Showing papers by "Princeton Plasma Physics Laboratory published in 2013"

Multiphysics simulations: Challenges and opportunities

Abstract: We consider multiphysics applications from algorithmic and architectural perspectives, where “algorithmic” includes both mathematical analysis and computational complexity, and “architectural” includes both software and hardware environments. Many diverse multiphysics applications can be reduced, en route to their computational simulation, to a common algebraic coupling paradigm. Mathematical analysis of multiphysics coupling in this form is not always practical for realistic applications, but model problems representative of applications discussed herein can provide insight. A variety of software frameworks for multiphysics applications have been constructed and refined within disciplinary communities and executed on leading-edge computer systems. We examine several of these, expose some commonalities among them, and attempt to extrapolate best practices to future systems. From our study, we summarize challenges and forecast opportunities.

A long-pulse high-confinement plasma regime in the Experimental Advanced Superconducting Tokamak

Abstract: A high-confinement plasma that is potentially useful for controlled fusion has now been sustained for over 30 s. The Experimental Advanced Superconducting Tokamak in Hefei, China, achieved this record pulse length by first confining the plasma using lithium-treated vessel walls, and then maintaining it with a so-called lower hybrid current drive.

Filamentation Instability of Counterstreaming Laser-Driven Plasmas

Abstract: Filamentation due to the growth of a Weibel-type instability was observed in the interaction of a pair of counterstreaming, ablatively driven plasma flows, in a supersonic, collisionless regime relevant to astrophysical collisionless shocks. The flows were created by irradiating a pair of opposing plastic (CH) foils with 1.8 kJ, 2-ns laser pulses on the OMEGA EP Laser System. Ultrafast laser-driven proton radiography was used to image the Weibel-generated electromagnetic fields. The experimental observations are in good agreement with the analytical theory of the Weibel instability and with particle-in-cell simulations.

An overview of intrinsic torque and momentum transport bifurcations in toroidal plasmas

Abstract: An overview of the physics of intrinsic torque is presented, with special emphasis on the phenomenology of intrinsic toroidal rotation in tokamaks, its theoretical understanding, and the variety of momentum transport bifurcation dynamics. Ohmic reversals and electron cyclotron heating-driven counter torque are discussed in some detail. Symmetry breaking by lower single null versus upper single null asymmetry is related to the origin of intrinsic torque at the separatrix. (Some figures may appear in colour only in the online journal)

Kinetic theory of plasma sheaths surrounding electron-emitting surfaces.

Abstract: A one-dimensional kinetic theory of sheaths surrounding planar, electron-emitting surfaces is presented which accounts for plasma electrons lost to the surface and the temperature of the emitted electrons. It is shown that ratio of plasma electron temperature to emitted electron temperature significantly affects the sheath potential when the plasma electron temperature is within an order of magnitude of the emitted electron temperature. The sheath potential goes to zero as the plasma electron temperature equals the emitted electron temperature, which can occur in the afterglow of an rf plasma and some low-temperature plasma sources. These results were validated by particle in cell simulations. The theory was tested by making measurements of the sheath surrounding a thermionically emitting cathode in the afterglow of an rf plasma. The measured sheath potential shrunk to zero as the plasma electron temperature cooled to the emitted electron temperature, as predicted by the theory.

Technical challenges in the construction of the steady-state stellarator Wendelstein 7-X

Abstract: The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

Energetic particle instabilities in fusion plasmas

Abstract: Remarkable progress has been made in diagnosing energetic particle instabilities on present-day machines and in establishing a theoretical framework for describing them. This overview describes the much improved diagnostics of Alfven instabilities and modelling tools developed world-wide, and discusses progress in interpreting the observed phenomena. A multi-machine comparison is presented giving information on the performance of both diagnostics and modelling tools for different plasma conditions outlining expectations for ITER based on our present knowledge.

Liquid lithium divertor characteristics and plasma–material interactions in NSTX high-performance plasmas

Abstract: Liquid metal plasma-facing components (PFCs) have been proposed as a means of solving several problems facing the creation of economically viable fusion power reactors. To date, few demonstrations exist of this approach in a diverted tokamak and we here provide an overview of such work on the National Spherical Torus Experiment (NSTX). The Liquid Lithium Divertor (LLD) was installed and operated for the 2010 run campaign using evaporated coatings as the filling method. The LLD consisted of a copper-backed structure with a porous molybdenum front face. Nominal Li filling levels by the end of the run campaign exceeded the porosity void fraction by 150%. Despite a nominal liquid level exceeding the capillary structure and peak current densities into the PFCs exceeding 100 kA m−2, no macroscopic ejection events were observed. In addition, no substrate line emission was observed after achieving lithium-melt temperatures indicating the lithium wicks and provides a protective coating on the molybdenum porous layer. Impurity emission from the divertor suggests that the plasma is interacting with oxygen-contaminated lithium whether diverted on the LLD or not. A database of LLD discharges is analysed to consider whether there is a net effect on the discharges over the range of total deposited lithium in the machine. Examination of H-97L indicates that performance was constant throughout the run, consistent with the hypothesis that it is the quality of the surface layers of the lithium that impact performance. The accumulation of impurities suggests a fully flowing liquid lithium system to obtain a steady-state PFC on timescales relevant to NSTX.

Comparisons of linear and nonlinear plasma response models for non-axisymmetric perturbations

Abstract: With the installation of non-axisymmetric coil systems on major tokamaks for the purpose of studying the prospects of ELM-free operation, understanding the plasma response to the applied fields is a crucial issue. Application of different response models, using standard tools, to DIII-D discharges with applied non-axisymmetric fields from internal coils, is shown to yield qualitatively different results. The plasma response can be treated as an initial value problem, following the system dynamically from an initial unperturbed state, or from a nearby perturbed equilibrium approach, and using both linear and nonlinear models [A. D. Turnbull, Nucl. Fusion 52, 054016 (2012)]. Criteria are discussed under which each of the approaches can yield a valid response. In the DIII-D cases studied, these criteria show a breakdown in the linear theory despite the small 10 3 relative magnitude of the applied magnetic field perturbations in this case. For nonlinear dynamical evolution simulations to reach a saturated nonlinear steady state, appropriate damping mechanisms need to be provided for each normal mode comprising the response. Other issues arise in the technical construction of perturbed flux surfaces from a displacement and from the presence of near nullspace normal modes. For the nearby equilibrium approach, in the absence of a full 3D equilibrium reconstruction with a controlled comparison, constraints relating the 2D system profiles to the final profiles in the 3D system also need to be imposed to assure accessibility. The magnetic helicity profile has been proposed as an appropriate input to a 3D equilibrium calculation and tests of this show the anticipated qualitative behavior.

ISABELA for effective in situ compression of scientific data

Abstract: Exploding dataset sizes from extreme‐scale scientific simulations necessitates efficient data management and reduction schemes to mitigate I/O costs. With the discrepancy between I/O bandwidth and computational power, scientists are forced to capture data infrequently, thereby making data collection an inherently lossy process. Although data compression can be an effective solution, the random nature of real‐valued scientific datasets renders lossless compression routines ineffective. These techniques also impose significant overhead during decompression, making them unsuitable for data analysis and visualization, which require repeated data access.

Negative plasma potential relative to electron-emitting surfaces.

Abstract: Most works on plasma-wall interaction predict that with strong electron emission, a nonmonotonic ``space-charge-limited'' (SCL) sheath forms where the plasma potential is positive relative to the wall. We show that a fundamentally different sheath structure is possible where the potential monotonically increases toward a positively charged wall that is shielded by a single layer of negative charge. No ion-accelerating presheath exists in the plasma and the ion wall flux is zero. An analytical solution of the ``inverse sheath'' regime is demonstrated for a general plasma-wall system where the plasma electrons and emitted electrons are Maxwellian with different temperatures. Implications of the inverse sheath effect are that (a) the plasma potential is negative, (b) ion sputtering vanishes, (c) no charge is lost at the wall, and (d) the electron energy flux is thermal. To test empirically what type of sheath structure forms under strong emission, a full plasma bounded by strongly emitting walls is simulated. It is found that inverse sheaths form at the walls and ions are confined in the plasma. This result differs from past particle-in-cell simulation studies of emission which contain an artificial ``source sheath'' that accelerates ions to the wall, leading to a SCL sheath at high emission intensity.

First observations of ELM triggering by injected lithium granules in EAST

Abstract: The first results of edge-localized mode (ELM) pacing using small spherical lithium granules injected mechanically into H-mode discharges are reported. Triggering of ELMs was accomplished using a simple rotating impeller to inject sub-millimetre size granules at speeds of a few tens of meters per second into the outer midplane of the EAST fusion device. During the injection phase, ELMs were triggered with near 100% efficiency and the amplitude of the induced ELMs as measured by Dα was clearly reduced compared to contemporaneous naturally occurring ELMs. In addition, a wide range of granule penetration depths was observed. Moreover, a substantial fraction of the injected granules appeared to penetrate up to 50% deeper than the 3 cm nominal EAST H-mode pedestal width. The observed granule penetration was, however, less deep than suggested by ablation modelling carried out after the experiment. The observation that ELMs can be triggered using the injection of something other than frozen hydrogenic pellets allows for the contemplation of lithium or beryllium-based ELM pace-making on future fusion devices. This change in triggering paradigm would allow for the decoupling of the ELM-triggering process from the plasma-fuelling process which is currently a limitation on the performance of hydrogen-based ELM mitigation by injected pellets.

Plasma current, position and shape feedback control on EAST

Abstract: In this paper, a linear model for plasma current, position and shape control based on the plasma rigid motion assumption is presented and implemented in an EAST tokamak simulator. The simulator models the plasma, poloidal field (PF) coils, and power supplies, and is used to verify the control algorithm and optimize control parameters and PF coil current trajectories. Plasma position and shape control has been achieved during the last several EAST operation campaigns due to successful decoupling of plasma current, plasma position and shape. The control logic used and experimental results are described in detail. Diverted plasma shapes, including double null, upper and lower single null, and with elongation up to 2.0, triangularity in the range 0.4–0.6 and X point control accuracy of 1 cm, were successfully controlled. Smooth shape transition in the current ramp-up ensures that volt–seconds are saved and that plasma disruptions are avoided. Such control capability provides the basis for future high performance plasma operation.

Improved understanding of physics processes in pedestal structure, leading to improved predictive capability for ITER

Abstract: Joint experiment/theory/modelling research has led to increased confidence in predictions of the pedestal height in ITER. This work was performed as part of a US Department of Energy Joint Research Target in FY11 to identify physics processes that control the H-mode pedestal structure. The study included experiments on C-Mod, DIII-D and NSTX as well as interpretation of experimental data with theory-based modelling codes. This work provides increased confidence in the ability of models for peeling–ballooning stability, bootstrap current, pedestal width and pedestal height scaling to make correct predictions, with some areas needing further work also being identified. A model for pedestal pressure height has made good predictions in existing machines for a range in pressure of a factor of 20. This provides a solid basis for predicting the maximum pedestal pressure height in ITER, which is found to be an extrapolation of a factor of 3 beyond the existing data set. Models were studied for a number of processes that are proposed to play a role in the pedestal ne and Te profiles. These processes include neoclassical transport, paleoclassical transport, electron temperature gradient turbulence and neutral fuelling. All of these processes may be important, with the importance being dependent on the plasma regime. Studies with several electromagnetic gyrokinetic codes show that the gradients in and on top of the pedestal can drive a number of instabilities.

Progress in Simulating Turbulent Electron Thermal Transport in NSTX

Abstract: Nonlinear simulations based on multiple NSTX discharge scenarios have progressed to help differentiate unique instability mechanisms and to validate with experimental turbulence and transport data. First nonlinear gyrokinetic simulations of microtearing turbulence in a high-beta NSTX H-mode discharge predict experimental levels of electron thermal transport that are dominated by magnetic flutter and increase with collisionality, roughly consistent with energy confinement times in dimensionless collisionality scaling experiments. Electron temperature gradient (ETG) simulations predict significant electron thermal transport in some low- and high-beta discharges when ion scales are suppressed by E × B shear. Although the predicted transport in H-modes is insensitive to variation in collisionality (inconsistent with confinement scaling), it is sensitive to variations in other parameters, particularly density gradient stabilization. In reversed shear L-mode discharges that exhibit electron internal transport barriers, ETG transport has also been shown to be suppressed nonlinearly by strong negative magnetic shear, s ≪ 0. In many high-beta plasmas, instabilities which exhibit a stiff beta dependence characteristic of kinetic ballooning modes (KBMs) are sometimes found in the core region. However, they do not have a distinct finite beta threshold, instead transitioning gradually to a trapped electron mode (TEM) as beta is reduced to zero. Nonlinear simulations of this ‘hybrid’ TEM/KBM predict significant transport in all channels, with substantial contributions from compressional magnetic perturbations. As multiple instabilities are often unstable simultaneously in the same plasma discharge, even on the same flux surface, unique parametric dependencies are discussed which may be useful for distinguishing the different mechanisms experimentally.

Increase of turbulence and transport with resonant magnetic perturbations in ELM-suppressed plasmas on DIII-D

Abstract: Long-wavelength turbulence increases dramatically in the outer regions of DIII-D plasmas with the application of resonant magnetic field perturbations (RMPs) that suppress edge-localized modes (ELMs). Correspondingly, transport increases and global energy confinement decreases in these low-collisionality RMP-ELM suppressed discharges. The core and pedestal density are sharply reduced, while ion and electron temperatures may change only slightly. Low wavenumber density turbulence (k⊥ρi < 1) in the range of 60–300 kHz, measured with beam emission spectroscopy, is modified and generally increases throughout the outer region (0.6 < ρ < 1.0) of the plasma in response to RMPs over a range of q95 values; ELM suppression, in contrast, occurs for a narrower range in q95. Radial magnetic field modulation experiments indicate that these turbulence modifications occur on a time scale of a few milliseconds or less near ρ = 0.85–0.95, significantly faster than transport time-scales and faster than the local pressure gradients and shearing rates evolve at these locations. As the internal coil current is modulated in a square-wave fashion from 3.2 to 4.2 kA, the turbulence magnitude varies in phase by 30% or more, while local density changes by only a few per cent. This dynamical behaviour suggests that the turbulence is directly affected by the RMP, which may partially or largely explain the resulting increased transport and stabilization of the pedestal against peeling–ballooning instabilities that are thought to drive ELMs.

Role of plasma response in displacements of the tokamak edge due to applied non-axisymmetric fields

Abstract: Linear, two-fluid, resistive modelling of the plasma response to applied non-axisymmetric fields shows significant displacement of edge temperature and density profiles The calculated displacements, often of 2 cm or more in H-mode pedestals with parameters appropriate to DIII-D, are due to the helical distortions resulting from stable edge modes being driven to finite amplitude by the applied fields In many cases, these displacements are greater in magnitude, and different in phase, than the distortions of the separatrix manifolds predicted from vacuum modelling Comparison of these results with experimental measurements from Thomson scattering and soft x-ray imaging finds good quantitative agreement In these experiments, the phase of the applied non-axisymmetric magnetic field was flipped or rotated in order to probe the non-axisymmetric features of the response The poloidal structures measured by x-ray imaging show clear indications of a helical response, as opposed to simply a change in the axisymmetric transport Inclusion of two-fluid effects and rotation are found to be important in obtaining quantitative agreement with Thomson scattering data Modelling shows screening of islands in the H-mode pedestal, but island penetration near the top of the pedestal where the electron rotation vanishes in plasmas with co-current rotation Enhanced transport due to these islands may provide a mechanism for maintaining the pedestal width below the stability threshold of edge-localized modes For typical DIII-D parameters, it is shown that the linear approximation is often near or beyond the limit of validity in the H-mode edge; however, the general agreement with experimental measurements indicates that these linear results nevertheless maintain good predictive value for profile displacements

Scrape-off layer properties of ITER-like limiter start-up plasmas in JET

Abstract: Recent experiments at JET combining reciprocating probe measurements upstream and infrared thermography at the plasma-facing components (PFC) on plasmas in limiter configurations show that the common approach to predicting the power load on the limiter underestimates the heat flux at the contact point by a factor 1.5–3. The current model and scaling laws used for predicting the power load onto the first wall during limiter current ramp-up/down in ITER are uncertain and a better understanding of the heat transport to the PFCs is required. The heat loads on PFCs are usually predicted by projecting the parallel heat flux associated with scrape-off layer (SOL) properties at the outer mid-plane (upstream) along the magnetic field lines to the limiter surface and deducing the surface heat flux through a cosine law, thus ignoring any local effect of the PFC on transport within the SOL. The underestimate of the heat flux is systematic in inner wall limiter configurations, independent of the plasma parameters, whereas in outer limiter configuration this is not observed, probably because of the much shorter SOL power decay length. Models that can explain this enhanced heat flux around the contact point are proposed and discussed although no definitive conclusion can be drawn.

Observation of ion acceleration and heating during collisionless magnetic reconnection in a laboratory plasma.

Abstract: The ion dynamics in a collisionless magnetic reconnection layer are studied in a laboratory plasma. The measured in-plane plasma potential profile, which is established by electrons accelerated around the electron diffusion region, shows a saddle-shaped structure that is wider and deeper towards the outflow direction. This potential structure ballistically accelerates ions near the separatrices toward the outflow direction. Ions are heated as they travel into the high-pressure downstream region.

Auroral particle precipitation characterized by the substorm cycle

Abstract: [1] Substorms release a large amount of energy, some of which is used to energize the precipitating particles in the polar region. Superposed epoch analysis was performed with 11 years of DMSP SSJ/4/5 data to characterize the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions. Although substorms only increase the ion pressure by 30%, they increase the power of the diffuse, monoenergetic, and wave electron aurora by 310%, 71%, and 170%, respectively. Substorms energize the ion aurora mainly in the 21:00–05:00 magnetic local time (MLT) sector. The dynamics of the diffuse electron aurora are different from those of the other two electron aurorae. The expansion phase duration is approximately 15 min for the monoenergetic and wave electron aurorae, whereas it is 1 h for the diffuse electron aurora. The monoenergetic and wave electron aurorae appear to complete the substorm cycle within a 5 h interval, whereas the diffuse electron aurora takes more than 5 h. The diffuse electron aurora power and energy flux start increasing at 15 min before the substorm onset, whereas those for the monoenergetic and wave electron aurorae start increasing at 1 h and 15 min before the onset. The increase in the monoenergetic electron aurora power and energy flux may result from the increase in the magnetotail stretching and region-1 field-aligned current during the growth phase. The monoenergetic electrons may also be associated with fast flows, which have been previously observed more frequently in the dusk-midnight sector.

Edge microstability of NSTX plasmas without and with lithium-coated plasma-facing components

Abstract: The pedestal structure in NSTX is strongly affected by lithium coatings applied to the PFCs. In discharges with lithium, the density pedestal widens, and the electron temperature (Te) gradient increases inside a radius of ?N???0.95, but is unchanged for ?N?>?0.95. The inferred effective electron thermal and particle profiles reflect the profile changes: is slightly increased in the near-separatrix region, and is reduced in the region ?N? ?0.95, both the pre- and with-lithium cases are calculated to be unstable to ETG modes, with higher growth rates with lithium. Both cases are also found to lie near the onset for kinetic ballooning modes, but in the second-stable region where growth rates decrease with increasing pressure gradient.

Non-local heat transport, rotation reversals and up/down impurity density asymmetries in Alcator C-Mod ohmic L-mode plasmas

Abstract: Several seemingly unrelated effects 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 and up/down impurity density asymmetries. These phenomena all abruptly transform at a critical value of the collisionality. At low densities in the linear ohmic confinement regime, with collisionality ?*???0.35 (evaluated inside of the q?=?3/2 surface), heat transport exhibits non-local behaviour, core toroidal rotation is directed co-current, edge impurity density profiles are up/down symmetric and a turbulent feature in core density fluctuations with k? up to 15?cm?1 (k??s???1) is present. At high density/collisionality with saturated ohmic confinement, electron thermal transport is diffusive, core rotation is in the counter-current direction, edge impurity density profiles are up/down asymmetric and the high k? turbulent feature is absent. The rotation reversal stagnation point (just inside of the q?=?3/2 surface) coincides with the non-local electron temperature profile inversion radius. All of these observations suggest a possible unification in a model with trapped electron mode prevalence at low collisionality and ion temperature gradient mode domination at high collisionality.

Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

Abstract: Research on the National Spherical Torus Experiment, NSTX, targets physics understanding needed for extrapolation to a steady-state ST Fusion Nuclear Science Facility, pilot plant, or DEMO. The unique ST operational space is leveraged to test physics theories for next-step tokamak operation, including ITER. Present research also examines implications for the coming device upgrade, NSTX-U. An energy confinement time, ?E, scaling unified for varied wall conditions exhibits a strong improvement of BT?E with decreased electron collisionality, accentuated by lithium (Li) wall conditioning. This result is consistent with nonlinear microtearing simulations that match the experimental electron diffusivity quantitatively and predict reduced electron heat transport at lower collisionality. Beam-emission spectroscopy measurements in the steep gradient region of the pedestal indicate the poloidal correlation length of turbulence of about ten ion gyroradii increases at higher electron density gradient and lower Ti gradient, consistent with turbulence caused by trapped electron instabilities. Density fluctuations in the pedestal top region indicate ion-scale microturbulence compatible with ion temperature gradient and/or kinetic ballooning mode instabilities. Plasma characteristics change nearly continuously with increasing Li evaporation and edge localized modes (ELMs) stabilize due to edge density gradient alteration. Global mode stability studies show stabilizing resonant kinetic effects are enhanced at lower collisionality, but in stark contrast have almost no dependence on collisionality when the plasma is off-resonance. Combined resistive wall mode radial and poloidal field sensor feedback was used to control n?=?1 perturbations and improve stability. The disruption probability due to unstable resistive wall modes (RWMs) was surprisingly reduced at very high ?N/li?>?10 consistent with low frequency magnetohydrodynamic spectroscopy measurements of mode stability. Greater instability seen at intermediate ?N is consistent with decreased kinetic RWM stabilization. A model-based RWM state-space controller produced long-pulse discharges exceeding ?N?=?6.4 and ?N/li?=?13. Precursor analysis shows 96.3% of disruptions can be predicted with 10?ms warning and a false positive rate of only 2.8%. Disruption halo currents rotate toroidally and can have significant toroidal asymmetry. Global kinks cause measured fast ion redistribution, with full-orbit calculations showing redistribution from the core outward and towards V?/V?=?1 where destabilizing compressional Alfv?n eigenmode resonances are expected. Applied 3D fields altered global Alfv?n eigenmode characteristics. High-harmonic fast-wave (HHFW) power couples to field lines across the entire width of the scrape-off layer, showing the importance of the inclusion of this phenomenon in designing future RF systems. The snowflake divertor configuration enhanced by radiative detachment showed large reductions in both steady-state and ELM heat fluxes (ELMing peak values down from 19?MW?m?2 to less than 1.5?MW?m?2). Toroidal asymmetry of heat deposition was observed during ELMs or by 3D fields. The heating power required for accessing H-mode decreased by 30% as the triangularity was decreased by moving the X-point to larger radius, consistent with calculations of the dependence of E???B shear in the edge region on ion heat flux and X-point radius. Co-axial helicity injection reduced the inductive start-up flux, with plasmas ramped to 1?MA requiring 35% less inductive flux. Non-inductive current fraction (NICF) up to 65% is reached experimentally with neutral beam injection at plasma current Ip?=?0.7?MA and between 70?100% with HHFW application at Ip?=?0.3?MA. NSTX-U scenario development calculations project 100% NICF for a large range of 0.6?

First results from H-mode plasmas generated by ICRF heating in the EAST

Abstract: Deuterium high-confinement (H-mode) plasmas, lasting up to 3.45 s, have been generated in the EAST by ion cyclotron range of frequency (ICRF) heating. H-mode access was achieved by coating the molybdenum-tiled first wall with lithium to reduce the hydrogen recycling from the wall. H-mode plasmas with plasma currents between 0.4 and 0.6 MA and axial toroidal magnetic fields between 1.85 and 1.95 T were generated by 27 MHz ICRF heating of deuterium plasma with hydrogen minority. The ICRF input power required to access the H-mode was 1.6–1.8 MW. The line-averaged density was in the range (1.83–2.3) × 1019 m−3. 200–500 Hz type-III edge localized mode activity was observed during the H-mode phase. The H-mode confinement factor, H98IPB(y, 2), was ~0.7.

Edge sheared flows and the dynamics of blob-filaments

Abstract: The edge and scrape-off layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and the dynamics of blob-filaments. Both numerical simulations and experimental data analysis are employed. The simulations use the fluid-based two-dimensional (2D) curvature-interchange model embedded in the SOLT code. A blob-tracking algorithm based on 2D time-resolved images from the gas puff imaging diagnostic has also been developed and applied to NSTX, Alcator C-Mod and simulation data. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations and quasi-steady turbulence simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. The simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale-length and direction of perpendicular (approximately poloidal) flows, the inferred Reynolds acceleration and residual stress, poloidal reversal of blob tracks, and blob trapping and/or ejection. Mechanisms related to blob motion, SOL currents and radial inhomogeneity are shown to be sufficient to explain the presence or absence of mean and oscillating zonal sheared flows in selected shots.

Inter-machine validation study of neoclassical transport modelling in medium- to high-density stellarator-heliotron plasmas

Abstract: A comparative study of energy transport for medium- to high-density discharges in the stellarator-heliotrons TJ-II, W7-AS and LHD is carried out. The specific discharge parameters are chosen to apply a recently concluded benchmarking study of neoclassical (NC) transport coefficients (Beidler et al 2011 Nucl. Fusion 51 076001) to perform this validation study. In contrast to previous experiments at low densities for which electron transport was predominant (Yokoyama et al 2007 Nucl. Fusion 47 1213), the current discharges also exhibit significant ion energy transport. As it affects the energy transport in 3D devices, the ambipolar radial electric field is addressed as well. For the discharges described, ion-root conditions, i.e. a small negative radial electric field were found. The energy transport in the peripheral region cannot be explained by NC theory. Within a 'core region'(r/a < 1/2 ~ 2/3), the predicted NC energy fluxes comply with experimental findings for W7-AS. For TJ-II, compliance in the core region is found for the particle transport and the electron energy transport. For the specific LHD discharges, the core energy transport complied with NC theory except for the electron energy transport in the inward-shifted magnetic configuration. The NC radial electric field tends to agree with experimental results for all devices but is measured to be more negative in the core of both LHD and TJ-II. As a general observation, the energy confinement time approaches the gyro-Bohm-type confinement scaling ISS04 (Yamada et al 2005 Nucl. Fusion 45 1684). This work is carried out within the International Stellarator-Heliotron Profile Database (www.ipp.mpg.de/ISS and http://ishpdb.nifs.ac.jp/index.html).

The effect of the fast-ion profile on Alfvén eigenmode stability

Abstract: Different combinations of on-axis and off-axis neutral beams are injected into DIII-D plasmas that are unstable to reversed shear Alfv?n eigenmodes (RSAE) and toroidal Alfv?n eigenmodes (TAE). The variations alter the classically expected fast-ion gradient ??f in the plasma interior. Off-axis injection reduces the amplitude of RSAE activity an order of magnitude. Core TAEs are also strongly stabilized. In contrast, at larger minor radius, the fast-ion gradient is similar for on- and off-axis injection and switching the angle of injection has a weaker effect on the stability of TAEs. The average mode amplitude correlates strongly with the classically expected profile but the measured profile relaxes to similar values independent of the fraction of off-axis beams. The observations agree qualitatively with a ?critical-gradient? model of fast-ion transport.

The Hamiltonian structure and Euler-Poincaré formulation of the Vlasov-Maxwell and gyrokinetic systems

Abstract: We present a new variational principle for the gyrokinetic system, similar to the Maxwell-Vlasov action presented in H. Cendra et al., [J. Math. Phys. 39, 3138 (1998)]. The variational principle is in the Eulerian frame and based on constrained variations of the phase space fluid velocity and particle distribution function. Using a Legendre transform, we explicitly derive the field theoretic Hamiltonian structure of the system. This is carried out with a modified Dirac theory of constraints, which is used to construct meaningful brackets from those obtained directly from Euler-Poincare theory. Possible applications of these formulations include continuum geometric integration techniques, large-eddy simulation models, and Casimir type stability methods.