Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session LO3: JET, Helical and Other Devices |
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Chair: Thomas Pedersen, Columbia University Room: Adam's Mark Hotel Governor's Square 15 |
Wednesday, October 26, 2005 2:00PM - 2:12PM |
LO3.00001: Detection of Alfv\'{e}n Cascades in Advanced JET Plasmas. B. Alper, S. Hacquin, S. Sharapov Advanced tokamak regimes with reversed magnetic shear and Internal Transport Barriers (ITBs) provide improved core energy confinement and are regarded as candidates for stationary operation in a fusion power plant. On JET, a close relation was established between frequency-sweeping Alfv\'{e}n Cascade (AC) eigenmodes excited by energetic ions and ITB triggering events. This led to the development of a technique to determine $q_{\min } (t)$ from the clustering in time of different toroidal mode number ACs. Novel microwave techniques have been used on JET for detecting the AC eigenmodes from density fluctuations that produce an unprecedented time and frequency resolution that exceeds those achieved with external Mirnov coils. These modes are best detected with the reflectometer operating in O-mode, as an interferometer, at a frequency just above the cut-off. With this technique, ACs have been detected in high-density JET discharges with high-power NBI and significant plasma rotation and also driven by sub-Alfv\'{e}nic NBI. ACs have also been found in the raw modulated signal from the JET (FIR) interferometer, facilitating their detection at higher density. [Preview Abstract] |
Wednesday, October 26, 2005 2:12PM - 2:24PM |
LO3.00002: Experimental observations of the effect of Ar seeding in JET H-Mode discharges Jef Ongena, Irina Voitsekhovitch, Pierre Dumortier, Andre Messiaen Ar seeding in H-Mode discharges in JET has resulted in simultaneous high radiation $\gamma$=P$_{rad}$/P$_{tot}\sim0.6$, high density n/n$_{GW}\sim1$, beta $\beta_{N}\sim1.9$ and confinement H$_{98(y,2)}\sim1$, stationary neutron yield and low $Z_{eff}\sim2.2$ for up to 5s. A quantitative comparison of the effect of Ar is shown by comparing (1) with discharges with same initial conditions and (2) with discharges with matched density during the Ar seeding. The main results are: (i) a decrease in the temperature of the divertor targets, observed by IR thermography and thermocouple measurements; (ii) at same density, an increase in the central ion temperature (from 2.5 to 3keV, with a reduction in $\chi_{i}$ by a factor 2) (iii) a change in the nature of the ELMs (mixed Type I/III); (iii) in plasmas with same initial conditions, a density increase is seen (by about 20$\%$) with Ar seeding, with only a small reduction (by about 5$\%$) in confinement enhancement H$_{98(y,2)}$, remaining close to 1. The tritium diffusion coefficient is reduced (by a factor of 2) with Ar, partly explained by the higher density reached. A comparison of the experimental results with modelling codes (TRANSP, UTC) will be included. [Preview Abstract] |
Wednesday, October 26, 2005 2:24PM - 2:36PM |
LO3.00003: Polarimetric measurements of line-integrated electron density at JET based on Cotton-Mouton effect in presence of large Faraday rotation Francesco Orsitto, Luca Zabeo, Edmondo Giovannozzi, Paolo Buratti Comparison between theory and polarimetry measurements have been carried out on selected JET discharges. In particular, an assessment of the line integrated plasma density along the vertical chords is done using the Cotton-Mouton effect in the presence of strong Faraday rotation. These measurements have been made for the first time in this regime, characteristic of a large tokamak, thus generalizing earlier experiments carried out under special conditions where the Faraday effect was negligible. The robustness of the measurement is demonstrated for the majority of JET plasma configurations, in particular for ITER-like plasmas. In the work, the rigorous numerical solution of the Stokes propagation equations (using dielectric tensor evaluated from equilibrium and Thomson scattering) are compared with the data available at JET from the polarimeter and interferometer. The evaluated phase shift and Faraday rotation angle of the emerging beam for the vertical chords of the JET polarimeter are compared with the corresponding measured quantities. The agreement with theory is satisfactory within the limits of experimental errors. The limits of the extraction of the line integrated electron density from the polarimeter data are discussed. Extrapolation to conditions of a burning plasma such as ITER is carried out using the theory checked on the JET database. [Preview Abstract] |
Wednesday, October 26, 2005 2:36PM - 2:48PM |
LO3.00004: Methods for Modeling Burning Plasmas and Validation using TFTR and JET DT Plasmas R.V. Budny, C. Kessel, K. Indireshkumar, D. McCune, J. Candy, R.E. Waltz Time-dependent integrated modeling is needed to increase our confidence in predictions for Tokamak burning plasmas. The PTRANSP project is modifying the TRANSP plasma analysis code to strengthen its predictive capabilities, including the GLF23 predictive model which has been benchmarked using gyrokinetic codes such as GYRO that simulate the turbulence induced transport of energy, angular momentum, and species.We use a prototype of PTRANSP to generate time-dependent models of ITER plasmas in several regimes such as the ELMy H and Hybrid modes.We performed checks to verify and validate the procedure. The PTRANSP plasma profiles are used as inputs in the GYRO code for simulating the nonlinearly-saturated ITG/TEM turbulence and transport. The turbulence depends sensitively on drive terms such as $\nabla(T_i)$ and suppression terms such as the external $E \times B$ flow shear which scales as $\nabla(E_r)$. We derive the $E_r$ profiles from predictions of the NNBI-induced toroidal velocity and the neoclassical poloidal velocity. Slight ($\simeq$ 15 {\%}) variations of $\nabla(T_i)$ and $\nabla(E_r)$ give transport consistent with the PTRANSP results. We compare these procedures with experimental results from DT plasmas produced in TFTR and JET. [Preview Abstract] |
Wednesday, October 26, 2005 2:48PM - 3:00PM |
LO3.00005: Axisymmetric Phase Space Structures Driven by Fast Ions in JET H.L. Berk, C.J. Boswell, S.E. Sharapov, T. Johnson, M.F.F. Nave, S.D. Pinches Theoretical analysis shows that frequency sweeping of kinetically driven instabilities, near marginal stability, may arise due to the spontaneous formation of phase-space structures.\footnote{Berk H.~L., \textit{et al.} Phys. Plasmas 6 3102 (1999)} The JET observation of axisymmetric (n=0) frequency sweeping modes induced by energetic particles (produced by high-field side off-axis ICRH) is indicative of the formation of such structures. A consistent explanation attributes the linear mode excitation to the geodesic acoustic mode which requires n=0 for mode existence. The SELFO Monte Carlo code shows that the fast ion distribution has a region in phase space where $\partial F / \partial E > 0$ with high field side ICRH and that the trapped particle bounce frequencies are found to coincide with the range of the chirped frequency span. Additional ion heating by tangential neutral beams makes the energetic particle distribution more isotropic, removing the instability drive and causing the quench of the mode. The understanding of the linear and nonlinear characteristics of the axisymmetric phase space structures gives a good example of plasma-wave interactions. [Preview Abstract] |
Wednesday, October 26, 2005 3:00PM - 3:12PM |
LO3.00006: New lost alpha diagnostics for JET Edward Cecil, Doug Darrow, Bob Ellis, Vasili Kiptily, Les Pedrick, Matthius Reich, Andreas Werner, Stefan Beaumel Two new devices have been installed in the JET vacuum vessel near the plasma boundary to investigate the loss of energetic fusion products in general and alpha particles in particular during the up coming JET experiments. These devices are (i) a well collimated scintillator which is optically coupled to a charge coupled device and (ii) a set of multichannel thin foil Faraday collectors. Both devices have been designed to withstand the harsh radiation, thermal, and mechanical stresses of the near plasma environment. The design and operating principles of these devices have been discussed elsewhere [1,2], Photography of the devices as installed and preliminary operation during plasma commissioning pulses will be presented. This work is supported by US DOE contracts DE-AC02-76CH33073 and DE-FG03-95ER54303 and is conducted under EFDA by IPP and PPPL. 1. S. Beaumel et al. Rev Sci Instruments \textbf{75}, 3563 (2004) 2. D.S. Darrow et al. Rev Sci Instruments \textbf{75}, 3566 (2004) [Preview Abstract] |
Wednesday, October 26, 2005 3:12PM - 3:24PM |
LO3.00007: Tests of the modified JET ITER-Like ICRH Antenna High Power Prototype F.W. Baity, R.H. Goulding, M. Brown, A. Fadnek, K.D. Freudenberg, B.E. Nelson, D.A. Rasmussen, D.O. Sparks, J.C. Hosea, G.D. Loesser, F. Durodi\'{e}, M. Vrancken, M. Nightingale, R. Walton The second round of tests of the JET ITER-Like Ion Cyclotron Resonance Heating Antenna High Power Prototype (HPP) has recently been completed. The purpose of the device is to test the advanced features of the actual antenna, designed to couple $\geq$ 7.1 MW into an ELMy H-Mode Plasma on JET. Before the most recent tests, modifications were made to the current straps, antenna enclosure, limiter tiles, and capacitors. The maximum voltage at the capacitors achieved during long pulse (10 s) operation has been increased from 23 kV (peak) to $\sim$ 40 kV. The capacitor voltage achieved during .05 s pulses with limiter tiles installed has increased from 24 kV to $\sim$ 50 kV. These and other results, reasons for the improved performance, and implications for the operation of the JET ITER-Like Antenna will be discussed. [Preview Abstract] |
Wednesday, October 26, 2005 3:24PM - 3:36PM |
LO3.00008: A Predictive Simulation Tool for Plasma Facing Antennas R. Maggiora, V. Lancellotti, D. Milanesio, V. Kyrytsya, L. Valitutti, G. Vecchi, A. Parisot, S.J. Wukitch TOPICA is an innovative tool for the simulation of plasma facing antennas that incorporates commercial-grade 3D graphic interfaces and an accurate description of the plasma. The coaxial feeding line or waveguide are modeled as such; computation and visualization of relevant parameters (input scattering parameters, current and field distributions, etc.) complete the suite. The approach to the problem is based on an integral-equation formulation for the self-consistent evaluation of the current distribution on the conductors. The environment has been subdivided in two coupled region: the plasma region and the vacuum region. The two problems are linked self-consistently by representing the field continuity in terms of equivalent (unknown) sources. In the vacuum region all the calculations are executed in the spatial domain, and this allows triangular-facet description of the arbitrarily shaped conductors and associated currents; in the plasma region a spectral representation of the fields is used, which allows to enter the plasma effect via impedance matrix. This work is devoted to an extensive set of comparisons between system parameters measured during operation and simulated. The comparison demonstrates a very good agreement, leading to a validation of TOPICA as a reliable predictive tool. [Preview Abstract] |
Wednesday, October 26, 2005 3:36PM - 3:48PM |
LO3.00009: Time Evolution of Density Profiles in ET R.J. Taylor, J.-L. Gauvreau, P.-A. Gourdain, D.J. Lafonteese, L.W. Schmitz We observe a persistent particle pinch in ET (R = 5 m, B = 0.25 T, A = 5, a = 1 m, I$_{p}$ = 60 kA), in which the ion dynamics is classical and the electron dynamics is subdominant. The ion banana dynamics drives a poloidal rotation which sets up a radial potential, trapping the bulk ions in an electrostatic well. The trapping rate agrees with the neoclassical ion mobility. The density peaking results in an MHD unstable configuration, which empties the trap rapidly with no significant current channel disruption. This process repeats on a 1 second time scale with large beta swings. Analysis shows that the radial pinch can dominate the radial particle diffusion in quiescent plasma. This is attributed to the large size of ET and contributes to an enhanced neo-Alcator confinement even though the toroidal magnetic field is very low (0.2 Tesla). This behavior has been modeled using neo-Alcator electrons and neo-classical ions. Time evolution density profile predictions for ITER, assuming no MHD instabilities, will be given. [Preview Abstract] |
Wednesday, October 26, 2005 3:48PM - 4:00PM |
LO3.00010: Comprehensive modeling of dust particle transport in tokamak plasmas with code DUSTT S.I. Krasheninnikov, A. Yu. Pigarov, T.K. Soboleva Recently, the presence of dust particles in tokamak plasma and the role of dust in material re-deposition, core contamination, and tritium inventory brought significant attention. Recent experimental observation of dust in fusion devices will be reviewed. The novel physical model for dust transport simulation and the newly developed 3D code DUSTT is discussed. The DUSTT code takes into account both the dust dynamics due to comprehensive dust-plasma interactions as well as the effects of dust charging, heating and evaporation. The code allows tracking of test dust particle in realistic plasma background calculated with the edge-plasma transport code UEDGE. The results on dust transport in NSTX, DIII-D, and ITER tokamaks are presented. The possible effect of dust on divertor plasma parameters is analyzed. An assessment of core plasma contamination due to dust is given. [Preview Abstract] |
Wednesday, October 26, 2005 4:00PM - 4:12PM |
LO3.00011: Overview of TJ-II experiments. Kieran McCarthy We present an overview of experiments in the TJ-II stellarator. Global confinement studies reveal a positive energy confinement dependence on rotational transform and density together with parametric dependences on wall conditions. Spontaneous and bias-induced improved confinement transitions are seen. Configuration scan experiments show interplay between magnetic structure, transport and electric fields. While DC radial electric fields are comparable with neoclassical estimates, neoclassical/turbulent bifurcation and kinetic effect mechanisms are needed to explain the impact of magnetic topology on flows and radial electric fields. Studies of the influence of magnetic configuration on core and edge instabilities show the role of low-order rationals and of density and heating power thresholds to trigger quasi-coherent modes and ELM-like instabilities. Hydrocarbon fuelling in configurations with a low-order rational value in the rotational transform close to the LCFS shows impurity screening related to the divertor effect. [Preview Abstract] |
Wednesday, October 26, 2005 4:12PM - 4:24PM |
LO3.00012: Influence of magnetic topology on transport and stability in Stellarators Francisco Castej\'{o}n, Akihide Fujisawa, Katsumi Ida, Joseph N. Talmadge, Teresa Estrada, Daniel L\'{o}pez-Bruna, Carlos Hidalgo Stellarators are suited to study the effect of magnetic topology on transport and stability, since they present good control of their magnetic configurations and rotational transform profiles. Four stellarators are considered: TJ-II, a shearless flexible heliac, CHS and LHD, a small and a large heliotrons with shear, and HSX a quasihelically symmetric device. Transport barriers are created by low order rationals in LHD and TJ-II in plasma core or at the edge, being the appearance of a positive and sheared electric field, Er, the key ingredient. Hence, it is demonstrated that low order rationals are not always deleterious but can be beneficial. The evolution of Er in CHS and TJ-II shows the onset of a bifurcation, triggered by the rational or by the change from ion to electron root. The effect of viscosity on Er is explored in HSX configurations with different magnetic ripples and the effect of island dynamics on barriers is studied in TJ-II and LHD. [Preview Abstract] |
Wednesday, October 26, 2005 4:24PM - 4:36PM |
LO3.00013: Spatial structure of the MHD instabilities at the relaxation events in the Large Helical Device Satoshi Ohdachi, Kazuo Toi, Fumitake Watanabe, Satoru Sakakibara, Kiyomasa Watanabe, Yoshiro Narushima, Kazumichi Narihara In order to realize a helical reactor, achievement of the high beta plasma is a one of the most important issue. However, in the Large Helical Device, the beta value is sometimes restricted by sawtooth-like relaxation events, which localized on the various rational surfaces of which rotational transform 1/q is 1/2, 2/3 and 1/1. The spatial structures of the MHD activities are visualized mainly by the fast-framing tangentially viewing soft X-ray camera system. It is found that the island-like structures evolve slowly (within several ms) during the events, together with much faster (several kHz) oscillations. The dynamics of the MHD events and its effects on the plasma confinements will be discussed. [Preview Abstract] |
Wednesday, October 26, 2005 4:36PM - 4:48PM |
LO3.00014: Spontaneous generation of solitary structures in tokamaks R.M.G.M. Trines, Robert Bingham, L.O. Silva, J.T. Mendonca, P.K. Shukla, W.B. Mori The interaction between broadband drift mode turbulence and zonal flows has been studied through the wave-kinetic approach [1]. Simulations have been conducted in which a particle-in-cell representation is used for the quasi-particles, while a fluid model is employed for the plasma. Simulation results show the spontaneous emergence of solitary structures about an ion gyroradius wide, drifting down the background density gradient. The interactions have been studied in a plasma edge configuration which has important implications for transport phenomena in tokamak plasmas. \newline \newline [1] R. Trines et al., Phys. Rev. Lett. 94, 165002 (2005). [Preview Abstract] |
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