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 JO4: C-Mod Tokamak |
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Chair: David Anderson, University of Wisconsin Room: Ballroom E |
Tuesday, November 15, 2011 2:00PM - 2:12PM |
JO4.00001: Highlights of the Alcator C-Mod Research Campaign Martin Greenwald Alcator C-Mod has completed an experimental campaign focusing on broad scientific issues with particular emphasis on ITER needs and requests. Experiments with no NBI torque have investigated spontaneous flow reversal, creation of transport barriers aided by the shear of intrinsic rotation and a variety of RF flow drive schemes. Studies of I-mode have found conditions where a wide operating regime opens up, allowing easy access to long-lived, high-performance discharges with L-mode like particle confinement. We are validating the EPED and BOUT++ models for pedestal height/width and ELM onset using extended parameter scans in ELMy H-mode. The challenge of high-Z impurity generation with ICRF is being addressed first by deployment of a novel antenna whose current straps and antenna box are perpendicular to the total magnetic field -second by studies of the modification of edge impurity transport, where fine-scale E$_{r}$ structures in the SOL in the presence of ICRF heating have been found. LH current drive has produced non-inductive reversed shear regimes at n $\sim $ 5x10$^{19}$ which exhibit electron temperature ITBs. The first observations have been made of in-tokamak production of divertor tungsten nano-structures (fuzz), which had previously been seen only in linear laboratory experiments. [Preview Abstract] |
Tuesday, November 15, 2011 2:12PM - 2:24PM |
JO4.00002: Progress in characterizing pedestal stability on Alcator C-Mod J.W. Hughes, R.M. Churchill, I. Cziegler, E.M. Davis, A. Dominguez, P. Ennever, D. Ernst, A.E. Hubbard, B. Lipschultz, Y. Ma, J.R. Walk, S.M. Wolfe, P.B. Snyder, T. Osborne, X. Xu, L. Sugiyama Experimental studies on Alcator C-Mod explore pedestal structure and edge relaxation mechanisms primarily in three high confinement regimes: ELMy H-mode, EDA H-mode and I-mode. Extensive scans of B$_{T}$, I$_{P}$, n$_{e}$, P$_{net}$ and shaping parameters have been carried out in these regimes, allowing the characterization of the operating space for ELMs as well as benign pedestal relaxation mechanisms (e.g. quasi- and weakly coherent modes). Well resolved edge profiles and accurate equilibrium reconstructions are produced from typical discharges, enabling evaluation of edge stability using various computational tools, such as ELITE, BOUT++, M3D and GS2. Relationships among dominant edge instabilities, radial transport and pedestal structure will be discussed. [Preview Abstract] |
Tuesday, November 15, 2011 2:24PM - 2:36PM |
JO4.00003: Modification of H-mode threshold power via the variation of outer strike point location Y. Ma, J.W. Hughes, R.M. Churchill, B. Lipschultz, B. LaBombard, C. Lau, E.S. Marmar In a recent Alcator C-Mod experiment performed in $B_{T}$=5.4T,
I$_{p}$=0.9MA, $ |
Tuesday, November 15, 2011 2:36PM - 2:48PM |
JO4.00004: Comparison of edge fluctuation measurements and energy and particle transport for I-mode plasmas on Alcator C-Mod A. Dominguez, E.S. Marmar, J.L. Terry, J.W. Hughes, I. Cziegler, P. Ennever, T. Golfinopoulos , A.E. Hubbard, B. LaBombard, B. Lipschultz, M. Porkolab, A.E. White, D.G. Whyte The I-mode regime features high energy confinement and low particle confinement, making it of interest both as a potential reactor regime as well as a physics case study for particle and energy channel separation. It has been observed that the I-mode is accompanied by two significant changes in edge fluctuations: The presence of a weakly coherent mode (WCM) at $\sim$250kHz and the suppression of broadband fluctuations at $\sim$50-150kHz during L-mode to I-mode transitions. Recently, experiments have been conducted which make use of D$\alpha$ emission measurements close to the last closed flux surface (LCFS) in order to estimate the ionization source profile and, consequently, the particle transport across the LCFS. In addition, estimates of the energy transport across the LCFS are acquired using the TRANSP code. Results and analysis will be presented studying the features of the edge fluctuations and their connection to the energy and particle transport across the LCFS. [Preview Abstract] |
Tuesday, November 15, 2011 2:48PM - 3:00PM |
JO4.00005: Gyrokinetic analysis of the linear ohmic confinement regime in Alcator C-Mod M. Porkolab, J. Dorris, P. Ennever, C. Fiore, A. Hubbard, M. Greenwald, Y. Ma, Y. Podpaly, M.L. Reinke, J. Rice, J.C. Rost, N. Tsujii, J. Candy, G.M. Staebler, R. Waltz In earlier studies of ohmically heated low density C-Mod plasmas, TRANSP results indicated ion heat diffusivities significantly smaller than those predicted by GYRO [1]. In the present work we present new experimental data where the ion temperature profiles have been measured by x-ray crystal spectroscopy, and the radial electric field is deduced from toroidal flow measurements. TGLF, the trapped gyro-Landau fluid model code was used to model the turbulent transport as a function of density, Z$_{i}$ and Z$_{eff}$, and the results indicate that the second (impurity) ion species with moderate Z$_{i}$ ($\le $ 8) reproduce the measured ion and electron thermal diffusivities in the range 0.5 $<$r/a$<$ 0.8. The nonlinear flux tube code GYRO predicts similar results [2]. The global GYRO based synthetic PCI diagnostic predicts Doppler shifted frequencies in agreement with observations. \\[4pt] [1] L. Lin, et al, Plasma Phys. Contr. Fusion 51, 065006 (2009).\\[0pt] [2] M. Porkolab, et al, EPS 2011 , Strasbourg, France, P5.109. [Preview Abstract] |
Tuesday, November 15, 2011 3:00PM - 3:12PM |
JO4.00006: Poloidal variation of high-Z impurity density driven by hydrogen minority heating on Alcator C-Mod Matthew Reinke, Ian Hutchinson, John Rice, Nathan Howard In the Alcator C-Mod tokamak, strong, steady-state variations of molybdenum density within a flux surface are routinely observed in plasmas using ICRH. In/out asymmetries up to a factor of 2 are observed with inboard and outboard accumulation occurring at different minor radius locations. Scanning the D(H) resonance layer off-axis is shown to alter the magnitude and direction of the asymmetry. These poloidal variations are important in determining the radial impurity transport and can be attributed to the impurity's high charge and large mass in parallel force balance. The large mass enhances the centrifugal force, causing outboard accumulation while the high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the plasma potential. Ion-impurity friction is shown to not play a role and it is demonstrated that a poloidal potential variation due to trapping of cyclotron heated minority ions is the likely cause of inboard accumulation. Quantitative comparisons between existing parallel impurity transport theories and experimental results are in good agreement when ICRH effects can be ignored and agree to within 50{\%} when attempting to account for the effects of cyclotron heating. [Preview Abstract] |
Tuesday, November 15, 2011 3:12PM - 3:24PM |
JO4.00007: Reduced-model (SOLT) simulations of an EDA H-mode shot at Alcator C-Mod D.A. Russell, D.A. D'Ippolito, J.R. Myra, B. LaBombard, J.L. Terry, S.J. Zweben Reduced-model scrape-off layer turbulence (SOLT) simulations of an Enhanced D-Alpha (EDA) H-mode observed at C-Mod were conducted to explore observed variations in scrape-off-layer (SOL) width. The amplitude of a mean poloidal flow was varied to control the level of turbulence in the simulation and to reproduce the observed heat flux across the separatrix. SOL width decreased with increasing input power and with increasing separatrix temperature in both experiment and simulation, consistent with the strong temperature dependence of \textit{collision-limited} parallel heat flux. A persistent quasi-coherent mode (QCM) dominates the SOLT turbulence. The wavelength of the SOLT QCM is comparable to that of the QCM consistently observed on C-Mod during EDA operation. The SOLT QCM consists of a quasi-stationary string of vortices, located just inside the separatrix, poloidally convected by the mean flow and occasionally emitting blobs into the SOL. The mode frequency is dominated by the Doppler shift of this convected pattern. Analysis reveals underlying drift-interchange and Kelvin-Helmholtz instabilities. [Preview Abstract] |
Tuesday, November 15, 2011 3:24PM - 3:36PM |
JO4.00008: Theoretical investigation of the quasi-coherent mode J.R. Myra, D.A. Baver, D.A. D'Ippolito, D.A. Russell, M.V. Umansky, J.W. Hughes, B. LaBombard It is of interest to identify candidates and physical mechanisms for the quasi-coherent (QC) mode, which is thought to be an essential ingredient for obtaining EDA operation on Alcator C-Mod. We employ the 2DX edge eigenvalue code for this purpose because it allows convenient comparison of a wide variety of physics models, and incorporates realistic divertor geometry and global profiles in the edge and SOL region. Neither drift-resistive modes (driven by plasma gradients and curvature), nor perpendicular and parallel Kelvin Helmholtz (KH) modes (driven by radial shear in velocity) provide linear candidates with growth rates that show a strong peak in the experimentally observed range of wave-numbers. Similar remarks also apply to a nonlinear SOLT simulation model [D. A. Russell et al, this meeting] which, nevertheless, produces a QC-like oscillation. A qualitative model presented here suggests that nonlinear mode-coupling together with certain spectral features are critical ingredients for the QC mode. [Preview Abstract] |
Tuesday, November 15, 2011 3:36PM - 3:48PM |
JO4.00009: Parallel correlation of turbulent fluctuations in the SOL of Alcator C-Mod O. Grulke, J.L. Terry, B. LaBombard, I. Cziegler, S.J. Zweben Turbulent fluctuations in the scrape-off layer (SOL) of fusion devices generally show an intermittent character with large-amplitude fluctuations in plasma density and potential. Gas-puff imaging (GPI) allows observation of the spatiotemporal dynamics of SOL fluctuations and reveals that those events are associated with the formation of spatiotemporal fluctuation structures. The structures propagate radially outwards due to their self-consistent dipolar potential, thereby contributing significantly to the cross-field transport. They are predominantly located on the outboard side and are essentially absent in the inboard SOL. The present study investigates their parallel dynamics by correlating outboard midplane fluctuations with those in the region of high magnetic shear close to the lower X-point. After producing a magnetic geometry that magnetically connects a reciprocating probe to the 2D view of the GPI at the midplane (with a connection length $\sim$3m), maximum correlation amplitudes of $\sim$95\% are observed between the emission fluctuations and the density and potential fluctuations measured at the probe. The time delay of maximum correlation is on average smaller than 1$\mu$s, which is much shorter than expected for parallel density diffusion. [Preview Abstract] |
Tuesday, November 15, 2011 3:48PM - 4:00PM |
JO4.00010: Search for Edge Zonal Flows in Alcator C-Mod Stewart Zweben, James Terry, Matteo Agostini, Ted Golfinopoulos, Olaf Grulke, Robert Hager, Jerry Hughes, David Pace Time-resolved measurements of the flow speed of edge turbulence were made using fast camera data from the gas puff imaging (GPI) diagnostic located near the outer midplane separatrix of C-Mod. The zonal flow was estimated using a cross-correlation technique to be the radially-resolved poloidal velocity of the turbulence averaged over the poloidal GPI field of view. The frequency spectrum of this poloidal flow is normally broadband and intermittent over $\sim $2-20 kHz, with a radial correlation length which decreases with the line-averaged density. In some cases with ICRH heating there was a near-coherent poloidal velocity oscillation at 6-7 kHz, which was highly correlated with a poloidal magnetic field oscillation at the same frequency. The relationship of this coherent zonal flow component with the GAM frequency and the L-H transition will be described. [Preview Abstract] |
Tuesday, November 15, 2011 4:00PM - 4:12PM |
JO4.00011: LHCD on Alcator C-Mod Orso Meneghini, Syunichi Shiraiwa, Ian Faust, Ronald Parker, Gregory Wallace Non-inductive discharges and reversed shear profiles with eITB were achieved on C-Mod using LHCD at moderate density ($\bar{n}_e<10^{20}$ m$^{-3}$). However an anomalous loss of CD efficiency is observed at higher density, limiting the development of AT regimes. Fullwave simulations using the LHEAF code reproduce the experimental drop in hard X-Ray emissivity, revealing a different physical picture than what was developed previously using raytracing codes, which relied on a strongly collisional SOL to reproduce the experiments. These results may be explained by the combined effect of an $n_\parallel$ upshift (localizing the damping in the plasma periphery) and radial diffusion of fast electrons. The implications are profound and suggest that the key to improving the LHCD performance at high density is to operate at high Te and to push the power deposition profile inward to avoid prompt loss of fast electrons. Increased hard X-Ray emissivity was indeed confirmed for high Te=4.5keV, Bt=8T, Ip=1.2MA plasmas. AT scenario development has motivated the design of an off-midplane antenna which exploits velocity space synergy with the existing launcher to improve performance at higher density. [Preview Abstract] |
Tuesday, November 15, 2011 4:12PM - 4:24PM |
JO4.00012: Co- and Counter-current Rotation Induced by Lower Hybrid Current Drive in Tokamak Plasmas Yuri Podpaly, John Rice, Ron Parker, Matthew Reinke, Luis Delgado-Aparicio, Darin Ernst, Syun'ichi Shiraiwa, Orso Meneghini, Greg Wallace, John Walk, Chi Gao Lower Hybrid Current Drive (LHCD) induced rotation has been observed in Alcator C-Mod plasmas as well as in other devices. Recent experiments at Alcator C-Mod have for the first time identified the plasma conditions that determine the LHCD driven rotation direction, co- or counter-current, of the main ion species. This effect is found to depend strongly on the plasma current: low current plasmas have co-current rotation and higher current plasmas exhibit counter-current rotation. Experiments were performed to explore this dependence and changes in rotation were observed to approach 40 km/s at $<\!n_{e}\!>$=0.66e20 m$^{-3}$; the LHCD rotation reversal point, $\Delta$v=0, was also identified. There appears to be a magnetic field configuration effect with the favorable (unfavorable) $\nabla$B configuration having a rotation reversal point around $\sim$400 kA ($\sim$550 kA). In both co- and counter- current cases, rotation profiles show that the momentum originates near the core of the plasma. Analyses of plasma behavior and gyrokinetic simulations were performed and results are shown. [Preview Abstract] |
Tuesday, November 15, 2011 4:24PM - 4:36PM |
JO4.00013: The Fine-scale Structure of the Radial Electric Field in the Scrape-Off-Layer during ICRF Heating in Alcator C-Mod J.L. Terry, I. Cziegler, S.J. Wukitch, Y. Lin, S.J. Zweben By observing the radial structure in the poloidal dynamics of the SOL turbulence, we find a fine-scale radial structure in the broadband turbulence poloidal phase velocities (V$_{\theta })$ with the application of ICRF power (P$_{RF}>$0.6 MW). The radial profiles are very different from typical profiles in Ohmic plasmas. Since V$_{\theta }$(r) in the SOL is dominated by V$_{ExB}$, this structure implies a fine-scale E$_{r}$ profile is formed in the presence of the ICRF. The existence of such spatial structure is not predicted to be a result of the ICRF since the radial size-scale is much smaller than the fast-wave perpendicular wavelength ($\sim $10 cm). Such an E$_{r}$ profile may help to explain the increased impurity content observed with ICRF heating and will be important for SOL flows and transport. Using 2D Gas-Puff-Imaging we find that, in the $\sim $3 cm region outside the separatrix, the dominant propagation direction for V$_{\theta }$ reverses up to 3 times. This is found beyond the region where the antenna near-fields are present; the fast wave is propagating there. While the mechanism is still unknown, it must be consistent with the following: the structure is dependent on which antenna is energized; it changes with q$_{95}$ and is present when the RF frequency is 78 or 50 MHz. [Preview Abstract] |
Tuesday, November 15, 2011 4:36PM - 4:48PM |
JO4.00014: Comparison of ICRF Full-wave Simulation with Phase Contrast Imaging in Alcator C-Mod N. Tsujii, M. Porkolab, P.T. Bonoli, Y. Lin, J.C. Wright, S.J. Wukitch, E.F. Jaeger, R.W. Harvey Waves in the ion cyclotron range of frequencies (ICRF) are used for heating plasmas and current and flow drive. Full-wave simulations are essential to describe the wave propagation and absorption processes accurately in a tokamak geometry and validation of these codes is required. Phase Contrast Imaging (PCI) has been used in Alcator C-Mod to directly measure the mode converted waves [1]. PCI measures line-integrated electron density fluctuations, including those due to RF waves. The measurements were compared to full-wave simulations TORIC and AORSA/CQL3D through synthetic diagnostic [2]. While in the strong single pass absorption regime the agreement between measurements and code predictions is reasonably good, in the weak single pass absorption regime the measured amplitudes are significantly smaller than code predictions.\\[4pt] [1] E.~Nelson-Melby, et~al, Phys. Rev. Lett. 90, 155004 (2003).\\[0pt] [2] N.~Tsujii, et~al, 19th Topical Conf. on RF Power in Plasmas, Newport, RI, 2011. [Preview Abstract] |
Tuesday, November 15, 2011 4:48PM - 5:00PM |
JO4.00015: Fast-ions on Alcator C-Mod: Comparisons between Simulation and Experiment for Equilibrium and Evolving Distributions A. Bader, P.T. Bonoli, R.S. Granetz, R.R. Parker, S.J. Wukitch, R.W. Harvey, E.F. Jaeger Analysis of fast-ion distributions $>$ 100 $T_{e}$ is important for both ITER and a fusion reactor, as both will have a significant population of fusion-born alpha particles. Therefore, it is imperative to benchmark current simulation codes with measurements of highly energetic fast-ions on current devices. Experimental measurements on ICRF heated H-minority plasmas on Alcator C-Mod are performed with an upgraded Compact Neutral Particle Analyzer (CNPA). The CNPA consists of vertically-viewing photodiode detectors with viewing chords from $r/a=\sim $0.18 to 0.6 and energies from 200 keV to 2 MeV. The CNPA measurements are compared to the output of a synthetic diagnostic that has been added to the Fokker-Planck solver CQL3D. CQL3D is coupled with the full-wave solver AORSA to evolve a self-consistent fast-ion distribution. Good agreement is obtained between simulation and experiment for fast ion distributions that are in equilibrium. However, there are discrepancies between the experimental and simulated results for the time-dependent evolution of the fast-ion distribution indicating a violation of quasi-linear theory. [Preview Abstract] |
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