Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session YI1: Fast Ions, Momentum Input, RF, and Current Drive |
Hide Abstracts |
Chair: Zhihong Lin, University of California, Irvine Room: Acadia |
Friday, October 31, 2014 9:30AM - 10:00AM |
YI1.00001: Novel Reactor Relevant RF Actuator Schemes for the Lower Hybrid and the Ion Cyclotron Range of Frequencies Invited Speaker: Paul Bonoli This paper presents a fresh physics perspective on the onerous problem of coupling and successfully utilizing ion cyclotron range of frequencies (ICRF) and lower hybrid range of frequencies (LHRF) actuators in the harsh environment of a nuclear fusion reactor. The ICRF and LH launchers are essentially first wall components in a fusion reactor and as such will be subjected to high heat fluxes. The high field side (HFS) of the plasma offers a region of reduced heat flux together with a quiescent scrape off layer (SOL). Placement of the ICRF and LHRF launchers on the tokamak HFS also offers distinct physics advantages: The higher toroidal magnetic field makes it possible to couple faster phase velocity LH waves that can penetrate farther into the plasma core and be absorbed by higher energy electrons, thereby increasing the current drive efficiency. In addition, re-location of the LH launcher off the mid-plane (i.e., poloidal ``steering'') allows further control of the deposition location. Also ICRF waves coupled from the HFS couple strongly to mode converted ion Bernstein waves and ion cyclotron waves waves as the minority density is increased, thus opening the possibility of using this scheme for flow drive and pressure control. Finally the quiescent nature of the HFS scrape off layer should minimize the effects of RF wave scattering from density fluctuations. Ray tracing / Fokker Planck simulations will be presented for LHRF applications in devices such as the proposed Advanced Divertor Experiment (ADX) and extending to ITER and beyond. Full-wave simulations will also be presented which demonstrate the possible combinations of electron and ion heating via ICRF mode conversion. [Preview Abstract] |
Friday, October 31, 2014 10:00AM - 10:30AM |
YI1.00002: Measurements of LHCD current profile and efficiency for simulation validation on Alcator C-Mod Invited Speaker: Robert T. Mumgaard Lower hybrid current drive (LHCD) is an effective tool to significantly modify the magnetic equilibrium by driving off-axis, non-inductive current. On Alcator C-Mod, an upgraded Motional Stark Effect (MSE) diagnostic enables the current profile to be accurately reconstructed during plasmas with strong LHCD and a hard X-ray camera measures the fast electron Bremsstrahlung profile. LHCD is applied for \textgreater 4 current relaxation times, producing fully-relaxed magnetic equilibria in plasmas with non-inductive current fraction up to unity at currents up to 1.0 MA. C-Mod has developed an extensive database of LHCD performance, spanning a wide range in plasma current, launched n$_{\vert \vert }$, LHCD power, T$_{e}$ and plasma density. This dataset provides a unique platform for validation of LHCD current drive simulations with the plasma shape, density, field and LH frequency range envisioned for ITER and future reactors. In these conditions the measured current drive efficiencies are similar to that assumed for ITER with values up to 0.4*10$^{20}$A/Wm$^{2}$ despite being in a weak single-pass absorption regime. The driven current is observed to be off-axis, broadening the current profile, raising q$_{0}$ above 1, suppressing sawteeth, decreasing/reversing the magnetic shear and sometimes destabilizing MHD modes and/or triggering internal transport barriers. Measurements indicate increased efficiency at increased temperature and plasma current but with a complicated dependence on launched n$_{\vert \vert }$. The MSE-constrained reconstructions show a loss in current drive efficiency as the plasma density is increased above \textless n$_{e}$\textgreater $=$1.0x10$^{20}$ m$^{-3}$ consistent with previous observations of a precipitous drop in hard x-ray emission. Additionally, the measured driven current profile moves radially outward as the density is increased. Ray tracing simulations using GENRAY-CQL3D qualitatively reproduce these trends showing the rays make many passes through the plasma at high density and predicting a narrower current and HXR profile with than that observed in the experiment. This work is supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466. [Preview Abstract] |
Friday, October 31, 2014 10:30AM - 11:00AM |
YI1.00003: Taming the ICRF Antenna - Plasma Edge Interaction using Novel Field-Aligned ICRF Antenna on Alcator C-Mod Invited Speaker: Yijun Lin For ICRF antenna utilization in future fusion reactors, taming the antenna-plasma edge interaction while robustly coupling RF power is a critical challenge. Using a novel field-aligned (FA) ICRF antenna where the antenna straps are perpendicular to the total magnetic field, we have shown dramatically improved ICRF antenna performance. The FA antenna has significantly reduced antenna impurity sources, core impurity contamination and radiated power compared to conventional toroidally aligned antennas. The FA antenna also has load tolerance to plasma transients and significantly reduced RF-enhanced heat flux. The emerging physics picture is that the FA antenna minimizes generation of slow wave fields (E//B polarization). This reduction in slow wave lowers the local RF sheath around the ICRF antenna, and thus lowers the impurity source at local antenna structure. Simplified antenna simulations show a strong reduction in slow wave fields. The reduction of the slow wave field also impacts the antenna load tolerance. With the slow wave present, the antenna impedance is strongly modified by the slow wave coupling between antenna straps and this coupling is dependent upon the local density. With reduced slow wave coupling, the antenna reactive impedance is defined by the strap geometry and independent of the plasma whereas the real impedance is determined by the fast wave coupling. Experimentally we have found that the FA antenna loading has similar trends versus plasma current and densities to TA antennas, but the FA antenna reflection coefficient has significantly reduced variation, particularly during L-H and H-L transitions, and ELMs. Further comparisons of the FA and TA antennas are underway with an extensive array of diagnostics to characterize the RF plasma edge interaction and the latest results will be presented. [Preview Abstract] |
Friday, October 31, 2014 11:00AM - 11:30AM |
YI1.00004: Fast Ion Transport Studies in DIII-D High $\beta_N$ Steady-State Scenarios Invited Speaker: C.T. Holcomb DIII-D research is identifying paths to optimize energetic particle (EP) transport in high $\beta_N$ steady-state tokamak scenarios. Operation with $q_{min} >2$ is predicted to achieve high $\beta_N$, confinement, and bootstrap fraction. However DIII-D experiments have shown that Alfv\'en eigenmodes (AE) and correlated EP transport can limit the performance of some $q_{min} >2$ plasmas. Enhanced EP transport occurs in plasmas with $q_{min}=\,$2-2.5, $q_{95}=\,$5-7, and relatively long slowing down time. Strong AEs are present, the confinement factor $H_{89}=\,$1.6-1.8 and $\beta_N$ is limited to $\sim$3 by the available power. These observations are consistent with EP transport models having a critical gradient in $\beta_f$. However, adjusting the parameters can recover classical EP confinement or improve thermal confinement so that $H_{89}>2$. One example is a scenario with $\beta_P$ and $\beta_N \approx 3.2$, $q_{min} >3$ and $q_{95}\approx 11$ developed to test control of long pulse, high heat flux operation on devices like EAST. This has an internal transport barrier at $\rho\approx 0.7$, bootstrap fraction $>$75\%, density limit fraction $\approx$1, and $H_{89}\ge2$. In these cases AE activity and EP transport is very dynamic - it varies between classical and anomalous from shot to shot and within shots. Thus these plasmas are close to a threshold for enhanced EP transport. This may be governed by a combination of a relatively low $\nabla\beta_{fast}$ due to good thermal confinement and lower beam power, short slowing down time, and possibly changes to the $q$-profile. Another example is scenarios with $q_{min}\approx$1.1. These typically have classical EP confinement and good thermal confinement. Thus by using its flexible parameters and profile control tools DIII-D is comparing a wide range of steady-state scenarios to identify the key physics setting EP transport [Preview Abstract] |
Friday, October 31, 2014 11:30AM - 12:00PM |
YI1.00005: Ion Loss as an Intrinsic Momentum Source in Tokamaks Invited Speaker: J.A. Boedo A series of coupled experiments in DIII-D and simulations provide strong support for the kinetic loss of thermal ions from the edge as the mechanism for toroidal momentum generation in tokamaks. Measurements of the near-separatrix parallel velocity of D$^+$ with Mach probes show a 1-2 cm wide D$^+$ parallel velocity peak at the separatrix reaching 40-60 km/s, up to half the thermal velocity, always in the direction of the plasma current. The magnitude and width of the velocity layer are in excellent agreement with a first-principle, collissionless, kinetic computation of selective particle loss due to the loss cone [1] including for the first time the measured radial electric field, $E_r$ in steady state. C$^{6+}$ rotation in the core, measured with charge exchange recombination (CER) spectroscopy is correlated with the edge D$^+$ velocity. XGC0 computations [2], which include collisions and kinetic ions and electrons, show results that agree with the measurements, and indicate that two mechanisms are relevant: 1) ion orbit loss and 2) a growing influence of the Pfirsch-Schluter mechanism in H-mode gradients. The inclusion of the measured $E_r$ in the loss-cone model [1] drastically affects the width and magnitude of the velocity profile and improves agreement with the Mach probe measurements. A fine structure in $E_r$ is found, still of unknown origin, featuring large (10-20 kV/m) positive peaks in the SOL and at, or slightly inside, the separatrix of low power L- or H-mode conditions. This high resolution probe measurement of Er agrees with CER measurements where the techniques overlap. The flow is attenuated in higher collisionality conditions, consistent with a depleted loss-cone mechanism. \vskip6pt \noindent [1] J.S.\ deGrassie et al., Nucl.\ Fusion {\bf 52}, 013010 (2011).\par \noindent [2] C.S.\ Chang et al., Phys.\ Plasmas {\bf 11}, 5626 (2004). [Preview Abstract] |
Friday, October 31, 2014 12:00PM - 12:30PM |
YI1.00006: Energy Channeling and Coupling of Neutral-beam-driven Compressional Alfv\'{e}n Eigenmodes to Kinetic Alfv\'{e}n Waves in NSTX Invited Speaker: Elena Belova Experimental observations from the National Spherical Torus Experiment (NSTX) have linked strong activity of global (GAEs) and compressional (CAEs) Alfven eigenmodes with a flattening of the electron temperature profile in beam heated plasmas in NSTX [1]. Previous theoretical studies attributed this effect to an enhanced electron transport due to these modes [2]. This work presents the first self-consistent simulations of neutral-beam-driven CAEs demonstrating an important alternative, an efficient energy channeling mechanism that will occur for any unstable CAE in NSTX or other toroidal devices. Three-dimensional hybrid MHD-particle simulations using the HYM code for an NSTX discharge (141398) show unstable CAEs for a range of toroidal mode numbers, n$=$4-9, and frequencies below the ion cyclotron frequency. It is found that an essential feature of CAE modes in NSTX is their coupling to kinetic Alfven waves (KAW) that occurs on the high-field side at the Alfven resonance location. The radial width of the KAW is found to be comparable to the fast ion Larmor radius. The beam-driven CAE can mode-convert to KAW, channeling energy from the beam ions at the injection region near the magnetic axis to the location of the resonant mode conversion at the edge of the beam density profile. This mechanism can explain the reduced heating of the plasma core in NSTX. The NBI power transferred to one CAE has been estimated as up to P$=$0.4MW, based on measured displacement amplitudes and HYM calculated mode structure. The energy flux from the CAE to the KAW and dissipation at the resonance location can have a direct effect on the temperature profile with changes in core electron temperature up to several hundred eV. It is shown that strong CAE/KAW coupling follows from the dispersion relation, and will occur for any unstable CAE in NSTX or other toroidal devices, independent of toroidal mode number or mode frequency. \\[4pt] [1] D. Stutman, et al., Phys. Rev. Lett. \textbf{102}, 115002 (2009).\\[0pt] [2] N.N. Gorelenkov, D. Stutman, K. Tritz et al., Nucl. Fusion \textbf{50} 084012 (2010). [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