Showing papers in "Plasma Physics and Controlled Fusion in 2012"
TL;DR: The Sustained Spheromak Physics Experiment (SSPX) as discussed by the authors was a high-temperature (Te up to 0.5 keV), coaxial helicity injection (CHI) formed by coaxial helical injection, with plasma duration of a few milliseconds following the high-current formation stage.
Abstract: The Sustained Spheromak Physics Experiment (SSPX) was a high-temperature (Te up to 0.5 keV) spheromak formed by coaxial helicity injection (CHI) and with plasma duration of a few milliseconds following the high-current formation stage. Clean walls and low impurity operation were obtained by a combination of baking, discharge cleaning and titanium deposition on the walls, allowing the generation of high-quality plasmas. Resistive-magnetohydrodynamic simulations, benchmarked to the experiment, were used to elucidate the physics. The detailed characteristics of the nφ = 1 toroidal mode associated with CHI were determined as was the physics of the nonlinear current drive and magnetic reconnection that formed and sustained the spheromak. If the helicity injection rate was reduced following formation the plasma became relatively quiescent and magnetic surfaces formed. The measured thermal diffusivity in the core was as low as ∼1 m2 s−1. However, reconnection events during buildup or sustainment of the plasma current by CHI were found to open magnetic surfaces throughout the plasma allowing rapid energy loss to the walls. As a result, experiments and simulations in SSPX found no path to simultaneous sustainment by CHI and good energy confinement. Additional physics results are also presented in this review.
482 citations
TL;DR: Based on the drift-reduced Braginskii equations, the Global Bragginskii Solver (GBS) as mentioned in this paper is able to model the scrape-off layer (SOL) plasma turbulence in terms of the interplay between the plasma outflow from the tokamak core, the turbulent transport, and the losses at the vessel.
Abstract: Based on the drift-reduced Braginskii equations, the Global Braginskii Solver, GBS, is able to model the scrape-off layer (SOL) plasma turbulence in terms of the interplay between the plasma outflow from the tokamak core, the turbulent transport, and the losses at the vessel. Model equations, the GBS numerical algorithm, and GBS simulation results are described. GBS has been first developed to model turbulence in basic plasma physics devices, such as linear and simple magnetized toroidal devices, which contain some of the main elements of SOL turbulence in a simplified setting. In this paper we summarize the findings obtained from the simulation carried out in these configurations and we report the first simulations of SOL turbulence. We also discuss the validation project that has been carried out together with the GBS development.
193 citations
TL;DR: In this article, a fast numerical algorithm for treatment of the field particle operator was developed for the case of multi-species plasmas, where a Legendre series expansion in ξ (the cosine of the pitch angle) was combined with a novel Laguerre spectral method in energy to ameliorate the rapid numerical precision loss.
Abstract: The complete linearized Fokker–Planck collision operator has been implemented in the drift-kinetic code NEO (Belli and Candy 2008 Plasma Phys. Control. Fusion 50 095010) for the calculation of neoclassical transport coefficients and flows. A key aspect of this work is the development of a fast numerical algorithm for treatment of the field particle operator. This Eulerian algorithm can accurately treat the disparate velocity scales that arise in the case of multi-species plasmas. Specifically, a Legendre series expansion in ξ (the cosine of the pitch angle) is combined with a novel Laguerre spectral method in energy to ameliorate the rapid numerical precision loss that occurs for traditional Laguerre spectral methods. We demonstrate the superiority of this approach to alternative spectral and finite-element schemes. The physical accuracy and limitations of more commonly used model collision operators, such as the Connor and Hirshman–Sigmar operators, are studied, and the effects on neoclassical impurity poloidal flows and neoclassical transport for experimental parameters are explored.
168 citations
TL;DR: In this article, it was shown that the ordinary, collisionless trapped-electron mode is stable in large parts of parameter space in stellarators that have been designed so that the parallel adiabatic invariant decreases with radius.
Abstract: An overview is given of physics differences between stellarators and tokamaks, including magnetohydrodynamic equilibrium, stability, fast-ion physics, plasma rotation, neoclassical and turbulent transport and edge physics. Regarding microinstabilities, it is shown that the ordinary, collisionless trapped-electron mode is stable in large parts of parameter space in stellarators that have been designed so that the parallel adiabatic invariant decreases with radius. Also, the first global, electromagnetic, gyrokinetic stability calculations performed for Wendelstein 7-X suggest that kinetic ballooning modes are more stable than in a typical tokamak.
138 citations
TL;DR: In this article, the role of magnetic fields in complex complex (complex) plasma experiments is discussed, and the magnetized dusty plasmas experiment device is described. But the experimental results are limited.
Abstract: This paper discusses the role of magnetic fields in dusty (complex) plasma experiments. It first provides a description of the conditions necessary for a dusty plasma to become fully magnetized. The paper then briefly reviews a series of experimental studies that illustrate how magnetic fields are applied to dusty plasmas—from experiments that use magnetic fields to control the background plasma to those that have strong enough magnetic fields to directly modify the confinement and dynamics of the charged microparticles. The paper will then discuss the newest experiment that is currently under development at Auburn University, the magnetized dusty plasma experiment device. The paper concludes with a discussion of important outstanding physics and technical issues that will define the next generation of experiments.
133 citations
TL;DR: In this paper, a particle-in-cell approach based on a synergistic approach of theoretical modeling, experiments and kinetic simulations based on the particle in-cell (PIC) approach is proposed for the control of ion properties in capacitive radio frequency (RF) discharge plasmas.
Abstract: Capacitive radio frequency (RF) discharge plasmas have been serving hi-tech industry (eg chip and solar cell manufacturing, realization of biocompatible surfaces) for several years Nonetheless, their complex modes of operation are not fully understood and represent topics of high interest The understanding of these phenomena is aided by modern diagnostic techniques and computer simulations From the industrial point of view the control of ion properties is of particular interest; possibilities of independent control of the ion flux and the ion energy have been utilized via excitation of the discharges with multiple frequencies ‘Classical’ dual-frequency (DF) discharges (where two significantly different driving frequencies are used), as well as discharges driven by a base frequency and its higher harmonic(s) have been analyzed thoroughly It has been recognized that the second solution results in an electrically induced asymmetry (electrical asymmetry effect), which provides the basis for the control of the mean ion energy This paper reviews recent advances on studies of the different electron heating mechanisms, on the possibilities of the separate control of ion energy and ion flux in DF discharges, on the effects of secondary electrons, as well as on the non-linear behavior (self-generated resonant current oscillations) of capacitive RF plasmas The work is based on a synergistic approach of theoretical modeling, experiments and kinetic simulations based on the particle-in-cell approach
106 citations
TL;DR: In this article, the effect of superthermality on the characteristics of electrostatic plasma waves is discussed. But the authors focus on nonlinear excitations (solitons), in the form of solitary waves (pulses), shocks and envelope solitons, and employ standard methodological tools of nonlinear plasmadynamical analysis.
Abstract: Space plasmas provide abundant evidence of highly energetic particle population, resulting in a long-tailed non-Maxwellian distribution. Furthermore, the first stages in the evolution of plasmas produced during laser–matter interaction are dominated by nonthermal electrons, as confirmed by experimental observation and computer simulations. This phenomenon is efficiently modelled via a kappa-type distribution. We present an overview, from first principles, of the effect of superthermality on the characteristics of electrostatic plasma waves. We rely on a fluid model for ion-acoustic excitations, employing a kappa distribution function to model excess superthermality of the electron distribution. Focusing on nonlinear excitations (solitons), in the form of solitary waves (pulses), shocks and envelope solitons, and employing standard methodological tools of nonlinear plasmadynamical analysis, we discuss the role of excess superthermality in their propagation dynamics (existence laws, stability profile), geometric characteristics and stability. Numerical simulations are employed to confirm theoretical predictions, namely in terms of the stability of electrostatic pulses, as well as the modulational stability profile of bright- and dark-type envelope solitons.
101 citations
TL;DR: In this paper, the authors used two Langmuir probes moving in parallel from the exit of the driver up to the extraction region neighbourhood of the IPP RF negative hydrogen ion source prototype.
Abstract: Axially resolved measurements of plasma parameters were performed by two Langmuir probes moving in parallel from the exit of the driver (where the plasma is generated) up to the extraction region neighbourhood of the IPP RF negative hydrogen ion source prototype. At the driver exit, the plasma parameters show an unexpected inhomogeneity in the presence of the magnetic field: a cold and dense plasma flows out of the top part of the driver while a hot and low density plasma flows from the bottom part. A local relation between the top and bottom parameters is derived from the conservation of the energy flux.
81 citations
TL;DR: In this paper, the penetration dynamics of the resonant magnetic perturbation (RMP) field is simulated in the full toroidal geometry, under realistic plasma conditions in MAST experiments.
Abstract: The penetration dynamics of the resonant magnetic perturbation (RMP) field is sim-
ulated in the full toroidal geometry, under realistic plasma conditions in MAST experiments.
The physics associated with several aspects of the RMP penetration - the plasma response
and rotational screening, the resonant and non-resonant torques and the toroidal momentum
balance - are highlighted. In particular, the plasma response is found to significantly amplify
the non-resonant component of the RMP field for some of the MAST plasmas. A fast rotating
plasma, in response to static external magnetic fields, experiences a more distributed electro-
magnetic torque due to the resonance with continuum waves in the plasma. At fast plasma
flow (such as for the MAST plasma), the electromagnetic torque is normally dominant over
the neoclassical toroidal viscous (NTV) torque. However, at sufficiently slow plasma flow,
the NTV torque can play a significant role in the toroidal momentum balance, thanks to the
precession drift resonance enhanced, so called superbanana plateau regime.
80 citations
TL;DR: The full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions, yielding higher plasma temperatures after the thermal quench and thus longer current quench times.
Abstract: The new full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions. The main difference is a significantly lower fraction (by up to a factor of 5) of energy radiated during the disruption process, yielding higher plasma temperatures after the thermal quench and thus longer current quench times. Thus, a larger fraction of the total energy was conducted to the wall resulting in larger heat loads. Active mitigation by means of massive gas injection became a necessity to avoid beryllium melting already at moderate levels of thermal and magnetic energy (i.e. already at plasma currents of 2 MA). A slower current quench, however, reduced the risk of runaway generation. Another beneficial effect of the ILW is that disruptions have a negligible impact on the formation and performance of the subsequent discharge.
79 citations
TL;DR: In this article, a computational method is presented to determine the tokamak actuator time evolution (trajectories) required to optimally reach a given point in the operating space while satisfying a set of constraints.
Abstract: A computational method is presented to determine the tokamak actuator time evolution (trajectories) required to optimally reach a given point in the tokamak operating space while satisfying a set of constraints. Usually, trajectories of plasma auxiliary heating, current drive and plasma current required during the transient phases of a tokamak shot to reach a desired shape of the plasma temperature and safety factor (q) profiles are determined by trial-and-error by physics operators. In this paper, these trajectories are calculated by solving a non-linear, constrained, finite-time optimal control problem.The optimization problem contains a physics model of the non-linear plasma profile dynamics, a cost function to be minimized, and a set of constraints on the actuators and plasma quantities. The method is tested by optimizing the trajectories of Ip, heating and current drive power to obtain a typical hybrid plasma q profile at the end of the current ramp-up phase, while minimizing both the Ohmic flux swing and the distance from a stationary condition, and requiring q > 1 and edge Vloop > 0 at all times. The optimized trajectories feature an Ip overshoot similar to that used in existing experiments, and are shown to perform significantly better than a set of non-optimized trajectories, allowing stationary profiles to be obtained at the beginning of the flat-top phase. Additional information is obtained, including the parameter sensitivity of the optimal solution, a linear model describing the linearized dynamics of the profiles around the optimal trajectory, as well as a classification of the actuator trajectories based on the critical constraint which limits their value at a given time. This provides a solid basis for subsequent closed-loop feedback controller design. The tools presented in this paper could be useful to improve existing tokamak operational scenarios, to prepare operation of future machines and optimize their design.
TL;DR: In this paper, a parallel high-Z impurity transport theory is extended to account for cyclotron effects and shown to agree with experimentally measured impurity density asymmetries.
Abstract: In the Alcator C-Mod tokamak, strong, steady-state variations of molybdenum density within a flux surface are routinely observed in plasmas using hydrogen minority ion cyclotron resonant heating. In/out asymmetries, up to a factor of 2, occur with either inboard or outboard accumulation depending on the major radius of the minority resonance layer. These poloidal variations can be attributed to the impurity's high charge and large mass in the neoclassical parallel force balance. The large mass enhances the centrifugal force, causing outboard accumulation while the high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the plasma potential. Quantitative comparisons between existing parallel high-Z impurity transport theories and experimental results for r/a < 0.7 show good agreement when the resonance layer is on the high-field side of the tokamak but disagree substantially for low-field side heating. Ion-impurity friction is insufficient to explain the experimental results, and the accumulation of impurity density on the inboard side of flux surface is shown to be driven by a poloidal potential variation due to magnetic trapping of non-thermal, cyclotron heated minority ions. Parallel impurity transport theory is extended to account for cyclotron effects and shown to agree with experimentally measured impurity density asymmetries.
TL;DR: In this paper, the authors examined the resulting changes to neoclassical flow and current in H-mode edge pedestals and found that the parallel and poloidal flows deviate strongly from the best available conventional neoclassic prediction, with large poloidal variation of a different form than in the local theory.
Abstract: In transport barriers, particularly H-mode edge pedestals, radial scale lengths can become comparable to the ion orbit width, causing neoclassical physics to become radially nonlocal. In this work, the resulting changes to neoclassical flow and current are examined both analytically and numerically. Steep density gradients are considered, with scale lengths comparable to the poloidal ion gyroradius, together with strong radial electric fields sufficient to electrostatically confine the ions. Attention is restricted to relatively weak ion temperature gradients (but permitting arbitrary electron temperature gradients), since in this limit a δf (small departures from a Maxwellian distribution) rather than full-f approach is justified. This assumption is in fact consistent with measured inter-ELM H-Mode edge pedestal density and ion temperature profiles in many present experiments, and is expected to be increasingly valid in future lower collisionality experiments. In the numerical analysis, the distribution function and Rosenbluth potentials are solved for simultaneously, allowing use of the exact field term in the linearized Fokker–Planck–Landau collision operator. In the pedestal, the parallel and poloidal flows are found to deviate strongly from the best available conventional neoclassical prediction, with large poloidal variation of a different form than in the local theory. These predicted effects may be observable experimentally. In the local limit, the Sauter bootstrap current formulae appear accurate at low collisionality, but they can overestimate the bootstrap current in the plateau regime. In the pedestal ordering, ion contributions to the bootstrap and Pfirsch–Schluter currents are also modified.
TL;DR: In this paper, a new technique has been developed to produce plasmas with improved confinement relative to the H-98,H-y2 scaling law on the JET tokamak.
Abstract: A new technique has been developed to produce plasmas with improved confinement relative to the H-98,H-y2 scaling law (ITER Physics Expert Groups on Confinement and Transport and Confinement Modelling and Database ITER Physics Basics Editors and ITER EDA 1999 Nucl. Fusion 39 2175) on the JET tokamak. In the mid-size tokamaks ASDEX upgrade and DIII-D heating during the current formation is used to produce a flat q-profile with a minimum close to 1. On JET this technique leads to q-profiles with similar minimum q but opposite to the other tokamaks not to an improved confinement state. By changing the method utilizing a faster current ramp with temporary higher current than in the flattop (current overshoot) plasmas with improved confinement (H-98,H-y2 = 1.35) and good stability (beta(N) approximate to 3) have been produced and extended to many confinement times only limited by technical constraints. The increase in H-98,H-y2-factor is stronger with more heating power as can be seen in a power scan. The q-profile development during the high power phase in JET is reproduced by current diffusion calculated by TRANSP and CRONOS. Therefore the modifications produced by the current overshoot disappear quickly from the edge but the confinement improvement lasts longer, in some cases up to the end of the heating phase.
TL;DR: In this article, the residual gases of discharges with and without nitrogen seeding at ASDEX upgrade were investigated by mass spectrometry, and significant formation of ammonia was observed during nitrogen-seeded H-mode discharges.
Abstract: The seeding of gaseous impurities will be mandatory for ITER to protect the tungsten divertor from local heat loads. A promising candidate for radiative cooling seems to be nitrogen, but reactions with hydrogen may influence tokamak operation. In particular, the formation of ammonia can become a serious issue for gas plants, cryo pumps and wall conditioning. Therefore, the residual gases of discharges with and without nitrogen seeding at ASDEX Upgrade were investigated by mass spectrometry. For the deconvolution of the measured spectra a method was developed that takes into account different protium concentrations in different compounds. The applied absolute calibration of the mass spectrometers allowed a quantitative analysis. Significant formation of ammonia was observed during nitrogen-seeded H-mode discharges. Up to 8% of the seeded nitrogen atoms were detected in the form of ammonia molecules. Ammonia was present not only in the residual gas of the nitrogen-seeded discharge itself, but also in the residual gases of subsequent unseeded discharges. For calibration purposes ammonia was injected into the plasma vessel of ASDEX Upgrade without plasma operation. A significant part of the ammonia was retained in the vessel. The simultaneous observation of partly deuterated ammonia strongly indicates an interaction between metal walls and ammonia.
TL;DR: In this paper, the edge moments of the toroidal current density profile were identified in an MHD equilibrium reconstruction code using only external magnetic measurements, which was demonstrated analytically for simple plasma shapes and applied to experimental data on the ASDEX Upgrade tokamak where CLISTE reconstructions from magnetic data are shown to be consistent with those obtained from a more complete set of diagnostic data.
Abstract: The flux surface topology of a toroidal plasma bounded by a magnetic separatrix allows edge moments of the toroidal current density profile to be identified in an MHD equilibrium reconstruction code using only external magnetic measurements. This is demonstrated analytically for simple plasma shapes and applied to experimental data on the ASDEX Upgrade tokamak where CLISTE reconstructions from magnetic data are shown to be consistent with those obtained from a more complete set of diagnostic data. An independent demonstration of edge current profile recoverability is obtained by analysing the reconstruction errors for a database of Monte Carlo-generated equilibria.
TL;DR: In this article, a new ray-tracing code named C3PO has been developed to study the propagation of arbitrary electromagnetic radio-frequency (RF) waves in magnetized toroidal plasmas.
Abstract: A new ray-tracing code named C3PO has been developed to study the propagation of arbitrary electromagnetic radio-frequency (rf) waves in magnetized toroidal plasmas. Its structure is designed for maximum flexibility regarding the choice of coordinate system and dielectric model. The versatility of this code makes it particularly suitable for integrated modeling systems. Using a coordinate system that reflects the nested structure of magnetic flux surfaces in tokamaks, fast and accurate calculations inside the plasma separatrix can be performed using analytical derivatives of a spline-Fourier interpolation of the axisymmetric toroidal MHD equilibrium. Applications to reverse field pinch magnetic configuration are also included. The effects of 3D perturbations of the axisymmetric toroidal MHD equilibrium, due to the discreteness of the magnetic coil system or plasma fluctuations in an original quasi-optical approach, are also studied. Using a Runge–Kutta–Fehlberg method for solving the set of ordinary differential equations, the ray-tracing code is extensively benchmarked against analytical models and other codes for lower hybrid and electron cyclotron waves.
TL;DR: In this paper, the authors summarized recent progress in the theory of a snowflake divertor, a possible path to reduce both steady-state and intermittent heat loads on the divertor plates to an acceptable level.
Abstract: This paper summarizes recent progress in the theory of a snowflake divertor, a possible path to reduce both steady-state and intermittent heat loads on the divertor plates to an acceptable level. The most important feature of a SF divertor is the presence of a large zone of a very weak poloidal magnetic field around the poloidal field (PF) null. Qualitative explanation of a variety of new features characteristic of a SF divertor is provided based on simple scaling relations. The main part of the paper is focused on the concept of spreading of the heat flux by curvature-driven convection near the PF null. References to experimental results from the NSTX and TCV tokamaks are provided.
TL;DR: In this paper, it was shown that the ion heating energy is proportional to the square of the reconnection magnetic field component, which indicates the significant ion heating effect of magnetic reconnection, leading to a new high-field reconnection heating experiment for fusion plasmas.
Abstract: Recently, the TS-3 and TS-4 tokamak merging experiments revealed significant plasma heating during magnetic reconnection. A key question is how and where ions and electrons are heated during magnetic reconnection. Two-dimensional measurements of ion and electron temperatures and plasma flow made clear that electrons are heated inside the current sheet mainly by the Ohmic heating and ions are heated in the downstream areas mainly by the reconnection outflows. The outflow kinetic energy is thermalized by the fast shock formation and viscous damping. The magnetic reconnection converts the reconnecting magnetic field energy mostly to the ion thermal energy in the outflow region whose size is much larger than the current sheet size for electron heating. The ion heating energy is proportional to the square of the reconnection magnetic field component . This scaling of reconnection heating indicates the significant ion heating effect of magnetic reconnection, which leads to a new high-field reconnection heating experiment for fusion plasmas.
TL;DR: In this article, the reduction of the H/(H+D) ratio and particle recycling in the EAST superconducting tokamak has been studied and several new milestones of EAST performance were achieved.
Abstract: Reductions in H content and particle recycling are important for the improvement of ion cyclotron range of frequency (ICRF) minority heating efficiency and the enhancement of plasma performance of the EAST superconducting tokamak. During recent years several techniques of surface conditioning such as baking, glow discharge cleaning/ICRF discharge cleaning, surface coatings, such as boronization, siliconization and lithium coating, have all been attempted in order to reduce the H/(H+D) ratio and particle recycling in EAST. Even though boronization and siliconization were both reasonably effective methods to improve plasma performance, lithium coatings were observed to reduce the H content and particle recycling to levels low enough to allow the attainment of enhanced plasma parameters and operating modes on EAST. For example, by accomplishing lithium coating using either vacuum evaporation or the real-time injection of fine lithium powder, the H/(H+D) ratio could be routinely decreased to about 5%, which significantly improved ICRF minority heating efficiency during the autumn campaign of 2010. Due to the reduced H/(H+D) ratio and lower particle recycling, and a reduced H-mode power threshold, improved plasma confinement and the first EAST H-mode plasma were obtained. Furthermore, with increasing accumulation of deposited lithium, several new milestones of EAST performance, such as a 6.4 s-long H-mode, a 100 s-long plasma duration and a 1 MA plasma current, were achieved in the 2010 autumn campaign.
TL;DR: The ion temperature in turbulent plasma filaments is measured by a retarding field analyser during L-mode discharges in the ASDEX upgrade tokamak as mentioned in this paper, which is 3-4 times that of the background ions and 50-70% of the ion temperature at the separatrix.
Abstract: The ion temperature in turbulent plasma filaments Ti fil is measured by a retarding field analyser during L-mode discharges in the ASDEX Upgrade tokamak. At 2 cm outside the separatrix Ti fil ≈ 80–110 eV, which is 3–4 times that of the background ions and 50–70% of the ion temperature at the separatrix. Ti fil is reproduced by a fluid model of the parallel filament transport assuming the radial filament propagation speed of 400–1000 m s−1 in the near scrape-off layer, consistent with earlier experimental estimates. The conditional sampling used in experiment to measure Ti fil is tested on artificial time series obtained from a gyrofluid turbulence simulation.
TL;DR: In this paper, the linear and nonlinear properties of large-amplitude electron-acoustic waves are investigated in a magnetized plasma comprising two distinct electron populations (hot and cold) and immobile ions.
Abstract: The linear and nonlinear properties of large-amplitude electron-acoustic waves are investigated in a magnetized plasma comprising two distinct electron populations (hot and cold) and immobile ions. The hot electrons are assumed to be in a non-Maxwellian state, characterized by an excess of superthermal particles, here modeled by a kappa-type long-tailed distribution function. Waves are assumed to propagate obliquely to the ambient magnetic field. Two types of electrostatic modes are shown to exist in the linear regime, and their properties are briefly analyzed. A nonlinear pseudopotential-type analysis reveals the existence of large-amplitude electrostatic solitary waves and allows for an investigation of their propagation characteristics and existence domain, in terms of the soliton speed (Mach number). The effects of the key plasma configuration parameters, namely the superthermality index and the cold electron density, on the soliton characteristics and existence domain, are studied. The role of obliqueness and magnetic field is discussed.
TL;DR: In this paper, the L-mode interchange turbulence in the edge and scrape-off-layer (SOL) of the tight aspect ratio tokamak MAST is investigated numerically.
Abstract: The L-mode interchange turbulence in the edge and scrape-off-layer (SOL) of the tight aspect ratio tokamak MAST is investigated numerically. The dynamics of the boundary plasma are studied using the 2D drift-fluid code ESEL, which has previously shown good agreement with large aspect ratio machines. In this context, a MAST-TCV comparison is presented in order to link the present analysis to well documented references. Next, scans of various edge parameters, such as density, temperature and current, are performed in the simulations with the aim of characterizing the profiles, fluctuation level and statistics of the edge/SOL density and temperature. In addition, we also discuss how the system changes when the length of the divertor leg is modified. This allows one to better understand the regime of operation of the Super-X divertor which will be implemented on MAST-Upgrade. The results obtained qualitatively agree with experimental observations. In particular, a universal behaviour of the fluctuation statistics is found for disparate edge conditions. Furthermore, the density and temperature decay lengths are inversely proportional to the plasma current and the edge temperature, while they are rather insensitive to the edge density (not to be confused with the line-averaged density).
TL;DR: In this article, the distortion of the plasma boundary when three-dimensional resonant magnetic perturbations (RMPs) are applied has been measured in MAST H-mode plasmas.
Abstract: The distortion of the plasma boundary when three-dimensional resonant magnetic perturbations (RMPs) are applied has been measured in MAST H-mode plasmas. When the n = 3 RMPs are applied to control edge-localized modes (ELMs), the plasma experiences a strong toroidal corrugation. The displacement of the plasma boundary is measured at various toroidal locations and found to be of the order of 5% of the minor radius for an applied field magnitude which mitigates ELMs. The empirically observed corrugation of the plasma edge position agrees well with three-dimensional ideal plasma equilibrium reconstruction.
TL;DR: In this article, a review of recent experiments performed on the University of Iowa dc discharge dusty plasma device on various aspects of dust acoustic waves is presented, including the observation and interpretation of large amplitude (nonlinear) dust acoustic wave; evolution of large-scale acoustic waves into shocks; and spontaneous formation of stationary, stable dust structures in moderately coupled dusty plasma (dust structurization).
Abstract: A review is presented of recent experiments performed on the University of Iowa dc discharge dusty plasma device on various aspects of dust acoustic waves. A brief introduction to the physics of dusty plasmas and the dust acoustic wave is first presented. Three experiments are then described: (i) observation and interpretation of large amplitude (nonlinear) dust acoustic waves; (ii) evolution of large amplitude dust acoustic waves into shocks, and comparison to numerical shock solutions of the generalized hydrodynamic equations and (iii) the spontaneous formation of stationary, stable dust structures in a moderately coupled dusty plasma (dust structurization).
TL;DR: A pedestal database was built using data from type-I ELMy H-modes of ASDEX Upgrade, DIII-D and JET as discussed by the authors, and the two-line method is a bilinear fit which shows better reproducibility of pedestal parameters than a modified hyperbolic tangent fit.
Abstract: A pedestal database was built using data from type-I ELMy H-modes of ASDEX Upgrade, DIII-D and JET. ELM synchronized pedestal data were analysed with the two-line method. The two-line method is a bilinear fit which shows better reproducibility of pedestal parameters than a modified hyperbolic tangent fit. This was tested with simulated and experimental data. The influence of the equilibrium reconstruction on pedestal parameters was investigated with sophisticated reconstructions from CLISTE and EFIT including edge kinetic profiles. No systematic deviation between the codes could be observed. The flux coordinate system is influenced by machine size, poloidal field and plasma shape. This will change the representation of the width in different coordinates, in particular, the two normalized coordinates ΨN and r/a show a very different dependence on the plasma shape. The scalings derived for the pedestal width, Δ, of all machines suggest a different scaling for the electron temperature and the electron density. Both cases show similar dependence with machine size, poloidal magnetic field and pedestal electron temperature and density. The influence of ion temperature and toroidal magnetic field is different on each of and . In dimensionless form the density pedestal width in ΨN scales with , the temperature pedestal width with . Both widths also show a strong correlation with the plasma shape. The shape dependence originates from the coordinate transformation and is not visible in real space. The presented scalings predict that in ITER the temperature pedestal will be appreciably wider than the density pedestal.
TL;DR: In this paper, a nonlinear drift-MHD (magnetohydrodynamical) magnetic island theory was proposed to predict the error-field penetration threshold in low density, ohmically heated, tokamak plasmas.
Abstract: A theory is developed to predict the error-field penetration threshold in low density, ohmically heated, tokamak plasmas. The novel feature of the theory is that the response of the plasma in the vicinity of the resonant surface to the applied error-field is calculated from nonlinear drift-MHD (magnetohydrodynamical) magnetic island theory, rather than linear layer theory. Error-field penetration, and subsequent locked mode formation, is triggered once the destabilizing effect of the resonant harmonic of the error-field overcomes the stabilizing effect of the ion polarization current (caused by the propagation of the error-field-induced island chain in the local ion fluid frame). The predicted scaling of the error-field penetration threshold with engineering parameters is (br/BT)crit ∼ ne B −1.8
TL;DR: In this article, two retarding field energy analysers (RFEAs) have been used in the mega amp spherical tokamak (MAST) to measure the ion temperature at both the midplane and the divertor in ohmic L-mode plasmas with a range of densities.
Abstract: Knowing the ion temperature in the scrape-off layer (SOL) of tokamaks is of great importance for understanding the heat flux to plasma facing components. Few measurements are available for SOL ion temperatures compared with electron temperatures due to the relative complexity of the measurement. Two retarding field energy analysers (RFEAs) have been used in the mega amp spherical tokamak (MAST) to measure the ion temperature at both the midplane and the divertor in ohmic L-mode plasmas with a range of densities. Midplane SOL Ti was found to be higher than Te by a factor of about 2 or greater. Divertor Ti measurements at the target showed Ti???Te having considered the effect of plasma flows on the divertor RFEA measurements.
TL;DR: In this paper, non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion.
Abstract: Non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion. These experiments use cryogenic fuel ice layers, consisting of mixtures of tritium and deuterium with large amounts of hydrogen to control the neutron yield and to allow fielding of an extensive suite of optical, x-ray and nuclear diagnostics. The thermonuclear fuel layer is contained in a spherical plastic capsule that is fielded in the center of a cylindrical gold hohlraum. Heating the hohlraum with 1.3 MJ of energy delivered by 192 laser beams produces a soft x-ray drive spectrum with a radiation temperature of 300 eV. The radiation field produces an ablation pressure of 100 Mbar which compresses the capsule to a spherical dense fuel shell that contains a hot plasma core 80 µm in diameter. The implosion core is observed with x-ray imaging diagnostics that provide size, shape, the absolute x-ray emission along with bangtime and hot plasma lifetime. Nuclear measurements provide the 14.1 MeV neutron yield from fusion of deuterium and tritium nuclei along with down-scattered neutrons at energies of 10–12 MeV due to energy loss by scattering in the dense fuel that surrounds the central hot-spot plasma. Neutron time-of-flight spectra allow the inference of the ion temperature while gamma-ray measurements provide the duration of nuclear activity. The fusion yield from deuterium–tritium reactions scales with ion temperature, which is in agreement with modeling over more than one order of magnitude to a neutron yield in excess of 1014 neutrons, indicating large confinement parameters on these first experiments.
TL;DR: In this paper, the authors carried out precision optimization of inertial confinement fusion ignition scale implosions and achieved hohlraum temperatures in excess of the 300 eV ignition goal with hot-spot symmetry and shock timing near ignition specs.
Abstract: We have carried out precision optimization of inertial confinement fusion ignition scale implosions. We have achieved hohlraum temperatures in excess of the 300 eV ignition goal with hot-spot symmetry and shock timing near ignition specs. Using slower rise pulses to peak power and extended pulses resulted in lower hot-spot adiabat and higher main fuel areal density at about 80% of the ignition goal. Yields are within a factor of 5–6 of that required to initiate alpha dominated burn. It is likely we will require thicker shells (+15–20%) to reach ignition velocity without mixing of ablator material into the hot spot.