Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session PO4: Reversed Field Pinch |
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Chair: Fatima Ebrahimi, University of New Hampshire Room: Ballroom E |
Wednesday, November 16, 2011 2:00PM - 2:12PM |
PO4.00001: Screening Effect of Plasma Flow on RMP Penetration in EXTRAP T2R Lorenzo Frassinetti, Erik Olofsson, Per Brunsell, Sheena Menmuir, James Drake The penetration of resonant magnetic perturbations (RMP) can be screened by plasma flow and the understanding of this phenomenon is important for ELM mitigation techniques. This work studies the screening effect in EXTRAP T2R. EXTRAP T2R is equipped with a feedback system able to suppress all error fields and to produce one or more external perturbations in a controlled fashion. The EXTRAP T2R feedback system is used to generate a RMP that interacts with the dynamics of its corresponding tearing mode (TM). The level of RMP penetration is quantified by analyzing the RMP effect on the TM amplitude and velocity. To study the screening effect, the flow is changed by applying a second perturbation that is non resonant (non-RMP). This produces the flow reduction without perturbing significantly the other parameters. By modifying the amplitude of the non-RMP, an experimental study of the flow effect on the RMP penetration is performed. Experimental results are compared with the model described in [Fitzpatrick R \textit{et al.}, Phys. Plasmas 8, 4489 (2001)]. [Preview Abstract] |
Wednesday, November 16, 2011 2:12PM - 2:24PM |
PO4.00002: Keda Torus eXperiment (KTX): A new Reversed Field Pinch program in China Wandong Liu, Weixing Ding, Chijin Xiao, Hong Qin, Hong Li, Jian Zheng, Shude Wan, Jinlin Xie, Changxuan Yu As a natural extension of China Magnetic Confinement Fusion program, an advanced RFP device, Keda Torus eXperiment is now been proposed in University of Science and Technology of China. The significant parameters of KTX are: the major radius 1.4 m, the minor radius 0.4 m, the plasma current 1 MA, the plasma pulse 30-100 ms and the plasma density 2$\times $10$^{19}$m$^{-3}$. In the design phase, lots of considerations have been paid to ensure the unique features of KTX. The aspect ratio is particularly selected to achieve the single helical mode. It is also good for the scaling research of the RFP confinement. The thin modular shell structure with the combination of stainless steel and copper is good for the potential research of different wall conditions, and such structure guarantees a better proximity to the plasma. The coil configuration has been optimized for future researches of Oscillation Field Current Drive and Pulsed Parallel Current Drive. KTX will address the ITER relevant issues and some fundamental issues in space and astro-plasmas. [Preview Abstract] |
Wednesday, November 16, 2011 2:24PM - 2:36PM |
PO4.00003: Studies of fast ion confinement in the MST Reversed Field Pinch D. Liu, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, S. Eilerman, M.D. Nornberg, J.S. Sarff, J. Waksman, G. Fiksel , V.V. Belykh, P. Deichuli, V.I. Davydenko, A.A. Ivanov, S. Polosatkin, N. Stupishin Studies of fast ion confinement in the MST Reversed Field Pinch (RFP) are performed with a 1 MW hydrogen neutral beam injector (NBI), doped with 3-5{\%} deuterium fuel for creation of beam-target fusion neutrons. It is found that fast ions born from co-current NBI are well confined and roughly consistent with classical behavior in spite of the RFP's stochastic magnetic field. The measured neutron decay times following a $\sim $5 ms NBI pulse approximately agree with the prediction of classical slowing-down theory. The neutron flux at the beam turn-off time increases with plasma density and temperature, also as expected. The estimated fast ion confinement times range from several times to ten times the thermal particle confinement time, and charge-exchange with background neutrals seems to be the dominant fast ion loss mechanism. The slowing-down of fast protons is observed with a multi-energy-channel neutral particle analyzer. These results corroborate previous experiments with very short and low power neutral beam pulses, and are largely in agreement with TRANSP modeling which predicts a centrally peaked fast ion density profile with peak value up to 15{\%} of the plasma density. *Work supported by USDOE. [Preview Abstract] |
Wednesday, November 16, 2011 2:36PM - 2:48PM |
PO4.00004: Fast Particle Resonant Modes on MST J.J. Koliner, C.B. Forest, J.S. Sarff, J.K. Anderson, L. Lin, W.X. Ding, D.L. Brower, D.A. Spong The interaction between fast particles and Alfv\'{e}n eigenmodes (AE's) is an important process critical to magnetically confined fusion plasmas. An effort is in progress to understand AE's through theory and experiment on MST, a reversed-field pinch. Coupling of energetic particle dynamics to one or more continuum modes can introduce undamped AE's. This coupling can drive modes unstable, a condition pertinent to NBI on MST and fusion alpha particles in future RFP devices. Computational studies for MST have predicted toroidal AE's with frequencies in the 200-300 kHz range and global structure. Alfv\'{e}n-wave-frequency modes have been observed with up to 1 MW of NBI. Toroidal and poloidal arrays of magnetic loops are utilized to find edge amplitudes, frequencies and mode numbers. Frequencies are in the 60-150 kHz range with n=4 and n=5, m=1. The strongest coherent activity scales inversely with density, as expected for AE's, but does not scale with magnetic field strength. Additional experiments to investigate resonance conditions and beam energy scaling have been performed. An FIR interferometer-polarimeter has been used to find internal structure of the detected modes through correlation analysis. Supported by USDoE and NSF. [Preview Abstract] |
Wednesday, November 16, 2011 2:48PM - 3:00PM |
PO4.00005: Magnetic reconnection and Alfv\'{e}n eigenmodes in the RFX-mod reversed-field pinch plasmas Silvia Spagnolo, Matteo Zuin, Roberto Cavazzana, Gianluca De Masi, Emilio Martines, Barbara Momo, Monica Spolaore, Nicola Vianello The effect of the spontaneous magnetic reconnection events, characterizing reversed-field pinch plasmas, on Alfv\'{e}n eigenmodes (AE) is studied in the RFX-mod device by means of insertable edge magnetic probes. In particular, reconnection is observed to induce a sudden growth of the amplitude of the long wavelength (m/n=1/0), high frequency (around 1 MHz), Alfv\'{e}nic activity (appearing as two peaks in the frequency power spectra), interpreted in terms of two coupled Global AE, being associated to the presence of a minimum in the Alfv\'{e}n continuum. The dynamical behavior of the amplitude of such modes suggests some hints on a possible driving mechanism in ohmically heated plasmas, which should be linked to the formation of a high energy particles tail during reconnection. Moreover, the behavior of mode frequencies allows a radial localization at the edge of the plasma. [Preview Abstract] |
Wednesday, November 16, 2011 3:00PM - 3:12PM |
PO4.00006: Optimization of helical equilibria control in RFX-mod Lidia Piron, Davide Fabris, Lionello Marrelli, Paolo Piovesan, Anton Soppelsa, Paolo Zanca Recent experiments in RFX-mod have demonstrated that m=1,n=7 helical equilibria can be sustained by imposing helical boundary conditions by means of magnetic feedback [1]. The optimization of these equilibria consists on the one hand in the identification of the optimal feedback parameters in the mode controller and the amplitude and phase of applied helical magnetic fields; on the other hand in the characterization of the wall dynamic response to external magnetic fields, produced by active coils. The 1,7 mode dynamics has been initially investigated with the RFXlocking code, adopting an optimization approach similar to the one described in [2]. Moreover, dry shots have revealed that, when doing magnetic feedback, magnetic field errors are produced by 3D wall structures. A dynamic decoupler algorithm, which accounts for the mutual couplings between active coils and magnetic sensors, has been implemented and tested in real time. Experimental results show that error fields can be strongly suppressed thanks to this tool. \\[4pt] [1] P. Piovesan \textit{et al}, 2011 Plasma Phys. Control. Fusion \textbf{53} 084005 \\[0pt] [2] L. Piron \textit{et al}, 2010 Nucl. Fusion \textbf{50} 115011 [Preview Abstract] |
Wednesday, November 16, 2011 3:12PM - 3:24PM |
PO4.00007: Helical coordinate systems for SHAx RFP states Barbara Momo, Emilio Martines, Dominique Franck Escande, Fulvio Auriemma, Alessandro Fassina, Paolo Innocente, Rita Lorenzini, David Terranova, Paolo Zanca SHAx states are the helical RFP states, characterized by the dominance of the (m,n)=(1,7) mode and the presence of some residual `secondary' modes in the MHD spectrum, that have been observed at high plasma current in the RFX-mod device. New helical coordinate systems are defined in the SHEq code, where SHAx states are modeled as pure Single Helicity (SH) states. The helical flux is a good flux function, and is therefore used as the `radial' coordinate. The goal is to find straight-field-line coordinates, with angles defined w.r.t the helical axis, in a toroidal geometry: it is done beginning from the canonical form of the magnetic field and computing a transformation to action-angle coordinates. This enables to compute the helical safety factor related to helical equilibria. From the experimental point of view, this can be used for helical MHD spectra analysis and better understanding of experimental observations like transport barriers usually associated to SHAx states. [Preview Abstract] |
Wednesday, November 16, 2011 3:24PM - 3:36PM |
PO4.00008: Parametric dependence and control of 3D helical structures in MST B.E. Chapman, D.J. Den Hartog, J.A. Goetz, M.B. McGarry, E. Parke, J.A. Reusch, J.S. Sarff, W.F. Bergerson, D.L. Brower, W.X. Ding, L. Lin, F. Auriemma, S. Cappello, P. Franz, P. Innocente, R. Lorenzini, E. Martines, B. Momo, P. Piovesan, M. Puiatti, M. Spolaore, D. Terranova, P. Zanca MST plasmas with a central 3D helical structure are now routinely achievable. As in RFX-mod, the stellarator-like structure arises in otherwise toroidally axisymmetric plasmas when the innermost-resonant m = 1 tearing mode grows to large amplitude and dominates the core-resonant-mode spectrum. The tearing mode spectrum in MST varies strongly with toroidal plasma current. Peaked spectra with a helical structure occur most reliably at high Ip (0.6 MA), while at low Ip (0.2 MA), the spectra are quite flat. This trend is like that in RFX-mod and may reflect a dependence on the Lundquist number. Preliminary MST data also show that manipulation of the inductive electric field has a significant impact, driving the dominant mode to large amplitude more robustly, and reducing the other modes. At high Ip, Te(0) $>$ 1 keV with a possible several-fold increase in the energy confinement time. Supported by USDOE. [Preview Abstract] |
Wednesday, November 16, 2011 3:36PM - 3:48PM |
PO4.00009: Main gas particle transport in RFP helical equilibria Fulvio Auriemma, William Bergeson, Brett Chapman, Wiexing Ding, David Brower, Lorella Carraro, Paolo Innocente, Liang Lin, Rita Lorenzini, Barbara Momo, David Terranova Self organized helical states, where the innermost resonant tearing mode grows to large amplitude, routinely appear in all the RFP machines. With a sufficiently large dominant mode, the plasma's original magnetic axis is replaced with a new helical axis, bringing about the single helical axis (SHAx) state. A 3D magnetic topology can thereby occur in a toroidally axisymmetric device and this topology has been shown in RFX-mod (the Italian RFP) to exhibit improved energy confinement features ($\tau _E $ increases more than a factor 3) and beneficial effects on particle confinement, too. In this work, we will compare the main gas particle transport in SHAx plasmas in RFX-mod and MST. This analysis is based in part on multi-chord measurements of the density profile and modeling with the ASTRA code, accounting for the helical topology of the magnetic surfaces in the core. [Preview Abstract] |
Wednesday, November 16, 2011 3:48PM - 4:00PM |
PO4.00010: Progress in low-aspect-ratio RFP studies in RELAX Sadao Masamune, A. Sanpei, K. Oki, D. Fukabori, K. Deguchi, S. Nakaki, H. Himura, N. Mizuguchi, D. Den Hartog, R. Paccagnella Equilibrium analyses have shown that the aspect ratio A is one of the important parameters for optimization of the RFP configuration because it affects the q profile or fraction of the pressure-driven bootstrap current. RELAX (A=R/a=0.5m/0.25m) is a low-A RFP machine to explore the potential advantages of low-A configurations. The discharge and plasma parameters in RELAX to date are as follows: plasma current up to 100kA, discharge duration of up to 2.5ms, electron density in the range from 0.2 to 2.0$\times$ 10$^{19}$ m$^{-3}$, and electron temperature $<$100eV from double-filtered soft-X ray (SXR) measurements. Characteristic plasma behaviors in very shallow reversal and extremely deep reversal regions have been reported. A non-linear 3-D MHD simulation for A=2 RFP configurations has revealed that the quasi-single helicity (QSH) state with experimentally observed m/n=1/4 structure can be realized in shallow reversal cases. New diagnostics are being installed in RELAX: a soft-X ray (SXR) imaging system for the study of 3-D structural studies on MHD instabilities, Thomson scattering system, and 140GHz interferometer for higher density regimes. The vacuum vessel has been covered with 64 saddle coils for RWM stabilization but with limited number of power supplies at present. [Preview Abstract] |
Wednesday, November 16, 2011 4:00PM - 4:12PM |
PO4.00011: Measurements of fluctuations in density and potential using a heavy ion beam probe in improved confinement MST discharges D.R. Demers, P.J. Fimognari, P.M. Schoch, X. Chen A heavy ion beam probe (HIBP) is used to diagnose the interior of Madison Symmetric Torus improved confinement discharges. Drift turbulence may be important in this regime, which is produced through current density profile control and exhibits reduced magnetic fluctuation activity. Characterization of electrostatic turbulence with the HIBP has begun using spatially localized measurements acquired concurrently from two sample volumes. The measurements are affected by characteristics of the 3D beam trajectory, sample volumes, and RFP discharges; these necessitate alternative methods of data analysis and operation. The spectra of fluctuations in density and potential measured out to 500 kHz are broadband with most power below 100 kHz and peak at frequencies associated with tearing modes. Results also suggest that the phase between fluctuations in density and potential is near 180 degrees in the 0-40 kHz range, but smaller at 100-300 kHz. Examination of the Boltzman relation is also presented. (Work supported by US DoE.) [Preview Abstract] |
Wednesday, November 16, 2011 4:12PM - 4:24PM |
PO4.00012: Relation between edge dynamics and magnetic topology in RFX-mod Gianluca De Masi, Barbara Momo, Matteo Agostini, Fulvio Auriemma, Roberto Cavazzana, Alessandro Fassina, Lionello Marrelli, Emilio Martines, Stefano Munaretto, Paolo Piovesan, Gianluca Spizzo, Nicola Vianello The spontaneous transition to the Quasi Single Helicity states, observed in the RFX-mod Reversed Field Pinch plasma at high plasma current, has been favoured by the full exploitation of its advanced feedback control system. The plasma-wall interaction in these states appears to be regulated by the peculiar magnetic topology, in which one single dominant tearing mode (m=1, n=7) imposes a rotating helical pattern to the whole plasma. This has also a direct effect on the m=0 islands, arising at the edge due to the presence of a q=0 surface, that display a similar n=7 periodicity and keep the more internal confined plasma away from the wall. In this contribution we study the relation between edge topology and global plasma parameters with special attention to the role of the m=0 islands using a large set of edge diagnostic systems. In particular, we consider an active approach to the edge topology modification, by applying on the m=0, n=7 harmonic a non-zero reference amplitude through the feedback control system. [Preview Abstract] |
Wednesday, November 16, 2011 4:24PM - 4:36PM |
PO4.00013: Measurements of broadband electrostatic and magnetic turbulence in the MST reversed-field pinch D.J. Thuecks, A.F. Almagri, Y. Ren, J.S. Sarff, P.W. Terry The dominant fluctuations in the reversed field pinch arise from large scale tearing instabilities, but a broadband spectrum is observed. Recent measurements in MST suggest that the shorter wavelength magnetic fluctuations (spatially resolved to the ion gyro-radius scale) arise via a nonlinear cascade driven by the tearing modes. This turbulence is highly anisotropic, with fluctuation power being spread broadly in the direction perpendicular to the background magnetic field. Additionally, the scaling of the wavenumber spectrum reveals both inertial and dissipation ranges consistent with strong MHD turbulence expectations. Here we report measurements of broadband electrostatic fluctuations covering a similar range of physical scales. The wavenumber spectra for $\bf{E}$ and $n_e$ fluctuations exhibit a clear power-law scaling suggestive of an inertial range, but there is no clear exponential fall-off indicative of dissipation. The electrostatic fluctuations are also anisotropic and appear to be consistent with the predicted relation $k_{||}=k_\perp^{2/3}$. Cross-correlation measurements are underway to determine if the electrostatic and magnetic fluctuations are related, for example as would be expected for an Alfv\'enic cascade. NSF and DOE support this work. [Preview Abstract] |
Wednesday, November 16, 2011 4:36PM - 4:48PM |
PO4.00014: Electron Temperature features of RFP DAX states Alessandro Fassina, Paolo Franz, Alberto Ruzzon, Alberto Alfier, Marco Gobbin, Lionello Marrelli, Emilio Martines, Barbara Momo RFP states characterized by the presence of an hot helical structure in the plasma core have shown a significative improvement in the plasma performances. In this work we focused on DAX (Double AXis) states, in which the hot island is surrounded by a separatrix and does not cross the plasma centroid. These states, with respect to SHAx --or Single Helical Axis, [1]- , show smaller thermal structures, but the $\nabla $T$_{e}$ strength suggests a drastic local reduction of energy transport. The analysis relies on data obtained by the Main Thomson Scattering [2] and by the multichord double filter SXR spectrometer [3]. The general scaling properties of local $\nabla $T$_{e}$ are presented and the results are compared with SHAx datasets; overall confinement changing is analyzed relying both on T$_{e}$ and n$_{e}$ data. Finally, being data remapping on helical coordinates a widely used tool in SHAx analysis, limits and possibilities of this technique for DAX states are discussed. \\[4pt] [1] Lorenzini R. et al$.$, Nature Physics \textbf{5}, 570 - 574 (2009)\\[0pt] [2] Alfier A., Pasqualotto R., Rev. Sci. Instrum. \textbf{78}, 1 (2007)\\[0pt] [3] Bonomo F. , Rev. Sci. Instrum. \textbf{77, }10F313 (2006) [Preview Abstract] |
Wednesday, November 16, 2011 4:48PM - 5:00PM |
PO4.00015: Theory for relaxed plasma states with pressure gradient Jinlin Xie, Hong Qin, Wandong Liu, Yao Zhou, Chang Liu In the Taylor-Woltjer theory for relaxed plasma states, the pressure gradient vanishes. This is because only one global constrain, i.e., the global helicity, is applied in the minimization procedure for the energy. However, for experimental studies of self-organized, relaxed plasma states in reverse field pinch devices, the pressure gradient always exists. Applying a set of more realistic constrains, such as the local mass conservation and vorticity conservation during the minimization procedure for the energy, we have developed a new theory for relaxed plasma states which allows more interesting structures, including the pressure gradient. [Preview Abstract] |
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