Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session JO3: Alcator C-Mod, KSTAR, Ignitor |
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Chair: Stewart Zweben, Princeton Plasma Physics Laboratory Room: Reunion A |
Tuesday, November 18, 2008 2:00PM - 2:12PM |
JO3.00001: Overview of Alcator C-Mod Research Earl Marmar Recent C-Mod investigations include: toroidal and poloidal mode-conversion ICRF flow drive; LHCD experiments at ITER relevant magnetic field and density demonstrating off-axis current drive, H-mode pedestal regulation, and counter-current flow drive; increased density peaking at low collisionality; stiffness of the H-mode density pedestal, suggesting an edge critical gradient; poloidal propagation of edge turbulence structures, and changes in character close to the x-point; deuterium retention in metal (Mo) PFCs showing higher than expected retention; ICRF-sheath mechanisms for impurity generation; enhanced energy confinement with edge Te pedestal L-mode particle confinement and no ELMs; current ramping experiments to help understand expected control requirements in ITER; runaway electron dynamics during disruptions. Improved measurements across all plasma regimes are enabled by new/upgraded diagnostics, including: active CXRS for Ti, flows and electric fields; MSE for j(r); ultra-high speed CCD cameras; advanced high-field side scanning Langmuir and mach probes; scanning surface science station. [Preview Abstract] |
Tuesday, November 18, 2008 2:12PM - 2:24PM |
JO3.00002: Overview of LHCD Experiments on Alcator C-Mod J.R. Wilson, C. Kessel, S. Scott, R.R. Parker, P. Bonoli, A. Hubbard, J. Hughes, A. Ince-Cushman, J. Ko, O. Meneghini, M. Porkolab, J. Rice, A. Schmidt, S. Shiraiwa, G. Wallace, J. Wright LHCD on Alcator C-Mod is being used in plasmas with parameters similar to those expected on ITER. LHCD experiments have also produced intriguing results related to momentum transport and edge pedestal physics. Quantitative comparisons between local measurements and theory/simulation have been performed, confirming the off-axis localization of the current drive, as well as its magnitude and location dependence on the launched n$_{\vert \vert }$ spectrum and electron temperature. Applying LHCD during the current ramp saves volt-seconds and delays the peaking of the current profile. Simultaneous operation with ICRF has been achieved utilizing antennas not closely magnetically connected to the LH launcher. Counter current toroidal rotation during LHCD has been observed in both L and H-mode plasmas. In H-mode plasmas the edge pedestal collisionality is reduced even though the overall pressure profile of the pedestal is unaffected. [Preview Abstract] |
Tuesday, November 18, 2008 2:24PM - 2:36PM |
JO3.00003: Evidence of Spectral Control over Lower Hybrid Power Deposition on Alcator C-Mod A.E. Schmidt, P.T. Bonoli, R.R. Parker, M. Porkolab, G. Wallace, J.C. Wright, J.R. Wilson, R.W. Harvey, A.P. Smirnov The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod can produce spectra with a wide range of peak parallel refractive index (n$\vert \vert )$. Theory predicts that LH power deposition location is strongly dependent on n$\vert \vert $. An experiment in which LH power is square-wave modulated on a time scale much faster than the current relaxation time does not significantly alter the poloidal magnetic field inside the plasma and thus allows for realistic modeling and consistent plasma conditions for different n$\vert \vert $ spectra. Inverted hard x-ray profiles show clear changes in LH-driven fast electron location with differing n$\vert \vert $. Boxcar binning of hard x-rays during LH power modulation allows for $\sim $ 1 ms time resolution, which is sufficient to resolve the build-up, steady-state, and slowing-down of fast electrons. Ray-tracing/Fokker-Plank modeling in combination with a synthetic hard x-ray diagnostic has been used to determine a fast electron diffusion coefficient for various plasma conditions. This work is supported by the US DOE awards DE-FC02-99ER54512 and DE-AC02-76CH03073. [Preview Abstract] |
Tuesday, November 18, 2008 2:36PM - 2:48PM |
JO3.00004: Measurement of current density profile modification in LHCD experiments in Alcator C-Mod Jinseok Ko, Steve Scott, Andrea Schmidt, Syun'ichi Shiraiwa, Paul Bonoli, Ron Parker, John Wright, Greg Wallace Recent identification and resolution of problems in the Motional Stark Effect (MSE) diagnostic in Alcator C-Mod have made it possible to measure current density profile modifications in Lower Hybrid Current Drive (LHCD) experiments. A spurious drift in measured pitch angle has been traced to thermal stress- induced birefringence on the in-vessel lenses and is avoided by measuring the change in pitch angle between a LHCD phase and a baseline Ohmic phase within a discharge. The MSE measurements imply clear off-axis current drive and the dependence on parallel refractive index in the location and the magnitude of the driven current. The measurements are compared with GENRAY/CQL3D modeling. [Preview Abstract] |
Tuesday, November 18, 2008 2:48PM - 3:00PM |
JO3.00005: Observation of Self Generated Flows in Tokamak Plasmas With Lower Hybrid Driven Current Alexander Ince-Cushman, John Rice, Matthew Reinke, Manfred Bitter, Kenneth Hill, Martin Greenwald, Gregory Wallace, Ronald Parker, Jerry Hughes, Paul Bonoli, Catherine Fiore, Shunichi Shiraiwa, Amanda Hubbard, Stephen Wolfe, Ian Hutchinson, Randy Wilson, Earl Marmar In Alcator C-Mod discharges lower hybrid waves have been shown to induce a counter-current change in toroidal rotation of up to 60 km/s in the central region of the plasma (r/a $\sim <$ 0.4). This modification of the toroidal rotation profile develops on a time scale comparable to the current redistribution time ($\sim $100ms) but longer than the energy and momentum confinement times ($\sim $20ms). Comparison of co-and counter-current injected waves indicates that current drive (as opposed to heating) is responsible for the rotation profile modifications. Furthermore, the changes in central rotation velocity induced by lower hybrid current drive (LHCD) are well correlated with changes in normalized internal inductance. The application of LHCD has been shown to generate strongly sheared rotation profiles and a negative increment in the radial electric field profile consistent with a fast electron pinch. Supported by USDoE award DE-FC02-99ER54512. [Preview Abstract] |
Tuesday, November 18, 2008 3:00PM - 3:12PM |
JO3.00006: Effects of LHCD on H-mode transition, pedestal and confinement on Alcator C-Mod A.E. Hubbard, P.T. Bonoli, C.L. Fiore, J.W. Hughes, L. Lin, Y. Lin, R. Parker, M. Porkolab, A.E. Schmidt, G. Wallace, S. Wolfe, S. Wukitch, A.C.C. Sips, C. Kessel, J.R. Wilson Advanced scenario experiments on Alcator C-Mod seek to combine current profile control using lower hybrid current drive (LHCD) with the high confinement of H-mode plasmas. There is little prior experience with LHCD in H-modes at the high field (5.4 T) and density ($>$10$^{20}$ m$^{-3})$ typical of C-Mod, and ITER. We find good LH coupling in H-modes, though relatively low driven current is expected at the LH power available to date (up to 1.2 MW). An unexpected result was a reduction in density and increase in T$_{ped}$ in some LHCD experiments, indicative of an increase in edge particle transport. In other experiments aimed at exploring the ``hybrid'' scenario, LHCD was used to modify j(r) before H-modes, delaying sawteeth until well into the current flat top. The timing of ICRF and H-modes with respect to q$_{0}$=1 affected both L-H transition dynamics and the H-mode pedestal and confinement, producing transiently higher stored energy than in discharges without LHCD. [Preview Abstract] |
Tuesday, November 18, 2008 3:12PM - 3:24PM |
JO3.00007: Relationship between Edge Gradients and Plasma Flows in Alcator C-Mod B. LaBombard, N. Smick, A. Graf, K. Marr, R. McDermott, M. Reinke, M. Greenwald, J.W. Hughes, B. Lipschultz, J.L. Terry, D.G. Whyte The edge pressure profiles in a tokamak are remarkably `stiff', suggestive of a system near marginal stability. Electromagnetic turbulence appears to be an underlying cause [1]: edge pressure gradients normalized by the square of the poloidal magnetic field strength (i.e., the MHD ballooning parameter, $\alpha _{MHD})$ are invariant in plasmas with the same normalized collisionality. Yet, despite this relationship, x-point topology exerts a surprisingly strong influence; higher $\alpha _{MHD}$ are obtained when \textit{Bx}$\nabla B$ points toward the active x-point [2] -- Is this behavior connected to the strong flows and/or toroidal rotation that is observed in the edge, which changes with x-point topology [3]? Recently, an expanded set of diagnostics has been used to explore edge flows in detail for upper/lower x-points with forward/reversed magnetic fields. Systematic differences in parallel flows and toroidal rotation are evident. Cross-field flows measured by scanning Mach probes reveal an enhanced (diminished) velocity shear layer near the separatrix for \textit{Bx}$\nabla B$ pointing toward (away from) the active x-point. [1] Nucl. Fusion \textbf{45} (2005) 1658; [2] Phys. Plasmas \textbf{15} (2008) 056106; [3] Nucl. Fusion \textbf{44} (2004) 1047. [Preview Abstract] |
Tuesday, November 18, 2008 3:24PM - 3:36PM |
JO3.00008: Observations and Interpretation of the Spatial Structure of Edge Turbulence near the X-point Region and Outboard Midplane of Alcator C-Mod J.L. Terry, S.J. Zweben, B. LaBombard, I. Cziegler, M.V. Umansky, D.P. Stotler Movies of edge turbulence at both the outboard midplane and the region just outboard of the typical LSN X-point location in C-Mod have been obtained using Gas-Puff-Imaging together with fast-framing cameras. Intermittent turbulent structures, typically referred to as blobs or filaments, are observed in both locations. Near the midplane the filaments are roughly circular in cross-section with radial and poloidal correlation lengths of $\sim$ 1 cm, while in the X-point region they are highly elongated in cross-section in a direction approximately normal to the local flux surfaces. In addition, filament velocities in this region are $\sim$ 3x faster than the radial velocities at the midplane and in a direction that is roughly radially outward. The observations are consistent with the picture that the large scale features of the filaments ($k_{\perp} \rho_s>0.1$) map along field lines as a consequence of the rapid parallel diffusion of the potential perturbations. A 3D simulation using the BOUT turbulence code reproduces many of the spatial features observed in the experiment. [Preview Abstract] |
Tuesday, November 18, 2008 3:36PM - 3:48PM |
JO3.00009: Rapid Changes of Turbulence Propagation Direction in the Edge of Alcator C-Mod Istvan Cziegler, James Terry The velocity fields and scale structure of the edge and near SOL turbulence in the outboard midplane region of ohmic L-mode plasmas have been characterized using Gas-Puff-Imaging measurements. Poloidal resolution was provided by a vertical array of views and radial profiles were constructed from slow spatial scans of the magnetic separatrix across this vertical array. The observed dispersions show a clear radial structure [1] with turbulence propagating in the ion-diamagnetic direction $(1.5-2$~km/s) at and outside the separatrix, and in the electron-diamagnetic direction $(3.5-4$~km/s) at and inside the separatrix. We have investigated the crossover region around the separatrix where for observation durations $\agt 1 $~ms two counter-propagating velocities are observed. We find that for shorter observation durations rapid changes in the propagation direction are revealed, with the speed of propagation remaining largely invariant. We present data on the statistical behavior of both the propagation velocities and the temporal-spatial structure of the edge turbulence. [Preview Abstract] |
Tuesday, November 18, 2008 3:48PM - 4:00PM |
JO3.00010: Studies of Turbulence and Transport in Alcator C-Mod Ohmic Plasmas with Phase Contrast Imaging and Comparisons with GYRO L. Lin, M. Porkolab, E.M. Edlund, J.C. Rost, M. Greenwald, D. Mikkelsen Recent results from C-Mod ohmic plasmas using phase contrast imaging (PCI)\footnote{M. Porkolab et al., IEEE Trans. Plasma Sci. \textbf{34}, 229 (2006).} will be presented. The experiments cover the ``neo-Alcator'' (linear confinement time scaling with density) to the ``saturated ohmic'' regime.\footnote{R. R. Parker et al., Nuclear Fusion, \textbf{25}, 1127 (1985).} We have also compared measured turbulence with GYRO\footnote{J. Candy et al., Phys. Rev. Lett., \textbf{91}, 045001 (2003).} predictions through a synthetic PCI diagnostic method. The key role played by ITG has been verified, including measurements of turbulent wave propagation in the ion diamagnetic direction. It is found that the intensity of ITG increases with density, in agreement between simulation and experiments. The absolute fluctuation intensity agrees with simulation within experimental error (+/-60{\%}). The impact of these results on measured scaling of confinement time with density will be discussed, including possible disagreements at low densities where $\chi _{e}$ dominates over $\chi _{i}$. [Preview Abstract] |
Tuesday, November 18, 2008 4:00PM - 4:12PM |
JO3.00011: Flux Surface Asymmetries in VUV/SXR Emission on Alcator C-Mod Matthew Reinke, Ian Hutchinson The radiated power loss in many Alcator C-Mod ICRF heated plasmas is dominated by intrinsic molybdenum line emission arising from high-Z plasma facing components. Thus broadband emissivity is approximately proportional to Mo density and can be used to explore its spatial structure. Because of their large mass, Mo ions experience sufficient centrifugal force to sustain parallel impurity pressure gradients, even for C-Mod's modest intrinsic rotation (main-ion Mach numbers up to 0.3). 2D emissivity profiles are presented, calculated using both 50 micron Be-filtered and bare AXUV photodiode data. Slowly rotating L-mode plasmas have nearly uniform radiation patters while H-modes, which rotate faster, are asymmetric with the outboard emissivity 50{\%} higher than inboard. This asymmetry is shown not to scale with core toroidal rotation frequency as predicted by theories that balance parallel pressure gradient, inertia and electric field forces. Impurity-ion friction is shown to be important for high-Z impurities in C-Mod plasmas and experimental data is compared to theories that incorporate this effect. [Preview Abstract] |
Tuesday, November 18, 2008 4:12PM - 4:24PM |
JO3.00012: ARRIBA: A novel in-situ plasma surface interaction diagnostic for magnetic fusion devices Soren Harrison, Dennis Whyte The results of the development and design of a novel plasma surface interactions diagnostic for fusion experiments are described. The Alpha Radioisotope Remote Ion Beam Analysis (ARRIBA) diagnostic is designed to provide in-situ, time and depth-resolved measurement of element concentrations and H/D/T fuel retention at any surface inside a magnetic fusion device, including those surfaces exposed to significant heat loads such as found in the divertor. A mechanical system retracts and flips cylindrical material samples (diameter $\sim $10mm) during a plasma discharge, such that each sample becomes exposed to the plasma. The opposing sample is protected from plasma exposure by the tile and is mechanically positioned for surface analysis. Surface analysis is accomplished using non-destructive ion beam analysis (IBA) techniques: Rutherford Backscattering (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection (ERD). We describe the initial laboratory performance tests of the ARRIBA mechanical prototype, the resolution of laboratory erosion/deposition measurements, and the sensitivity of hydrogenic detection measurements. Additionally, we discuss the status of the planned installation on the C-mod tokamak. [Preview Abstract] |
Tuesday, November 18, 2008 4:24PM - 4:36PM |
JO3.00013: Overview of KSTAR 1st Plasma Campaign Si-Woo Yoon, S.H. Hahn, Y.S. Bae, W.C. Kim, J. Kim, Y.K. Oh, J.Y. Kim, H.L. Yang, D. Mueller, J.A. Leuer, D.A. Humphreys, A.W. Hyatt, N.W. Eidietis, G.L. Jackson, M.L. Walker For the 1st discharge of KSTAR, the available poloidal magnetic flux and the loop voltage from the PF coils are limited and therefore, the 2nd harmonic (84 GHz, 500kW) ECH is introduced for reliable breakdown and providing an additional plasma heating during the current ramp-up and the initial field-null configuration is optimized to maximize the plasma current with significant charging-up of the outer poloidal coils. During the campaign, the maximum achievable plasma current is around 130kA in circular-shaped discharges with a relatively low gas-puff to minimize the resistive Vs consumption. In this work, several aspects during KSTAR 1st campaign will be addressed briefly including breakdown and current ramp-up, scan of ECH pre- ionization, MHD signature and the effect of ECH, and the discharge reproducibility issues. [Preview Abstract] |
Tuesday, November 18, 2008 4:36PM - 4:48PM |
JO3.00014: Ignitor and the High Density Approach to Fusion Reactors* F. Bombarda, B. Coppi The optimal path to ignition that can be followed by experiments based on existing technologies and knowledge of plasma physics relies on the high plasma density regimes that are at the basis of the Ignitor design ($R_0\cong1.32$ m, $a\times b\cong0.47\times 0.83 \rm m^2$, $B_T\cong$13 T, $I_p\cong$11 MA). Their value has been rediscovered recently following experiments by the helical LHD facility that have systematically produced plasmas with $n_0\leq 10^{21} \rm m^{- 3}$. Consequently, conceptual power producing reactors that would operate with plasma parameters close to those of Ignitor when reaching ignition have been envisioned. The main purpose of the Ignitor experiment is, in fact, that of establishing the reactor physics in regimes close to ignition, where the thermonuclear instability can set in with all its associated non linear effects. ``Extended limiter'' and double X-point configurations have been analyzed and relevant transport simulations show that similar burning plasma conditions can be attained with both. The machine core design has been essentially completed, but the recent development of a new intermediate temperature superconducting material (MgB$_2$) has led to its adoption for the largest poloidal field coils, producing a vertical field component of 4 T. The properties of this material make it possible to envision its future use for coils producing higher magnetic fields and open new options in the design of novel experimental devices. \\ $^*$Sponsored in part by ENEA of Italy and by the U.S. D.O.E. [Preview Abstract] |
Tuesday, November 18, 2008 4:48PM - 5:00PM |
JO3.00015: ICRH Physics in the Ignitor Experiment A. Cardinali, R.V. Budny, B. Coppi The ICRH system adopted for Ignitor can operate with a large frequency band (80-120 MHz) that is consistent with toroidal magnetic fields in the interval 9-13 T. The broad range of delivered power (4-12 MW) is suitable to investigate different aspects of burning plasma dynamics. The ICRH physics relevant to the plasmas produced by Ignitor is reviewed. The calculated Power Deposition Profiles (PDP) when the ICRH is used to control the plasma relevant parameters in the igniting scenario has been used as an input for the transport code PTRANSP. In particular, PDP's calculated by means of the toroidal full wave code ``TORIC'' show that a small fraction of $^3$He (1-2\%) improve the wave absorption on ions at the plasma center, while a considerable fraction of the coupled power is damped on the electrons, in a broad range of plasma radii, considering the $n_ {\|}$-spectrum radiated by the antenna. The evolution of the plasma parameter profiles simulated by PTRANSP using two different transport models are presented pointing out the role of the density profiles on fusion reaction rates. In particular, given the flexibility of the ICRH system, it is possible to control the plasma temperature, and consequently the thermonuclear instability that occurs at ignition, with modest amounts of ICRH power ($\leq 8$ MW). [Preview Abstract] |
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