Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session PO6: Pegasus, NSTX-U, and International Experiments |
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Chair: Ted Strait, General Atomics, Inc. Room: 201/202 |
Wednesday, November 18, 2015 2:00PM - 2:12PM |
PO6.00001: Expanding Non-solenoidal Startup with Local Helicity Injection to Increased Toroidal Field and Helicity Injection Rate J.M. Perry, J.L. Barr, G.M. Bodner, M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, B.T. Lewicki, J.A. Reusch, D.J. Schlossberg, G.R. Winz Local helicity injection (LHI) is a non-solenoidal startup technique under development on the Pegasus ST. Plasma currents up to 0.18 MA have been initiated by LHI in conjunction with poloidal field induction. A 0-D power balance model has been developed to predict plasma current evolution by balancing helicity input against resistive dissipation. The model is being validated against a set of experimental measurements and magnetic reconstructions with radically varied plasma geometric evolutions. Outstanding physics issues with LHI startup are the scalings of confinement and MHD activity with helicity injection rate and toroidal field strength, as well as injector behavior at high field. Preliminary results from the newly-installed Thomson scattering system suggest core temperatures of a few hundred eV during LHI startup. Measurements are being expanded to multiple spatial points for ongoing confinement studies. A set of larger-area injectors is being installed in the lower divertor region, where increased toroidal field will provide a helicity injection rate over 3 times that of outboard injectors. In this regime helicity injection will be the dominant current drive. Experiments with divertor injectors will permit experimental differentiation of several possible confinement models, and demonstrate the feasibility of LHI startup at high field. [Preview Abstract] |
Wednesday, November 18, 2015 2:12PM - 2:24PM |
PO6.00002: Full flux closure and equilibrium state during simulations of Coaxial Helicity Injection in NSTX-U F. Ebrahimi, R. Raman A large-volume flux closure during transient Coaxial Helicity Injection (CHI) experiments in NSTX-U is demonstrated through resistive MHD simulations. Several major updates, including the location of CHI poloidal coils, are planned to improve the CHI start-up phase in NSTX-U. Simulations in the NSTX-U configuration with fixed coil currents show that with strong flux shaping the injected open field lines (injector flux) could rapidly reconnect and form a large volume of closed flux surfaces. This is achieved by driving parallel current in the injector flux coil and oppositely directed currents in the flux shaping coils to form a narrow flux footprint and push the injector flux. As the helicity and plasma are injected into the device, the oppositely directed field lines in the injector region (a) are forced to reconnect and form a current sheet (b) or spontaneously to reconnect when the elongated current sheet becomes MHD unstable. Simulations in NSTX-U also show that the magnetic pressures around the enclosed flux surface would support a steady configuration to allow a good start-up equilibrium after the injector voltage is turned off. Supported by DOE-FG02-12ER55115. [Preview Abstract] |
Wednesday, November 18, 2015 2:24PM - 2:36PM |
PO6.00003: Global MHD Mode Stabilization and Control for Tokamak Disruption Avoidance S.A. Sabbagh, J.W. Berkery, J.M. Bialek, J.M. Hanson, Y.S. Park, R.E. Bell, D.A. Gates, S.P. Gerhardt, I. Goumiri, B. Grierson, C. Holcomb The near-complete elimination of plasma disruptions in fusion-producing tokamaks is the present ``grand challenge'' for stability research. Meeting this goal requires multiple approaches, important components of which are prediction, stabilization, and control of global MHD instabilities. Research on NSTX and its upgrade is synergizing these elements to make quantified progress on this challenge. Initial results from disruption characterization and prediction analyses describe physical disruption event chains in NSTX. Analysis of NSTX and DIII-D experiments show that stabilization of global modes is dominated by precession drift and bounce orbit resonances respectively. Stability therefore depends on the plasma rotation profile. A model-based rotation profile controller for NSTX-U using both neutral beams and neoclassical toroidal viscosity is shown in simulations to evolve profiles away from unstable states. Active RWM control is addressed using dual field component sensor feedback and a model-based RWM state-space controller. Comparison of measurements and synthetic diagnostics is examined for off-normal event handling. A planned 3D coil system upgrade can allow RWM control close to the ideal n~$=$~1 with-wall limit. [Preview Abstract] |
Wednesday, November 18, 2015 2:36PM - 2:48PM |
PO6.00004: On the conditions for the onset of nonlinear chirping structures in NSTX Vinicius Duarte, Mario Podesta, Herbert Berk, Nikolai Gorelenkov The nonlinear dynamics of phase space structures is a topic of interest in tokamak physics in connection with fast ion loss mechanisms. The onset of phase-space holes and clumps has been theoretically shown to be associated with an explosive solution of an integro-differential, nonlocal cubic equation that governs the early mode amplitude evolution in the weakly nonlinear regime. The existence and stability of the solutions of the cubic equation have been theoretically studied as a function of Fokker-Planck coefficients [M. K. Lilley, B. N. Breizman and S. E. Sharapov, Phys. Rev. Lett. 102, 195003 (2009)] for the idealized case of a single resonant point of a localized mode. From realistic computations of NSTX mode structures and resonant surfaces, we calculate effective pitch angle scattering and slowing-down (drag) collisional coefficients and analyze NSTX discharges for different cases with respect to chirping experimental observation. Those results are confronted to the theory that predicts the parameters region that allow for chirping to take place. [Preview Abstract] |
Wednesday, November 18, 2015 2:48PM - 3:00PM |
PO6.00005: Substantial Fast-Wave Power Flux in the SOL of a Cylindrical Model; Comparison with Coaxial Modes R.J. Perkins, N. Bertelli, J.C. Hosea, C.K. Phillips, G. Taylor, J.R. Wilson The NSTX high-harmonic fast-wave (HHFW) heating system can lose a significant amount of power along magnetic fields lines in the SOL to the divertor regions under certain conditions [1]. A cylindrical cold-plasma model, with parameters resembling those of NSTX, shows the existence of modes with relatively large RF field amplitudes in the low-density annulus [2], similar to recent results found with the full-wave simulation AORSA [3]. Here, we compare and contrast these modes against ``coaxial modes,'' modes that resemble TEM modes found in coaxial cables. We also compute the 3D Poynting flux as a function of length along the cylinder for comparison to NSTX. Such work is part of an effort to include the proper edge damping into full-wave codes so that they can reproduce the losses observed in NSTX and predict their importance for ITER. This work was supported by DOE Contract No. DE-AC02-09CH11466.\\[4pt] [1] J.C. Hosea et al., Phys. Plasmas 15 (2008) 056104.\\[0pt] [2] R.J. Perkins et al., 41st EPS Conf. Plasma Phys., Berlin June 23-17 2015, P2.059.\\[0pt] [3] N. Bertelli et al., Nucl. Fusion 54 (2012) 083004. [Preview Abstract] |
Wednesday, November 18, 2015 3:00PM - 3:12PM |
PO6.00006: H-mode and Edge Physics on the Pegasus ST: Progress and Future Directions M.W. Bongard, G.M. Bodner, J.L. Barr, M.G. Burke, R.J. Fonck, E.T. Hinson, D.M. Kriete, B.T. Lewicki, J.M. Perry, J.A. Reusch, D.J. Schlossberg, K.E. Thome, G.R. Winz Ohmic H-modes are routinely attained on the Pegasus ST, in part due to the low L-H power threshold $P_{LH} $ arising from low-$B_{T} $ operation at $A\sim 1$. Characteristics of H-mode include: improved $\tau_{e} $, consistent with $H_{98} \sim 1$; edge current and pressure pedestal formation; and the occurrence of ELMs. Experiments in the past year have examined magnetic topology and density dependencies of $P_{LH} $ in detail. $P_{LH} $ exceeds ITER L-H scaling values by 10--20$\times $, with ${P_{LH} } \mathord{\left/ {\vphantom {{P_{LH} } {P_{ITPA08} }}} \right. \kern-\nulldelimiterspace} {P_{ITPA08} }$ increasing sharply as $A\to 1$. No $P_{LH} $-minimizing density has been found. Unlike at high-$A$, $P_{LH} $ is insensitive to limited or diverted magnetic topologies to date. The low $B_{T} $ and modest pedestal values at $A\sim 1$ afford unique edge diagnostic accessibility to investigate ELMs and their nonlinear dynamics. $J_{edge} (R,t)$ measured through a Type I ELM shows a complex pedestal collapse and filament ejection. These studies are being extended to higher $I_{p} $ and longer pulse length with LHI startup to conserve Ohmic V-s and improve MHD stability. A modest-cost upgrade to the facility will enable detailed validation studies of nonlinear ELM dynamics and ELM control. This initiative will upgrade the centerstack, increasing $B_{T} $ by $\times 3$, Ohmic V-s by $\times 4$, and pulse lengths to 100 ms at $A<1.3$, as well as deploy a comprehensive 3D magnetic perturbation coil system with full poloidal coverage from frame coils and helical centerstack windings. [Preview Abstract] |
Wednesday, November 18, 2015 3:12PM - 3:24PM |
PO6.00007: The role of plasma response in divertor footprint modification by 3D fields in NSTX Joonwook Ahn, Kimin Kim, Gustavo Canal, Kaifu Gan, Travis Gray, Adam McLean, Jong-Kyu Park, Filippo Scotti In NSTX, the divertor footprints of both heat and particle fluxes are found to be significantly modified by externally applied 3D magnetic perturbations. Striations on the divertor surface, indicating separatrix splitting and formation of magnetic lobes, are observed for both $n \quad =$ 1 and $n \quad =$ 3 perturbation fields. These striations can lead to localized heating of the divertor plates and to the re-attachment of detached plasmas, both of which have to be avoided in ITER for successful heat flux management. In this work, the role of plasma response on the formation of separatrix splitting has been investigated in the ideal framework by comparing measured heat and particle flux footprints with field line tracing calculations with and without contributions from the plasma response calculated by the ideal code IPEC. Simulations show that, $n \quad =$ 3 fields are slightly shielded by the plasma, with the measured helical pattern of striations in good agreement with the results from the vacuum approximation. The $n \quad =$ 1 fields are, however, significantly amplified by the plasma response, which provides a better agreement with the measurements. Resistive plasma response calculations by M3D-C$^{\mathrm{1}}$ are also in progress and the results will be compared with those from the ideal code IPEC. *This work was supported by DoE Contracts: DE-AC05-00OR22725, DE-AC52-07NA27344 and DE-AC02-09CH11466. [Preview Abstract] |
Wednesday, November 18, 2015 3:24PM - 3:36PM |
PO6.00008: Maximizing power dissipation by impurity seeding on JET with metal plasma facing components Marco Wischmeier, Alexander Huber, Christopher Lowry, Costanza Maggi, Matthew Reinke A reactor such as DEMO will operate at considerably higher total heating power even when compared to ITER. This will require mitigating a much higher power flux density in the Scrape-Off Layer. A highly detached divertor will be required for maximizing the lifetime of the eroding plasma facing components, PFCs, in the divertor as well as for operating within the engineering limits expected for the power handling components. A dissipation of $\sim$ 95{\%} of the total heating power will be needed, with more than 70{\%} being radiation on closed field lines. On JET with metal PFCs highly radiative conditions with N$_{2}$, Ne, both combined and Ar as radiators were approached in H-mode plasmas. For all seeding species radiative power fractions larger than 70{\%} were achieved under stable discharge conditions with a concentration of the radiation in the X-point region. Detachment along both divertor plates was complete. A degradation of the pedestal profile was compensated by steeper core profiles. [Preview Abstract] |
Wednesday, November 18, 2015 3:36PM - 3:48PM |
PO6.00009: Resolving the uncertainties of non-axisymmetric fields in tokamaks Yongkyoon In, J. Seol, W.H. Ko, S.G. Lee, S.W. Yoon, H.H. Lee, Y.M. Jeon, J. Kim, J.G. Bak, H. Park, J.K. Park, G.S. Yun Recent study suggests that KSTAR could be a benefactor of the extremely low level of intrinsic error field in n$=$1 resonant magnetic perturbation (RMP) driven edge localized modes (ELM) control [1]. Specifically, when the $n=$1 RMP currents increases in order to suppress/mitigate ELMs, a kink-resonant mode-locking is not usually invoked in KSTAR, unlike in other devices [2]. Besides we have discovered that the mid-plane RMP appeared much more effective than the off-midplane RMPs in affecting the ELMs with strong density pump-outs and enhanced ELM frequency. Presently, the enhanced understanding of non-axisymmetric field in tokamaks has been in great need, in particular, for the ITER RMP requirements [3]. As the prevailing design of in-vessel RMP coils in ITER is similar to that in KSTAR, we are keen to resolve the uncertainties of the non-axisymmetric fields on transport and stability, and their limits, contributing directly to ITER and beyond. \\[4pt] [1] Y. In \textit{et al}, NF (2015)\\[0pt] [2] Y.M. Jeon \textit{et al}, PRL(2012)\\[0pt] [3] A. Loarte \textit{et al}, NF (2014) [Preview Abstract] |
Wednesday, November 18, 2015 3:48PM - 4:00PM |
PO6.00010: 0-D Particle Balance Modeling for the Long-term Density Response and Control in KSTAR June-Woo Juhn, S.H. Hong, S.H. Hahn, Y.S. Hwang Long-term density response of KSTAR plasmas has been reproduced with a 0-D particle balance equations. The \textit{long-term} response of density means at least a few seconds of the density decay time during the absent of fuels as well as the full discharge length of the KSTAR within the record of 47s. The model includes reactions between deuterium ions, atoms, molecules and wall contents based on the Maddion's model which is one of the most comprehensive 0-D particle balance and validated with the MAST experiments. In order to reflect the long term response, the model is modified including outgassing-like particle emission from the first walls as the form of wall contents over residence time i.e. $N_w/\tau_w$. The model result in excellent agreement with the experimental density response basically in 0.3MA Ohmic limiter plasmas yielding the global particle confinement time $\tau_i$ about 30ms. The model also reproduces the density behavior in the higher $I_p$ diverted plasmas, including ELMy H-modes with the explicit ELM density drop. The model can be used for the improvement of the density feedback control system by comparing the voltage-controlled gas puffing with flux-controlled one for example. [Preview Abstract] |
Wednesday, November 18, 2015 4:00PM - 4:12PM |
PO6.00011: Measurement of helicon wave coupling for current drive and anticipated role for high beta KSTAR plasmas S.J. Wang, H.J. Kim, M. Joung, J.H. Jeong, J.H. Kim, Y.S. Bae, J.G. Kwak, H.H. Wi, H.-S Kim Helicon wave current drive has been suggested for efficient off-axis current drive in high electron beta tokamak plasmas [1]. Fast wave drives centrally peaking current in the frequency range up to several ion cyclotron harmonics in the present tokamaks, such as KSTAR. Increasing fast wave frequency up to LH resonance frequency at the plasma edge, the spiral propagation of wave at the outer region of plasma lengthens the wave path to the plasma center. Also, optical thickness increases with frequency. It is expected that these effects produce efficient off-axis power deposition depending on the electron beta and magnetic field pitch. A low power TWA for helicon wave was installed and tested in KSTAR tokamak which is aiming for the steady-state high beta plasma requiring off-axis current drive. The power coupling properties of TWA at various plasma conditions will be presented. In addition to the coupling efficiency, issues such as load sensitivity and unwanted slow wave coupling will be addressed. Also, the simulation of plasma performance with the combination of helicon wave current drive and other conventional heating and current drive power in KSTAR will be discussed.\\[4pt] [1] V. L. Vdovin, Plasma Phys. Reports 39 (2013) 115. [Preview Abstract] |
Wednesday, November 18, 2015 4:12PM - 4:24PM |
PO6.00012: Enhancement of KSTAR plasma control for expanding operational space Sang-hee Hahn, Y.M. Jeon, H. Han, H.S. Ahn, J. Kim, Y.J. Kim, M. Joung, M.H. Woo, D. Mueller, N.W. Eidietis, M. Lanctot, D.A. Humphreys, A.W. Hyatt, A.S. Welander, M.L. Walker, E. Kolemen, Y.S. Park, S.A. Sabbagh In order to expand the operational space with stationary high performances, new approaches on the magnetic control design are necessary. A few examples on recent achievements at KSTAr are presented here: The Introduction of the in-vessel radial control (IRC) provides a fundamental change on baseline axisymmetric magnetic controls. Analysis on dedicated simulations\slash experiments for the vertical stabilization control margin gave an insight for improvement of the vertical position control. In order to enhance flexibility on the non-axisymmetric 3D field physics studies, the KSTAR RMP coil systems have been upgraded in 2015 provide more variety on the available 3D field profile. Integration of real-time heating device control enabled more elaborate kinetic controls since 2013. Real-time TM suppression is introduced as an example of the integrated control, which will be linked to stability control in the high-beta regime relevant to ITER success. [Preview Abstract] |
Wednesday, November 18, 2015 4:24PM - 4:36PM |
PO6.00013: MSE commissioning and other major diagnostic updates on KSTAR Jinseok Ko The motional Stark effect (MSE) diagnostic based on the photoelastic-modulator (PEM) approach has been commissioned for the Korea Superconducting Tokamak Advanced Research (KSTAR). The 25-channel MSE system with the polarization-preserving front optics and precise tilt-tuning narrow bandpass filters provides the spatial resolution less than 1 cm in most of the plasma cross section and about 10 millisecond of time resolution. The polarization response curves with the daily Faraday rotation correction provides reliable pitch angle profiles for the KSTAR discharges with the MSE-optimized energy combination in the three-ion-source neutral beam injection. Some major diagnostic advances such as the poloidal charge exchange spectroscopy, the improved Thomson-scatting system, and the divertor infrared TV are reported as well. [Preview Abstract] |
Wednesday, November 18, 2015 4:36PM - 4:48PM |
PO6.00014: First flux surface measurements on W7-X Thomas Sunn Pedersen, Matthias Otte, Christoph Biedermann, Sergey Bozhenkov, Torsten Braeuer, Samuel Lazerson Wendelstein 7-X [1] is rapidly approaching first plasma operation. The full operational B-field of 2.5 T has been reached using the 70 superconducting coils. The first flux surface measurements have recently been successfully performed. This talk will describe the W7-X flux surface measurement system [2], and show and analyze the first results from this diagnostic, which, at the time of writing this abstract, can be summarized as follows: Confirmation of the existence of nested, closed flux surfaces, first measurements of iota, and detection of the expected internal 5/6 island chain of the OP1.1 configuration. The data obtained so far agree with expectations, and provide a first confirmation of the accuracy of the coil geometry and assembly, as well as diagnostic installation [3]. They also confirm that, with respect to the magnetic topology, plasma operation can start. Plans for, and potentially first results of, measurements of any remnant field errors, will be reported separately at this meeting [4].\\[4pt] [1] H.-S. Bosch et al., Nucl. Fusion 53, 126001 (2013). \newline [2] H.-S. Bosch et al., IEEE transactions on plasma science, 42(3), (2014) \newline [3] T. Braeuer et al., submitted to IEEE~Transactions on Plasma Science\newline [4] S. A. Lazerson et [Preview Abstract] |
Wednesday, November 18, 2015 4:48PM - 5:00PM |
PO6.00015: Modelling and analysis of flux surface mapping experiments on W7-X Samuel Lazerson, Matthias Otte, Sergey Bozhenkov, Thomas Sunn Pedersen, Torsten Br\"auer, David Gates, Hutch Neilson The measurement and compensation of error fields in W7-X [1] will be key to the device achieving high beta steady state operations. Flux surface mapping utilizes the vacuum magnetic flux surfaces, a feature unique to stellarators and heliotrons, to allow direct measurement of magnetic topology [2], and thereby allows a highly accurate determination of remnant magnetic field errors. As will be reported separately at this meeting [3], the first measurements confirming the existence of nested flux surfaces in W7-X have been made. In this presentation, a synthetic diagnostic for the flux surface mapping diagnostic is presented. It utilizes Poincar\'e traces to construct an image of the flux surface consistent with the measured camera geometry, fluorescent rod sweep plane, and emitter beam position. Forward modeling of the high-iota configuration will be presented demonstrating an ability to measure the intrinsic error field using the U.S. supplied trim coil system on W7-X [4], and a first experimental assessment of error fields in W7-X will be presented. [1] Bosch H S, et al., IEEE Trans. on Plas. Sci 42, 3 (2014) [2] Otte M, et al., AIP Conf. Proc. 993, 3-10 (2008) [3] Pedersen T S, et al., this meeting. [4] Rummel T, et al., IEEE Trans. on Appl. Supercond. 24, 4200904 (2014) [Preview Abstract] |
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