Showing papers by "Princeton Plasma Physics Laboratory published in 2006"
TL;DR: In this paper, local shearing box simulations of turbulence driven by the magnetorotational instability (MRI) in a collisionless plasma are described, where the MHD version of ZEUS is modified to evolve an anisotropic pressure tensor.
Abstract: We describe local shearing box simulations of turbulence driven by the magnetorotational instability (MRI) in a collisionless plasma. Collisionless effects may be important in radiatively inefficient accretion flows, such as near the black hole in the Galactic center. The MHD version of ZEUS is modified to evolve an anisotropic pressure tensor. A fluid closure approximation is used to calculate heat conduction along magnetic field lines. The anisotropic pressure tensor provides a qualitatively new mechanism for transporting angular momentum in accretion flows (in addition to the Maxwell and Reynolds stresses). We estimate limits on the pressure anisotropy due to pitch angle scattering by kinetic instabilities. Such instabilities provide an effective "collision" rate in a collisionless plasma and lead to more MHD-like dynamics. We find that the MRI leads to efficient growth of the magnetic field in a collisionless plasma, with saturation amplitudes comparable to those in MHD. In the saturated state, the anisotropic stress is comparable to the Maxwell stress, implying that the rate of angular momentum transport may be moderately enhanced in a collisionless plasma.
214 citations
TL;DR: The ARIES-AT as discussed by the authors is a simple low-pressure design consisting of SiC composite boxes with a SiC insert for flow distribution that does not carry any structural load.
208 citations
TL;DR: In this paper, new viewpoints and unifying concepts are presented, which facilitate understanding of zonal flow physics, via theory, computation and their confrontation with the results of laboratory experiment.
Abstract: Zonal flows, which means azimuthally symmetric band-like shear flows, are ubiquitous phenomena in nature and the laboratory. It is now widely recognized that zonal flows are a key constituent in virtually all cases and regimes of drift wave turbulence, indeed, so much so that this classic problem is now frequently referred to as "drift wave-zonal flow turbulence." In this review, new viewpoints and unifying concepts are presented, which facilitate understanding of zonal flow physics, via theory, computation and their confrontation with the results of laboratory experiment. Special emphasis is placed on identifying avenues for further progress. © 2006 American Institute of Physics.
184 citations
TL;DR: Quantitative agreement between experiment and theory is found when the effect of toroidally trapped particles is included and the measured decrease of the plasma toroidal angular momentum profile is compared to calculations of nonresonant drag torque based on the theory of neoclassical toroidal viscosity.
Abstract: Dissipation of plasma toroidal angular momentum is observed in the National Spherical Torus Experiment due to applied nonaxisymmetric magnetic fields and their plasma-induced increase by resonant field amplification and resistive wall mode destabilization. The measured decrease of the plasma toroidal angular momentum profile is compared to calculations of nonresonant drag torque based on the theory of neoclassical toroidal viscosity. Quantitative agreement between experiment and theory is found when the effect of toroidally trapped particles is included.
182 citations
TL;DR: Predictions of the measured temperature and density perturbation profiles as well as deltaT(e)/deltan(e) from the ideal magnetohydrodynamic code NOVA are in close agreement with experiment.
Abstract: The spatial structure of toroidal Alfven eigenmodes and reversed shear Alfven eigenmodes in DIII-D is obtained from electron-cyclotron-emission measurements. Peak measured temperature perturbations are of similar magnitude for both toroidal Alfven eigenmodes and reversed shear Alfven eigenmodes and found to be deltaT(e)/T(e) approximately equal to 0.5%. Simultaneous measurements of density fluctuations using beam-emission spectroscopy indicate deltan(e)/n(e) approximately equal to 0.25%. Predictions of the measured temperature and density perturbation profiles as well as deltaT(e)/deltan(e) from the ideal magnetohydrodynamic code NOVA are in close agreement with experiment.
148 citations
TL;DR: Measurements of Dalpha emission indicate that global recycling coefficients decrease to approximately 0.3, the lowest documented for a magnetically confined hydrogen plasma.
Abstract: Extensive lithium wall coatings and liquid lithium plasma-limiting surfaces reduce recycling, with dramatic improvements in Ohmic plasma discharges in the Current Drive Experiment-Upgrade. Global energy confinement times increase by up to 6 times. These results exceed confinement scalings such as $\mathrm{ITER}98\mathrm{P}(y,1)$ by $2--3$ times, and represent the largest increase in energy confinement ever observed for an Ohmic tokamak plasma. Measurements of ${D}_{\ensuremath{\alpha}}$ emission indicate that global recycling coefficients decrease to approximately 0.3, the lowest documented for a magnetically confined hydrogen plasma.
144 citations
TL;DR: In this article, resistive wall mode (RWM) physics is studied to understand mode stabilization in high toroidal and normalized plasma beta, and βN ≡ 10 8 ǫ aB0/Ip through boundary and profile optimization.
Abstract: The National Spherical Torus Experiment (NSTX) has demonstrated the advantages of low aspect ratio geometry in accessing high toroidal and normalized plasma beta, and βN ≡ 10 8〈βt〉 aB0/Ip. Experiments have reached βt = 39% and βN = 7.2 through boundary and profile optimization. High βN plasmas can exceed the ideal no-wall stability limit, βNno-wall, for periods much greater than the wall eddy current decay time. Resistive wall mode (RWM) physics is studied to understand mode stabilization in these plasmas. The toroidal mode spectrum of unstable RWMs has been measured with mode number n up to 3. The critical rotation frequency of Bondeson-Chu, Ωcrit = ωA/(4q2), describes well the RWM stability of NSTX plasmas when applied over the entire rotation profile and in conjunction with the ideal stability criterion. Rotation damping and global rotation collapse observed in plasmas exceeding βNno-wall differs from the damping observed during tearing mode activity and can be described qualitatively by drag due to neoclassical toroidal viscosity in the helically perturbed field of an ideal displacement. Resonant field amplification of an applied n = 1 field perturbation has been measured and increases with increasing βN. Equilibria are reconstructed including measured ion and electron pressure, toroidal rotation and flux isotherm constraint in plasmas with core rotation ω/ωA up to 0.48. Peak pressure shifts of 18% of the minor radius from the magnetic axis have been reconstructed.
139 citations
TL;DR: Huttenlocher et al. as mentioned in this paper used the combination of electric probes, a radial array of views measuring Dα emission, and two-dimensional imaging of Dα emissions in the scrape-off layer of Alcator C-Mod.
Abstract: Radially propagating spatiotemporal fluctuation structures are observed in the scrape-off layer of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, (1994)] using the combination of electric probes, a radial array of views measuring Dα emission, and two-dimensional imaging of Dα emission. For a specific magnetic-field configuration the electric probe and the Dα array measured plasma density and potential fluctuations along the same magnetic-flux tube. Calculations of the cross-correlation functions of Dα intensity fluctuations with ion saturation current fluctuations and floating potential fluctuations, respectively, reveal that the potential associated with fluctuation structures is of dipole type, consistent with fundamental models for radial blob propagation. Radial and poloidal velocities of fluctuation structures are obtained by two-dimensional spatiotemporal turbulence imaging using an ultrafast framing camera observing the Dα emission intensity in the poloidal plane. In the poloidal directio...
137 citations
TL;DR: The resistive-wall mode is actively stabilized in the National Spherical Torus Experiment in high-beta plasmas rotating significantly below the critical rotation speed for passive stability and in the range predicted for the International Thermonuclear Experimental Reactor.
Abstract: The resistive-wall mode is actively stabilized in the National Spherical Torus Experiment in high-beta plasmas rotating significantly below the critical rotation speed for passive stability and in the range predicted for the International Thermonuclear Experimental Reactor. Variation of feedback stabilization parameters shows mode excitation or suppression. Stabilization of toroidal mode number unity did not lead to instability of toroidal mode number two. The mode can become unstable by deforming poloidally, an important consideration for stabilization system design.
125 citations
TL;DR: High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas.
Abstract: High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas. The combination of kink and local pressure driven instabilities leads to a small poloidally localized puncture in the magnetic surface at both the low and the high field sides of the poloidal plane. This observation closely resembles the ``fingering event'' of the ballooning mode model with the high-$m$ mode only predicted at the low field side.
113 citations
TL;DR: A review of the use of gamma ray spectrometry as a diagnostic of nuclear reaction rates and nuclear reaction product densities in high temperature fusion plasmas is presented in this paper.
Abstract: A review of the use of gamma ray spectrometry as a diagnostic of nuclear reaction rates and nuclear reaction product densities in high temperature fusion plasmas is presented. In this review we will discuss the historic genesis of the concept, a brief overview of the relevant nuclear physics, the experimental techniques utilized in the measurements and some of the analytical techniques required to extract the diagnostic information from the basic measurements. Of particular interest is the ability to measure the population of confined fast alpha particles in future burning plasma experiments.
TL;DR: It is shown that in the resistive limit, HMRI is a weakly destabilized inertial oscillation propagating in a unique direction along the axis, and features of HMRI that make it less attractive for experiments and for resistive astrophysical disks are reported.
Abstract: Hollerbach and Ruediger have reported a new type of magnetorotational instability (MRI) in magnetized Taylor-Couette flow in the presence of combined axial and azimuthal magnetic fields. The salient advantage of this 'helical' MRI (HMRI) is that marginal instability occurs at arbitrarily low magnetic Reynolds and Lundquist numbers, suggesting that HMRI might be easier to realize than standard MRI (axial field only), and that it might be relevant to cooler astrophysical disks, especially those around protostars, which may be quite resistive. We confirm previous results for marginal stability and calculate HMRI growth rates. We show that in the resistive limit, HMRI is a weakly destabilized inertial oscillation propagating in a unique direction along the axis. But we report other features of HMRI that make it less attractive for experiments and for resistive astrophysical disks. Large axial currents are required. More fundamentally, instability of highly resistive flow is peculiar to infinitely long or periodic cylinders: finite cylinders with insulating endcaps are shown to be stable in this limit, at least if viscosity is neglected. Also, Keplerian rotation profiles are stable in the resistive limit regardless of axial boundary conditions. Nevertheless, the addition of a toroidal field lowers thresholds for instability even inmore » finite cylinders.« less
TL;DR: High temporal and spatial resolution two-dimensional images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas and the observed experimental 2D images are only partially in agreement with the expected patterns from each model.
Abstract: High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model.
TL;DR: A motional Stark effect diagnostic has been utilized to reconstruct the parallel current density profile in a spherical-torus plasma for the first time, and a current profile anomaly is observed during saturated interchange-type instability activity.
Abstract: A motional Stark effect diagnostic has been utilized to reconstruct the parallel current density profile in a spherical-torus plasma for the first time. The measured current profile compares favorably with neoclassical theory when no large-scale magnetohydrodynamic instabilities are present in the plasma. However, a current profile anomaly is observed during saturated interchange-type instability activity. This apparent anomaly can be explained by redistribution of neutral beam injection current drive and represents the first observation of interchange-type instabilities causing such redistribution. The associated current profile modifications contribute to sustaining the central safety factor above unity for over five resistive diffusion times, and similar processes may contribute to improved operational scenarios proposed for ITER.
TL;DR: In this article, a particle-in-cell code was developed for simulations of plasmas of Hall thruster discharges, where an external, accelerating electric field directed parallel to the wall and an external magnetic field directed normal to the walls were applied.
Abstract: A particle-in-cell code has been developed for simulations of plasmas of Hall thruster discharges. The simulated system is a plasma slab bounded by dielectric walls with secondary electron emission. An external, accelerating electric field directed parallel to the walls and an external magnetic field directed normal to the walls are applied. The strongly anisotropic non-Maxwellian electron velocity distribution function is obtained in simulations. The average energy of electron motion parallel to the walls is defined by collisional heating in the accelerating electric field. This energy is much higher than the average energy of electron motion normal to the walls, which is determined by the energy of electrons produced in ionization and by scattering of electrons by neutral atoms. The electron distribution function for velocity components normal to the walls is depleted for energies above the near-wall plasma potential. The effects of Coulomb collisions on the electron velocity distribution function and electron wall losses are studied
TL;DR: In this article, the parametric dependences of both the global and thermal energy confinement times at low aspect ratio in high power National Spherical Torus Experiment discharges were analyzed.
Abstract: Systematic and statistical studies have been conducted in order to develop an understanding of the parametric dependences of both the global and thermal energy confinement times at low aspect ratio in high power National Spherical Torus Experiment discharges. The global and thermal confinement times of both L- and H-mode discharges can exceed values given by H-mode scalings developed for conventional aspect ratio. Results of systematic scans in the H-mode indicate that the confinement times exhibit a nearly linear dependence on plasma current and a power degradation weaker than that observed at conventional aspect ratio. In addition, the dependence on the toroidal magnetic field is stronger than that seen in conventional aspect ratio tokamaks. This latter trend is also evident in statistical analyses of the available dataset. These statistical studies also indicate a weaker parametric dependence on plasma current than found in the systematic scans, due to correlations among the predictor variables. Regressions based on engineering variables, when transformed to dimensionless physics variables, indicate that the dependence of BτE on βt can range from being negative to null. Regressions based directly on the dimensionless physics variables are inexact because of large correlations among these variables. Scatter in the confinement data, at otherwise fixed operating parameters, is found to be due to variations in ELM activity, low frequency density fluctuations and plasma shaping.
TL;DR: In this paper, two modifications have been proposed to suppress the spread of beamlet-bundle which may be caused by beamlet−beamlet interaction in the multi-aperture grid due to the space charge force.
Abstract: The 500 keV negative-ion based neutral beam injector for JT-60U started operations in 1996. The availability of the negative ion based neutral beam injection system has been improved gradually by modifying the ion source and optimizing its operation parameters. Recently, the extension of the pulse duration up to 30 s has been intended to study quasi-steady state plasma on JT-60U. The most serious issue is to reduce the heat load on the grids for long pulse operation. Two modifications have been proposed to reduce the heat load. One is to suppress the spread of beamlet-bundle which may be caused by beamlet–beamlet interaction in the multi-aperture grid due to the space charge force. Indeed, the investigation of the beam deflection, which was measured by the infrared camera on the target plate set 3.5 m away from the grid, indicates that the spread of beamlet-bundle is in proportion to the current density. Field-shaping plates were attached on the extraction grid to modify the local electric field. The plate thickness was optimized to steer the beamlet deflection. The other is to reduce the stripping loss, where the electron of the negative ion beam is stripped and accelerated in the accelerator and then collides with the grids. The ion source was modified to reduce the pressure in the accelerator column to suppress the beam-ion stripping loss. To date, long pulse injection of 19 s of 1.5–1.6 MW at a high energy beam of 360 keV, 9–10 A for D− has been obtained by one ion source with these modifications.
TL;DR: In this article, the authors investigated the mechanisms responsible for the radial transport and loss of fast ions and identified the HAGIS code, which describes the interaction of the fast ion and the toroidal Alfven eigenmodes (TAEs) observed.
Abstract: The confinement of fast particles is of crucial importance for the success of future burning plasma experiments. On JET, the confinement of ion cyclotron resonant frequency (ICRF) accelerated fast hydrogen ions with energies exceeding 5 MeV has been measured using the characteristic γ-rays emitted through their inelastic scattering with carbon impurities, 12C(p,p'γ)12C. Recent experiments have shown a significant decrease in this γ-ray emission (by a factor of 2) during so-called tornado mode activity (core-localized toroidal Alfven eigenmodes (TAEs) within the q = 1 surface) in sawtoothing plasmas. This is indicative of a significant loss or extensive re-distribution of these (>5 MeV) particles from the plasma core. In this paper, mechanisms responsible for the radial transport and loss of these fast ions are investigated and identified using the HAGIS code, which describes the interaction of the fast ions and the TAE observed. The calculations show that the overlap of wave-particle resonances in phase-space leads to an enhanced radial transport and loss. On both JET and ASDEX Upgrade, new fast ion loss detectors have been installed to further investigate the loss of such particles. On JET, fast ion loss detectors based around an array of Faraday cups and a scintillator probe have been installed as part of a suite of diagnostic enhancements. On ASDEX Upgrade, a new fast ion loss detector has been mounted on the mid-plane manipulator allowing high resolution measurements in pitch angle, energy and time. This has enabled the direct observation of fast ion losses during various magnetohydrodynamics (MHD) phenomena to be studied in detail. Edge localised mode (ELM) induced fast ion losses have been directly observed along with the enhancement of fast ion losses from specific areas of phase-space in the presence of neoclassical tearing modes (NTMs) and TAEs.
TL;DR: Several different tokamak plasma instabilities and the control approaches for their stabilization are described and the progress achieved at toKamak devices around the world is described.
Abstract: This paper describes several different tokamak plasma instabilities and the control approaches for their stabilization. It also describes the progress achieved at tokamak devices around the world
TL;DR: Evidence is presented for a multitude of discrete frequency Alfvén waves in the core of magnetically confined high-temperature fusion plasmas andoretical analysis confirms a dominant interaction of the modes with particles in the thermal ion distribution traveling well below the AlfVén velocity.
Abstract: Evidence is presented for a multitude of discrete frequency Alfven waves in the core of magnetically confined high-temperature fusion plasmas. Multiple diagnostic instruments confirm wave excitation over a wide spatial range from the device size at the longest wavelengths down to the thermal ion Larmor radius. At the shortest scales, the poloidal wavelengths are comparable to the scale length of electrostatic drift wave turbulence. Theoretical analysis confirms a dominant interaction of the modes with particles in the thermal ion distribution traveling well below the Alfven velocity.
TL;DR: In this article, the authors considered the advanced tokamak as the basis for a fusion power plant and proposed the ARIES-AT design with an aspect ratio of A ≡ R / a = 4.0, an elongation and triangularity of κ = 2.20, δ = 0.90, and a toroidal beta of β = 9.1 %.
TL;DR: Ono et al. as mentioned in this paper proposed that when multiple modes with resonances closely spaced in phase space reach sufficient amplitude so that the fast ion trajectories overlap, very rapid nonlinear growth can occur.
Abstract: Super-thermal fast ions provide a source of free energy to excite instabilities, which in turn can enhance the loss of fast ions. It has been proposed that when multiple modes with resonances closely spaced in phase space reach sufficient amplitude so that the fast ion trajectories overlap, very rapid non-linear growth can occur. The modification of the fast ion distribution by this loss may in turn excite additional, otherwise stable, modes, leading to an 'avalanche' effect greatly enhancing the transport of fast ions (Berk H.L., Breizman B.N., Fitzpatrick J. and Wong H.V. 1995 Nucl. Fusion 35 1661). It has been proposed that in ITER (ITER Physics Basis Editors et al 1999 Nucl. Fusion 39 2137), the transport of fast ions will be through a similar interaction of many modes. In NSTX (Ono M. et al 2000 Nucl. Fusion 40 557) bursts of multiple TAE-like instabilities are correlated with fast ion losses in a manner which qualitatively resembles avalanche behaviour.
TL;DR: In this paper, a 2 kW Hall thruster with segmented electrodes made of carbon fiber velvet was operated in the discharge voltage range of 200-700 V. The arcing between the floating velvet electrodes and the plasma was visually observed, especially during the initial conditioning time, which lasted for about 1 h.
Abstract: Carbon fiber velvet material provides exceptional sputtering resistance properties exceeding those for graphite and carbon composite materials. A 2 kW Hall thruster with segmented electrodes made of this material was operated in the discharge voltage range of 200–700 V. The arcing between the floating velvet electrodes and the plasma was visually observed, especially, during the initial conditioning time, which lasted for about 1 h. The comparison of voltage versus current and plume characteristics of the Hall thruster with and without segmented electrodes indicates that the magnetic insulation of the segmented thruster improves with the discharge voltage at a fixed magnetic field. The observations reported here also extend the regimes wherein the segmented Hall thruster can have a narrower plume than that of the conventional nonsegmented thruster.
TL;DR: In this paper, the authors investigated the timescale for the formation of cold-dense plasma sheet ions with an event in which the interplanetary magnetic field (IMF) was northward for almost one day.
Abstract: [1] The timescale for the formation of cold-dense plasma sheet ions was investigated with an event in which the interplanetary magnetic field (IMF) was northward for almost one day. The plasma sheet dawn and dusk flanks appear to reach cold dense states (n > 1 cm−3; T < 2 keV) within a few hours after IMF northward turning. Closer to the center (midnight meridian), the ion temperatures reach < 2 keV within a few hours of IMF northward turning, but the ion densities do not reach above 1 cm−3 for at least ∼8 hours after IMF northward turning. The connection between solar wind ions and plasma sheet cold-component ions is demonstrated. The plasma sheet dawn flank ions appear to lag the solar wind ions by about 3 hours. This study confirms the previous statistical results: (a) the densification of the plasma sheet can be attributed to the influx of the cold-component (magnetosheath/solar wind origin) ions; and (b) the cooling of the plasma sheet can be attributed not only to the influx of the solar wind ions, but also to the cooling of the hot components. Order of magnitude calculations of the plasma sheet filling rate from reconnection and diffusion suggest that both entry mechanisms could result in roughly comparable filling rates. Hence, the dawn-dusk asymmetries would be key in distinguishing the roles of the various proposed entry mechanisms.
TL;DR: A bioinformatic web server for detecting amino acid sites or regions of a protein under positive selection by sliding a 3D window across one reference structure by estimating the ratio of non-synonymous to synonymous substitution rates (KA/KS).
Abstract: We present a bioinformatic web server (SWAKK) for detecting amino acid sites or regions of a protein under positive selection. It estimates the ratio of non-synonymous to synonymous substitution rates (KA/KS) between a pair of protein-coding DNA sequences, by sliding a 3D window, or sphere, across one reference structure. The program displays the results on the 3D protein structure. In addition, for comparison or when a reference structure is unavailable, the server can also perform a sliding window analysis on the primary sequence. The SWAKK web server is available at http://oxytricha.princeton.edu/SWAKK/.
TL;DR: In this paper, the authors studied the resistivity of a reconnecting current sheet in the presence and absence of the guide field and found that the resistivities were in a good agreement with the parallel and transverse Spitzer values, respectively.
Abstract: Plasma resistivity has been studied experimentally in a reconnecting current sheet. Resistivities during collisional reconnection, when the electron mean free path is much shorter than the current sheet thickness, in the presence and absence of the guide field are found to be in a good agreement with the parallel and transverse Spitzer values, respectively.
TL;DR: In this paper, the authors developed a multiple interface variational model, comprising multiple Taylor-relaxed plasma regions separated by ideal magnetohydrodynamic (MHD) barriers.
Abstract: We develop a multiple interface variational model, comprising multiple Taylor-relaxed plasma regions separated by ideal magnetohydrodynamic (MHD) barriers. A principal motivation is the development of a mathematically rigorous ideal MHD model to describe intrinsically three-dimensional equilibria, with non-zero internal pressure. A second application is the description of transport harriers as constrained minimum energy states. As a first example, we calculate the plasma solution in a periodic cylinder, generalizing the analysis of the treatment of Kaiser and Uecker (2004 Q. J. Mech, Appl. Math. 57. 1-17). who treated the single interface in cylindrical geometry, Expressions for the equilibrium field are generated, and equilibrium states computed. Unlike other Taylor relaxed equilibria, for the equilibria investigated here, only the plasma core necessarily has reverse magnetic shear. We show the existence of tokamak-like equilibria, with increasing safety factor and stepped-pressure profiles.
TL;DR: In this article, the effects of the EPM on fast ion transport and consequent fast ion loss in the compact helical system (CHS) were revealed, and it was shown that EPM excited by co-circulating fast ions in an outward-shifted configuration is a mode of m/n = 3/2 and can enhance fast-ion loss when its magnetic fluctuation amplitude exceeds 4 × 10−5 T at the magnetic probe position.
Abstract: The purpose of this work is to reveal the effects of the energetic particle mode (EPM) on fast-ion transport and consequent fast-ion loss in the compact helical system (CHS) For this purpose, fast particle diagnostics capable of following fast events originating from the EPM (f < 100 kHz) and from the toroidicity-induced Alfven eigenmode (TAE) (f = 100–200 kHz) are employed in CHS Experiments show that the EPM excited by co-circulating fast ions in an outward-shifted configuration is identified as a mode of m/n = 3/2 and can enhance fast-ion loss when its magnetic fluctuation amplitude exceeds ~4 × 10−5 T at the magnetic probe position The lost fast-ion probe (LIP) located at the outboard side of the torus indicates that bursting EPMs lead to periodically enhanced losses of co-going fast ions having smaller pitch angles in addition to losses of marginally co-passing fast ions Coinciding with EPM bursts, the Hα light detector viewing the peripheral region at the outboard side also shows large pulsed increases similar to that of the LIP whereas the detector viewing the peripheral region at the inboard side does not This is also evidence that fast ions are expelled to the outboard side due to the EPM The charge-exchange neutral particle analyser indicates that only fast ions whose energy is close to the beam injection energy Eb are strongly affected by EPM, suggesting in turn that observed EPMs are excited by fast ions having energy close to Eb
TL;DR: In this paper, the magnetic configurations of the Wendelstein 7-AS (W7-AS) stellarator and the Large Helical Device (LHD) and their optimization for high-beta operation within the flexibility of the devices are characterized.
Abstract: Substantial progress has been achieved in raising the plasma beta in stellarators and helical systems by high-power neutral beam heating, approaching reactor-relevant values. The achievement of high-beta operation is closely linked with configuration effects on the confinement and with magnetohydrodynamic (MHD) stability. The magnetic configurations of the Wendelstein 7-AS (W7-AS) stellarator and of the Large Helical Device (LHD) and their optimization for high-beta operation within the flexibility of the devices are characterized. A comparative description of the accessible operational regimes in W7-AS and LHD is given. The finite-beta effects on the flux surfaces depend on the degree of configuration optimization. In particular, a large Shafranov shift is accompanied by formation of islands and stochastic field regions as found by numerical equilibrium studies. However, the observed pressure gradients indicate some mitigation of the effects on the plasma confinement, presumably because of the high collisionality of high-beta plasmas and island healing effects (LHD). As far as operational limits by pressure-driven MHD instabilities are concerned, only weak confinement degradation effects are usually observed, even in linearly unstable regimes. The impact of the results concerning high-beta operation in W7-AS and LHD on the future stellarator program will be discussed, including the relationship to tokamak research. Some of the future key issues appear to be the following: the control of the magnetic configuration (including toroidal current control), the modification of confinement and MHD properties toward the low-collisional regime, and the compatibility of high-beta regimes with power and particle exhaust requirements to achieve steady-state operation.