Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session TO4: JET, EAST, RFX and FRCs |
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Chair: John Sarff, University of Wisconsin Room: Rosen Centre Hotel Salon 1/2 |
Thursday, November 15, 2007 9:30AM - 9:42AM |
TO4.00001: High beta-N experiments at JET Clive Challis JET has investigated the performance potential and limitations of highly triangular plasmas relevant to fully non-inductive tokamak operation. The q-profile shape has been varied from cases with highly negative core magnetic shear to low shear with q0 close to 1, allowing the effect on confinement and stability to be studied. Operation with beta-N above the no-wall `limit' has been demonstrated for durations comparable with the resistive time and direct measurements of the no-wall beta have been developed as a tool for systematic performance optimization. Regimes have been developed with ITBs at reduced plasma current and toroidal field (1.2-1.5MA/2.3-2.7T) to obtain high values of beta-N and beta-P with either impurity seeding or quasi-double-null plasma configurations used to mitigate ELMs. The importance of the q-profile shape for performance optimization has been demonstrated in plasmas without ITBs (1.2MA/1.8T) with low values of minimum q (1-2) providing access to the highest beta-N (above 3). [Preview Abstract] |
Thursday, November 15, 2007 9:42AM - 9:54AM |
TO4.00002: Toward the stationary operation at high $\beta_{N}$ at JET: experiments and modelling I. Voitsekhovitch, C.D. Challis, C. Giroud, R.V. Budny , D. McCune, P. Buratti, E. Joffrin, T.C. Luce, M. Murakami Recently, a route to stationary MHD stable operation at high $\beta _{N}$ has been explored at JET by optimising the current ramp up, preheat start time and the waveform of NBI power in high triangularity plasmas. Transient $\beta _{N}\approx $3.5 and stationary $\beta _{N}\approx $3 have been achieved. The results of the analysis of current drive and transport in these discharges can be summarised as: (a) 50-70{\%} of current is driven non-inductively (NI); (b) a half of this current is due to the bootstrap (BS) current; (c) broad BS current profile is produced since the ITB was deliberately avoided; (d) a large contribution to the BS current is due to the $\nabla $Te term; (e) the GLF23 model predicts the temperature profiles within 20{\%} of discrepancy with the data, this model has been benchmarked in ASTRA, ONE-TWO and TRANSP. Based on this analysis the optimisation of high $\beta _{N}$ scenario towards fully NI stationary operation by optimising the BS current is investigated in predictive modelling coupling the transport, current diffusion and NBI and including the feedback control of $\beta _{N}$. A projection of this regime to ITER will be discussed. [Preview Abstract] |
Thursday, November 15, 2007 9:54AM - 10:06AM |
TO4.00003: Reversed shear Alfv\'en Eigenmodes in the frequency range of the triangularity induced gap on JET G.J. Kramer, G.Y. Fu, R. Nazikian, R.V. Budny, N.N. Gorelenkov, C.Z. Cheng, B. Alper, S.D. Pinches, F. Rimini, S.E. Sharapov, P. de Vries, K-D. Zastrow, V. Zoita In reversed magnetic shear plasmas a class of Alfv\'en eigenmodes (AE) can exist, the Reversed shear Alfv\'en eigen modes (RSAE). They are often observed in Tokamaks and are located just above the local maximum of the lower TAE continuum gap at the shear reversal point. Similar maxima exist in the higher order Alfv\'en gaps such as the EAE and NAE gap. In this presentation we will show from ideal MHD simulations and analytical theory that modes similar to the RSAE can exist under certain conditions in those higher order gaps. In burning plasmas modes in the AE gaps can be harmful for the confinement of fusion born alpha particles which can get lost before they thermalize thereby reducing the efficiency of a fusion reactor. We will show experimental evidence for RSAEs in the NAE gap in JET discharges. The JET NAE-RSAEs are identified from state of the art MHD simulations with the NOVA code in which the experimentally observed equilibrium parameters were used. [Preview Abstract] |
Thursday, November 15, 2007 10:06AM - 10:18AM |
TO4.00004: EAST First Plasma -- Design, Simulation {\&} Experimental Results J.A. Leuer, D.A. Humphreys, A.W. Hyatt, G.L. Jackson, R.D. Johnson, B.G. Penaflor, D.A. Piglowski, M.L. Walker, A.S. Welander, D. Mueller, B.J. Xiao, Q.P. Yuan, H.Z. Wang First plasma was achieved in EAST, the world's first highly-shaped fully superconducting tokamak, in Sept. 2006 [1]. We describe the design of the first plasma scenario and present results of the first plasma campaign in which this scenario was used successfully. Tools and methods used to optimize the breakdown and initial plasma current ramp follow previous analyses performed for ITER [2]. Testing of the EAST plasma control system (PCS) realtime software against simulations was crucial in designing the scenario. Open loop, resistor-based startup followed by PF current control produced 220 kA plasma current in a circular limiter configuration, with feedback control of plasma current and radial and vertical position. These modeling and design methods are expected to be important to future devices like KSTAR and ITER. \newline [1] B. Wan, et al., Plasma Sci. {\&} Tech. \textbf{9}, 125 (2007). \newline [2] J.A. Leuer, et al., Proc 15th IEEE Symp. on Fusion Eng. (1993) p.~629. [Preview Abstract] |
Thursday, November 15, 2007 10:18AM - 10:30AM |
TO4.00005: ABSTRACT WITHDRAWN |
Thursday, November 15, 2007 10:30AM - 10:42AM |
TO4.00006: The ITER ICRF Antenna Design with TOPICA Daniele Milanesio, Riccardo Maggiora, Orso Meneghini, Giuseppe Vecchi TOPICA (Torino Polytechnic Ion Cyclotron Antenna) code is an innovative tool for the 3D/1D simulation of Ion Cyclotron Radio Frequency (ICRF), i.e. accounting for antennas in a realistic 3D geometry and with an accurate 1D plasma model [1]. The TOPICA code has been deeply parallelized and has been already proved to be a reliable tool for antennas design and performance prediction. A detailed analysis of the 24 straps ITER ICRF antenna geometry has been carried out, underlining the strong dependence and asymmetries of the antenna input parameters due to the ITER plasma response. We optimized the antenna array geometry dimensions to maximize loading, lower mutual couplings and mitigate sheath effects. The calculated antenna input impedance matrices are TOPICA results of a paramount importance for the tuning and matching system design. Electric field distributions have been also calculated and they are used as the main input for the power flux estimation tool. The designed optimized antenna is capable of coupling 20 MW of power to plasma in the 40 -- 55 MHz frequency range with a maximum voltage of 45 kV in the feeding coaxial cables. \newline \newline [1] V. Lancellotti et al., Nuclear Fusion, \textbf{46} (2006) S476-S499 [Preview Abstract] |
Thursday, November 15, 2007 10:42AM - 10:54AM |
TO4.00007: Finite Electron Temperature Effects on Interferometric Measurements in Fusion Plasmas V.V. Mirnov, W.X. Ding, D.L. Brower Finite electron temperature effects on interferometry and polarimetry measurments in burning plasma are considered. In the limit when the wave frequency is much higher than the electron cyclotron frequency, the dispersion relation is derived to lowest order in $T_{e}/m_{e}c^2 \ll 1$. Previous analysis of the problem included non-relativistic dispersive corrections only [1]. We show that the relativistic effects are equally important and result in a change of the sign of the thermal correction to the Faraday rotation angle. Experimental observation of the ``sign'' effect could be important verification of fundamental relativistic physics in high-temperature plasma devices. The implication of the thermal corrections for ITER interferometer diagnostics is discussed. \newline \newline [1] S.E.Segre, V.Zanza, Physics of Plasmas, ${\bf 9}$, 2919 (2002) [Preview Abstract] |
Thursday, November 15, 2007 10:54AM - 11:06AM |
TO4.00008: Direct Measurement of the Electron Bernstein Wave Absorption and Current Drive at the WEGA Stellarator. Heinrich Laqua, Stefan Marsen, Matthias Otte, Yuriy Podoba, Josef Preinhaelter, Jakub Urban At the WEGA, which is a classical five period l=2 stellarator with a major radius of 0.72m and an aspect ratio of 6, electron Bernstein wave (EBW) heating by OXB-mode conversion was established for a frequency of 2.45GHz. Typical electron densities of $10^{18}m^{-3}$, which is 12 times the cut-off density, with an electron temperature of $\le $ 20eV were achieved for 30s operation with an ECRH power of up to 26kW. The low temperature plasma allows the investigation both, the EBW power deposition and current drive profiles by probes. The first was performed by high frequency (12kHz) power modulation and coherent detection of the generated heat waves with Langmuir probes. The second was measured with a $\dot {B}$-loop. The total EBW current was of up to 45A and the related normalised current drive efficiency $\zeta$ was 0.48. The EBW propagation is strongly sensitive on the magnetic configuration. Both, the variation of $\nabla $B along B and the shear can be changed individually, which makes WEGA an unique test bed for EBW propagation. As a result of the shear variation an EBW current reversal was found. The results were modelled by 3D ray-tracing calculations for the different magnetic configurations. [Preview Abstract] |
Thursday, November 15, 2007 11:06AM - 11:18AM |
TO4.00009: Electron temperature measurements and heat transport improvement in the RFX-mod experiment. Alberto Alfier, Federica Bonomo, Paolo Franz, Lionello Marrelli, Roberto Pasqualotto, Paolo Piovesan, Gianluca Spizzo, Silvia Valeria Annibaldi Electron temperature profiles at about 1keV have been measured in the RFX-mod experiment during the recent high plasma current campaign (I$_{p}>$1.2MA, n$_{e}\sim $4$\cdot $10$^{19})$: peaked Te profiles, obtained through the Thomson scattering diagnostic, are characterized by a steep gradient in the core during the quasi-single helicity (QSH) state. The formation of well defined magnetic flux surfaces during QSH states determines a reduction of thermal heat conductivity, whose estimate is essential to quantify this transport improvement. We apply the M1TeV code [1] to various experimental scenarios in order to estimate heat diffusivity, then also calculating electron confinement time: in this study, we consider the effect of the increase of plasma current and also of eventual external current drive. \newline [1] F.Porcelli \textit{et al.}, Phys. Rev. Lett. 82, 1458 (1999). [Preview Abstract] |
Thursday, November 15, 2007 11:18AM - 11:30AM |
TO4.00010: Enhanced confinement, magnetic topology and diffusion in RFX-mod for poloidal current drive David Terranova, Alessandra Canton, Paolo Innocente, Rita Lorenzini, Paolo Zanca, Alberto Alfier, Federica Bonomo The oscillating poloidal currrent drive (OPCD) technique proved to be the best way for inducing enhanced confinement regimes in the RFX- mod reversed field pinch. New OPCD experiments were carried out with plasma currents up to 1.4 MA with the virtual shell system operated in the clean mode control scheme. A significant improvement in plasma performaces has been obtained over a large section of the plasma volume (as shown by the electron temperature profiles) by inducing modifications in the radial profile of magnetic fluctuations. The enhanced regimes are studied also in terms of magnetic topology and diffusion by using a field line tracing code based on the radial profiles of tearing mode eigenfunctions reconstructed in toroidal geometry from external magnetic field measurements. Experiments were carried out also with the injection of pellets synchronized with the poloidal position of the OPCD-induced island. Pellet deposition and three dimensional trajectroy are correlated with the magnetic field structure in these enhanced regimes. [Preview Abstract] |
Thursday, November 15, 2007 11:30AM - 11:42AM |
TO4.00011: High Current Regimes in RFX-mod Marco Valisa, T. Bolzonella, P. Innocente, L. Marrelli, S. Martini, S. Ortolani, R. Paccagnella, M.E. Puiatti, M. Spolaore, P. Zanca RFX-mod has explored for the first time the confinement properties of the RFP configuration at plasma currents up to 1.5 MA. Such an accomplishment has been possible for the successful feedback control of the magnetic boundary, whereby an extensive mesh of individually controlled saddle coils have kept below 1 cm the maximum radial excursion of the last magnetic surface resulting from the overlapping of the many m=0 and m=1 MHD modes. Operation at high current has significantly expanded the database for scaling studies. Increasing current the ohmic input power is to be increased, the toroidal loop voltage does not drop significantly and the plasma wall interaction increases in intensity. On the other hand, in stationary regimes, poloidal beta does not decrease with current and is 15{\%} around n/n$_G$=0.5. In the high current regimes the probability to obtain Quasi Single Helicity states increases and the plasma volume inside the associated transport barrier increases. Both transient and stationary performance records for RFX have been obtained. [Preview Abstract] |
Thursday, November 15, 2007 11:42AM - 11:54AM |
TO4.00012: Filamented current density structures measured in the edge region of the RFX-mod experiment M. Spolaore, N. Vianello, R. Cavazzana, E. Martines, G. Serianni, E. Spada, M. Zuin, V. Antoni, M. Agostini, P. Scarin Coherent structures emerging from turbulence background have been detected in the edge region of the RFX-mod Reversed Field Pinch fusion device and are believed to significantly contribute to the particle transport. In order to gain insight into their presence and features a new and original probe system has been used: the system consists of two sets of electric and magnetic probes toroidally spaced by 88 mm. Each set is equipped with a 2-D array of Langmuir probes and a radial array of 3-axial magnetic coils. Magnetic and electrostatic fluctuations can then be measured simultaneously and on the same location with a high time resolution; statistical methods have been applied in order to detect structure-related bursts in the turbulence. It has been found that in the cross-field plane the bursts in density fluctuations correspond to structures, often referred in literature as `blobs', and are associated to current density filaments, which are mainly oriented along the magnetic field. Work is in progress to compare the current density bursts, as deduced by the magnetic field circulation, with those due to the diamagnetic current density, related to pressure gradient fluctuations. [Preview Abstract] |
Thursday, November 15, 2007 11:54AM - 12:06PM |
TO4.00013: Dynamic Formation of FRCs on the PHD Experiment John Slough, Samuel Andreason, Hiroshi Gota, Chris Pihl The goal of the Pulsed High Density (PHD) experiment is to reach break-even conditions through the Magneto-Kinetic (MK) compression of the Field Reversed Configuration (FRC). The distributed nature of the MK process provides for highly efficient coupling of bank energy into FRC thermal energy. The FRC is self-compressed to a hot, high density, burning plasmoid. An expansion phase after the burn allows the direct recovery of electrical energy from the heated FRC through induction. The entire process is similar to a reciprocating Brayton engine with no need for flux sustainment. The initial experimental work on PHD has been focused on generating a FRC that has sufficient lifetime, temperature and flux to reach fusion conditions on compression. To achieve this, the largest FRC experiment to date is being constructed. Dynamic formation is employed to produce the FRC during translation resulting in a very high flux, high velocity plasmoid. This process is also thought to generate large sheared axial flows with significant toroidal flux. Experiments with the device operating at half power have achieved initial target parameters with equilibrium temperatures of 300 eV and 15 mWb of flux. Modifications to achieve full power have been completed with the addition of a drift chamber for detailed equilibrium measurements. Results from the first dynamically formed FRCs will be presented. [Preview Abstract] |
Thursday, November 15, 2007 12:06PM - 12:18PM |
TO4.00014: Compression of Dynamically Formed and Merged FRCs George Votroubek, John Slough, Arthur Blair, Chris Pihl, Samuel Andreason, Richard Milroy, Thomas Weber In order to explore the high energy density approach to fusion, experimental studies of field reversed configuration (FRC) plasma dynamic formation/ acceleration, collisional merging, and compression to high density have been performed in the Inductive Plasma Acceleratior (IPA) experiment at MSNW in Redmond, WA. Two identical IPA sources eject FRCs at $\sim $ 250 km/s into a central chamber where they collide, merge, and decay with flux lifetimes of $\sim $ 40 $\mu $s. The merged FRC is then magnetically compressed via a 125 kJ bank up to a field of $\sim $1.5 T resulting in a high density plasma. The experimental device now in operation will be discussed. Results from the merging experiments will be presented, as well as results from compression studies. These results will be compared to 2D MHD numerical calculations. [Preview Abstract] |
Thursday, November 15, 2007 12:18PM - 12:30PM |
TO4.00015: FRC rotation control using an electric field Edward Ruden, Michael Frese A Field Reversed Configuration (FRC) spontaneously gains angular momentum about its $z$-axis over time until an instability with azimuthal mode number $n=2$ develops. This is potentially a limiting factor for particle confinement in FRC's. Ions diffusing beyond the separatrix having a preferred angular momentum is one cause of rotation. The boundary conditions where the open magnetic field lines outside the separatrix exit the vacuum chamber resulting in a viscous torque being applied to the FRC at the separatrix is another. Controlling the axial electric field via equipotential conducting rings at a fused quartz tube's inner surface where the open field lines exit the vacuum to prevent rotation is considered. Torque on the FRC due to otherwise passive boundary conditions there may thereby be avoided, and spin-up due to particle diffusion countered. A steady state analysis of FRC rotation due to the boundary potential distribution of a perfectly conducting extended MHD plasma obeying generalized Ohm's law, in addition to numerical simulations of the process are presented. [Preview Abstract] |
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