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 RO3: CMOD, ASDEX, TORE-SUPRA, RFX |
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Chair: Steve Allen, Lawrence Livermore National Laboratory Room: Adam's Mark Hotel Governor's Square 15 |
Thursday, October 27, 2005 2:00PM - 2:12PM |
RO3.00001: Overview of Recent Alcator C-Mod Results Steven Scott Alcator {\sc c-mod} has operated with all-metal plasma facing components following removal of boron nitride protective tiles and residual surface boron. {\sc rf}-heated {\sc h}-mode performance with molybdenum walls achieves only {\sc h}89$\sim 1 $ with high radiated power. {\sc ecdc} boron deposition reduces Mo influx and radiated power and doubles $\tau_E$. Brief boronizations (10-30 minute) significantly affect the subsequent shot with beneficial effects that increase with application time. Disruption mitigation studies at high plasma energy density using high-pressure helium and neon gas demonstrate that neon, but not helium, speeds up the current quench and reduces the halo current. The major radius scaling of error field required for locked modes is $\tilde{B}/B \propto R^{0.59}$ based on dimensional analysis of its variation with $B_T$ and $\bar{n}_e$ and implies $\tilde B_{21}/B$ of $\sim0.8\times10^{- 4}$ for {\sc iter} at its ohmic density. The frequency and $k_ {\theta}$ spectrum of Alfv\'{e}n cascades measured by phase contrast imaging and magnetic coils during early {\sc rf} heating indicates that the $q$-profile remains reversed up to 0.18~seconds, offering a good target plasma for off-axis {\sc lhcd}. Discrete {\sc elm}s observed at large lower triangularity ($\geq 0.75$) exhibit a rapidly-growing precursor oscillation that is localized radially to the vicinity of the top of the pedestal. Outside the {\sc lcfs}, {\sc elm}s propagate radially outward with a complicated spatial structure resembling `blobs'. [Preview Abstract] |
Thursday, October 27, 2005 2:12PM - 2:24PM |
RO3.00002: B-field Scaling of Locked Mode Error Field Threshold in Alcator C-Mod R.S. Granetz, S.M. Wolfe, I.H. Hutchinson, T. Hender One of the most ITER-relevant locked mode issues is the scaling of the threshold for error field locking, $\tilde B/B$, versus machine size, $R$. One way to obtain this is by comparing the locking threshold on tokamaks of different size. But the size scaling can also be obtained from just a single machine, utilizing dimensionless constraint arguments that relate it to $B$-field and density scaling. This alternative approach may provide a result with smaller uncertainty. On Alcator C-Mod this $B$-field scaling experiment has been carried out while also matching the ITER $q_{95}$ ($\sim3.2$) and $n/n_G$ (0.17, which is the ITER ohmic value and is the most relevant for locked modes). $B_T$ was varied shot-to-shot from 3 to 7 tesla while the applied 2/1 error field was ramped during the steady-state portion of each shot to find the locking threshold. The result gives a $B$-field scaling exponent of -1.13, which in turn yields an exponent for the size scaling ($R$) of 0.59. This implies a locking threshold for $\tilde B_{21}/B$ of $\sim0.8\times10^{-4}$ in ITER at its ohmic density. This $B$-field scaling experiment in C-Mod included a point at the actual ITER field (5.3 T), in addition to the ITER $q_{95}$ and $n/n_G$, which further reduces the extrapolation uncertainty. [Preview Abstract] |
Thursday, October 27, 2005 2:24PM - 2:36PM |
RO3.00003: Radiation loss and confinement performance of Alcator C-Mod. I. Hutchinson, M. Reinke, J. Irby, Y. Lin, B. Lipschultz, E. Marmar, J. Rice, J. Terry Ultimate performance of tokamaks with high-Z plasma facing components is of great interest for ITER. C-Mod has had molybdenum divertor and limiter since its first operation, and produced some excellent results. However, recent detailed experiments show that Mo radiation is in many cases, especially in H-modes when impurities tend to accumulate, a major limitation on confinement performance. The radiation is predominantly in the outer half of the plasma, but nevertheless causes the central temperature, as well as the edge transport barrier height to decrease. When accumulation is severe, the H-mode is eventually lost when the core radiation exceeds about 60{\%} of the total input power. Boronization can suppress the Mo to a negligible level, but the effect wears off quite quickly in some cases. [Preview Abstract] |
Thursday, October 27, 2005 2:36PM - 2:48PM |
RO3.00004: Between-Shot Boronization on Alcator C-Mod E. Marmar, I. Hutchinson, J. Irby, Y. Lin, B. Lipschultz, M. Reinke, J. Rice, J. Terry Boronization is a standard wall conditioning technique used in C-Mod. The boron layer is usually deposited using an Electron Cyclotron Discharge (ECDC) run in 10{\%}B$_{2}$D$_{6}$/90{\%}He gas. Normally a $\sim $8 hour boronization is applied about once every week to cover the all-metal plasma facing components and vessel surfaces. Assuming uniform coverage, the rate of B deposition is $\sim $20 nm/hour, with typical coverage of $\sim $150 nm from a single overnight treatment. Boronization has important effects on plasma operation, including reduction of core radiation and improvement of global energy confinement. The effects of a single boronization typically last for between 10 and 100 plasma discharges, depending on plasma conditions and auxiliary heating power. To better understand the effects of boronization and to improve discharge consistency, we are investigating between-shot boronization. Initial experiments show that as little as 10 minutes of ECDC boronization significantly affects the subsequent tokamak plasma shot, and the beneficial effects increase monotonically with application time, at least up to 30 minutes. These effects also wear off in 1 or 2 shots, enabling the assessment of variations in resonance scan and gas parameters during ECDC. Glow discharge boronization is also being considered as a possible between-shot technique. [Preview Abstract] |
Thursday, October 27, 2005 2:48PM - 3:00PM |
RO3.00005: Disruption Studies On C-MOD: Mitigation {\&} Hydrogen/Deuterium Fuel Recovery D.G. Whyte, M. Bakhtiari, R. Grantez, V. Izzo, J. Terry, B. Lipschultz, T. Jernigan Due to its high magnetic field, high plasma energy density and metallic first wall, the Alcator C-Mod tokamak provides a critical testbed for developing and testing disruption mitigation scenarios for ITER. We will report the first results using high-pressure gas injection of noble gases into C-Mod. These results will be compared to previous results from DIII-D, and possible implications for the penetration of gas jet impurities in ITER will be discussed. We also report on the successful demonstration of recovery of hydrogenic fuel by exploiting transient heating of the wall from planned disruptions, a technique proposed for ITER. Disruptions are shown to be effective at reducing the hydrogen/deuterium isotope ratio, as required for ion cyclotron H minority heating. The routine use of disruptions was successful at preventing net retention of fueled deuterium gas in the wall without detrimental effects to C-Mod operation, and thus shows initial promise for reducing tritium retention in ITER. [Preview Abstract] |
Thursday, October 27, 2005 3:00PM - 3:12PM |
RO3.00006: Reversed Shear Alfv\'{e}n Eigenmodes in the Presence of Current Ramps and ICRF Minority Tails in Alcator C-Mod* M. Porkolab, E. Edlund, C. Boswell, N. Basse, Y. Lin, L. Lin, J.A. Snipes, S.J. Wukitch, G.J. Kramer Alfv\'{e}n cascades (ACs), or equivalently, reverse shear Alfv\'{e}n eigenmodes (RSAE) have been observed in Alcator C-Mod during current ramps with intense ICRF minority heating$^{1}$. Both phase contrast imaging (PCI) and magnetic pick-up coils have been used to study the frequency spectrum. PCI can also give an estimate of the k$_{\theta }$-spectrum. From such data, the evolution of the q-profile has been deduced. The results indicate that up to 0.18 seconds reversed shear should be present with peak electron temperatures approaching 5 keV, offering a reasonable target plasma for off-axis lower hybrid current drive (LHCD) to maintain the reversed shear for AT studies. Without LHCD the current profiles relax to monotonic q-profiles and sawteeth set in by 0.3 sec. Using the ideal MHD code NOVA-K$^{2}$, the frequency chirping characteristics of these modes have been modeled with good accuracy and the code predicts accurately the minimum frequency $\Delta \omega $, which may be interpreted as the geodesic deformation of the Alfv\'{e}n continuum at low frequency$^{3}$. (1) M. Porkolab et al, Proc. 15$^{th}$ Conf. High RF Power in Plasmas, Park City, UT, (2005). (2) C. Z. Cheng, Phys. Rep. 211, \textbf{1} (1992). (3) M.S. Chu, J. M. Greene, L.L. Lao et al., Phys. Fluids B \textbf{4}, 3713 (1992). *Work supported by USDOE Contract DE-AC02-76-CH0-3073 and Cooperative Agreement No. DE-FC02-99-ER54512. [Preview Abstract] |
Thursday, October 27, 2005 3:12PM - 3:24PM |
RO3.00007: Extension of Pedestal Scaling Studies on the Alcator C-Mod Tokamak T.M. Biewer, J.W. Hughes, A.E. Hubbard, C.S. Chang, S.H. Ku, D.A. Mossessian During the operation of the Alcator C-Mod tokamak three common classifications of H-mode are observed: 1) ELM-free H-mode, 2) ELMy H-mode, and 3) so-called ``enhanced D alpha'' (EDA) H-mode accompanied by a quasi-coherent mode (QCM) edge relaxation mechanism. In all cases, the transition to H-mode occurs concurrently with the formation of edge temperature and density pedestals. Pedestal characteristics are dependent on operational machine parameters, and vary with the type of H-mode regime that is achieved. Recent experiments, with a particular plasma shape, are being extended over the full range of C-Mod operational parameter space in an effort to classify the range over which each type of H-mode occurs, and to examine the behavior of plasma performance with achieved pedestal parameters. In particular, the experiments reported here emphasize access to ELM-free H-modes in C-Mod. Pedestal scalings in this regime are particularly useful for cross-machine comparisons, since other machines like DIII-D and JT60-U have generated substantial database scalings in the ELM-free regime. Moreover, calculated neoclassical transport predictions of pedestal width scaling can be tested. [Preview Abstract] |
Thursday, October 27, 2005 3:24PM - 3:36PM |
RO3.00008: Operational Phase Space of the Edge Plasma in Alcator C-Mod B. LaBombard, T. Biewer, M. Greenwald, J.W. Hughes, B. Lipschultz, N. Smick, J.L. Terry Earlier C-Mod experiments [1] uncovered some clear connections between the operational space of the near scrape-off layer (SOL) and that expected from electromagnetic fluid drift turbulence theory -- at fixed values of normalized collisionality (`diamagnetic parameter', $\alpha$d, pressure gradients increase with plasma current squared, holding the MHD ballooning parameter, $\alpha$MHD , unchanged. Thus, the edge plasma state occupies a narrow band within this two-parameter phase-space; both L and H-mode profiles appear controlled by a critical-gradient transport paradigm. Here, we examine a wider parameter range in lower and upper single null (LSN/USN) discharges and with an improved edge diagnostic set: Langmuir-Mach probes on high and low-field SOLs and D$\alpha$ imaging on the low-field SOL. Preliminary results indicate a sensitivity of L-mode gradients to magnetic topology, favoring higher sensitivity of L-mode gradients to magnetic topology, favoring higher $\alpha$MHD for LSN compared to USN. These observations potentially connect to the different SOL plasma flows and velocity shears that arise in LSN versus USN [2]. [1] B. LaBombard, et al., submitted to Nuclear Fusion [2] Nucl. Fusion \textbf{44} (2004) 1047, Phys. Plasmas \textbf {12} (2005) 056111. [Preview Abstract] |
Thursday, October 27, 2005 3:36PM - 3:48PM |
RO3.00009: Investigation of ELMs on Alcator C-Mod J.L. Terry, A. Hubbard, J.A. Snipes, J.W. Hughes, S. Wukitch, Y. Lin C-Mod typically operates in regimes without large ELMs. Recently, discrete ELMs have been routinely produced by making plasmas with large lower triangularity (i.e. $>$0.75), compared to the more typical C-Mod values $<$0.6. The ELM character was substantially modified as the triangularity was reduced, changing from discrete ELMs of $\sim$60 $\mu$sec duration, to H-to-L mode back transitions, lasting $\sim$4msec. The discrete ELMs are most apparent when the density is just above the low-density H-mode threshold, $n_e\sim8x10^{19}m^{-3}$. Pedestal $T_e$ up to 1 keV was measured early in the H-mode phase. The spatial structure and propagation of the discrete ELMs are studied using fast-framing ($\sim$250 kHz) cameras and other high resolution optical diagnostics. The magnetics and the optical diagnostics show a rapidly-growing precursor oscillation (100-200 kHz just prior to the ELM crash) that is localized radially to around the top of the pedestal. Outside the LCFS the enhanced emission from the ELM propagates radially outward with a complicated spatial structure, similar in many respects to `blobs.' [Preview Abstract] |
Thursday, October 27, 2005 3:48PM - 4:00PM |
RO3.00010: Electron Multipactor Discharges in Vacuum Transmission Lines for VHF Timothy P. Graves, B. LaBombard, S. Wukitch, I. Hutchinson A multipactor discharge is a resonant condition for electrons in an alternating electric field. This discharge can be disruptive to RF circuits, cavities, and resonators. The Coaxial Multipactor Experiment (CMX) investigates these discharges in parallel plate and coaxial transmission line geometries with goals of measuring the electron energy distributions at frequencies from 40 to 150 MHz. CMX has a unique experimental setup which allows the transmission line to pass continuously through a vacuum region. Retarding potential analyzers with secondary electron suppression measure the electron current as a function of bias voltage. Results for both parallel plate and coaxial multipactor discharges provide the first detailed measurements of the electron energy distributions with varying frequency. Results show that coaxial multipactoring can be much more destructive to the RF circuit than parallel plate multipactoring under certain conditions, as is the case at 80 MHz and 4 inch coaxial line. This is the scenario on the C-MOD ICRH system, making it highly susceptible to coaxial multipactoring. [Preview Abstract] |
Thursday, October 27, 2005 4:00PM - 4:12PM |
RO3.00011: Control of core MHD Instabilities by ECCD in ASDEX Upgrade M. Maraschek $^1$, G. Gantenbein $^2$, S. G\"unter $^1$, F. Leuterer $^1$, A. M\"uck $^3$, H. Zohm $^1$ Neoclassical Tearing Modes limiting the achievable $\beta_N$, are of great interest for fusion. The control of MHD activity triggering NTMs and the removal or mitigation of NTMs is the main target. Electron Cyclotron Current Drive at the q=1 surface is able to control sawteeth, the main trigger for NTMs and increased $\beta_N$ values could be achieved by this NTM avoidance. Excited NTMs can be stabilized with moderate ECCD power at the resonant surface. The required power for the different NTMs based on the marginal $\beta_p^{marg}$ will be discussed. A scan of the ECCD deposition width ($d < W_{marg} ... d > W_{marg}$) in comparison to the marginal island size $W_{marg}$ and the driven current has been done, confirming the theoretical prediction of the current density $I/d$ as the figure of merit. As for ITER $W_{marg} < W < d$ might occur and non-modulated ECCD might be insufficient, a comparison with modulated ECCD, depositing only in the islands O-point, %will be shown. has been performed. The possibility to control the Frequently Interupted Regime (FIR-NTM) with ECCD will be discussed. Typically this regime leads to a confinement recovery at high $\beta_N$ values. The present status of these schemes at ASDEX Upgrade and the perspectives for ITER will be shown.\newline \newline $^1$MPI for Plasmaphy., EURATOM As., D-Garching; $^2$Inst. for Plasmares., D-Stuttgart; $^3$CRPP-EPFL, EURATOM As., CH-Lausanne. [Preview Abstract] |
Thursday, October 27, 2005 4:12PM - 4:24PM |
RO3.00012: ELM Mitigation by Noble Gas Injection Ahmed Hassanein, Isak Konkashbaev Edge-localized modes (ELMs) remain a serious concern for plasma-facing components (PFCs) in the divertor during normal H-mode operation of future tokamaks. ELMs could result in excessive divertor erosion and plasma contamination. Use of high-pressure noble gas is a simple and robust method to mitigate the effects of ELM on the divertor plate. The noble gas cloud above the divertor surface absorbed and reradiated $\approx $100{\%} of the deposited species energy, correspondingly decreasing heat and particle load onto the divertor plate. This noble gas cloud acts similarly to the well-known vapor shield for low-temperature materials (Be, Li). The high-temperature divertor materials as W has small rate of vapor formation above the divertor surface even for ELM heat load of hundreds MW/m$^{2}$, thus puffing radiative gas becomes as necessary mitigated method. This method of using massive noble gas injection was checked on DIII-D for disruption effects mitigation. Nitrogen seeding in the JET gas box divertor Mk-IIAP was used to reduction of the temperature in the front of the divertor target, an indication of ELM mitigation. The possibility of ELM-effect mitigation by a neon cloud puffed above the tungsten divertor surface was studied by the 2DIM MHD code, taking into account radiation transport in the whole diapason of photons, both as lines and continuum. Conditions of strong mitigations of the ITER-relevant ELMs was found. Corresponding neon radiation fluxes onto the opposite PFC are defined for different types of divertors. [Preview Abstract] |
Thursday, October 27, 2005 4:24PM - 4:36PM |
RO3.00013: Extraction of coherent bursts from turbulent edge plasma in Tokamak Tore-Supra using orthogonal wavelets Marie Farge, Kai Schneider, Pascal Devynck A wavelet-based method to extract coherent bursts out of turbulent signals is presented. The signal is projected onto an orthogonal wavelet basis, a threshold is applied to the wavelet coefficients, and the denoised signal is reconstructed in physical space. The threshold value is recursively determined and thus no adjustable parameters are required. The signal is split into two orthogonal components, a coherent and an incoherent one, whose properties can be studied independently. Statistical diagnostics based on the wavelet representation are introduced to compare the scaling behaviour and intermittency of the total signal and its coherent and incoherent components. The extraction method is applied to ion density of edge plasma measured in the scrape-off layer of the tokamak Tore Supra, Cadarache, France. We show that this procedure disentangles the coherent bursts, which contain most of the density variance, are intermittent and long-time correlated with non--Gaussian statistics, from an incoherent background noise, which is much weaker, non-intermittent and almost decorrelated with quasi-Gaussian statistics. [Preview Abstract] |
Thursday, October 27, 2005 4:36PM - 4:48PM |
RO3.00014: First results with thin shell and active mode control in the RFX-mod experiment Stefano Martini Plasma experiments resumed in December 2004 on RFX-mod. The machine now has a thin (3mm) Cu shell with one overlapped poloidal gap and one toroidal gap. Penetration time for Bv has been lowered from 450 to 50 ms and shell/plasma proximity from b/a = 1.24 to 1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide a more flexible control of the toroidal field. Newly designed graphite tiles protect the vessel from highly localized power deposition. The MHD Mode Control System, MHD-MCS, a set of 192 external saddle coils controlled by a digital feedback system, will act on field errors, radial fields due to dynamo modes and resistive wall modes (RWMs). First experiments at 300-600 kA without active mode control have been very encouraging, since performance similar to that of the thick shell machine has been readily achieved both in terms of pulse length and plasma confinement. The loop voltage is higher by 2-3 V, likely because of enhanced plasma-wall interaction due to the faster penetration of radial field components of locked MHD modes and to systematic field errors. Such radial fields, as well as errors due to RWMs, will be corrected by the MHD-MCS, which should further improve the performance, demonstrating the possibility to operate a large RFP without a thick conducting shell, and obtain access to enhanced RFP scenarios. Commissioning of the MHD-MCS is under way and first results will be presented at the Conference. [Preview Abstract] |
Thursday, October 27, 2005 4:48PM - 5:00PM |
RO3.00015: Turbulent energy transfer in electromagnetic turbulence: hints from a Reversed Field Pinch plasma N. Vianello, V. Antoni, E. Spada, M. Spolaore, R. Cavazzana, G. Serianni, H. Bergsaker, M. Cecconello, J. Drake The relationship between electromagnetic turbulence and sheared plasma flow in a Reversed Field Pinch is addressed. $\mathbf{E}\times\mathbf{B}$ sheared flows and turbulence at the edge tends to organize themeselves near marginal stability, suggesting an underlying energy exchange process between turbulence and mean flow. In MHD this process is well described through the quantity $P$ which represents the energy transfer (per mass and time unit) from turbulence to mean fields. In the edge region of RFP configuration, where magnetic field is mainly poloidal and the mean $\mathbf{E}\times\mathbf{B}$ is consequently toroidal, the quantity $P$ results: $$ P =\left [ -\frac{\langle\tilde{b}_r\tilde{b}_{\phi}\rangle}{\overline{\rho}\mu_0} + \langle \tilde{v}_r\tilde{v}_{\phi} \rangle\right ]\frac{\partial \overline{V}_{\phi}}{\partial r} $$ where $\overline{V}_{\phi}$ is the mean $\mathbf{E}\times\mathbf{B}$ toroidal flow, $\overline{\rho}$ the mean mass density and $\tilde{b}$ and $\tilde{v}$ the fluctuations of velocity and magnetic field respectively. Both the radial profiles and the temporal evolution of $P$ have been measured in the edge region of Extrap-T2R Reversed Field Pinch experiment. The results support the existence of oscillating energy exchange process between fluctuations and mean flow. [Preview Abstract] |
Thursday, October 27, 2005 5:00PM - 5:12PM |
RO3.00016: MHD Instabilities and Their Effects on Plasma Column Behavior in Tokamak. Pejman Khorshid, Long Wang, Mahmoud Ghoranneviss In the edge plasma of the CT-6B tokamak the poloidal rotation velocity based on MHD behavior has been studied. We found that mode number and rotation frequency of plasma column are different in angle position, so that the phase data detected from Mirnov coils array located in poloidal angle on the inner side of chamber is more than outer side which it can be because of toroidal magnetic field effects. It shows that plasma column behaves as non-Rigid body plasma. A relative correction coefficient has been suggested for optical diagnostic purposes. It is observed that when mode number achieves to m=2/n=1, plasma treats like rigid body and almost it leads to a major disruption. The results have been compared with measurements via Mach probe and Doppler shift of H-alpha line spectroscopy. In addition, during positive limiter bias application the poloidal magnetic field oscillations first decreased and then after a delay time it increased with a faster speed. With contribution of viscous force effects in the radial force balance equation for limiter biasing, in terms of the non-stationary model, it allows us to identify the understanding physics responsible for change in the Mirnov oscillations that could be related to poloidal rotation velocity and radial electric field. [Preview Abstract] |
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