Main-Ion Intrinsic Toroidal Rotation Profile Driven by Residual Stress Torque from Ion Temperature Gradient Turbulence in the DIII-D Tokamak.

Intrinsic toroidal rotation of the deuterium main ions in the core of the DIII-D tokamak is observed to transition from flat to hollow, forming an off-axis peak, above a threshold level of direct electron heating. Nonlinear gyrokinetic simulations show that the residual stress associated with electrostatic ion temperature gradient turbulence possesses the correct radial location and stress structure to cause the observed hollow rotation profile. Residual stress momentum flux in the gyrokinetic simulations is balanced by turbulent momentum diffusion, with negligible contributions from turbulent pinch. The prediction of the velocity profile by integrating the momentum balance equation produces a rotation profile that qualitatively and quantitatively agrees with the measured main-ion profile, demonstrating that fluctuation-induced residual stress can drive the observed intrinsic velocity profile.

Pedestal bifurcation and resonant field penetration at the threshold of edge-localized mode suppression in the DIII-D Tokamak.

Rapid bifurcations in the plasma response to slowly varying n=2 magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of n=2 fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied n=2 field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.

Access to a new plasma edge state with high density and pressures using the quiescent H mode.

A path to a new high performance regime has been discovered in tokamaks that could improve the attractiveness of a fusion reactor. Experiments on DIII-D using a quiescent H-mode edge have navigated a valley of improved edge peeling-ballooning stability that opens up with strong plasma shaping at high density, leading to a doubling of the edge pressure over the standard H mode with edge localized modes at these parameters. The thermal energy confinement time increases as a result of both the increased pedestal height and improvements in the core transport and reduced low-k turbulence. Calculations of the pedestal height and width as a function of density using constraints imposed by peeling-ballooning and kinetic-ballooning theory are in quantitative agreement with the measurements.

Active spectroscopic measurements of the bulk deuterium properties in the DIII-D tokamak (invited).

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.

Observation of peak neoclassical toroidal viscous force in the DIII-D tokamak.

Observation of a theoretically predicted peak in the neoclassical toroidal viscosity (NTV) force as a function of toroidal plasma rotation rate Ω is reported. The NTV was generated by applying n=3 magnetic fields from internal coils to low Ω plasmas produced with nearly balanced neutral beam injection. Locally, the peak corresponds to a toroidal rotation rate Ω(0) where the radial electric field E(r) is near zero as determined by radial ion force balance.

Experimental investigation of the role of fluid turbulent stresses and edge plasma flows for intrinsic rotation generation in DIII-D H-mode plasmas.

The first measurements of turbulent stresses and flows inside the separatrix of a tokamak H-mode plasma are reported, using a reciprocating multitip Langmuir probe at the DIII-D tokamak. A strong co-current rotation layer at the separatrix is found to precede intrinsic rotation development in the core. The measured fluid turbulent stresses transport toroidal momentum outward against the velocity gradient and thus try to sustain the edge layer. However, large kinetic stresses must exist to explain the net inward momentum transport leading to co-current core plasma rotation. The importance of such kinetic stresses is corroborated by the success of a simple orbit loss model, representing a purely kinetic mechanism, in the prediction of features of the edge corotation layer.

Masking a CCD camera allows multichord charge exchange spectroscopy measurements at high speed on the DIII-D tokamak.

Charge exchange spectroscopy is one of the standard plasma diagnostic techniques used in tokamak research to determine ion temperature, rotation speed, particle density, and radial electric field. Configuring a charge coupled device (CCD) camera to serve as a detector in such a system requires a trade-off between the competing desires to detect light from as many independent spatial views as possible while still obtaining the best possible time resolution. High time resolution is essential, for example, for studying transient phenomena such as edge localized modes. By installing a mask in front of a camera with a 1024 × 1024 pixel CCD chip, we are able to acquire spectra from eight separate views while still achieving a minimum time resolution of 0.2 ms. The mask separates the light from the eight spectra, preventing spatial and temporal cross talk. A key part of the design was devising a compact translation stage which attaches to the front of the camera and allows adjustment of the position of the mask openings relative to the CCD surface. The stage is thin enough to fit into the restricted space between the CCD camera and the spectrometer endplate.

Deuterium velocity and temperature measurements on the DIII-D tokamak.

Newly installed diagnostic capabilities on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 46, 6114 (2002)] enable the measurement of main ion (deuterium) velocity and temperature by charge exchange recombination spectroscopy. The uncertainty in atomic physics corrections for determining the velocity is overcome by exploiting the geometrical dependence of the apparent velocity on the viewing angle with respect to the neutral beam.

Quiescent H-mode plasmas with strong edge rotation in the cocurrent direction.

For the first time in any tokamak, quiescent H-mode (QH-mode) plasmas have been created with strong edge rotation in the direction of the plasma current. This confirms the theoretical prediction that the QH mode should exist with either sign of the edge rotation provided the magnitude of the shear in the edge rotation is sufficiently large and demonstrates that counterinjection and counteredge rotation are not essential for the QH mode. Accordingly, the present work demonstrates a substantial broadening of the QH-mode operating space and represents a significant confirmation of the theory.

Dust measurements in tokamaks (invited).

Dust production and accumulation present potential safety and operational issues for the ITER. Dust diagnostics can be divided into two groups: diagnostics of dust on surfaces and diagnostics of dust in plasma. Diagnostics from both groups are employed in contemporary tokamaks; new diagnostics suitable for ITER are also being developed and tested. Dust accumulation in ITER is likely to occur in hidden areas, e.g., between tiles and under divertor baffles. A novel electrostatic dust detector for monitoring dust in these regions has been developed and tested at PPPL. In the DIII-D tokamak dust diagnostics include Mie scattering from Nd:YAG lasers, visible imaging, and spectroscopy. Laser scattering is able to resolve particles between 0.16 and 1.6 microm in diameter; using these data the total dust content in the edge plasmas and trends in the dust production rates within this size range have been established. Individual dust particles are observed by visible imaging using fast framing cameras, detecting dust particles of a few microns in diameter and larger. Dust velocities and trajectories can be determined in two-dimension with a single camera or three-dimension using multiple cameras, but determination of particle size is challenging. In order to calibrate diagnostics and benchmark dust dynamics modeling, precharacterized carbon dust has been injected into the lower divertor of DIII-D. Injected dust is seen by cameras, and spectroscopic diagnostics observe an increase in carbon line (CI, CII, C(2) dimer) and thermal continuum emissions from the injected dust. The latter observation can be used in the design of novel dust survey diagnostics.

Characterization of cross-section correction to charge exchange recombination spectroscopy rotation measurements using co- and counter-neutral-beam views.

Measurements of rotation using charge exchange recombination spectroscopy can be affected by the energy dependence of the charge exchange cross section. On DIII-D, the associated correction to the rotation can exceed 100 kms at high temperatures. In reactor-relevant low rotation conditions, the correction can be several times larger than the actual plasma rotation and therefore must be carefully validated. New chords have been added to the DIII-D CER diagnostic to view the counter-neutral-beam line. The addition of these views allows determination of the toroidal rotation without depending on detailed atomic physics calculations, while also allowing experimental characterization of the atomic physics. A database of rotation comparisons from the two views shows that the calculated cross-section correction can adequately describe the measurements, although there is a tendency for "overcorrection." In cases where accuracy better than about 15% is desired, relying on calculation of the cross-section correction may be insufficient.

Observation of plasma rotation driven by static nonaxisymmetric magnetic fields in a tokamak.

We present the first evidence for the existence of a neoclassical toroidal rotation driven in a direction counter to the plasma current by nonaxisymmetric, nonresonant magnetic fields. At high beta and with large injected neutral beam momentum, the nonresonant field torque slows down the plasma toward the neoclassical "offset" rotation rate. With small injected neutral beam momentum, the toroidal rotation is accelerated toward the offset rotation, with resulting improvement in the global energy confinement time. The observed magnitude, direction, and radial profile of the offset rotation are consistent with neoclassical theory predictions.

Intense geodesic acousticlike modes driven by suprathermal ions in a tokamak plasma.

Intense axisymmetric oscillations driven by suprathermal ions injected in the direction counter to the toroidal plasma current are observed in the DIII-D tokamak. The modes appear at nearly half the ideal geodesic acoustic mode frequency, in plasmas with comparable electron and ion temperatures and elevated magnetic safety factor (q_{min}>or=2). Strong bursting and frequency chirping are observed, concomitant with large (10%-15%) drops in the neutron emission. Large electron density fluctuations (n[over ]_{e}/n_{e} approximately 1.5%) are observed with no detectable electron temperature fluctuations, confirming a dominant compressional contribution to the pressure perturbation as predicted by kinetic theory. The observed mode frequency is consistent with a recent theoretical prediction for the energetic-particle-driven geodesic acoustic mode.

Momentum-transport studies in high E x B shear plasmas in the National Spherical Torus Experiment.

Experiments have been conducted at the National Sperical Torus Experiment (NSTX) to study both steady state and perturbative momentum transport. These studies are unique in their parameter space under investigation, where the low aspect ratio of NSTX results in rapid plasma rotation with ExB shearing rates high enough to suppress low-k turbulence. In some cases, the ratio of momentum to energy confinement time is found to exceed five. Momentum pinch velocities of order 10-40 m/s are inferred from the measured angular momentum flux evolution after nonresonant magnetic perturbations are applied to brake the plasma.

Reduced critical rotation for resistive-wall mode stabilization in a near-axisymmetric configuration.

Recent DIII-D experiments with reduced neutral beam torque and minimum nonaxisymmetric perturbations of the magnetic field show a significant reduction of the toroidal plasma rotation required for the stabilization of the resistive-wall mode (RWM) below the threshold values observed in experiments that apply nonaxisymmetric magnetic fields to slow the plasma rotation. A toroidal rotation frequency of less than 10 krad/s at the q=2 surface (measured with charge exchange recombination spectroscopy using C VI) corresponding to 0.3% of the inverse of the toroidal Alfvén time is sufficient to sustain the plasma pressure above the ideal MHD no-wall stability limit. The low-rotation threshold is found to be consistent with predictions by a kinetic model of RWM damping.

Radial structure of Alfvén eigenmodes in the DIII-D tokamak through electron-cyclotron-emission measurements.

The spatial structure of toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes in DIII-D is obtained from electron-cyclotron-emission measurements. Peak measured temperature perturbations are of similar magnitude for both toroidal Alfvén eigenmodes and reversed shear Alfvén 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.

Multitude of core-localized shear Alfvén waves in a high-temperature fusion plasma.

Evidence is presented for a multitude of discrete frequency Alfvén 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 Alfvén velocity.

Turbulent transport reduction and randomization of coherent fluctuations by zonal flows in toroidal plasma.

Fluctuation-driven particle flux is greatly reduced in the plasma radial region where zonal flows are present in the H-1 toroidal heliac. This occurs without reduction in the fluctuation level. Statistical properties of fluctuations are significantly modified in this region. It is shown that the randomization of phases of coherent structures by zonal flows is responsible for the observed effect. This mechanism of transport reduction complements theoretically predicted random shearing of turbulence by zonal flows and does not require the fluctuations suppression.

Experimental evidence of self-regulation of fluctuations by time-varying flows.

We report the first extended experimental results indicating that radially localized time-varying potential structures, which possess many of the characteristics of zonal flows, are generated by strong fluctuations. Experiments performed in the H-1 heliac show that these poloidally symmetric flows are nonlinearly coupled to other fluctuations and are responsible for significant modifications in fluctuations and in the fluctuation-driven transport.

Nonambipolarity of fluctuation-driven fluxes and its effect on the radial electric field.

The nonambipolarity of the fluctuation-driven particle transport is demonstrated experimentally. Convective radial transport of electrons by fluctuations is found to be significantly stronger than that of ions, leading to a mean fluctuation-driven radial current balanced in steady state by other bipolar particle losses. Fluctuation suppression leads to a sudden disappearance of this current and results in significant modification to the radial electric field. The observed change in the electric field is in good agreement with the measured fluctuation-driven flux.