Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session GO6: NSTX-U & Pegasus |
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Chair: Matthew Thompson, Tri Alpha Energy, Inc. Room: 230 C |
Tuesday, November 1, 2016 9:30AM - 9:42AM |
GO6.00001: Overview of Initial NSTX-U Experimental Operations Devon Battaglia Initial operation of the National Spherical Torus Experiment Upgrade (NSTX-U) has satisfied a number of commissioning milestones, including demonstration of discharges that exceed the field and pulse length of NSTX. ELMy H-mode operation at the no-wall $\beta_{\mathrm{N}}$ limit is obtained with Boronized wall conditioning. Peak H-mode parameters include: I$_{\mathrm{p}}=$ 1 MA, B$_{\mathrm{T0}}=$ 0.63 T, W$_{\mathrm{MHD}}=$ 330 kJ, $\beta_{\mathrm{N}}=$ 4, $\beta _{\mathrm{N}}$/l$_{\mathrm{i}}=$ 6, $\kappa =$ 2.3, $\tau _{\mathrm{E,tot}}$ \textgreater 50 ms. Access to high-performance H-mode scenarios with long MHD-quiescent periods is enabled by the resilient timing of the L-H transition via feedback control of the diverting time and shape, and correction of the dominant n$=$1 error fields during the I$_{\mathrm{p}}$ ramp. Stationary L-mode discharges have been realized up to 1 MA with 2 s discharges achieved at I$_{\mathrm{p}}=$ 650 kA. The long-pulse L-mode discharges enabled by the new central solenoid supported initial experiments on error field measurements and correction, plasma shape control, controlled discharge ramp-down, L-mode transport and fast ion physics. Increased off-axis current drive and reduction of fast ion instabilities has been observed with the new, more tangential neutral beamline. The initial results support that access to increased field, current and heating at low-aspect-ratio expands the regimes available to develop scenarios, diagnostics and predictive models that inform the design and optimization of future burning plasma tokamak devices, including ITER. [Preview Abstract] |
Tuesday, November 1, 2016 9:42AM - 9:54AM |
GO6.00002: Initial error field correction studies in the National Spherical Torus Experiment Upgrade C. E. Myers, S. P. Gerhardt, J. E. Menard, N. M. Ferraro, J.-K. Park, R. E. Bell, B. P. LeBlanc, M. Podesta, S. A. Sabbagh Non-axisymmetries in the magnetic field coils and/or the passive conducting structures of tokamaks produce small but important `error fields' that can strongly degrade plasma performance. This paper reports on initial error field correction (EFC) studies in the National Spherical Torus Experiment Upgrade (NSTX-U). The NSTX-U device is equipped with an array of six independently powered EFC coils that produce non-axisymmetric radial magnetic fields at the midplane. These coils are used to perform compass scans where an $n=1$ magnetic perturbation is ramped in amplitude at constant phase until the plasma rotation locks. Successive discharges are taken with different toroidal phases to determine the optimum EFC required to avoid locking. Multiple compass scans confirm that substantial EFC coil current (1200 A-turns) is required to achieve optimum performance in the flattop of beam-heated L-mode discharges. Interestingly, the $q=2$ surface is locked throughout these L-modes such that the applied magnetic perturbations lock the sawtoothing core of the plasma. Additional compass scans early in the discharge indicate that the required EFC evolves as a function of time. Efforts to identify the various error field sources, including metrology of the main vertical field coils, are ongoing. [Preview Abstract] |
Tuesday, November 1, 2016 9:54AM - 10:06AM |
GO6.00003: Results of using the NSTX-U Plasma Control System for scenario development M.D. Boyer, D.J. Battaglia, D.A. Gates, S. Gerhardt, J. Menard, D. Mueller, C.E. Myers, J. Ferron, S. Sabbagh To best use the new capabilities of NSTX-U (e.g., higher toroidal field and additional, more distributed heating and current drive sources) and to achieve the operational goals of the program, major upgrades to the Plasma Control System have been made. These include improvements to vertical control, real-time equilibrium reconstruction, and plasma boundary shape control and the addition of flexible algorithms for beam modulation and gas injection to control the upgraded actuators in real-time, enabling their use in algorithms for stored energy and profile control. Control system commissioning activities have so far focused on vertical position and shape control. The upgraded controllers have been used to explore the vertical stability limits in inner wall limited and diverted discharges, and control of X-point and strike point locations has been demonstrated and is routinely used. A method for controlling the mid-plane inner gap, a challenge for STs, has also been added to improve reproducible control of diverted discharges. A supervisory shutdown handling algorithm has also been commissioned to ramp the plasma down and safely turn off actuators after an event such as loss of vertical control. Use of the upgrades has contributed to achieving 1MA, 0.65T scenarios with greater than 1s pulse length. [Preview Abstract] |
Tuesday, November 1, 2016 10:06AM - 10:18AM |
GO6.00004: Initial transport validation studies using NSTX-U L-mode plasmas Walter Guttenfelder, D. Battaglia, R.E. Bell, M.D. Boyer, N. Crocker, A. Diallo, N. Ferraro, S.P. Gerhardt, S.M. Kaye, B.P. LeBlanc, D. Liu, J.E. Menard, D. Mueller, C. Myer, M. Podesta, R. Raman, Y. Ren, S. Sabbagh, D. Smith A variety of stationary L-mode plasmas have been successfully developed in NSTX-U for physics validation studies. The plasmas span a range of density (1-4$\times $10$^{\mathrm{19}}$ m$^{\mathrm{-3}})$, plasma current (0.65-1.0 MA), and neutral beam heating power ($\le $4 MW), taking advantage of new, more tangential neutral beam sources to vary rotation profiles. Transport analysis (TRANSP) and turbulence measurements (BES, reflectometry) of these plasmas will be illustrated and compared with initial microstability and transport predictions. In particular, the normalized beta of these L-modes range between $\beta_{\mathrm{N}}=$1-2, providing a valuable bridge in parameter space between (i) H-modes at comparable beta in conventional tokamaks (R/a$\sim $3, $\beta_{\mathrm{N}}\sim $2), where transport models have been largely developed and tested, and (ii) low-aspect-ratio H-modes at higher beta (R/a$\sim $1.5-1.7, $\beta_{\mathrm{N}}\sim $5), where transport models are less tested and challenged by stronger electromagnetic and equilibrium effects. This work is supported by US DOE contract DE-AC02-09CH11466. [Preview Abstract] |
Tuesday, November 1, 2016 10:18AM - 10:30AM |
GO6.00005: Beam ion confinement on NSTX-U D. Liu, W. W. Heidbrink, G. Z. Hao, M. Podesta, D. S. Darrow, E. D. Fredrickson A second and more tangential neutral beam line is a major upgrade component of the National Spherical Torus Experiment -- Upgrade (NSTX-U) with the purpose of improving neutral beam current drive efficiency and providing more flexibility in current/pressure profile control. Good beam ion confinement is essential to achieve the anticipated improvements in performance. In the planned beam ion confinement experiment, various short and long (relative to fast ion slowing-down time) neutral beam (NB) pulses from six neutral beam sources will be injected into center-stack limited L-mode plasmas to characterize the beam ion confinement and distribution function produced by the new and the existing NBI lines. The neutron rate decay after the turn-off of short NB pulses will be used to estimate the beam ion confinement time and to investigate its dependence on NB source/geometry, injection energy, and plasma current. The tangential and vertical Fast-Ion D-Alpha (FIDA) diagnostics and multi-view Solid State Neutral Particle Analyzer (SSNPA) arrays will be used to measure beam ion slowing-down distribution function and spatial profile during the injection of relatively long NB pulses. Beam ion prompt losses will be monitored with a scintillator Fast Lost Ion Probe (sFLIP) diagnostic. The experimental data and comparisons with classical predictions from NUBEAM modeling will be presented. [Preview Abstract] |
Tuesday, November 1, 2016 10:30AM - 10:42AM |
GO6.00006: Suppression of Alfv\'{e}nic modes with off-axis NBI* Eric Fredrickson, R Bell, A Diallo, B Leblanc, M Podesta, F Levinton, H Yuh, D Liu GAE are seen on NSTX-U in the frequency range from 1 to 3 MHz with injection of the more perpendicular, NSTX neutral beam sources. A new result is that injection of any of the new, more tangential, neutral beam sources with tangency radii larger than the magnetic axis suppress this GAE activity. Simulations of beam deposition and slowing down with the TRANSP code indicate that these new sources deposit fast ions with 0.9 \textless pitch \textless 1. The observations are consistent with the theory of resonant drive for ctr-propagating GAE [Gorelenkov, NF 43 (2003) 228], which predicts only fast ions with large larmor radius are destabilizing, i.e., very tangential fast ions will be stabilizing. Estimates of these parameters support this qualitative prediction. Experiments have shown complete suppression of GAE with \textless 30{\%} of the total injected beam power in the stabilizing sources. This may provide a tool to control or mitigate GAE instabilities by tailoring the NB injection profile through a mix of the available NB sources.\new line Work supported by U.S. DOE Contract DE-AC02-09CH11466. [Preview Abstract] |
Tuesday, November 1, 2016 10:42AM - 10:54AM |
GO6.00007: Disruption Event Characterization and Forecasting in NSTX-U S.A. Sabbagh, J.W. Berkery, J.M. Bialek, Y.S. Park, R.E. Bell, M.D. Boyer, S.P. Gerhardt, C. Myers, J.D. Riquezes Disruption avoidance with high reliability is a critical need for future fusion-producing tokamaks. Reaching this challenging goal will require multiple approaches including an understanding of the connection between events leading to disruptions. Automated disruption event chain characterization and forecasting (DECAF) analysis has started for NSTX-U by first analyzing NSTX data. The relative timing of global MHD instabilities such as resistive wall modes (RWM), rotating MHD modes, vertical displacement events, and several other events including mismatching the desired plasma current, loss of boundary control, and safety factor reduction is considered. In a database of plasmas exhibiting global MHD mode activity, the RWM and loss of boundary control events are found in all cases, and vertical displacements events are found in 90{\%} of cases. The earliest RWM events occur within 20 conducting wall current diffusion times of full current quench disruptions in 60{\%} of the plasmas and apparent false positives are often identified as minor disruptions. Common disruption event chains are identified, with new insights gained on the connection of mode activity to events such as reaching high Greenwald density fraction. $^{\mathrm{\ast }}$Supported by US DOE Contracts DE-FG02-99ER54524 and DE-AC02-09CH11466. [Preview Abstract] |
Tuesday, November 1, 2016 10:54AM - 11:06AM |
GO6.00008: Advances in boronization on NSTX-Upgrade C.H. Skinner, W. Blanchard, D. Cai, M. Jaworski, F. Bedoya, J.P. Allain, F. Scotti, B.E. Koel Boronization has been effective in reducing plasma impurities and enabling access to higher density, higher confinement plasmas in many magnetic fusion devices. The National Spherical Torus eXperiment, NSTX, has recently undergone a major upgrade to NSTX-U in order to develop the physics basis for a ST-based Fusion Nuclear Science Facility (FNSF) with capability for double the toroidal field, plasma current, and NBI heating power and increased pulse duration from 1 - 1.5 s to 5 - 8 s. A new deuterated tri-methyl boron conditioning system was implemented together with a novel surface analysis diagnostic (MAPP). We report on the spatial distribution of the boron deposition versus discharge pressure, gas injection and electrode location. The oxygen concentration of the plasma facing surface was measured by in-vacuo XPS and increased both with plasma exposure and with exposure to trace residual gases. This increase was correlated with the rise of oxygen emission from the plasma. A dedicated experiment is planned to optimize the boronization process including XPS measurements of the plasma facing surface under specific plasma conditions. We will report on the results. [Preview Abstract] |
Tuesday, November 1, 2016 11:06AM - 11:18AM |
GO6.00009: In-vacuo studies of Boronization and Lithiumization in NSTX-U and relationship to plasma performance* Carlos Bedoya, Jean Paul Allain, Robert Kaita, Charles Skinner, Scotti Filippo, Bruce Koel A new plasma facing component (PFC) diagnostic, the MAPP probe, was installed on NSTX-U in the beginning of the 2015 campaign. MAPP was used to find qualitative correlations between PFC conditions and plasma performance. XPS data collected with MAPP suggests the formation of B$_{\mathrm{4}}$C following boron deposition (boronization). The depositions of these thin films seem to temporarily improve the plasma performance. The data shows how the atomic concentration of oxygen in the coatings rises from \textasciitilde 5{\%} to almost 30{\%} after exposures to tens of plasma discharges. This oxidation coincides with the decrease in plasma performance. Increments in the content of oxygen (OII line) in the plasma were also observed with visible light spectroscopy over the same time range. MAPP is also able to measure the chemical state of graphite as a result of lithium evaporation onto PFCs (lithiumization). This work will report on the effect on the surface chemistry of ATJ graphite of lithium deposition and plasma exposure in NSTX-U. As it was the case with boronization, the relationship between plasma performance and PFC conditioning with lithium is investigated. [Preview Abstract] |
Tuesday, November 1, 2016 11:18AM - 11:30AM |
GO6.00010: Energy Exchange Dynamics across L-H transitions in NSTX A. Diallo, S. Banerjee, S. Zweben, T. Stoltzfus-Dueck This work is motivated by the need to test L-H transition paradigms (e.g., predator-prey, and ExB flow suppression) and explore possible new L-H transition dynamics. We present analysis of the L-H transition on three sets (NBI, RF, and Ohmic) of NSTX discharges using the gas-puff-imaging diagnostics for high temporal and spatial resolutions. The analysis studies the edge velocities and energy dynamics across the L-H transition using an implementation of the orthogonal decomposition programming for high temporal resolution velocity fields. In the database NSTX discharges, the production term (computed 1 cm inside the separatrix) is negative, pointing to transfer from the DC flows to the fluctuations, even immediately before the L-H transition. This suggests that depletion of turbulent fluctuation energy via transfer to the mean flow may not play a key role in the L-H transition. The thermal free energy is consistently much larger than the kinetic energy produced by the mean poloidal flow across the L-H transitions. These observations are inconsistent with the predator-prey model. The paper will describe the analysis including error estimations. Furthermore, analysis of the radial correlation dynamics across the L-H transition will be discussed. [Preview Abstract] |
Tuesday, November 1, 2016 11:30AM - 11:42AM |
GO6.00011: Inter-ELM and ELM-free divertor heat flux broadening induced by EHO in NSTX. Kaifu Gan, Joon-Wook Ahn, Rajesh Maingi, Travis Gray, Stewart Zweben, Eric Fredrickson, Gustavo Canal, Brian Wirth Recent study on multi-machine database of inter-ELM divertor heat flux indicates that the midplane Scrape-off Layer power fall-off length for ITER is expected to be very narrow$^{\mathrm{1}}$, \textasciitilde 1mm, which will induce small plasma-wetted area (Awet) and high peak heat flux (q$_{\mathrm{peak}})$. In NSTX, edge harmonic oscillation (EHO) was observed during certain inter-ELM and ELM-free periods of H-mode operation. The EHO is observed to significantly increase Awet, by up to a factor of 3, and decrease q$_{\mathrm{peak}}$ accordingly. The Awet increases with the amplitude of EHO. Multiple peaks appeared in the heat flux profile in the presence of EHO. An EHO-induced filament around separatrix rotating in the counter-current direction was also observed by gas puff imaging diagnostic. The increase of Awet is suspected to be caused by the rotating current filaments; the toroidally rotating filaments could have changed the edge magnetic topology and broadened heat flux profile by strike points splitting. Experimental observation of the temperal evolution of multiple peaks in heat flux profile shows consistent trend with the toroidal rotation of EHO. $^{\mathrm{1}}$T. Eich et al., Nucl. Fusion \textbf{53} (2013) 093031 [Preview Abstract] |
Tuesday, November 1, 2016 11:42AM - 11:54AM |
GO6.00012: Stabilizing Effect of Resistivity towards ELM-free H-mode Discharge in Lithium-conditioned NSTX Debabrata Banerjee, Ping Zhu, Rajesh Maingi The stabilizing effect of edge resistivity on the edge localized modes (ELMs) has been recently recovered through analyzing NSTX experimental profiles of Lithium-conditioned ELM-free H-mode discharge. Comparative studies of ELM-free and a reference NSTX ELMy-H mode equilibriums have been performed using both resistive and 2-fluid MHD models implemented in the initial value extended MHD code NIMROD. Our results indicate that in addition to the pedestal profile refinement in electron pressure, the inclusion of enhanced resistivity due to the increase in the effective electric charge number $Z_{eff}$, which is observed after Lithium-conditioning in experiment, is further required to account for the full stabilization of the low-$n$ edge localized modes. Such a stabilization from the enhanced edge resistivity only becomes effective when the two-fluid diamagnetic and finite-Larmor-radius (FLR) effects are considered in the MHD model. [Preview Abstract] |
Tuesday, November 1, 2016 11:54AM - 12:06PM |
GO6.00013: Advancing Non-Solenoidal Startup on the Pegasus ST J.A. Reusch, J.L. Barr, G.M. Bodner, M.W. Bongard, M.G. Burke, R.J. Fonck, J.L. Pachicano, J.M. Perry, N.J. Richner, C. Rodriguez Sanchez, D.J. Schlossberg The Pegasus experiment utilizes compact, edge-localized current sources ($A_{inj} \sim 2-4$ cm$^{\mathrm{2}}$, $I_{inj} \sim 10$ kA, $V_{inj} \sim 1$ kV) for non-solenoidal local helicity injection (LHI) startup. Recent campaigns are comparing two injector geometries that vary the differing relative contributions of DC helicity input and non-solenoidal inductive voltages. A predictive 0-D model that treats the plasma as a resistive element with time-varying inductance and enforces $I_{p} $ limits from Taylor relaxation was tested with inward growth of the plasma current channel using injectors on the outboard midplane. Strong inductive drive arises from plasma shape evolution and poloidal field (PF) induction. A major unknown in the model is the resistive dissipation, and hence the electron confinement. $T_{e} (R)$ profile measurements in LHI show centrally-peaked $T_{e} >100$ eV while the plasma is coupled to the injectors, suggesting LHI confinement is not strongly stochastic. A second campaign utilizes new injectors in the lower divertor region. This geometry trades subtler relaxation field programming and reduced PF induction for higher HI rates. Present efforts are developing relaxation methods at high $B_{T} $, with relaxation at $B_{T,inj} >0.15$ T achieved to date via higher $I_{inj} $ and PF manipulation. Conceptual design studies of coaxial helicity injection (CHI) and ECH heating systems for Pegasus have been initiated to explore direct comparison of LHI to CHI with and without ECH assist. [Preview Abstract] |
Tuesday, November 1, 2016 12:06PM - 12:18PM |
GO6.00014: Electron Temperature Evolution During Local Helicity Injection on the Pegasus Toroidal Experiment D.J. Schlossberg, J.L. Barr, G.M. Bodner, M.W. Bongard, R.J. Fonck, J.M. Perry, J.A. Reusch, C. Rodriguez Sanchez Understanding the electron temperature ($T_{e} )$ evolution during local helicity injection (LHI) is critical for scaling up this non-solenoidal startup technique to MA-class devices. The first comprehensive $T_{e} $ measurements during LHI reveal centrally-peaked profiles with $T_{e} >100$ eV for plasma current $I_{p} >120$ kA, toroidal field $\sim 0.15$ T, and electron density $n_{e} \sim 10^{19}$ m$^{\mathrm{-3}}$. $T_{e} $ rises and is sustained from just after magnetic relaxation through the plasma decoupling from edge-localized injectors. Results are presented for two injector edge locations: outboard midplane and inboard divertor. Outboard midplane injection couples LHI with inductive drive from poloidal field ramps and radial compression during inward plasma growth. Comparisons of $T_{e} $ at different LHI-to-inductive drive ratios show some profile flattening for higher LHI drive fraction. The latter, constant-shape discharges were necessarily lower performance, with $I_{p} \sim 50$ kA and reduced $T_{e,max} $. Inboard divertor injection achieves higher $I_{p} $ using minimal inductive drive and thus isolates effects of LHI drive on $T_{e} $. Initial results in this configuration show $T_{e} $ rising rapidly at the injector location as the discharge grows, settling to a roughly flat profile $\sim 100$ eV. Thus far, both scenarios provide relatively stable discharges with moderate $n_{e} $ and high-$T_{e} $, suitable for coupling to auxiliary current drive. Detailed studies of confinement dynamics and discharge optimization are planned for the near future. [Preview Abstract] |
Tuesday, November 1, 2016 12:18PM - 12:30PM |
GO6.00015: Impurity contamination effects on the interaction of Li and Sn Films on W (poly) Oluseyi Fasoranti, Bruce Koel Plasma-solid interactions under fusion divertor conditions lead to continuous material erosion and may result in performance degradation of the plasma-facing components. Liquid metals such as Li and Sn may help to circumvent this issue due to their ability for self-recovery and heat-flux management. This has driven interest in understanding plasma-liquid metal interactions. We have shown in our lab that surface science experiments can examine discrete aspects of plasma-surface interactions by enabling clean and controlled deposition of metal films. We will review our recent results on the thermal stability of ultrathin Li and Sn films on a polycrystalline W surface using surface diagnostic methods such as Temperature Programmed Desorption, Auger Electron Spectroscopy, and Ion Scattering Spectroscopy. These studies examine Li-W and Sn-W interfaces and investigate the impact of impurities. We will discuss relevant issues such as the differences in oxygen uptake between solid and liquid lithium films and the effects of post-oxidation, as well as pre-adsorbed surface oxygen and carbon, on the thermal stability of these films. We plan to present additional studies of deuterium ion uptake and retention on Li and Sn films. [Preview Abstract] |
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