Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session GO4: C MOD Tokamak |
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Chair: Steve Allen, LLNL/General Atomics Room: Regency VI |
Tuesday, November 3, 2009 9:30AM - 9:42AM |
GO4.00001: Overview of Recent Research on the Alcator C-Mod Tokamak Amanda Hubbard Recent experiments on Alcator C-Mod exploit high power density RF heating and current drive and metal walls, focusing on key issues for ITER. These include hydrogenic retention in metal walls, and impurity generation by RF sheaths. New edge diagnostics and coated tiles have been added to study sheath effects, as well as divertor and SOL heat flux widths. SOL turbulence has been compared to gyrofluid simulations. Improved lower hybrid and ion cyclotron models, both ray tracing and full wave, are benchmarked against experiments. Assessment of LHCD at high densities, and rotation generated by both mode converted ICRF and LHCD, are of particular interest. Transport studies include assessment of rotation in internal transport barriers, particle transport and peaking in low collisionality discharges, and comparison of core turbulence with GYRO. The ``improved L-Mode'' regime, featuring a T$_{e}$ pedestal without a particle barrier, has been extended to longer durations and reached H$_{ITER-98-Y2} \quad \sim $ 1.0. Disruption and gas jet mitigation experiments show runaways, seeded by LHCD, are suppressed by stochastic transport. [Preview Abstract] |
Tuesday, November 3, 2009 9:42AM - 9:54AM |
GO4.00002: Studies of enhanced energy confinement discharges with L-mode-like edge particle transport in Alcator C-Mod E. Marmar, B. Lipschultz, A. Dominguez, M. Greenwald, N. Howard, A. Hubbard, J. Hughes, B. LaBombard, R. McDermott, M. Reinke, D. Whyte, C. Kessel A regime of enhanced energy confinement on Alcator C-Mod exhibits an H-mode-like temperature pedestal, but with edge particle transport and density profiles like those in L-mode. Access to this ``improved L-mode" is achieved by operating just below the H-mode power threshold, with ion drift in the unfavorable direction, away from the x-point. Increases of power, up to a factor of three above that required for entry to H-mode with favorable drift, yield global confinement comparable to H-mode (H{\_}ITER-98$\sim $1.0), but with no density or impurity accumulation, and no need for ELMs to regulate the edge. Edge fluctuations seen with magnetics typically cover the range from 80 kHz to 150 kHz, somewhat broader than the quasi-coherent mode responsible for edge particle transport in EDA H-mode. The resulting high temperature, low density edge pedestal has low collisionality, and modeling indicates this could be an attractive target for Lower Hybrid Current Drive in advanced scenarios. [Preview Abstract] |
Tuesday, November 3, 2009 9:54AM - 10:06AM |
GO4.00003: Analysis of Rotation and Transport Data in C-Mod ITB Plasmas C.L. Fiore, J.E. Rice, M.L. Reinke, Y. Podpaly, I.O. Bespamyatnov, W.L. Rowan Internal transport barriers (ITBs) spontaneously form near the half radius of Alcator C-Mod plasmas when the EDA H-mode is sustained for several energy confinement times in either off-axis ICRF heated discharges or in purely ohmic heated plasmas. These plasmas exhibit strongly peaked density and pressure profiles, static or peaking temperature profiles, peaking impurity density profiles, and thermal transport coefficients that approach neoclassical values in the core. It has long been observed that the intrinsic central plasma rotation that is strongly co-current following the H-mode transition slows and often reverses as the density peaks as the ITB forms. Recent spatial measurements demonstrate that the rotation profile develops a well in the core region that decreases continuously as central density rises while the value outside of the core remains strongly co-current. This results in the formation of a steep potential gradient/strong electric field at the location of the foot of the ITB density profile. The resulting E X B shearing rate is also quite significant at the foot. These analyses and the implications for plasma transport and stability will be presented. [Preview Abstract] |
Tuesday, November 3, 2009 10:06AM - 10:18AM |
GO4.00004: Studies of Filament Formation during Lithium Pellet Injection in Alcator C-Mod B. Bose, E. Marmar, D.R. Mikkelsen Using a high speed CCD camera, (frame rate up to 500 kHz) and a stereoscopic imaging system, the detailed three dimensional evolution of filaments formed in the ablation clouds of injected lithium pellets has been studied on the Alcator C-Mod tokamak. After formation, the filaments move primarily in the poloidal direction during the first $10 \mu s$ after their formation. During the '07 and '08 campaigns the stereo-imaging system was employed to make a survey of the characteristics of filament trajectories in a variety of background plasmas and was then used to determine the filaments' statistical properties. The distributions of poloidal velocities of the filaments show no statistical variation with local density, temperature or magnetic shear, but do show significant variation with the local safety factor and plasma collisionality. The filament velocity distributions and the spatial scales of the filament trajectories are compared with simulated turbulent ExB flows from the nonlinear gyrokinetic solver GYRO. Simulations using plasma profiles just prior to the injection do not generate large enough fluctuations in ExB drift to be consistent with experimental data from the imaging diagnostic. Additional simulations using estimates of the strongly perturbed temperature and density profiles will \mbox{also be discussed}. [Preview Abstract] |
Tuesday, November 3, 2009 10:18AM - 10:30AM |
GO4.00005: Measurement of the adiabatic index through the temperature scaling of reversed shear Alfv{\'e}n eigenmodes E.M. Edlund, M. Porkolab, Y. Lin, N. Tsujii, S.J. Wukitch, L. Lin, G.J. Kramer Reversed shear Alfv{\'e}n eigenmodes (RSAEs) have been excited in Alcator C-Mod during the current ramp phase at ITER relevant densities of $\textrm{n}_{e0} \leq 1.5 \: \times \: 10^{20} \: \textrm{m}^{-3}$ with (2-5) MW of ICRH power absorbed by H minority heating $[1]$. We have studied the scaling of the minimum frequency of the RSAEs by varying the temperature of the electrons and majority ions and compare the results to theoretical scalings from the code \textsc{NOVA} $[2]$ and an analytic dispersion relation $[3]$. Taking the adiabatic index ($\gamma$) as a free parameter, a best fit to the data indicates $\gamma = 1.40 \pm 0.15$, excluding the ideal gas limit of $\gamma = 5/3$. A limiting value of $\gamma = 3/2$ is predicted from consideration of the energy and pressure of shear Alfven waves $[4]$. Kinetic electron response is considered as a possible correction to the theoretical treatment. Work supported by DOE under DE-FG02-94-ER54235 and DE-FC02-99- ER54512.\\[4pt] [1] M. Porkolab \textit{et al}., IEEE Trans. Plasma Sci. \textbf{34}, 229 (2006).\\[0pt] [2] C.Z. Cheng and M.S. Chance, J. Comput. Phys. \textbf{71}, 124 (1987).\\[0pt] [3] B.N. Breizman \textit{et al}., Phys. Plasmas \textbf{12}, 112506 (2005).\\[0pt] [4] C.F. McKee and E.G. Zweibel, Astro. J. \textbf{440}, 686 (1995). [Preview Abstract] |
Tuesday, November 3, 2009 10:30AM - 10:42AM |
GO4.00006: ICRF Mode Conversion Flow Drive on Alcator C-Mod Y. Lin, J.E. Rice, S.J. Wukitch, M.J. Greenwald, A.E. Hubbard, A. Ince-cushman, L. Lin, E.S. Marmar, M. Porkolab, M.L. Reinke, N. Tsujii, J.C. Wright Plasma flow driven by externally launched waves could be important for tokamaks. We have observed both toroidal (V$_{\phi })$ and poloidal (V$_{\theta })$ flows driven via an ICRF mode conversion (MC) process in D($^{3}$He) plasmas on Alcator C-Mod [1, 2]. Strong V$_{\phi }$in the co-current direction is observed by x-ray spectroscopy in L-mode plasmas heated with 50 MHz RF power at B$_{t0}\sim $ 5.1 T and n$_{3He}$/n$_{e}\sim $ 8{\%}. The central V$_{\phi }$scales with the applied RF power and is at least a factor of 2 more than the intrinsic plasma rotation. The MC ion cyclotron wave (MC ICW) is detected by a phase contrast imaging system. The MC physics have been studied by 2-D full wave TORIC simulation. We hypothesize that an MC scenario where MC slow waves are strongly damped on ions may be necessary for MC flow drive. Momentum transport modeling has been applied to infer the total flow drive force. Results from flow drive optimization experiments will be reported. The feasibility of creating such an MC scenario, thus potentially driving plasma flow, in larger tokamaks like JET and ITER will also be discussed. \\[4pt] [1] Y. Lin et al, Phys. Rev. Lett. 101, 235002 (2008). \\[0pt] [2] Y. Lin et al, Phys. Phys. 16, 056102 (2009). [Preview Abstract] |
Tuesday, November 3, 2009 10:42AM - 10:54AM |
GO4.00007: Evaluation of ICRF heated discharges with boron coated molybdenum tiles S.J. Wukitch, B. LaBombard, Y. Lin, B. Lipschultz, E. Marmar, R. Ochoukov, M.L. Reinke, D.G. Whyte One of the primary utilization challenges is to minimize impurity production associated with ICRF operation. In present experiments, boronization is frequently applied to control impurities and its lifetime has been observed to be proportional to number of RF Joules injected. We have also observed that RF sheaths localized to field lines connected to the active antenna were responsible for the boron erosion and subsequent impurity generation. In an effort to identify important erosion and impurity source locations, we have vacuum plasma sprayed $\sim $100 $\mu $m of boron on molybdenum tiles from the outer divertor shelf, main plasma limiters, and the RF antennas. We have also modified the shape of the main plasma limiter and increased our spectroscopic monitoring diagnostics of the main plasma limiter. We have installed a set of probes to monitor the plasma potential and RF fields on field lines connected an antenna. Results from experiments investigating plasma performance and ICRF generated impurities will be presented. [Preview Abstract] |
Tuesday, November 3, 2009 10:54AM - 11:06AM |
GO4.00008: Observations of Lower Hybrid Wave Interactions in the Scrape-Off-Layer of a Diverted Tokamak G.M. Wallace, R.R. Parker, P.T. Bonoli, O. Meneghini, A.E. Schmidt, S. Shiraiwa, D.G. Whyte, J.C. Wright, S.J. Wukitch, R.W. Harvey, A.P. Smirnov, J.R. Wilson Experiments with LHCD on Alcator C-Mod indicate that, at high density, significant power is deposited directly in the SOL, thereby reducing the effective current drive efficiency. This phenomenon is observed in plasmas which are accessible to LH waves and below the density limit attributed to parametric decay as $\omega \to 2 \omega_{LH}$. Current densities of $\sim 500$ kA/m$^2$ are measured just outside the separatrix during high power LH operation above $\bar{n}_e \sim 1 \times 10^{20}$ m$^{-3}$. Line integrated hard X-ray emission also drops sharply above $\bar{n}_e \sim 1 \times 10^{20}$ m$^{-3}$. Ray tracing/Fokker-Planck simulations over-estimate the line integrated X-ray emissivity by a factor of $\sim 500$ at $\bar{n}_e = 1.4 \times 10^{20}$ m$^{-3}$ as compared to a factor of 3-4 at $\bar{n}_e = 0.6 \times 10^{20}$ m$^{-3}$. Simulations including a realistic SOL with collisional damping show substantial improvement in agreement with experiment. [Preview Abstract] |
Tuesday, November 3, 2009 11:06AM - 11:18AM |
GO4.00009: Closing the Poloidal Flow Loop in the High-Field Side Scrape-off Layer of Alcator C-Mod Noah Smick, Brian LaBombard Observations of near-sonic parallel flows towards the high-field side (HFS) divertor have suggested a ballooning-like transport drive on the low-field side. However, the mechanisms that close the implied particle flow loop have not been identified experimentally. To investigate this phenomenon, a magnetically-actuated scanning probe was operated on the HFS midplane during C-Mod's '07-'09 campaigns. This probe is capable of measuring the total flow vector in the SOL, including radial fluctuation-induced fluxes. ExB drift measurements confirm that the net poloidal flow is indeed directed towards the HFS divertor. Two mechanisms for closing the particle loop have been identified as important contributors. Measurements of the fluctuation-induced particle fluxes on the HFS show an inward pinch velocity of $\sim $10 m/s at or just inside the separatrix, sufficient to balance the midplane poloidal particle flux. Neutral penetration from inner divertor recycling has been quantified using a 1-D kinetic code, indicating that it can also account for a significant fraction of the total poloidal flux. [Preview Abstract] |
Tuesday, November 3, 2009 11:18AM - 11:30AM |
GO4.00010: Poloidal and Radial Characteristics of Edge Turbulence at the High-field-side Midplane of Alcator C-Mod J.L. Terry, I. Cziegler, B. LaBombard, S.J. Zweben Previous measurements have shown that edge turbulence on the high-field, ``good'' curvature side is much reduced (x 0.1-0.3) compared to that on the low-field, 'bad' curvature side. This has been shown by both Gas-Puff-Imaging (GPI) and scanning probe measurements. A recent upgrade to the high-field-side GPI view on C-Mod allows 2D (radial and poloidal) resolution of emission from the midplane region there. The 2D array of view spans a radial region $\sim$2.3 cm around the separatrix and a poloidal region from Z=-0.5 to 1.7 cm with $\sim$0.38 cm resolution. This allows investigation of the 2D characteristics of edge phenomena like blobs/filaments, the Quasi-Coherent Mode, ELMs, and broadband turbulence. Of particular interest are the existence and structure of both filaments and the QCM on the inboard midplane, and the dynamics and structure of both the ELM perturbation and the broadband turbulence there. Results of these investigations will be presented and compared to observations made on the low-field-side. [Preview Abstract] |
Tuesday, November 3, 2009 11:30AM - 11:42AM |
GO4.00011: Relationship of SOL turbulence to SOL width in Alcator C-Mod Stewart Zweben, James Terry, Brian LaBombard, Daren Stotler, D.P. Lundberg, R.H. Cohen, M. Umansky, D.A. Baver, D.A. D'Ippolito, J.R. Myra, D.A. Russell Measurements of SOL turbulence in Alcator C-Mod show a strong variation with poloidal angle [1], therefore it is likely this variation will affect the SOL width. The poloidal variation of the SOL turbulence will be measured as a function of the SOL collisionality, plasma shape, and normalized gyroradius, and the results will be compared with theoretical models for these variations. The radial transport due to the SOL turbulence will be estimated from the radial propagation and correlation length at each poloidal angle, and the results will be compared with the local temperature/density SOL widths measured by Langmuir probes and D$_{\alpha}$ emission. The expected relationship between these local SOL widths and the total heat flux SOL width at the divertor plate will be discussed.\\[4pt] [1] J.L. Terry, S.J. Zweben et al, J. Nucl. Mat. 390-391, 339 (2009) [Preview Abstract] |
Tuesday, November 3, 2009 11:42AM - 11:54AM |
GO4.00012: Initial results from the swept frequency O-mode correlation reflectometer system in Alcator C-Mod A. Dominguez, E. Marmar, G.J. Kramer, E. Valeo Turbulent eddies are believed to be largely responsible for increased cross-field particle and energy transport in confined plasma devices. One of the tools that has been used to characterize this turbulence is correlation reflectometry whereby the spatial extent of the eddies in the direction of propagation of the reflectometry waves can be determined. A swept frequency O-mode reflectometer has been installed on the Alcator C-Mod tokamak. The system has a midplane view of the plasma from the low field side and covers the frequency range of 110GHz-140GHz, corresponding to electron density cutoffs of 1.50-2.43 x$10^{20}m^{-3}$. In conjunction with two fixed frequency O-mode reflectometry channels at 110GHz and 140GHz, it is possible to determine radial correlation lengths, which can be mapped to the turbulent radial correlation lengths with the use of a 2-D synthetic diagnostic code[1]. Initial results from the analysis of results from various plasma conditions using this system will be presented.\\[4pt] [1] E.J. Valeo, G.J. Kramer, R. Nazikian, Plasma Phys. Control. Fus. 44 (2002) L1 [Preview Abstract] |
Tuesday, November 3, 2009 11:54AM - 12:06PM |
GO4.00013: Intrinsic Toroidal Rotation Reversal Comparison between TCV and CMOD B.P. Duval, A. Bortolon, J. Rice, M. Reinke, R. McDermott Toroidal reversal was observed on the TCV tokamak using a CXRS system with a diagnostic beam that did not perturb the plasma motion. Support experiments were performed on CMOD where the core rotation is measured with an X-ray imaging spectrometer and edge CHEERS so the measured rotation may again be considered ``intrinsic''. Rotation reversal was observed on both machines when q$\sim $3 across a density ramp. The plasma shape used on CMOD was repeated on TCV and the toroidal rotation for this marginally diverted discharge shows behavior similar to limited discharges on TCV. This paper will report the diagnostic measurements including preliminary PCI turbulence data that does not indicate a strong role of turbulence for rotation reversal as suggested by several theoretical approaches. In both machines there is evidence that the change in torque can not be entirely attributed to changes in the plasma edge necessitating a model that applied torque directly to the plasma bulk. [Preview Abstract] |
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