Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session BO7: C-Mod Tokamak |
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Chair: David Pace, General Atomics Room: 556AB |
Monday, October 29, 2012 9:30AM - 9:42AM |
BO7.00001: Highlights of Recent Alcator C-Mod Research Earl Marmar Alcator C-Mod focuses on broad scientific issues with particular emphasis on ITER needs and requests. First results from a high power, magnetic field-aligned ICRF antenna show significant reduction of high Z impurity contamination under various plasma conditions Detailed comparisons of core and edge fluctuations in L-mode and I-mode reveal that core turbulence is reduced in I-mode before the reduction of edge turbulence and the onset of the WCM, in stark contrast to the usual turbulence changes observed in H-mode on other tokamaks. The connection among rotation reversals, non-local heat transport, energy confinement saturation (the transition between the linear Ohmic confinement, and saturated Ohmic confinement, regimes) and changes in underlying turbulence has been demonstrated. Joint pedestal studies among C-Mod, DIII-D and NSTX reveal common features, including an upper limit on pedestal pressure in ELMy H-mode determined by peeling-ballooning modes (PBMs), and pedestal width scaling approximately as $\beta _{pol}^{1/2}$. A novel stochastic model for intermittent SOL plasma fluctuations has been constructed, is in excellent agreement with experiment, and reveals important details on the underlying physics. Design of the first actively heated tokamak tungsten divertor is described. [Preview Abstract] |
Monday, October 29, 2012 9:42AM - 9:54AM |
BO7.00002: Motivation and goals of the new heated outer divertor for Alcator C-Mod B. Lipschultz, J. Doody, R. Ellis, R. Granetz, S. Harrison, B. LaBombard, R. Vieira, H. Zhang, L. Zhou A precision-aligned, high-temperature outer divertor is being developed for Alcator C-Mod to enhance heatflux handling and to advance our knowledge and experience with high-Z Plasma Facing Components (PFCs) in a reactor-level power density environment. Several departures from the design of the current divertor will be implemented: Instead of 10 toroidal divertor segments that expand toroidally as they heat up, the divertor plate will be toroidally continuous, with no openings or leading edges in the high-heat flux region. It will expand in the radial direction when heated while maintaining good alignment with shallow field line angles ($\sim $ 2 degrees), a requirement for future divertors. Those characteristics will reduce both impurity sources and disruption forces. A second design goal is to be able to control the divertor temperature up to 600$^{\circ}$C by installing heaters in the structure. Given the Arrhenius relation between hydrogen diffusivity and temperature in tungsten (and molybdenum) this will open up a new area of study for tokamaks - exploration of the effect of PFC temperature on fuel retention. Temperature control may also open up a new area of study into the effect of changes in divertor recycling on fueling and core confinement. [Preview Abstract] |
Monday, October 29, 2012 9:54AM - 10:06AM |
BO7.00003: Multi-Channel Validation of the Gyrokinetic Transport Model in ITG and TEM Dominant Plasmas N.T. Howard, A.E. White, M. Greenwald, M.L. Reinke, D.R. Mikkelsen, M. Chilenski, A. Hubbard, J. Hughes, J.E. Rice, J. Candy The need to accurately determine the performance of future fusion devices motivates the development of a predictive model of plasma transport and confinement. On Alcator C-Mod, significant progress has been made in the validation of the gyrokinetic model, the current leading candidate for a predictive transport model. Recent experiments represent an ideal testing ground for the gyrokinetic model in the core of plasmas dominated by both ion (ITG) and electron (TEM) turbulence. In this presentation, results from global, nonlinear simulation using the gyrokinetic code, GYRO, are compared directly with unique measurements of particle impurity transport (provided by laser blow-off and spatially resolved x-ray emission profiles) and power balance levels of heat transport in the plasma core (0.3 $<$ r/a $<$ 0.7). To strengthen the quantitative comparison between model and experiment in the plasma core, this multi-channel validation effort consists of a rigorous investigation of experimental uncertainties and their propagation to derived and simulated transport levels. Results from both ion (k$_{\theta }\rho _{s} <$ 1.0) and electron scale (k$_{\theta }\rho _{s} >$1.0) simulation of plasma turbulence will be shown as part of an ongoing validation effort. [Preview Abstract] |
Monday, October 29, 2012 10:06AM - 10:18AM |
BO7.00004: First measurements of core electron temperature fluctuations in Alcator C-Mod via Correlation ECE C. Sung, A. White, N. Howard, C. Oi, J. Rice, C. Gao, P. Ennever, M. Porkolab, A. Hubbard, M. Greenwald A new Correlation Electron Cyclotron Emission (CECE) diagnostic has been installed at Alcator C-Mod to measure broadband, turbulent electron temperature fluctuations in the core region ($\rho <0.9$). This diagnostic has high poloidal resolution ($\omega_{0} < 0.5cm $), and can measure long wavelength turbulence ($k_{\theta}\rho_s < 0.4$). We obtained the first electron temperature fluctuation data in Ohmic and ICRH plasmas. It was observed that the core ($\rho \sim 0.8$) electron temperature fluctuation level decreases as the Ohmic confinement regime transitions from Linear Ohmic Confinement (LOC) to Saturated Ohmic Confinement (SOC), while edge fluctuation levels ($\rho > 0.9$) stay similar in amplitude. These first quantitative, local electron temperature fluctuation measurements in C-Mod demonstrate that core turbulence characteristics change between LOC and SOC. In order to interpret the data, linear stability analysis is being performed, and the relation between fluctuation level and other plasma parameters is being investigated. This talk will describe the new CECE diagnostic, the SOC/LOC measurements, and associated gyrokinetic analyses. [Preview Abstract] |
Monday, October 29, 2012 10:18AM - 10:30AM |
BO7.00005: Non-local Heat Transport, Rotation Reversals and Up/Down Impurity Density Asymmetries in Alcator C-Mod Ohmic L-mode Plasmas John Rice, Chi Gao, Matt Reinke, Patrick Diamond, Nate Howard, Hongjuan Sun, Istvan Cziegler, Amanda Hubbard, Yuri Podpaly, William Rowan, Jim Terry, Mark Chilenski, Luis Delgado-Aparicio, Paul Ennever, Darin Ernst, Martin Greenwald, Jerry Hughes, Yunxing Ma, Earl Marmar, Miklos Porkolab, Steve Wolfe Several seemingly unrelated phenomena in Alcator C-Mod Ohmic L-mode plasmas are shown to be closely connected: non-local heat transport, core toroidal rotation reversals, energy confinement saturation, up/down impurity density asymmetries and turbulence changes. These phenomena all abruptly transform at a critical value of the collisionality. [Preview Abstract] |
Monday, October 29, 2012 10:30AM - 10:42AM |
BO7.00006: The Effect of Main-Ion Dilution on Turbulence and Transport in Alcator C-Mod Paul Ennever, Miklos Porkolab, Matthew Reinke, John Rice, Chris Rost, Naoto Tsujii, Evan Davis, Darin Ernst, Catherine Fiore, Martin Greenwald, Amanda Hubbard, Jerry Hughes, Earl Marmar, Jeff Candy, Gary Staebler, Ron Waltz, Chris Holland Previous studies of turbulence using the reduced gyro-landau fluid code TGLF, and the gyrokinetic code GYRO, have predicted that in C-Mod ohmic plasmas a dilution of the main ions by a significant amount causes a reduction of turbulent transport in the ion channel [1]. This could be a factor in the LOC-SOC transition. To test this effect, experiments were performed where nitrogen was puffed into ohmic target plasmas with the density kept constant. This seeding reduced the turbulence in the ion diamagnetic direction as measured by phase contrast imaging. To determine impurity concentrations in the plasma, line brightnesses for the relevant impurity species (N, O, Ar, and Mo) were compared to Zeff from neoclassical conductivity and from continuum measurements. The turbulence and transport simulated with GYRO and TGLF were compared to that measured experimentally. Work supported by US DOE awards DE-FG02-94-ER54235 and DE-FC02-99-ER54512.\\[4pt] [1] M. Porkolab, et al, Bull. Am Phys. Soc. 56, no 12 139 (2011). [Preview Abstract] |
Monday, October 29, 2012 10:42AM - 10:54AM |
BO7.00007: Formation and stability of impurity ``snakes'' in tokamak plasmas Luis F. Delgado-Aparicio, L. Sugiyama, R. Granetz, J. Rice, Y. Podpaly, M.L. Reinke, D.A. Gates, E. Fredrickson, C. Gao, M. Greenwald, K. Hill, A. Hubbard, E. Marmar, N. Pablant, S. Scott, R. Wilson, R. Wilson, S. Wolfe, S. Wukitch Although (1,1) ``snake'' modes were discovered at JET more than 25 years ago, there are still basic unanswered questions regarding their formation, stability, and superb particle confinement, shown by surviving tens to hundreds of sawtooth cycles. High-resolution spectroscopic imaging diagnostics have facilitated the determination of the perturbed radiated power density and temperature oscillations inside the mode with unprecedented temporal and spatial resolution, making it possible to infer, for the first time, the perturbed profiles of the impurity density, plasma pressure, Zeff, and resistivity at the center of these helical modes. These new results indicate that snakes form as an asymmetry in the impurity ion density that undergoes a seamless transition from a small helically displaced kink density to a large crescent-shaped helical island structure inside $q<1$, with a regularly sawtoothing core. These new high-resolution observations show details of their evolutio [Preview Abstract] |
Monday, October 29, 2012 10:54AM - 11:06AM |
BO7.00008: Poloidal Variation of Impurity Density and Flows in the Pedestal Region R.M. Churchill, B. Lipscultz, C. Theiler, J.W. Hughes The pedestal region plays an important role in determining the overall performance of tokamak plasmas. Despite its importance, much of the physics determining the transport in, and structure of, the pedestal is still being developed. Experimental characterization of the pedestal region can help guide theory development and provide benchmark tests for codes. A unique set of CXRS views at both the low-field side (LFS) and high-field side (HFS) midplane of Alcator C-Mod provides measurements of the boron (Z=5) impurity density, temperature, and both poloidal and toroidal bulk velocity. These views allow studying how impurity density varies on a flux surface in the presence of strong gradients, and also how flows change. Experimental evidence of an in-out impurity density asymmetry will be discussed, for a range of plasmas with varying collisionality and Lni/LTi ratio. Total flow vectors will also be presented and the variation between LFS and HFS flows will be compared to that predicted by neoclassical theory. Supported by USDoE award DE-FC02-99ER54512. [Preview Abstract] |
Monday, October 29, 2012 11:06AM - 11:18AM |
BO7.00009: Initial Characterization of Magnetic Field-Aligned ICRF Antenna M.L. Garrett, S.J. Wukitch, A.N. James, P. Koert, C. Lau, Y. Lin, B. Lipschultz, D.R. Miller, R. Ochoukov, M.L. Reinke, J.L. Terry A principle challenge of ICRF heating in tokamaks with high-Z walls is the minimization of impurity contamination associated with ICRF operation. This impurity contamination is attributed to both source and transport mechanisms. The source mechanism introduces impurities by sputtering of PFCs by energetic ions. The transport mechanism alters core impurity concentration through spatial variation of plasma potentials in front of the antenna, establishing local E $\times $ B drifts that affect edge transport via the formation of convective cells. In each case, RF sheath formation driven by ICRF E$_{\vert \vert }$, is implicated as the root cause. A new magnetic field-aligned antenna was optimized to reduce RF sheath formation. Using FEM with a 3-D toroidal cold plasma model, E$_{\vert \vert }$ dependence on toroidal phasing and magnetic pitch angle were investigated. In each case, the field-aligned antenna had reduced integrated E-parallel relative to the existing non-aligned antenna geometry. The field-aligned antenna has been installed on Alcator C-Mod and initial characterization is complete. Both core molybdenum concentration and molybdenum source measurements at the antenna are lower during operation of the field-aligned antenna relative to the non-aligned antenna. Latest results will be presented. [Preview Abstract] |
Monday, October 29, 2012 11:18AM - 11:30AM |
BO7.00010: The effect of a field-aligned ICRF antenna on enhanced sheath potentials in Alcator C-Mod Roman Ochoukov, Dennis Whyte, Daniel Brunner, Michael Garrett, Brian LaBombard, Bruce Lipschultz, James Terry, Stephen Wukitch Radio frequency (RF) rectification of the plasma potential ($\Phi _{P})$ is a leading mechanism suspected of causing enhanced sputtering of high-Z metallic plasma facing components in ion cyclotron range-of frequencies (ICRF)-heated discharges on Alcator C-Mod. Previous emissive probe measurements on Alcator C-Mod showed that significant ICRF-enhanced $\Phi _{P}$'s ($>$100 V) are present on magnetic field lines that map and not map to active ICRF antennas. A ``field-aligned'' (FA) ICRF antenna has recently been installed in order to minimize these deleterious ICRF-driven effects. The FA configuration of the antenna minimizes oscillating electric fields parallel to the magnetic field, reducing RF rectification. Plasma potentials are measured with probes that directly map along a field line to the antenna. Gas puff imaging (GPI) of density fluctuation flows also infers radial electric fields (E$_{r}$ = -d$\Phi _{P}$/dr) in the SOL of ICRF-heated discharges. Initial GPI results show that E$_{r}$, and hence $\Phi _{P}$'s, are mitigated in discharges heated with the FA antenna and the reduction is the strongest in field-aligned discharges as compared to non-aligned ICRF antennas. Direct $\Phi _{P}$ measurements on field lines mapped to the active FA antenna will also be presented. [Preview Abstract] |
Monday, October 29, 2012 11:30AM - 11:42AM |
BO7.00011: Progress towards steady-state regimes with LHCD in Alcator C-Mod Ron Parker, Paul Bonoli, Orso Meneghini, Syun'ichi Shiraiwa, Greg Wallace, Randy Wilson, Seung Gyou Baek, Ian Faust, Bob Harvey, Amanda Hubbard, Alexander Smirnov Non-inductive discharges with $\overline n $ = 0.5x10$^{20}$ m$^{-3}$, $I_{p}$ = 0.5 MA and B$_{T}$ = 5.4 T have been obtained with LHCD in Alcator C-Mod. Sawteeth are suppressed and MSE-constrained EFIT profiles reveal modestly reversed shear, with q$_{0} \sim $ 2 and q$_{min} \sim $ 1.5. LHCD efficiency is in the range $\eta $ = 2.0 - 2.5x10$^{19}$ A/Wm$^{2}$, in line with the LHCD efficiency assumed for advanced regimes sustained in part by LH in ITER. Accessing ITER relevant steady-state regimes in C-Mod with $f_{BS} \sim $ 50{\%} requires increasing the density to $\overline n $ $\sim $ 1.5x10$^{20}$ m$^{-3}$ with $T_{e0} >$ 5 keV . However, at $T_{e0} <$ 3 keV, the efficiency and all indications of fast electron generation fall rapidly above $\overline n \sim $ 1x10$^{20}$ m$^{-3}$, well below the limit set by wave accessibility. Ray tracing and full wave simulations have been carried out to investigate the cause of the drop in efficiency and both indicate that the SOL can play an important role in absorbing LH waves, either in the SOL or near but inside the separatrix. Recently, decay instabilities on the high field side have been identified as an additional loss mechanism. Regardless of the absorption process, simulations show that fast electron production can be maintained up to $\overline n \sim $ 1.5x10$^{20}$ m$^{-3}$ by assuring strong single-pass absorption, a conclusion that has been verified in 8 T He plasmas with $T_{e0} \sim $ 5 keV. [Preview Abstract] |
Monday, October 29, 2012 11:42AM - 11:54AM |
BO7.00012: Comparison of lower-hybrid frequency spectra at the high-field and low-field side in Alcator C-Mod S.G. Baek, R.R. Parker, S. Shiraiwa, G. Wallace, D. Brunner, I. Faust, A. Hubbard, B. LaBombard, M. Porkolab, J. Terry, C. Lau, Y. Takase On Alcator C-Mod, the precipitous decrease of lower-hybrid (LH) current drive efficiency has been observed above the line averaged density of 1x10$^{20}$ m$^{-3}$ in relatively low temperature target plasmas (T$_{e0}$ $\sim $ 2 keV). A set of 6 spectral recorders with high repetition rate have been developed to examine the non-linear interactions between LH waves and plasmas. LH wave frequency spectra are measured using existing probes on Alcator C-Mod: one on the LH launcher, two on the inner wall, and three on the outer divertor. Interestingly, when the density is raised over 1x10$^{20}$ m$^{-3}$, the LH spectra measured at the inner wall show strong sidebands with spectral power that is comparable to that of the pump wave. The frequency spacing of the sidebands corresponds to the ion cyclotron frequency high field side, indicating that parametric decay instabilities occur near the inner scrape-off layers. These sidebands are observed to be much weaker when measured at the low field side. The LH spectra are generally broadened when measured away from the LH launcher, indicating LH waves may undergo scattering processes. These observations suggest that nonlinear processes can be also responsible for the observed degradation of the current drive efficiency in high density plasmas, in addition to collisional absorptions, ionizations, and full-wave effects. [Preview Abstract] |
Monday, October 29, 2012 11:54AM - 12:06PM |
BO7.00013: LH-induced SOL density profile asymmetries on Alcator C-Mod Cornwall Lau, G.R. Hanson, Brian Labombard, Yijun Lin, Roman Ochoukov, Ron Parker, Syun'ichi Shiraiwa, Jim Terry, Greg Wallace, John Wilgen, Steve Wukitch On Alcator C-Mod, the LHCD has a density limit where the current drive efficiency drops abruptly significantly below the classical density limit. One possible explanation is LH wave absorption in SOL. A suite of SOL diagnostics have been used to diagnose the LH-SOL interaction: a X-mode reflectometer installed at three poloidal locations adjacent to the LH launcher is used to measure the density profile, a visible video camera that images the LH launcher measures the emissivity, and gas puff imaging diagnostic magnetically connected to the LH launcher measures the poloidal velocity. These measurements have confirmed that the LH absorption in SOL drives ExB drifts that are consistent with a density vortex. The poloidal density profile asymmetries scale with LH power, launched spectrum and are correlated with the video camera emissivity measurements. Using a diffusive-convective transnport model, these density profiles modifications are shown to be consistent with the measured plasma flows. The impact of these poloidal density profile asymmetries on LH coupling and SOL absorption models will be discussed. [Preview Abstract] |
Monday, October 29, 2012 12:06PM - 12:18PM |
BO7.00014: Mirror Langmuir Probe measurements of fluctuation-induced heat and particle transport in the Alcator C-Mod boundary plasma B. LaBombard, D. Brunner, O.E. Garcia, M. Greenwald, J.W. Hughes, R. Kube, J.L. Terry, S. Zweben A ``Mirror Langmuir Probe'' technique [1] has recently been implemented on a four-electrode scanning Langmuir/Mach probe, allowing simultaneous measurements of electron temperature, density and plasma potential fluctuations at high bandwidth (1 MHz) at four separate spatial locations. This unique capability allows cross-field profiles of fluctuation-induced heat and particle transport fluxes to be recorded in the C-Mod boundary plasma for the first time. Wavenumber and frequency spectra of $\tilde{T}_e$, $\tilde{n}$ and $\tilde{\Phi}$ and their relative phase shifts can also be computed, providing key information on transport dynamics (drift-wave vs. interchange). Higher-order correlations, such as Reynolds stress components, may also be evaluated. By varying current, toroidal field, and line-averaged density, we have begun to explore the scaling of fluctuation-induced transport and its relationship to the empirical scalings of the heat flux widths seen in C-Mod L and H-mode plasmas. Of particular interest are the flux-gradient relationships near the last-closed flux surface and their connections to the ``critical gradient transport'' behavior that has been reported previously from observations of time-averaged $T_e$, $n$ profiles.\\[4pt] [1] Rev. Sci. Instrum. {\bf 78} (2007) 073501. [Preview Abstract] |
Monday, October 29, 2012 12:18PM - 12:30PM |
BO7.00015: First ion temperature profiles in the boundary of Alcator C-Mod Dan Brunner, Brian LaBombard, Michael Churchill, Bruce Lipschultz, Roman Ochoukov, Christian Theiler, Dennis Whyte The ion temperature is an important parameter in the boundary of magnetic fusion devices, playing a role in the sheath heat flux, total plasma pressure, plasma potential, and sound speed. High spatial resolution measurements of $T_{i}$ and $T_{e}$ profiles are needed to evaluate these quantities and to unfold the rates of cross-field heat transport in each species. To this end, we have developed two specialized scanning probes: an Ion Sensitive Probe (ISP) and a Retarding Field Analyzer (RFA). Alcator C-Mod is a challenging environment for probes, with parallel heat fluxes comparable to that expected in reactors, 100's MW/m$^{2}$, necessitating the use of refractory materials in compact, optimized geometries. The two probes, along with Charge-eXchange Recombination Spectroscopy (CXRS) measurements of B$^{5+}$ ions, allow important cross-checks to be performed. Preliminary results indicate good agreement between the ISP and RFA. Comparison with B$^{5+}$ CXRS is also favorable. In ohmic-heated, sheath-limited plasmas the ratio of $T_{i}$/$T_{e}$ is $\sim $3 (150 and 50 eV) at the seperatrix and increases to $\sim $5 (50 and 10 eV) at the limiter boundary. Experiments with conduction-limited plasma conditions are planned and will be reported. Implications for boundary electron and ion heat transport will also be discussed. [Preview Abstract] |
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