Showing papers by "Princeton Plasma Physics Laboratory published in 2001"
University of California, Los Angeles1, University of Texas at Austin2, Karlsruhe Institute of Technology3, Oak Ridge National Laboratory4, Lawrence Livermore National Laboratory5, Sandia National Laboratories6, Idaho National Laboratory7, Argonne National Laboratory8, General Atomics9, University of Wisconsin-Madison10, Princeton Plasma Physics Laboratory11, University of California, San Diego12, University of Tokyo13
TL;DR: In this paper, the authors explored novel concepts for fusion chamber technology that can substantially improve the attractiveness of fusion energy systems, including the potential for: (1) high power density capability; (2) higher plasma β and stable physics regimes if liquid metals are used; (3) increased disruption survivability; (4) reduced volume of radioactive waste; (5) reduced radiation damage in structural materials; and (6) higher availability.
319 citations
TL;DR: The National Compact Stellarator Experiment (NCSX) as discussed by the authors was designed to enhance the magnetohydrodynamic (MHD) stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks.
Abstract: Compact optimized stellarators offer novel solutions for confining high-β plasmas and developing magnetic confinement fusion. The three-dimensional plasma shape can be designed to enhance the magnetohydrodynamic (MHD) stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the low aspect ratio, high β limit, and good confinement of advanced tokamaks. Quasi-axisymmetric equilibria have been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4-4.4 and average elongation ~1.8. Even with bootstrap-current consistent profiles, they are passively stable to the ballooning, kink, vertical, Mercier, and neoclassical-tearing modes for β > 4%, without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at β = 4% (the rest is from the coils); thus the equilibrium is much less non-linear and is more controllable than similar advanced tokamaks. The enhanced stability is a result of `reversed' global shear, the spatial distribution of local shear, and the large fraction of externally generated transform. Transport simulations show adequate fast-ion confinement and thermal neoclassical transport similar to equivalent tokamaks. Modular coils have been designed which reproduce the physics properties, provide good flux surfaces, and allow flexible variation of the plasma shape to control the predicted MHD stability and transport properties.
188 citations
TL;DR: In this article, the possibility of studying MRI in a rotating liquid metal annulus (Couette flow) is explored by local and global stability analysis, and it is shown that MRI can be triggered in a moderately rapidly rotating table-top apparatus, using easy-to-handle metals such as gallium.
Abstract: Although the magnetorotational instability (MRI) has been widely accepted as a powerful accretion mechanism in magnetized accretion discs, it has not been realized in the laboratory. The possibility of studying MRI in a rotating liquid metal annulus (Couette flow) is explored by local and global stability analysis. Stability diagrams are drawn in dimensionless parameters, and also in terms of the angular velocities at the inner and outer cylinders. It is shown that MRI can be triggered in a moderately rapidly rotating table-top apparatus, using easy-to-handle metals such as gallium. Practical issues of this proposed experiment are discussed.
188 citations
TL;DR: In this paper, the evolution of the plasma equilibrium between discharges is analyzed with an automated version of the EFIT code and reconstructed magnetic signals are fitted to the corresponding measured values with low errors.
Abstract: Research in NSTX has been conducted to establish spherical torus plasmas to be used for high ?, auxiliary heated experiments. This device has a major radius R0 = 0.86?m and a midplane halfwidth of 0.7?m. It has been operated with toroidal magnetic field B0 ? 0.3?T and Ip ? 1.0?MA. The evolution of the plasma equilibrium is analysed between discharges with an automated version of the EFIT code. Limiter, double null and lower single null diverted configurations have been sustained for several energy confinement times. The plasma stored energy reached 92?kJ (?t = 17.8%) with neutral beam heating. A plasma elongation in the range 1.6 ? ? ? 2.0 and a triangularity in the range 0.25 ? ? ? 0.45 have been sustained, with values of ? = 2.6 and ? = 0.6 being reached transiently. The reconstructed magnetic signals are fitted to the corresponding measured values with low errors. Aspects of the plasma boundary, pressure and safety factor profiles are supported by measurements from non-magnetic diagnostics. Plasma densities have reached 0.8 and 1.2 times the Greenwald limit in deuterium and helium plasmas, respectively, with no clear limit encountered. Instabilities including sawteeth and reconnection events, characterized by Mirnov oscillations, and a perturbation of the Ip, ? and li evolutions, have been observed. A low q limit was observed and is imposed by a low toroidal mode number kink instability.
176 citations
TL;DR: In this article, a new sustained high performance regime, combining discrete edge and core transport barriers, has been discovered in the DIII-D tokamak, where edge localized modes are replaced by a steady oscillation that increases edge particle transport, thereby allowing particle control with no ELM-induced pulsed divertor heat load.
Abstract: A new sustained high-performance regime, combining discrete edge and core transport barriers, has been discovered in the DIII-D tokamak. Edge localized modes (ELMs) are replaced by a steady oscillation that increases edge particle transport, thereby allowing particle control with no ELM-induced pulsed divertor heat load. The core barrier resembles those usually seen with a low (L) mode edge, without the degradation often associated with ELMs. The barriers are separated by a narrow region of high transport associated with a zero crossing in the E x B shearing rate.
137 citations
TL;DR: The core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative and consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD.
Abstract: Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core ( r/a< or =0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative.
134 citations
TL;DR: In this article, the authors explored the secondary instability as a mechanism for zonal flow generation, transport barrier dynamics and avalanche formation in drift-ion temperature gradient turbulence and reported on recent developments in the theory of secondary instability.
Abstract: The article reports on recent developments in the theory of secondary instability in drift-ion temperature gradient turbulence. Specifically, the article explores secondary instability as a mechanism for zonal flow generation, transport barrier dynamics and avalanche formation. These in turn are related to the space-time statistics of the drift wave induced flux, the scaling of transport with collisionality and β, and the spatio-temporal evolution of transport barriers.
118 citations
TL;DR: In this article, the two theories for magnetic reconnection, one of Sweet and Parker, and the other of Petschek, are reconciled by exhibiting an extra condition in that of petschek which reduces his theory to that of sweet and Parker provided that the resistivity is constant in space.
Abstract: The two theories for magnetic reconnection, one of Sweet and Parker, and the other of Petschek, are reconciled by exhibiting an extra condition in that of Petschek which reduces his theory to that of Sweet and Parker, provided that the resistivity is constant in space. On the other hand, if the resistivity is enhanced by instabilities, then the reconnection rate of both theories is increased substantially, but Petschek’s rate can be faster. A different formula from the usual one is presented for enhanced Petschek reconnection.
114 citations
TL;DR: In this paper, the authors showed that the growth of the n = 1 resistive wall mode (RWM) can be influenced by an external magnetic field applied in closed loop feedback using a six element error field correction coil (C coil).
Abstract: Recent DIII-D experiments have shown that the growth of the n = 1 resistive wall mode (RWM) can be influenced by an external magnetic field applied in closed loop feedback using a six element error field correction coil (C coil). The RWM constitutes the primary limitation to normalized beta in recent DIII-D advanced tokamak plasma experiments. The toroidal rotation of DIII-D plasmas does not seem sufficient to completely suppress the RWM: a very slowly growing (growth rate γ<<1/τw) or saturated RWM is often observed at normalized beta above the no wall limit and this small RWM slows the rotation. As the rotation decreases, there is a transition to more rapid growth (γ~1/τw, where τw is the resistive time constant of the wall). The application of magnetic feedback is able to hold the RWM to a very small amplitude, prolonging the plasma duration above the no wall limit for times much longer than τw. These initial experimental results are being used to compare control algorithms, to benchmark models of the feedback stabilization process and to guide the design of an upgraded coil sensor system for stabilization of the RWM at normalized beta values closer to the ideal wall limit.
113 citations
TL;DR: In Tore Supra plasmas with fast wave electron heating, a critical threshold in the electron temperature gradient is clearly observed, i.e., a finite value of inverted DeltaT(e) for which the turbulent heat diffusivity vanishes.
Abstract: In Tore Supra plasmas with fast wave electron heating, a critical threshold in the electron temperature gradient (inverted DeltaT(e)) is clearly observed, i.e., a finite value of inverted DeltaT(e) for which the turbulent heat diffusivity vanishes. The radial profile of this critical gradient is experimentally determined from a set of discharges characterized by similar plasma parameters with fast wave powers ranging from 0.75 to 7.4 MW. The dependence of the electron heat flux on the gradient length is found to be offset linearly. The offset term increases linearly with the ratio of the local magnetic shear to the safety factor.
111 citations
TL;DR: In this article, a scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation in Alcator C-Mod plasmas.
Abstract: Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation, generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased together with the formation of an internal transport barrier (ITB). During the ITB phase, electrons and impurities peaked continuously for |r/a| ≤ 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.
TL;DR: In this paper, the magnetic field modulus |B| in magnetic co-ordinates is dominated by poloidally symmetric components and two distinct configuration types are considered: (1) those that achieve their drift optimization and rotational transform at low β and low bootstrap current by appropriate plasma shaping; and (2) those which have a greater reliance on plasma β and bootstrap currents for supplying the transform and obtaining quasi-poloidal symmetry.
Abstract: Physics issues are discussed for compact stellarator configurations which achieve good confinement by the fact that the magnetic field modulus |B| in magnetic co-ordinates is dominated by poloidally symmetric components. Two distinct configuration types are considered: (1) those which achieve their drift optimization and rotational transform at low β and low bootstrap current by appropriate plasma shaping; and (2) those which have a greater reliance on plasma β and bootstrap currents for supplying the transform and obtaining quasi-poloidal symmetry. Stability analysis of the latter group of devices against ballooning, kink and vertical displacement modes has indicated that stable β values on the order of 15% are possible. The first class of devices is being considered for a low β near term experiment that could explore some of the confinement features of the high β configurations.
TL;DR: In this article, the authors showed that the inward shift of the Large Helical Device (LHD) can improve the stability of the high-energy particle in the high n mode.
Abstract: Recent experimental results in the Large Helical Device have indicated that a large pressure gradient can be formed beyond the stability criterion for the Mercier (high-n) mode. While the stability against an interchange mode is violated in the inward-shifted configuration due to an enhancement of the magnetic hill, the neoclassical transport and confinement of high-energy particle are, in contrast, improved by this inward shift. Mitigation of the unfavourable effects of MHD instability has led to a significant extension of the operational regime. Achievements of the stored energy of I MJ and the volume-averaged beta of 3% are representative results from this finding. A confinement enhancement factor above the international stellarator scaling ISS95 is also maintained around 1.5 towards a volume-averaged beta, (beta), of 3%. Configuration studies on confinement and MHD characteristics emphasize the superiority of the inward-shifted geometry to other geometries. The emergence of coherent modes appears to be consistent with the linear ideal MHD theory; however, the inward-shifted configuration has reduced heat transport in spite of a larger amplitude of magnetic fluctuation than the outward-shifted configuration. While neoclassical helical ripple transport becomes visible for the outward-shifted configuration in the collisionless regime, the inward-shifted configuration does not show any degradation of confinement deep in the collisionless regime (nu* < 0.1). The distinguished characteristics observed in the inward-shifted configuration help in creating a new perspective of MHD stability and related transport in net current-free plasmas. The first result of the pellet launching at different locations is also reported.
TL;DR: In this article, the q-profile was modified by applying lower hybrid heating and current drive (LHCD) before the main heating pulse, to provide a region of reduced or negative magnetic shear in the plasma core.
Abstract: Optimized shear (OS) experiments in JET can generate an internal transport barrier (ITB) during a high power heating phase early in the plasma discharge. A strong link is generally observed between the formation of the barrier and the location of an integer q magnetic surface within a low magnetic shear (s = r/q(dq/dr)) region of the plasma. However, if the q-profile for such experiments is modified by applying lower hybrid heating and current drive (LHCD) before the main heating pulse, to provide a region of reduced or negative magnetic shear in the plasma core, ITBs can be formed the location of which does not exhibit any apparent association with a particular internal magnetic surface. Initial results suggest that q-profile modification using an LHCD prelude can also be used to reduce the heating power level required for ITB generation.
TL;DR: In this paper, the authors found instability with frequency chirping in the frequency range of the Alfven eigenmodes in the domain 0.1% < βh < 1% and vb||/vA ~1 with high energy neutral beam injection in JT-60U.
Abstract: Instabilities with frequency chirping in the frequency range of Alfven eigenmodes have been found in the domain 0.1% < βh < 1% and vb||/vA ~1 with high energy neutral beam injection in JT-60U. One instability with a frequency inside the Alfven continuum spectrum appears and its frequency increases slowly to the toroidicity induced Alfven eigenmode (TAE) gap on the timescale of an equilibrium change ( ≈ 200 ms). Other instabilities appear with a frequency inside the TAE gap and their frequencies change very quickly by 10-20 kHz in 1-5 ms. During the period when these fast frequency sweeping (fast FS) modes occur, abrupt large amplitude events (ALEs) often appear with a drop of neutron emission rate and an increase in fast neutral particle fluxes. The loss of energetic ions increases with a peak fluctuation amplitude of θ/Bθ. An energy dependence of the loss ions is observed and suggests a resonant interaction between energetic ions and the mode.
TL;DR: Neutral-beam-driven compressional Alfvén eigenmodes at frequencies below the ion cyclotron frequency have been observed and identified for the first time in the National Spherical Torus Experiment.
Abstract: Neutral-beam-driven compressional Alfv\'en eigenmodes at frequencies below the ion cyclotron frequency have been observed and identified for the first time in the National Spherical Torus Experiment. The modes are observed as a broad spectrum of nearly equally spaced peaks in the frequency range from $\ensuremath{\approx}0.2{\ensuremath{\omega}}_{\mathrm{ci}}$ to $\ensuremath{\approx}1.2{\ensuremath{\omega}}_{\mathrm{ci}}$. The frequency has a scaling with toroidal field and plasma density consistent with Alfv\'en waves. The modes have been observed with high bandwidth magnetic pickup coils and with a reflectometer.
TL;DR: In this article, the scrape-off layer (SOL) of Alcator C-Mod plasmas were measured using visible emission from the plasma edge, where line emissions from a local gas jet of He atoms were imaged along radially separated, toroidally viewing chords and provided local measurements of density fluctuations.
TL;DR: In this paper, the dependence of edge stability on plasma shape and local pressure gradients P' in the DIII-D and JT-60U tokamaks is studied.
Abstract: The dependence of edge stability on plasma shape and local pressure gradients P' in the DIII-D and JT-60U tokamaks is studied. The stronger plasma shaping in DIII-D allows the edge region of DIII-D discharges with type I (giant) ELMs to have access to the second region of stability for ideal ballooning modes and a larger edge pedestal pressure gradient P' than JT-60U type I ELM discharges. These JT-60U discharges are near the ballooning mode first regime stability limit. The DIII-D results support an ideal stability based working model of type I ELMs as low to intermediate toroidal mode number, n, MHD modes. The results from a stability analysis of JT-60U type I ELM discharges indicate that the predictions of this model are also consistent with JT-60U edge stability observations.
TL;DR: In this article, an analytic theory is presented for non-linear zonal dynamics described in terms of two axisymmetric potentials, which spatially depend only on a (magnetic) flux co-ordinate.
Abstract: The present work addresses the issue of identifying the major non-linear physics processes which may regulate drift and drift-Alfven turbulence using a weak turbulence approach. Within this framework, on the basis of the non-linear gyrokinetic equation for both electrons and ions, an analytic theory is presented for non-linear zonal dynamics described in terms of two axisymmetric potentials, δz and δA|| z, which spatially depend only on a (magnetic) flux co-ordinate. Spontaneous excitation of zonal flows by electrostatic drift microinstabilities is demonstrated both analytically and by direct 3-D gyrokinetic simulations. Direct comparisons indicate good agreement between analytic expressions of the zonal flow growth rate and numerical simulation results for ion temperature gradient driven modes. Analogously, it is shown that zonal flows may be spontaneously excited by drift-Alfven turbulence, in the form of modulational instability of the radial envelope of the mode as well, whereas in general, excitations of zonal currents are possible but have little feedback on the turbulence itself.
TL;DR: Coaxial helicity injection (CHI) on the National Spherical Torus Experiment (NSTX) has produced 240 kA of toroidal current without the use of the central solenoid.
Abstract: Coaxial helicity injection (CHI) on the National Spherical Torus Experiment (NSTX) has produced 240 kA of toroidal current without the use of the central solenoid. Values of the current multiplication ratio (CHI produced toroidal current/injector current) up to 10 were obtained, in agreement with predictions. The discharges, which lasted for up to 200 ms, limited only by the programmed waveform, are more than an order of magnitude longer in duration than any CHI discharges previously produced in a spheromak or a spherical torus.
TL;DR: In this article, a phase screen model for the general cases and the results are supported by detailed numerical 2-D realistic geometry full wave simulations for the specific case of the Alcator C-Mod tokamak.
Abstract: Plasma poloidal curvature can significantly extend microwave reflectometry responses to high k⊥ poloidal fluctuations. Reflectometry responses can be several orders of magnitude larger at high k⊥ than that predicted by analysis based on two dimensional (2-D) slab geometry. As a result, the responses may approach the 1-D geometrical optics limit. This superresolution leads to a major modification of the spectral resolution of reflectometry. The phenomenon is analysed using a phase screen model for the general cases and the results are supported by detailed numerical 2-D realistic geometry full wave simulations for the specific case of the Alcator C-Mod tokamak.
TL;DR: In this paper, the authors describe the results of an extensive numerical study of microwave reflectometry in tokamaks showing that the two dimensional structure of plasma fluctuations near the cut-off can be obtained from the phase of reflected waves.
Abstract: Understanding the mechanism of anomalous transport in magnetically confined plasmas requires the use of sophisticated diagnostic tools for the measurement of turbulent fluctuations. The article describes the results of an extensive numerical study of microwave reflectometry in tokamaks showing that the two dimensional structure of plasma fluctuations near the cut-off can be obtained from the phase of reflected waves. This requires the latter to be collected by an optical system making an image of the reflecting layer onto an array of microwave receivers, and the amplitude of fluctuations to be below a threshold that is set by the spectrum of poloidal wavenumbers. The conceptual design of an experimental scheme for the global visualization of turbulent fluctuations in tokamaks is described.
TL;DR: In this article, Ar gas was injected into ELMy H mode plasmas in JT-60U to obtain improved confinement in high radiation loss power fraction at an electron density of ~0.65nGW.
Abstract: In order to obtain improved confinement plasmas with high radiation at high density, Ar gas was injected into ELMy H mode plasmas in JT-60U. A confinement improvement of HH98(y,2) ≈ 1 was obtained with a high radiation loss power fraction (~80%) at an electron density of ~0.65nGW. The HH factor was about 50% higher than that in plasmas without Ar injection.
TL;DR: In this paper, the energy confinement and thermal transport characteristics of net current free plasmas in regimes with much smaller gyroradii and collisionality than previously studied have been investigated in the Large Helical Device (LHD).
Abstract: The energy confinement and thermal transport characteristics of net current free plasmas in regimes with much smaller gyroradii and collisionality than previously studied have been investigated in the Large Helical Device (LHD). The inward shifted configuration, which is superior from the point of view of neoclassical transport theory, has revealed a systematic confinement improvement over the standard configuration. Energy confinement times are improved over the International Stellarator Scaling 95 by a factor of 1.6 ±0.2 for an inward shifted configuration. This enhancement is primarily due to the broad temperature profile with a high edge value. A simple dimensional analysis involving LHD and other medium sized heliotrons yields a strongly gyro-Bohm dependence (τEΩ ∝ ρ*-3.8) of energy confinement times. It should be noted that this result is attributed to a comprehensive treatment of LHD for systematic confinement enhancement and that the medium sized heliotrons have narrow temperature profiles. The core stored energy still indicates a dependence of τEΩ ∝ ρ*-2.6 when data only from LHD are processed. The local heat transport analysis of discharges dimensionally similar except for ρ* suggests that the heat conduction coefficient lies between Bohm and gyro-Bohm in the core and changes towards strong gyro-Bohm in the peripheral region. Since the inward shifted configuration has a geometrical feature suppressing neoclassical transport, confinement improvement can be maintained in the collisionless regime where ripple transport is important. The stiffness of the pressure profile coincides with enhanced transport in the peaked density profile obtained by pellet injection.
TL;DR: In this article, the authors studied the problem of particle diffusion in the Galactic disk with particular attention paid to the particle scattering through the θ = (v∥/v) = 90° pitch angle in momentum space by wave-particle mirror interaction.
Abstract: We study the problem of cosmic-ray diffusion in the Galactic disk with particular attention paid to the problem of particle scattering through the θ =(v∥/v) = 90° pitch angle in momentum space by wave-particle mirror interaction (here v∥ is the cosmic-ray velocity parallel to the average Galactic magnetic field). We consider the case in which cosmic rays are the only source of magnetic turbulence, which originates as the relativistic particles try to stream through the interstellar plasma faster than the local Alfven speed. The wave growth rate is proportional to the cosmic-ray anisotropy, and the amplitude of hydromagnetic waves generated by this streaming instability is limited by the presence of various damping mechanisms. We study the propagation of cosmic rays in the different phases of the interstellar medium, in particular the coronal regions (where the main form of wave dissipation is nonlinear Landau damping) and the warm regions (where charge exchange between the ions and the neutral atoms gives rise to the dominant form of wave dissipation). We also account for ion-cyclotron damping of small wavelength waves. The effect of a spectrum of waves is to limit the anisotropy of the cosmic-ray distribution function and hence their drift velocity. We show that quasi-linear resonant scattering cannot account for particle diffusion everywhere in momentum space and that particles with θ ~ 90° must change their pitch angle by mirror interaction with long-wavelength waves generated by the θ ~ 0° particles. We match the quasi-linear scattering with the adiabatic mirroring in a small boundary layer in momentum space close to the θ ~ 90° point, and then we calculate the diffusion coefficient together with the cosmic-ray drift velocity. We show that scattering through the 90° point is very efficient, and we calculate the correction to the particle diffusion coefficient due to the presence of mirroring.
Princeton Plasma Physics Laboratory1, Oak Ridge National Laboratory2, University of California, Los Angeles3, Los Alamos National Laboratory4, University of Washington5, Columbia University6, Johns Hopkins University7, European Atomic Energy Community8, Massachusetts Institute of Technology9, General Atomics10, University of California, Davis11, University of California, San Diego12, Kyushu Tokai University13, Hiroshima University14, Lawrence Livermore National Laboratory15, University of Tokyo16, Sandia National Laboratories17
TL;DR: The National Spherical Torus Experiment (NSTX) as discussed by the authors was the first experiment to establish the fusion physics principles of the spherical torus (ST) concept, and the NSTX device began plasma operations in February 1999 and the plasma current Ip was successfully brought up to the design value of 1-MA on 14 December 1999.
Abstract: The main aim of the National Spherical Torus Experiment (NSTX) is to establish the fusion physics principles of the spherical torus (ST) concept. The NSTX device began plasma operations in February 1999 and the plasma current Ip was successfully brought up to the design value of 1 MA on 14 December 1999. The planned plasma shaping parameters, elongation κ = 1.6-2.2 and triangularity δ = 0.2-0.4, were achieved in inner wall limited, and single null and double null diverted configurations. The coaxial helicity injection (CHI) and high harmonic fast wave (HHFW) experiments were also initiated. CHI current of 27 kA produced up to 260 kA toroidal current without using an ohmic solenoid. With the injection of 2.3 MW of HHFW power, using 12 antennas connected to six transmitters, electrons were heated from a central temperature of 400 eV to 900 eV at a central density of 3.5 × 1013 cm-3, increasing the plasma energy to 59 kJ and the toroidal β, βT, to 10%. The NBI system commenced operation in September 2000. The initial results with two ion sources (PNBI = 2.8 MW) show good heating, producing a total plasma stored energy of 90 kJ corresponding to βT ≈ 18% at a plasma current of 1.1 MA.
TL;DR: Coaxial helicity injection has been investigated on the National Spherical Torus Experiment (NSTX) as mentioned in this paper, and initial experiments produced 130 kA of toroidal current without the use of the central solenoid.
Abstract: Coaxial helicity injection has been investigated on the National Spherical Torus Experiment (NSTX). Initial experiments produced 130 kA of toroidal current without the use of the central solenoid. The corresponding injector current was 20 kA. Discharges with pulse lengths up to 130 ms have been produced.
Oak Ridge National Laboratory1, University of Wisconsin-Madison2, Lehigh University3, University of California, San Diego4, University of Colorado Boulder5, Lawrence Livermore National Laboratory6, University of California, Los Angeles7, Princeton Plasma Physics Laboratory8, Georgia Institute of Technology9, University of Texas at Austin10, Massachusetts Institute of Technology11, University of Alberta12
TL;DR: In this paper, external impurity injection into L mode edge discharges in DIII-D has produced clear confinement improvement (a factor of 2 in energy confinement and neutron emission), reduction in all transport channels (particularly ion thermal diffusivity to the neoclassical level), and simultaneous reduction of long wavelength turbulence.
Abstract: External impurity injection into L mode edge discharges in DIII-D has produced clear confinement improvement (a factor of 2 in energy confinement and neutron emission), reduction in all transport channels (particularly ion thermal diffusivity to the neoclassical level), and simultaneous reduction of long wavelength turbulence. Suppression of the long wavelength turbulence and transport reduction are attributed to synergistic effects of impurity induced enhancement of E × B shearing rate and reduction of toroidal drift wave turbulence growth rate. A prompt reduction of density fluctuations and local transport at the beginning of impurity injection appears to result from an increased gradient of toroidal rotation enhancing the E × B shearing. Transport simulations carried out using the National Transport Code Collaboration demonstration code with a gyro-Landau fluid model, GLF23, indicate that E × B shearing suppression is the dominant transport suppression mechanism.
TL;DR: In this article, high density and high confinement for the ITER reference scenario at Q = 10 is summarized, with a new and more favourable behaviour observed at high densities, requiring further studies.
Abstract: Recent progress towards obtaining high density and high confinement in JET as required for the ITER reference scenario at Q = 10 is summarized. Plasmas with simultaneous confinement H-98(y.2) = 1 and densities up to n/n(Gw) similar to 1 are now routinely obtained. This has been possible (i) by using plasmas at high (delta similar to 0.5) and medium (delta similar to 0.3-0.4) triangularity with sufficient heating power to maintain Type I ELMs, (ii) with impurity seeded plasmas at high (delta similar to 0.5) and low (delta less than or equal to 0.2) triangularity, (iii) with an optimized pellet injection sequence, maintaining the energy confinement and raising the density, and (iv) by carefully tuning the gas puff rate leading to plasmas with peaked density profiles and good confinement at long time scales. These high performance discharges exhibit Type I ELMs, with a new and more favourable behaviour observed at high densities, requiring further studies. Techniques for a possible mitigation of these ELMs are discussed, and first promising results are obtained with impurity seeding in discharges at high triangularity. Scaling studies using the new data of this year show a strong dependence of confinement on upper triangularity, density and proximity to the Greenwald limit. Observed MHD instabilities and methods to avoid these in high density and high confinement plasmas are discussed.