the impurities are flushed from the plasma between pulses and the effect on the overall plasma is modest. For quantities above 10 mg/pulse the impurities are partially flushed, and Zeff increased from 1.3 to 3.2 over the series of pulses. For each injection, the stored energy initially drops, the radiated power transiently increases, and the radial density profile flattens on the edge as the material is assimilated. After the injected boron has been fully absorbed and the radiated power recovers, the stored energy increases above the previous baseline level by an amount linearly correlated with the injection quantity. The peak core ion temperature increases from 1.7 keV to a maximum of 2.9 keV for the largest injection amounts. During the injections, the ion temperature gradient steepens and a central plateau develops out to a minor radius of 0.2m, resulting in a decrease in the gradient scale length. This is accompanied by a reversal of the electric field at r \textgreater 0.4m, indicating the transport has switched to the ion root. The impurity confinement time increased by roughly a factor of 2. These observations are suggestive of a transient stabilization of ITG turbulence, similar to the enhanced confinement after cryogenic pellet injection. This provides further evidence that externally induced profile modification is a possible path to elevated W7-X performance metrics. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700