scispace - formally typeset
Search or ask a question

Showing papers by "Princeton Plasma Physics Laboratory published in 2012"


Journal ArticleDOI
TL;DR: Interesting phenomena such as a saturated evolution of edge T(e) and broadband changes of magnetic fluctuations were observed, suggesting the change of edge transport by the applied magnetic perturbations.
Abstract: Edge localized modes (ELMs) in high-confinement mode plasmas were completely suppressed in KSTAR by applying $n=1$ nonaxisymmetric magnetic perturbations. Initially, the ELMs were intensified with a reduction of frequency, but completely suppressed later. The electron density had an initial 10% decrease followed by a gradual increase as ELMs were suppressed. Interesting phenomena such as a saturated evolution of edge ${T}_{e}$ and broadband changes of magnetic fluctuations were observed, suggesting the change of edge transport by the applied magnetic perturbations.

291 citations


Journal ArticleDOI
TL;DR: The National Spherical Torus eXperiment (NSTX) is a MA-class spherical tokamak (ST) facility in the US actively developing the physics basis for an ST-based FNSF as discussed by the authors.
Abstract: The spherical tokamak (ST) is a leading candidate for a Fusion Nuclear Science Facility (FNSF) due to its compact size and modular configuration. The National Spherical Torus eXperiment (NSTX) is a MA-class ST facility in the US actively developing the physics basis for an ST-based FNSF. In plasma transport research, ST experiments exhibit a strong (nearly inverse) scaling of normalized confinement with collisionality, and if this trend holds at low collisionality, high fusion neutron fluences could be achievable in very compact ST devices. A major motivation for the NSTX Upgrade (NSTX-U) is to span the next factor of 3–6 reduction in collisionality. To achieve this collisionality reduction with equilibrated profiles, NSTX-U will double the toroidal field, plasma current, and NBI heating power and increase the pulse length from 1–1.5 s to 5–8 s. In the area of stability and advanced scenarios, plasmas with higher aspect ratio and elongation, high βN, and broad current profiles approaching those of an ST-based FNSF have been produced in NSTX using active control of the plasma β and advanced resistive wall mode control. High non-inductive current fractions of 70% have been sustained for many current diffusion times, and the more tangential injection of the 2nd NBI of the Upgrade is projected to increase the NBI current drive by up to a factor of 2 and support 100% non-inductive operation. More tangential NBI injection is also projected to provide non-solenoidal current ramp-up as needed for an ST-based FNSF. In boundary physics, NSTX measures an inverse relationship between the scrape-off layer heat-flux width and plasma current that could unfavourably impact next-step devices. Recently, NSTX has successfully demonstrated substantial heat-flux reduction using a snowflake divertor configuration, and this type of divertor is incorporated in the NSTX-U design. The physics and engineering design supporting NSTX Upgrade is described.

182 citations


Journal ArticleDOI
TL;DR: In this paper, the current conducted across the magnetic field via a rotating spoke has been directly measured for the first time in the E × B discharge of a cylindrical Hall thruster.
Abstract: Rotating spoke phenomena have been observed in a variety of Hall thruster and other E × B devices. It has been suggested that the spoke may be associated with the enhancement of the electron cross-field transport. In this paper, the current conducted across the magnetic field via a rotating spoke has been directly measured for the first time in the E × B discharge of a cylindrical Hall thruster. The spoke current was measured using a segmented anode. Synchronized measurements with a high speed camera and a four-segment anode allow observation of the current as a function of time and azimuthal position. Upwards of 50% of the total current is conducted through the spoke, which occupies a quarter of the Hall thruster channel area. To determine the transport mechanism, emissive and Langmuir probes were installed to measure fluctuating plasma potential, electron density, and temperature. A perturbed, azimuthal electric field and density are observed to oscillate in-phase with the rotating spoke. The resulting ...

169 citations


Journal ArticleDOI
TL;DR: In this paper, the EPED model was used to predict pedestal height and width for quiescent H-mode (QH) discharges with edge localized modes (ELMs) and found good quantitative agreement across a range of parameters.
Abstract: The EPED model predicts the H-mode pedestal height and width based upon two fundamental and calculable constraints: (1) onset of non-local peeling-ballooning modes at low to intermediate mode number, (2) onset of nearly local kinetic ballooning modes at high mode number. We present detailed tests of the EPED model in discharges with edge localized modes (ELMs), employing new high resolution measurements, and finding good quantitative agreement across a range of parameters. The EPED model is then applied for the first time to quiescent H-mode (QH), finding a similar level of agreement between predicted and observed pedestal height and width, and suggesting that the model can be used to predict the critical density for QH-mode operation. Finally, the model is applied toward understanding the suppression of ELMs with 3D resonant magnetic perturbations (RMP). Combining EPED with plasma response physics, a new working model for RMP ELM suppression is developed. We propose that ELMs are suppressed when a “wall”...

146 citations


Journal ArticleDOI
TL;DR: In this article, a new amplification method, weaving the three basic compression techniques, Chirped Pulse Amplification (CPA), Optical Parametric (OPCPA) and Plasma Compression by Backward Raman Amplification(BRA) in plasma, is proposed.

126 citations


Journal ArticleDOI
TL;DR: In this paper, a fully variational, unstructured, electromagnetic particle-in-cell integrator is developed for integration of the Vlasov-Maxwell equations using the formalism of discrete exterior calculus, and the field solver, interpolation scheme and particle advance algorithm are derived through minimization of a single discrete field theory action.
Abstract: A fully variational, unstructured, electromagnetic particle-in-cell integrator is developed for integration of the Vlasov-Maxwell equations. Using the formalism of discrete exterior calculus [Desbrun et al., e-print arXiv:math/0508341 (2005)], the field solver, interpolation scheme, and particle advance algorithm are derived through minimization of a single discrete field theory action. As a consequence of ensuring that the action is invariant under discrete electromagnetic gauge transformations, the integrator exactly conserves Gauss’s law.

122 citations


Journal ArticleDOI
TL;DR: In this article, the effect of gradually increasing lithium wall coatings on discharge characteristics, with the reference ELMy discharges obtained in boronized, i.e. non-lithiated conditions, was investigated.
Abstract: Lithium wall coatings have been shown to reduce recycling, suppress edge-localized modes (ELMs), and improve energy confinement in the National Spherical Torus Experiment (NSTX). Here we document the effect of gradually increasing lithium wall coatings on the discharge characteristics, with the reference ELMy discharges obtained in boronized, i.e. non-lithiated conditions. We observed a continuous but not quite monotonic reduction in recycling and improvement in energy confinement, a gradual alteration of edge plasma profiles, and slowly increasing periods of ELM quiescence. The measured edge plasma profiles during the lithium-coating scan were simulated with the SOLPS code, which quantified the reduction in divertor recycling coefficient from ?98% to ?90%. The reduction in recycling and fuelling, coupled with a drop in the edge particle transport rate, reduced the average edge density profile gradient, and shifted it radially inwards from the separatrix location. In contrast, the edge electron temperature (Te) profile was unaffected in the H-mode pedestal steep gradient region within the last 5% of normalized poloidal flux, ?N ; however, the Te gradient became steeper at the top of the H-mode pedestal for 0.8?

105 citations


Journal ArticleDOI
TL;DR: A bounded plasma where the hot electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation and it is found that no classical Debye sheath or space-charge-limited sheath exists.
Abstract: A bounded plasma where the hot electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation. It is found that no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled. Hence the unconfined plasma electrons travel unobstructed to the walls, causing extreme particle and energy fluxes. Each wall has a positive charge, forming a small potential barrier or "inverse sheath" that pulls some secondaries back to the wall to maintain the zero current condition.

92 citations


01 Oct 2012
TL;DR: A fully variational, unstructured, electromagnetic particle-in-cell integrator is developed for integration of the Vlasov-Maxwell equations, which exactly conserves Gauss’s law.
Abstract: A fully variational, unstructured, electromagnetic particle-in-cell integrator is developed for integration of the Vlasov-Maxwell equations. Using the formalism of discrete exterior calculus [Desbrun et al., e-print arXiv:math/0508341 (2005)], the field solver, interpolation scheme, and particle advance algorithm are derived through minimization of a single discrete field theory action. As a consequence of ensuring that the action is invariant under discrete electromagnetic gauge transformations, the integrator exactly conserves Gauss’s law.

82 citations


Journal ArticleDOI
TL;DR: In this paper, a molybdenum Liquid Lithium Divertor (LLD) surface was installed on the outer part of the lower divertor to test the effectiveness of maintaining the deuterium retention properties of a static liquid lithium surface.

76 citations


Journal ArticleDOI
TL;DR: In this paper, the canonical and kinetic momenta carried by a photon, as well as the two corresponding energy-momentum tensors of a wave, are derived from the first principles of Lagrangian mechanics.
Abstract: By restating geometrical optics within the field-theoretical approach, the classical concept of a photon (and, more generally, any elementary excitation) in an arbitrary dispersive medium is introduced, and photon properties are calculated unambiguously. In particular, the canonical and kinetic momenta carried by a photon, as well as the two corresponding energy-momentum tensors of a wave, are derived from the first principles of Lagrangian mechanics. As an example application of this formalism, the Abraham-Minkowski controversy pertaining to the definitions of these quantities is resolved for linear waves of arbitrary nature, and corrections to the traditional formulas for the photon kinetic energy-momentum are found. Several other applications of axiomatic geometrical optics to electromagnetic waves are also presented.

Journal ArticleDOI
TL;DR: In this paper, the spectral analysis and error propagation techniques associated with x-ray imaging crystal spectroscopy (XICS) have been used to accurately constrain particle, momentum, and heat transport studies in a tokamak for the first time.
Abstract: This research describes advancements in the spectral analysis and error propagation techniques associated with x-ray imaging crystal spectroscopy (XICS) that have enabled this diagnostic to be used to accurately constrain particle, momentum, and heat transport studies in a tokamak for the first time. Doppler tomography techniques have been extended to include propagation of statistical uncertainty due to photon noise, the effect of non-uniform instrumental broadening as well as flux surface variations in impurity density. These methods have been deployed as a suite of modeling and analysis tools, written in interactive data language (IDL) and designed for general use on tokamaks. Its application to the Alcator C-Mod XICS is discussed, along with novel spectral and spatial calibration techniques. Example ion temperature and radial electric field profiles from recent I-mode plasmas are shown, and the impact of poloidally asymmetric impurity density and natural line broadening is discussed in the context of the planned ITER x-ray crystal spectrometer.

Journal ArticleDOI
TL;DR: A condition for sheath instability due to secondary electron emission (SEE) is derived for low collisionality plasmas and explains three different instability phenomena observed in Hall thruster simulations including a newly found state with spontaneous ∼20 MHz oscillations.
Abstract: A condition for sheath instability due to secondary electron emission (SEE) is derived for low collisionality plasmas. When the SEE coefficient of the electrons bordering the depleted loss cone in energy space exceeds unity, the sheath potential is unstable to a negative perturbation. This result explains three different instability phenomena observed in Hall thruster simulations including a newly found state with spontaneous ~20MHz oscillations. When instabilities occur, the SEE propagating between the walls becomes the dominant contribution to the particle flux, energy loss and axial transport.

Journal ArticleDOI
TL;DR: In this paper, a new fully numerical method is presented which employs multiple Poincare sections to find quasiperiodic orbits of the Restricted Three-Body Problem (RTBP), which reduces the calculations required for searching two-dimensional invariant tori to a search for closed orbits.
Abstract: A new fully numerical method is presented which employs multiple Poincare sections to find quasiperiodic orbits of the Restricted Three-Body Problem (RTBP). The main advantages of this method are the small overhead cost of programming and very fast execution times, robust behavior near chaotic regions that leads to full convergence for given family of quasiperiodic orbits and the minimal memory required to store these orbits. This method reduces the calculations required for searching two-dimensional invariant tori to a search for closed orbits, which are the intersection of the invariant tori with the Poincare sections. Truncated Fourier series are employed to represent these closed orbits. The flow of the differential equation on the invariant tori is reduced to maps between the consecutive Poincare maps. A Newton iteration scheme utilizes the invariance of the circles of the maps onthesePoincaresectionsinordertofindtheFouriercoefficientsthatdefinethecirclestoany given accuracy. A continuation procedure that uses the incremental behavior of the Fourier coefficients between close quasiperiodic orbits is utilized to extend the results from a single orbit to a family of orbits. Quasi-halo and Lissajous families of the Sun-Earth RTBP around the L2 libration point are obtained via this method. Results are compared with the existing literature. A numerical method to transform these orbits from the RTBP model to the real ephemeris model of the Solar System is introduced and applied.

Journal ArticleDOI
TL;DR: In this paper, a local measurement of the azimuthal-radial component of the Reynolds stress tensor in a novel laboratory apparatus where Ekman e ects are minimized by flexible control of axial boundary conditions is performed.
Abstract: Subcritical transition to turbulence has been proposed as a source of turbulent viscosity required for the associated angular momentum transport for fast accretion in Keplerian disks. Previously cited laboratory experiments in supporting this hypothesis were performed either in a di erent type of flow than Keplerian or without quantitative measurements of angular momentum transport and mean flow profile, and all of them appear to su er from Ekman e ects, secondary flows induced by nonoptimal axial boundary conditions. Such Ekman e ects are expected to be absent from astronomical disks, which probably have stress-free vertical boundaries unless strongly magnetized. Aims. To quantify angular momentum transport due to subcritical hydrodynamic turbulence, if exists, in a quasi-Keplerian flow with minimized Ekman e ects. Methods.We perform a local measurement of the azimuthal-radial component of the Reynolds stress tensor in a novel laboratory apparatus where Ekman e ects are minimized by flexible control of axial boundary conditions. Results.We find significant Ekman e ects on angular momentum transport due to nonoptimal axial boundary conditions in quasi-Keplerian flows. With the optimal control of Ekman e ects, no statistically meaningful angular momentum transport is detected in such flows at Reynolds number up to two millions. Conclusions. Eithermore » a subcritical transition does not occur, or, if a subcritical transition does occur, the associated radial transport of angular momentum in optimized quasi-Keplerian laboratory flows is too small to directly support the hypothesis that subcritical hydrodynamic turbulence is responsible for accretion in astrophysical disks. Possible limitations in applying laboratory results to astrophysical disks due to experimental geometry are discussed.« less

Journal ArticleDOI
TL;DR: The simulation provides a comprehensive picture of mode conversion and shows the fundamental importance of the 3D nonlinear physics in transferring energy to large perpendicular k(y) modes, which can provide large transport across plasma boundaries in space and laboratory plasmas.
Abstract: We report the first three-dimensional (3D) ion particle simulation of mode conversion from a fast mode compressional wave to kinetic Alfv\'en waves (KAWs) that occurs when a compressional mode propagates across a plasma boundary into a region of increasing Alfv\'en velocity. The magnetic field is oriented in the $\stackrel{^}{z}$ direction perpendicular to the gradients in the background density and magnetic field ($\stackrel{^}{x}$ direction). Following a stage dominated by linear physics in which KAWs with large wave numbers ${k}_{x}{\ensuremath{\rho}}_{i}\ensuremath{\sim}1$ (with ${\ensuremath{\rho}}_{i}$ being the ion Larmor radius) are generated near the Alfv\'en resonance surface, the growth of KAW modes with ${k}_{y}{\ensuremath{\rho}}_{i}\ensuremath{\sim}1$ is observed in the nonlinear stage when the amplitude of KAWs generated by linear mode conversion becomes large enough to drive a nonlinear parametric decay process. The simulation provides a comprehensive picture of mode conversion and shows the fundamental importance of the 3D nonlinear physics in transferring energy to large perpendicular ${k}_{y}$ modes, which can provide large transport across plasma boundaries in space and laboratory plasmas.

Journal ArticleDOI
TL;DR: The neutral-beam induced D(α) emission spectrum contains a wealth of information such as deuterium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density, and local magnetic field strength magnitude from the Stark-split beam emission spectrum.
Abstract: The neutral-beam induced D(α) emission spectrum contains a wealth of information such as deuterium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density, and local magnetic field strength magnitude |B| from the Stark-split beam emission spectrum, and fast-ion D(α) emission (FIDA) proportional to the beam-injected fast ion density. A comprehensive spectral fitting routine which accounts for all photoemission processes is employed for the spectral analysis. Interpretation of the measurements to determine physically relevant plasma parameters is assisted by the use of an optimized viewing geometry and forward modeling of the emission spectra using a Monte-Carlo 3D simulation code.

01 Dec 2012
TL;DR: In this paper, the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions was analyzed with 11 years of DMSP SSJ/4/5 data.
Abstract: [1] Substorms release a large amount of energy, some of which is used to energize the precipitating particles in the polar region. Superposed epoch analysis was performed with 11 years of DMSP SSJ/4/5 data to characterize the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions. Although substorms only increase the ion pressure by 30%, they increase the power of the diffuse, monoenergetic, and wave electron aurora by 310%, 71%, and 170%, respectively. Substorms energize the ion aurora mainly in the 21:00–05:00 magnetic local time (MLT) sector. The dynamics of the diffuse electron aurora are different from those of the other two electron aurorae. The expansion phase duration is approximately 15 min for the monoenergetic and wave electron aurorae, whereas it is 1 h for the diffuse electron aurora. The monoenergetic and wave electron aurorae appear to complete the substorm cycle within a 5 h interval, whereas the diffuse electron aurora takes more than 5 h. The diffuse electron aurora power and energy flux start increasing at 15 min before the substorm onset, whereas those for the monoenergetic and wave electron aurorae start increasing at 1 h and 15 min before the onset. The increase in the monoenergetic electron aurora power and energy flux may result from the increase in the magnetotail stretching and region-1 field-aligned current during the growth phase. The monoenergetic electrons may also be associated with fast flows, which have been previously observed more frequently in the dusk-midnight sector.

Journal ArticleDOI
TL;DR: Menard et al. as mentioned in this paper explored a range of high-performance equilibrium scenarios achievable with neutral beam heating in the NSTX-Upgrade device, with three additional neutral beam sources with significantly larger current-drive efficiency.
Abstract: This paper explores a range of high-performance equilibrium scenarios achievable with neutral beam heating in the NSTX-Upgrade device (Menard J.E. 2012 Nucl. Fusion 52 083015). NSTX-Upgrade is a substantial upgrade to the existing NSTX device (Ono M. et al 2000 Nucl. Fusion 40 557), with significantly higher toroidal field and solenoid capabilities, and three additional neutral beam sources with significantly larger current-drive efficiency. Equilibria are computed with free-boundary TRANSP, allowing a self-consistent calculation of the non-inductive current-drive sources, the plasma equilibrium and poloidal-field coil currents, using the realistic device geometry. The thermal profiles are taken from a variety of existing NSTX discharges, and different assumptions for the thermal confinement scalings are utilized. The no-wall and ideal-wall n = 1 stability limits are computed with the DCON code. The central and minimum safety factors are quite sensitive to many parameters: they generally increase with large outer plasma-wall gaps and higher density, but can have either trend with the confinement enhancement factor. In scenarios with strong central beam current drive, the inclusion of non-classical fast-ion diffusion raises qmin, decreases the pressure peaking, and generally improves the global stability, at the expense of a reduction in the non-inductive current-drive fraction; cases with less beam current drive are largely insensitive to additional fast-ion diffusion. The non-inductive current level is quite sensitive to the underlying confinement and profile assumptions. For instance, for BT = 1.0 T and Pinj = 12.6 MW, the non-inductive current level varies from 875 kA with ITER-98y,2 thermal confinement scaling and narrow thermal profiles to 1325 kA for an ST specific scaling expression and broad profiles. Scenarios are presented which can be sustained for 8–10 s, or (20–30) τCR, at βN = 3.8–4.5. The value of qmin can be controlled at either fixed non-inductive fraction of 100% or fixed plasma current, by varying which beam sources are used, opening the possibility for feedback control of the current profile. In terms of quantities like collisionality, neutron emission, non-inductive fraction, or stored energy, these scenarios represent a significant performance extension compared with NSTX and other present spherical torii.

Journal ArticleDOI
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.

Journal ArticleDOI
TL;DR: In this paper, two and three magnetic flux ropes are created and studied in a well-diagnosed laboratory experiment and the divergence of the field lines within the quasi-separatrix layers and the field line motion is presented.
Abstract: Two and three magnetic flux ropes are created and studied in a well-diagnosed laboratory experiment. The twisted helical bundles of field lines rotate and collide with each other over time. In the two rope case, reverse current layers indicative of reconnection are observed. Using a high spatial and temporal resolution three-dimensional volume data set in both cases, quasi-separatrix layers (QSLs) are identified in the magnetic field. Originally developed in the context of solar magnetic reconnection, QSLs are thought to be preferred sites for reconnection. This is verified in these studies. In the case of three flux ropes there are multiple QSLs, which come and go in time. The divergence of the field lines within the QSLs and the field line motion is presented. In all cases, it is observed that the reconnection is patchy in space and bursty in time. Although it occurs at localized positions it is the result of the nonlocal behavior of the flux ropes.

Journal ArticleDOI
TL;DR: It is shown that classical calculations with statistical potentials derived from the exact quantum two-body density matrix produce results in far better agreement with pure-Coulomb quantum GLB than classical calculations performed with older existing Statistical potentials.
Abstract: We study the problem of electron-ion temperature equilibration in plasmas. We consider pure H at various densities and temperatures and Ar-doped H at temperatures high enough so that the Ar is fully ionized. Two theoretical approaches are used: classical molecular dynamics (MD) with statistical two-body potentials and a generalized Lenard-Balescu (GLB) theory capable of treating multicomponent weakly coupled plasmas. The GLB is used in two modes: (1) with the quantum dielectric response in the random-phase approximation (RPA) together with the pure Coulomb interaction and (2) with the classical ($\ensuremath{\hbar}\ensuremath{\rightarrow}0$) dielectric response (both with and without local-field corrections) together with the statistical potentials. We find that the MD results are described very well by classical GLB including the statistical potentials and without local-field corrections (RPA only); worse agreement is found when static local-field effects are included, in contradiction to the classical pure-Coulomb case with like charges. The results of the various approaches are all in excellent agreement with pure-Coulomb quantum GLB when the temperature is high enough. In addition, we show that classical calculations with statistical potentials derived from the exact quantum two-body density matrix produce results in far better agreement with pure-Coulomb quantum GLB than classical calculations performed with older existing statistical potentials.

Journal ArticleDOI
TL;DR: Advanced implicit algorithms with highly accurate spatial representations of the vector fields describing the plasma flow and magnetic fields and with improved methods of calculating anisotropic thermal conduction now makes possible simulations of fusion experiments using realistic values of plasma parameters and actual configuration geometry.

Journal ArticleDOI
TL;DR: It is shown that the HHFW power flows to these spirals along magnetic field lines passing through the SOL in front of the antenna, implying that theHHFW power couples across the entire width of the SOL rather than mostly at the antenna face.
Abstract: A significant fraction of high-harmonic fast-wave (HHFW) power applied to NSTX can be lost to the scrape-off layer (SOL) and deposited in bright and hot spirals on the divertor rather than in the core plasma. We show that the HHFW power flows to these spirals along magnetic field lines passing through the SOL in front of the antenna, implying that the HHFW power couples across the entire width of the SOL rather than mostly at the antenna face. This result will help guide future efforts to understand and minimize these edge losses in order to maximize fast-wave heating and current drive.

Journal ArticleDOI
TL;DR: In this article, the displacement of the plasma boundary in ELMing H-mode discharges with a 10 Hz rotating n = 2 external magnetic field perturbation in DIII-D was studied.
Abstract: Imaging beam emission spectroscopy has been used to study the displacement of the plasma boundary in ELMing H-mode discharges with a 10 Hz rotating n = 2 external magnetic field perturbation in DIII-D. The rotating magnetic field creates a helical displacement of the beam emission profile of ~2 cm on the low-field-side (LFS) midplane which rotates with the applied resonant magnetic perturbation. This shift in the beam emission profile is due primarily to a shift in the electron density profile, which is independently measured to be 1.9 cm on the LFS midplane. These boundary displacements exceed calculations for the displacement of the stable and unstable manifolds formed by the interaction of the magnetic perturbation with the divertor separatrix by a factor of 4–5, suggesting that the vacuum field model does not correctly model the effect of the magnetic perturbations even near the separatrix. The measured displacements are suggestive of a non-resonant kink response.

Journal ArticleDOI
TL;DR: Sawtooth control using electron cyclotron current drive (ECCD) has been demonstrated in ITER-like plasmas with a large fast ion fraction, wide q = 1 radius and long uncontrolled sawtooth period in DIII-D.
Abstract: Sawtooth control using electron cyclotron current drive (ECCD) has been demonstrated in ITER-like plasmas with a large fast ion fraction, wide q = 1 radius and long uncontrolled sawtooth period in DIII-D. The sawtooth period is minimized when the ECCD resonance is just inside the q = 1 surface. Sawtooth destabilization using driven current inside q = 1 avoids the triggering of performance-degrading neoclassical tearing modes (NTMs), even at much higher pressure than required in the ITER baseline scenario. Operation at βN = 3 without 3/2 or 2/1 NTMs has been achieved in ITER demonstration plasmas when sawtooth control is applied using only modest ECCD power. Numerical modelling qualitatively confirms that the achieved driven current changes the local magnetic shear sufficiently to compensate for the stabilizing influence of the energetic particles in the plasma core.

Journal ArticleDOI
TL;DR: Observations of a free-Shercliff-layer instability in a Taylor-Couette experiment using a liquid metal over a wide range of Reynolds numbers have implications for a range of rotating MHD systems in which similar shear layers may be produced.
Abstract: We report on observations of a free-Shercliff-layer instability in a Taylor-Couette experiment using a liquid metal over a wide range of Reynolds numbers, Re∼10(3)-10(6). The free Shercliff layer is formed by imposing a sufficiently strong axial magnetic field across a pair of differentially rotating axial end cap rings. This layer is destabilized by a hydrodynamic Kelvin-Helmholtz-type instability, characterized by velocity fluctuations in the r-θ plane. The instability appears with an Elsasser number above unity, and saturates with an azimuthal mode number m which increases with the Elsasser number. Measurements of the structure agree well with 2D global linear mode analyses and 3D global nonlinear simulations. These observations have implications for a range of rotating MHD systems in which similar shear layers may be produced.

Journal ArticleDOI
TL;DR: Using magnetic torque from n=3 fields to replace counter-Ip torque from neutral beam injection (NBI) has achieved long duration, counter-rotating QH-mode operation with NBI torque ranging from counterIp to up to coIp values of 1-1.3 Nm.
Abstract: Results from recent experiments demonstrate that quiescent H-mode (QH-mode) sustained by magnetic torque from non-axisymmetric magnetic fields is a promising operating mode for future burning plasmas. Using magnetic torque from n=3 fields to replace counter-Ip torque from neutral beam injection (NBI), we have achieved long duration, counter-rotating QH-mode operation with NBI torque ranging from counter-Ip to up to co-Ip values of 1-1.3 Nm. This co-Ip torque is 3 to 4 times the scaled torque that ITER will have. These experiments utilized an ITER-relevant lower single-null plasma shape and were done with ITER-relevant values of νped* and βNped. These discharges exhibited confinement quality H98y2=1.3, in the range required for ITER. In preliminary experiments using n=3 fields only from a coil outside the toroidal coil, QH-mode plasmas with low q95=3.4 have reached fusion gain values of G=βNH89/q952=0.4, which is the desired value for ITER. Shots with the same coil configuration also operated with net zero...

Journal ArticleDOI
TL;DR: In this paper, the ideal MHD code NOVA is used to calculate eigenmodes of DIII-D plasmas and perturbatively calculate the actual stability of the modes, including finite orbit width and finite Larmor radius effects.
Abstract: Neutral beam injection into reversed-magnetic shear DIII-D plasmas produces a variety of Alfv?nic activity including toroidicity-induced Alfv?n eigenmodes (TAEs) and reversed shear Alfv?n eigenmodes (RSAEs). With measured equilibrium profiles as inputs, the ideal MHD code NOVA is used to calculate eigenmodes of these plasmas. The postprocessor code NOVA-K is then used to perturbatively calculate the actual stability of the modes, including finite orbit width and finite Larmor radius effects, and reasonable agreement with the spectrum of observed modes is found. Using experimentally measured mode amplitudes, fast ion orbit following simulations have been carried out in the presence of the NOVA calculated eigenmodes and are found to reproduce the dominant energy, pitch and temporal evolution of the losses measured using a large bandwidth scintillator diagnostic. The same analysis techniques applied to a DT 8 MA ITER steady-state plasma scenario with reversed-magnetic shear and both beam ion and alpha populations show Alfv?n eigenmode instability. Both RSAEs and TAEs are found to be unstable with maximum growth rates occurring for toroidal mode number n?=?6 and the majority of the drive coming from fast ions injected by the 1?MeV negative ion beams. AE instability due to beam ion drive is confirmed by the non-perturbative code TAEFL. Initial fast ion orbit following simulations using the unstable modes with a range of amplitudes (?B/B?=?10?5?10?3) have been carried out and show negligible fast ion loss. The lack of fast ion loss is a result of loss boundaries being limited to large radii and significantly removed from the actual modes themselves.

Journal ArticleDOI
TL;DR: In this paper, a drift-kinetic particle code XGC0, equipped with a mass-momentum energy conserving collision operator, is used to study the edge bootstrap current in a realistic diverted magnetic field geometry with a self-consistent radial electric field.
Abstract: The edge bootstrap current plays a critical role in the equilibrium and stability of the steep edge pedestal plasma. The pedestal plasma has an unconventional and difficult neoclassical property, as compared with the core plasma. It has a narrow passing particle region in velocity space that can be easily modified or destroyed by Coulomb collisions. At the same time, the edge pedestal plasma has steep pressure and electrostatic potential gradients whose scale-lengths are comparable with the ion banana width, and includes a magnetic separatrix surface, across which the topological properties of the magnetic field and particle orbits change abruptly. A drift-kinetic particle code XGC0, equipped with a mass-momentum-energy conserving collision operator, is used to study the edge bootstrap current in a realistic diverted magnetic field geometry with a self-consistent radial electric field. When the edge electrons are in the weakly collisional banana regime, surprisingly, the present kinetic simulation confirm...