Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session JO4: NSTX-U and Pegasus |
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Chair: Brian Nelson, University of Washington Room: Plaza D |
Tuesday, November 12, 2013 2:00PM - 2:12PM |
JO4.00001: Advancing High Current Startup via Localized Helicity Injection in the \textsc{Pegasus} Toroidal Experiment E.T. Hinson, J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, J.M. Perry, A.J. Redd, D.J. Schlossberg Non-solenoidal startup via local helicity injection (LHI) and poloidal field induction is used to produce $I_{p} =0.17$ MA tokamak discharges. Impurity contamination has been reduced to negligible levels by use of conical frustum cathode geometry and local scraper limiters. Attainable currents are governed by global limits of helicity and energy balance, and Taylor relaxation. A simple lumped parameter model based on these limits is used to project discharge evolution, and indicates that attaining 1 MA in NSTX-U will require LHI-driven effective loop voltages to dominate contributions from $dL_{p} /dt$. This regime contrasts with results to date and will be tested at 0.3 MA in \textsc{Pegasus} with a new integrated multi-injector array. Injector impedance characteristics are consistent with magnetically-limited regimes observed in higher-power foilless diodes. Bursts of MHD are measured on time scales of order $\sim 100 \mu $s, and correlate with rapid equilibrium changes, discrete rises in $I_{p} $, redistribution of the toroidal current, ion heating ($T_{i} \sim 1$ keV), transient drops in injector voltage, and apparent $n=1$ line-tied kink activity at the injector. NIMROD simulations\footnote{J.B. O'Bryan \textit{et al.}, Phys. Plasmas \textbf{19}, 080701 (2012)} of high-field-side HI discharges in \textsc{Pegasus} are in qualitative agreement, suggesting $I_{p} $ buildup results from inward propagating toroidal current loops created by intermittent reconnection of injected current streams. [Preview Abstract] |
Tuesday, November 12, 2013 2:12PM - 2:24PM |
JO4.00002: Non-inductive Plasma Start-up and Current Ramp-up in NSTX-U R. Raman, T.R. Jarboe, S.C. Jardin, C.E. Kessel, D. Mueller, B.A. Nelson, F. Poli, G. Taylor Results from NSTX Transient Coaxial Helicity Injection (CHI) experiments have demonstrated generation of 300kA start-up currents, and when these discharges were coupled to induction they attained 1MA of plasma current consuming 65{\%} of the inductive flux of standard inductive-only discharges in NSTX. The NSTX-U device, which is now under construction at PPPL, will have numerous improvements to enhance transient CHI start-up. These are: (1) factor of two increase in toroidal field, (2) more than 2.5 times the injector flux, (3) increased CHI voltage, (4) full lithium coverage to reduce low-Z impurities and (5) 1 MW ECH system for increasing the electron temperature of CHI discharges to allow direct coupling to NBI current drive using a new second more tangential neutral beam system. In support of NSTX-U objectives for full non-inductive start-up and current ramp-up, the TSC code has been used for a full discharge simulation in which a transient CHI discharge is used as the front end of the non-inductive current ramp-up simulation. This work supported by U.S. DOE Contracts DE-AC02-09CH11466~and DE-FG02-99ER54519 AM08. [Preview Abstract] |
Tuesday, November 12, 2013 2:24PM - 2:36PM |
JO4.00003: Simulation of current-filament dynamics and relaxation in the Pegasus ST J.B. O'Bryan, C.R. Sovinec The dynamics of magnetic relaxation of during non-inductive startup in the Pegasus spherical tokamak are investigated with nonlinear MHD-based numerical computation. A new phenomenology, evident from our modeling, is the attraction and reconnection of adjacent passes of the helical filament, releasing axisymmetric rings of current [O'Bryan, Phys.\ Plas.\ 19, 080701 (2012)]. Accumulation of poloidal flux over multiple relaxation events leads to toroidal configurations with hollow average $J_{||}$ profiles. Similar to experimental results, the magnetic fluctuation during helicity injection is approximately 5\% of the toroidal field with significant activity in the 10-20 kHz range. The inclusion of two-fluid effects in Ohm's law produces qualitatively similar plasma evolution. Temperature and current profiles broaden and closed flux surfaces form rapidly upon cessation of the simulated current drive, leaving a tokamak-like discharge suitable for transition to other forms of current drive. Investigation of time-dependent evolution of externally driven poloidal flux is underway. [Preview Abstract] |
Tuesday, November 12, 2013 2:36PM - 2:48PM |
JO4.00004: The dependence of discharge performance on pre-discharge lithium evaporation in high triangularity H-mode discharges in NSTX R. Maingi, S.M. Kaye Lithium wall conditioning on graphite plasma facing components has been shown to reduce recycling and edge transport [1], improve energy confinement, and suppress edge localized modes in the NSTX. These benefits increased nearly continuously with the amount of pre-discharge lithium evaporation in medium triangularity discharges [2-5], consistent with cross-field transport increasing with collisionality [6]. Here we show similar results for a comparable dataset with increasing pre-discharge lithium evaporation in high triangularity, high performance discharges in NSTX. We observed a nearly monotonic reduction in recycling, neutral pressure, and increase in H-mode confinement factor with increasing evaporation. The ELM frequency was clearly reduced with increasing lithium, although complete ELM-free operation was not observed in this particular sequence. This suggests that the benefits of lithium conditioning should also apply to the highly shaped plasmas planned in NSTX-U. Comparisons between the high and (previously analyzed) medium triangularity boundary shapes will be presented. \\[4pt] [1] Canik J. M.\textit{, et al.} \textit{Phys. Plasmas} \textbf{18,} 056118 (2011).\\[0pt] [2] Maingi R.\textit{, et al.} \textit{PRL} \textbf{107,} 145004 (2011).\\[0pt] [3] Boyle D. P.\textit{, et al.} \textit{PPCF} \textbf{53,} 105011 (2011). \\[0pt] [4] Maingi, R., \textit{et al. Nucl. Fusion }\textbf{53,} 083001(2012).\\[0pt] [5] Boyle D. P.\textit{, et al.} \textit{J. Nucl. Mater.} \textbf{438,} S979 (2013). \\[0pt] [6] S.M. Kaye, \textit{et al. Nucl. Fusion }\textbf{53,} 063005 (2013). [Preview Abstract] |
Tuesday, November 12, 2013 2:48PM - 3:00PM |
JO4.00005: Changes to edge gyrokinetic stability with lithium coated PFCs in NSTX J.M. Canik, W. Guttenfelder, R. Maingi, R.E. Bell, B.P. LeBlanc, T.H. Osborne The application of lithium coatings to the NSTX PFCs has been shown to eliminate ELMs and increase energy confinement. The linear microstability properties in the edge region of plasmas without and with lithium coatings have been examined with the GS2 code. In the far edge, outside $\psi _{\mathrm{N}}$ $\sim$ 0.95, electron temperature gradient (ETG) modes are destabilized with lithium. Nonlinear simulations suggest that ETG transport may be large enough to prevent the electron temperature gradient from increasing, even as the density is reduced when lithium is introduced; experimentally, a clamping of the T$_{\mathrm{e}}$ profile is observed that contributes to the stabilization of ELMs by lithium coatings. Near the pedestal top, microtearing modes are unstable without lithium, and are stabilized by the locally increased density gradient with lithium; this stabilization may play a role in the reduced transport rate inferred in the same region and the increase in global energy confinement observed with lithium. These results point to the edge density profile as the controlling factor determining both ELM stability and energy confinement. [Preview Abstract] |
Tuesday, November 12, 2013 3:00PM - 3:12PM |
JO4.00006: H-mode Characterization and Edge Stability at Near-Unity Aspect Ratio in \textsc{Pegasus} Discharges K.E. Thome, J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, L.M. Peguero, J.M. Perry, D.J. Schlossberg, D.S. Thompson Unique features of operating at near-unity aspect ratio include: ready access to Ohmic H-mode; operation in the low collisionality regime with strong neoclassical effects; and ELM instabilities driven by peeling and peeling- ballooning modes. Ohmic H-mode is achieved in both limited and diverted configurations by using high-field-side fueling. The access to and characteristics of H-mode regimes as well as various ELM types in \textsc{Pegasus} is currently being explored. Characteristics of the L-H transition are: formation of an edge current pedestal; reversal of the direction of toroidal flow at the transition; doubling of the stored energy; and the presence of ELMs. Modest temperatures and pulse lengths in \textsc{Pegasus} allow the use of insertable probes to measure the properties of the edge plasma with high spatial and temporal resolution, even in ELMy H-mode. A current pedestal in the edge $J(R,t)$ profile is observed in H-mode but not in L-mode operation. This pedestal is destroyed during an ELM event cycle, but returns quickly after the ELM. Peeling modes, identified in the edge of L-mode plasmas with strong edge current, drive the formation of an edge current hole and ejection of a current-carrying filament consistent with electromagnetic blob theory. Similar behavior is indicated with ELMs in H-mode plasmas. [Preview Abstract] |
Tuesday, November 12, 2013 3:12PM - 3:24PM |
JO4.00007: Active resistive wall mode and plasma rotation control for disruption avoidance in NSTX S.A. Sabbagh, J.W. Berkery, J.M. Bialek, Y.S. Park, R.E. Bell, D.A. Gates, S.P. Gerhardt, I. Goumiri NSTX has produced high beta plasmas at levels needed for future ST devices. Research turns to understanding how the disruption probability due to global MHD and resistive wall modes (RWM) can be reduced to very low levels. Analysis of present data supports a multi-layered approach including rotation and current profile control for instability avoidance, and active mode control. Experiments using MHD spectroscopy show a decrease in measured n~$=$~1 mode stability as the marginal point is approached, with stability \textit{increasing} when $\beta_{\mathrm{N}}$/l$_{\mathrm{i}}$~\textgreater ~10. Experimental results are consistent with the theory that stabilizing kinetic effects can be enhanced at lower collisionality, $\nu $, and that instability can be avoided by steering the rotation profile toward ion precession drift resonance. The database of plasmas using neoclassical toroidal viscosity (NTV) to alter the rotation profile is further analyzed to understand the dependence on key parameters, including $\nu $, for use in active instability avoidance. Results from the active RWM state-space controller used in plasmas exceeding $\beta_{N}$~$=$~6.4 and $\beta _{N}/l_{i}$~$=$~13 are analyzed to determine improvements based on 3D effects. VALEN-3D analysis shows that RWM control is possible near the ideal with-wall stability limit with a planned internal control coil upgrade and proper sensor positioning. [Preview Abstract] |
Tuesday, November 12, 2013 3:24PM - 3:36PM |
JO4.00008: M3D-K Simulations of Toroidicity-induced Alfv\'{e}n Eigenmodes on NSTX D. Liu, G.Y. Fu, J.A. Breslau, E.D. Fredrickson, M. Podesta, N.A. Crocker, S. Kubota Energetic particle modes and Alfv\'{e}nic modes driven by super-alfv\'{e}nic beam ions are routinely observed in neutral beam heated plasmas on the National Spherical Torus Experiment (NSTX). These modes can significantly impact beam-ion transport, thus cause beam-ion redistribution or loss. Hybrid kinetic/MHD simulations of Toroidicity-induced Alfv\'{e}n Eigenmodes (TAEs) in NSTX plasmas have been carried out with the M3D-K code using experimental plasma profiles. The simulations show that unstable TAEs with n$=$2-5 can be excited by the beam ions. Mode frequency, structure and phase shift are roughly consistent with experimental measurements from a multi-channel reflectometer diagnostic. A sensitivity study on plasma rotation, q profile and equilibrium beam-ion distribution is performed. It is found that rotation has a significant destabilizing effect on mode stability at experimental level. The growth rate is also sensitive to q$_{\mathrm{min}}$ position and beam-ion distribution. But mode structure and peak position have weak dependence on these factors. To investigate the effects of beam ion distribution on mode stability, an interface between the fast ion Monte Carlo modeling code NUBEAM and M3D-K has been developed. The fast ion distribution in phase-space coordinates from NUBEAM is converted to a set of continuously and differentiable 2D cubic B-splines in the (P$_{\mathrm{\varphi }}$, E) space with a set of discrete bins in $\mu $ direction. The comparison between M3D-K simulations with analytic and NUBEAM fast ion distribution will be discussed. *\textit{Work supported by US DOE.} [Preview Abstract] |
Tuesday, November 12, 2013 3:36PM - 3:48PM |
JO4.00009: Mitigation of Alfv\'{e}nic MHD Activity in the NSTX Tokamak by Externally Applied Perturbation Fields Alessandro Bortolon, William W. Heidbrink, Gerrit J. Kramer, Jong-Kyu Park, Eric D. Fredrickson, Jeremy D. Lore, Mario Podesta Observations from NSTX experiments demonstrate that externally applied 3D magnetic fields can be used to alter the dynamic of bursting and chirping Alfv\'{e}n modes, driven by energetic beam ions [Bortolon et al., Phys. Rev. Letters, Vol. 110 (2013) 265008]. Pulses of static n$=$3 fields ($\delta $B/B $\sim$ 0.01 at the plasma edge) were applied during plasma discharges with persistent Global Alfv\'{e}n Eigenmode activity (n$=$7-8, 400-700 kHz). In response to the perturbations, the mode amplitude, the bursting period and the frequency sweep were reduced by a factor of 2-3. For modes of weaker bursting character, the magnetic perturbation induced a temporary transition to a saturated continuous mode. The $\sim$5{\%} drops of neutron emission rate during the pulses suggest perturbation of the fast ion population. Calculations of the perturbed fast-ion distribution function, made with the SPIRAL code, indicate that the 3D perturbation affects the orbits of fast ions that resonate with the bursting modes. The results represent an early demonstration of the possibility of controlling fast-ion instabilities by ``phase-space engineering'' of the fast-ion distribution function. [Preview Abstract] |
Tuesday, November 12, 2013 3:48PM - 4:00PM |
JO4.00010: Linear Stability and Nonlinear Dynamics of Fishbone in NSTX Feng Wang, Guoyong Fu, J.A. Breslau, Jinyuan Liu, Deyong Liu Plasms in spherical tokamaks such as NSTX, with a safety factor above unity and weakly reversed magnetic shear may be unstable to an ideal, non-resonant internal kink mode. This mode, termed the LLM in MAST, can saturate and persist. This indicates strong interaction of energetic beam ions with LLM. In this work, we perform linear and nonlinear simulations to investigate energetic particle effects on the non-resonant kink mode and excitation of fishbone for NSTX-like parameters and profiles. The global kinetic-MHD hybrid code M3D-K is used. Numerical results show that beam ions have a strong stabilizing effect on the kink mode at low values of $q_{min}$ and beam beta. However, at higher beam ion pressure, a fishbone-like mode is excited. The results show that the fishbone is preferentially excited at higher $q_{min}$ values, consistent with the observed appearance of fishbone before ``long-lived mode'' in NSTX and MAST experiments. Nonlinear simulations show that the fishbone saturates nonlinearly with strong downward frequency chirping, and beam distribution flattened. An m/n=2/1 magnetic island is induced nonlinearly, which could provide a trigger for the 2/1 NTM sometime observed after fishbone instability in NSTX. [Preview Abstract] |
Tuesday, November 12, 2013 4:00PM - 4:12PM |
JO4.00011: ELM heat flux study in NSTX Joon-Wook Ahn, Kaifu Gan, John Canik, Travis Gray, Jeremy Lore, Rajesh Maingi, Filippo Scotti, Lane Roquemore, Adam McLean, Vlad Soukhanivskii Dynamic evolution of heat flux profiles for various types of ELMs has been investigated in NSTX. The heat flux width ($\lambda_{\mathrm{q}})$ was found to increase (for type-V and low power type-III ELMs) or decrease (for type-I and high power type-III ELMs) during the ELM compared to the inter-ELM value. This is contrary to the result from conventional tokamaks such as JET and ASDEX-U where $\lambda_{\mathrm{q}}$ showed clear increase during the type-I ELM. It is also interesting to note that the ELM peak heat flux (q$_{\mathrm{peak}})$ rises with increasing ELM energy loss for the same plasma stored energy (W$_{\mathrm{MHD}})$ in NSTX, which appears to be primarily due to the reduced wetted area (A$_{\mathrm{wet}})$ and poses concern about the ELM heat flux for NSTX-U where higher plasma current and input power are likely to raise the ELM energy loss. This result is compared to the JET data that showed constant peak heat flux over a range of ELM energy losses if the pedestal energy remains the same. Data for several ELM types will be presented and its relation to the pedestal parameters as well as implication to the future machine will be also discussed. [Preview Abstract] |
Tuesday, November 12, 2013 4:12PM - 4:24PM |
JO4.00012: Fast-wave propagation in the edge of a cylindrical cold-plasma under NSTX-like conditions R.J. Perkins, N. Bertelli, D.L. Green, E.F. Jaeger, J.C. Hosea, C.K. Phillips, P.M. Ryan, G. Taylor, J.R. Wilson NSTX uses a high-harmonic fast-wave (HHFW) antenna for plasma heating and current drive, but a significant amount of HHFW power can be lost directly to the divertor [1]. This loss has several interesting and unexplained properties: the flow of HHFW power from the antenna to the divertor regions is mostly aligned along the magnetic field and occurs along all field lines between the antenna and the separatrix [2]. The underlying cause is related to fast-wave propagation in the scrape-off layer due to the strong correlation between heating efficiency and the onset density for perpendicular fast-wave propagation. To understand the role of the onset density, a cylindrical cold-plasma model is being developed to determine the amount of wave-propagation in the SOL and how closely aligned the Poynting flux is to the magnetic field. This modeling will assist work with RF codes [3] to include the proper edge damping into such codes so that they can reproduce the losses observed in NSTX and predict their importance for ITER. This work is supported by USDOE Contract No. DE-AC02-09CH11466. \\[4pt] [1] Hosea J.C. et al 2008 Phys. Plasmas 15 056104\\[0pt] [2] Perkins R.J. et al 2012 Phys. Rev. Lett. 109 045001\\[0pt] [3] Green D.L. et al 2011 Phys. Rev. Lett. 107 145001 [Preview Abstract] |
Tuesday, November 12, 2013 4:24PM - 4:36PM |
JO4.00013: Surface analysis of retention and lithium wetting of Molybdenum C.H. Skinner, A.M. Capece, J.P. Roszell, B.E. Koel A quantitative understanding of the adsorption and retention of deuterium by lithium-conditioned materials is needed to optimize the performance of Li-PFCs, especially for the long duration discharges anticipated in NSTX-U. For liquid Li PFCs, wetting by Li of the substrate is a key factor in the design. We report UHV surface science experiments on these topics. The effects of impurities and grain boundaries in TZM is shown by comparing single crystal Mo substrates to Mo alloy (TZM). The substrate is coated with a known monolayer-scale thickness of Li and exposed to D thermal neutrals and ions from 5 eV to 500 eV with controlled amounts of residual vacuum gases. The surface composition is measured with Auger electron spectroscopy and D uptake by thermal desorption spectroscopy. Microscale wetting of stainless steel and TZM by lithium will be measured by Auger elemental microimaging. The results will connect atomistic surface science models to plasma surface interactions expected in tokamaks. [Preview Abstract] |
Tuesday, November 12, 2013 4:36PM - 4:48PM |
JO4.00014: Erosion and re-deposition of lithium coatings on graphite and TZM molybdenum in support of NSTX-U divertor operations Tyler Abrams, M.A. Jaworski, R. Kaita, D.P. Stotler, G. De Temmerman, M.A. van den Berg, H.J. van der Meiden, T.W. Morgan It is expected that lithium films applied to graphite and TZM molybdenum tiles in NSTX-U will persist significantly longer than calculated from gross erosion rates due to prompt re-deposition. The time evolution of Li coatings on these substrates is being studied in Magnum-PSI, a linear plasma device capable of ion fluxes up to 10$^{\mathrm{25}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$ at electron temperatures \textless 5 eV. A series of 2-7 s plasma exposures at normal magnetic incidence were run on bare samples of each substrate then repeated after a deposition of 100-1000 nm of Li. During discharges on Li-coated graphite, Li-I emission begins decreasing after only five seconds. This indicates that rapid diffusion of Li into the bulk graphite substrate may be occurring. In contrast, emission from Li-coated TZM Mo persists for \textgreater 30 s. The total Li erosion fluence from the sample $\varphi_{\mathrm{Li}}$ was calculated and compared to the areal density of Li on the sample $\rho_{\mathrm{Li}}$ to estimate the re-deposition fraction $R \quad =$ 1 - $\rho _{\mathrm{Li}}$/$\varphi_{\mathrm{Li}}$. Initial calculations from experimental data indicate that $R$ \textgreater 0.85 for Li-coated TZM Mo at the center of the plasma column. An analytic/numerical model is in development to simulate the dominant mechanisms of particle loss and gain from thin Li films in the presence of plasma bombardment, the results of which will be presented. [Preview Abstract] |
Tuesday, November 12, 2013 4:48PM - 5:00PM |
JO4.00015: Thermoelectric-driven Flow of Liquid Lithium in Solid Metal Trenches: a New Plasma-Facing Component for Fusion Devices David N. Ruzic, Daniel Andruczyk, Davide Curreli, Soonwook Jung, Michael Peter Christenson, Kyle Lindquist, Peter Fiflis, Wenyu Xu The new LiMIT device (Lithium/Metal Infused Trenches) recently proposed [D. N. Ruzic, et al., Nucl. Fusion 51, 102002 (2011)] is an innovative plasma-facing component able to naturally self-adapt its cooling capabilities depending upon the heat flux from the plasma. The system uses the thermoelectric forces arising from the strong temperature gradients and magnetic fields of the divertor region, to obtain a JxB MHD drive of the liquid lithium into small solid-metal trenches. A number of cooling channels guarantee the temperature gradient between the hot plasma-facing liquid surface and the cool solid-metal assembly comprising the trenches. A prototype of the device has been built and tested in the electron-beam-based SLIDE facility at Illinois and diagnosed by means of an IR camera and embedded thermocouples. The flow velocity of liquid lithium has been measured using a fast-frame camera, monitoring the motion of small particles deposited on the liquid lithium surface. Velocities of the order of several centimeters per second have been observed, compatible with a simplified 1D model and more accurate 3D TE-MHD (Thermoelectric Magnetohydrodynamics) computations. LiMIT is planned to be tested at Magnum-PSI (The Netherlands), EAST (China) and in the new TELS facility at Illinois. [Preview Abstract] |
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