[en] Advanced tokamak research seeks to find the ultimate potential of the tokamak as a magnetic confinement system. Achieving this potential involves optimizing the plasma cross-sectional shape, current density, and pressure profiles for stability to magnetohydrodynamic (MHD) modes while simultaneously controlling the current density, pressure, and radial electric field profiles to minimize the cross field transport of plasma energy. In its ultimate, steady-state incarnation, the advanced tokamak also requires pressure profiles that have been adjusted to achieve the maximum possible bootstrap current, subject to the constraints of MHD stability. This simultaneous, nonlinear optimization of shape, current, pressure, and electric field profiles to meet multiple goals is a grand challenge to plasma physics. To keep the plasma at peak performance, active feedback control will almost certainly be required. Diagnostic measurements play a crucial role in advanced tokamak research both for developing the scientific understanding underlying the optimization and for serving as sensors for real time feedback control. One outstanding example of this is the way motional Stark effect (MSE) measurements of the internal magnetic field revolutionized work on current profile shaping. Improved diagnostic measurements are essential in testing theories which must be validated in order to apply advanced tokamak results to next step devices

[en] The spectrum of turbulent density fluctuations at long poloidal wavelengths in the edge plasma of the DIII-D tokamak peaks at nonzero radial wave number. The associated electric-potential fluctuations cause sheared tilde ExB flows primarily in the poloidal direction. These zonal flows have been predicted by theory and are believed to regulate the overall level of turbulence and anomalous transport. This study provides the first indirect experimental identification of zonal flows

[en] The transition from low-confinement (L-mode) to high-confinement (H-mode) plasmas has been directly produced by injecting frozen deuterium pellets in the DIII-D tokamak. H-mode transitions were produced at edge electron and ion temperatures below the L-mode values. This implies that a critical edge temperature is not necessary for H-mode transitions. The experimentally determined edge plasma parameters were well below those predicted by several theories of the H-mode transition to trigger the H-mode, indicating a need for revision of these theories

[en] The GLF23 transport model is used to dynamically follow bifurcations in the energy and toroidal momentum confinement in DIII-D discharges with an internal transport barrier. The temperatures and toroidal velocity profiles are evolved while self-consistently computing the effects of ExB shear stabilization during the formation and expansion of internal transport barriers. The barrier is predicted to form in a stepwise fashion through a series of sudden jumps in the core-electron and ion temperatures and toroidal rotation velocity. These results are consistent with experimental observations. In the simulations, the step transitions are a direct result of local ExB driven transport bifurcations

[en] There is general agreement that the creation of transport barriers in magnetized plasmas is associated with the reduction in turbulence-driven transport. The fundamental physics involved in barrier formation is the effect of equilibrium E x B shear and zonal flows on turbulence and transport. This paper focuses on three major issues in turbulence and transport barriers that were discussed at the Tenth IAEA Technical Committee Meeting on H-Mode Physics and Transport Barriers: (1) zonal flows and their effects on turbulence (2) spatial spreading of turbulence from regions of instability to regions of stability and (3) the effects of short wavelength turbulence. This work gives a short summary of experimental work bearing on each of the themes and raises fundamental questions to motivate future research in each of these areas

[en] Charge exchange spectroscopy is one of the key ion diagnostics on the DIII-D tokamak. It allows measurement of impurity densities, toroidal and poloidal rotation speeds, ion temperatures, and the radial electric field. For the 2000 experimental campaign, we have replaced the intensified photodiode array detectors on the edge portion of the system with advanced charge-coupled device (CCD) detectors mounted on faster (f/4.7) Czerny--Turner spectrometers equipped with toroidal mirrors. The combination has improved the photoelectron signal level by about a factor of 20 and the signal to noise by a factor of 2--8, depending on the absolute signal level and readout mode. A major portion of the signal level improvement comes from the improved quantum efficiency of the back-illuminated, thinned CCD detector (70% to 85% quantum efficiency for the CCD versus 10% for the image intensifier) with the remainder coming from the faster spectrometer. The CCD camera also allows shorter minimum integration times: 0.33 ms while archiving to computer memory and 0.15 ms using temporary storage on the CCD chip

[en] In electron cyclotron heated (ECH) H-mode discharges with neutral beam injection (NBI) pulses that are short compared with the fast ion scattering or slowing times, and it is observed that the plasma stores all the angular momentum delivered by the NBI torque impulse. The pulse length is also much shorter than the momentum confinement time of the plasma. Source computations with the Monte Carlo code TRANSP [R. J. Goldston, D. C. McCune, H. H. Towner et al., J. Comput. Phys. 43, 61 (1981)] show that during a pulse approximately 90% of this torque impulse is delivered via the collisionless fast radial current injection process, so that the plasma acquires the balancing toroidal acceleration through ion drift motion in the increasing ∂E/∂t where E is the electric field normal to the flux surfaces. The measured radial profile of the toroidal momentum increase matches the source, i.e., the computed torque impulse profile. We measure the bulk ion toroidal acceleration in helium discharges, as well as that of the primary impurity, carbon. These two species show a common acceleration, consistent with an incremental velocity due to an electric drift. This process is mediated by the dielectric response of the ions. The acceleration measurements are consistent with this being the neoclassical value of the dielectric constant, as computed from measured quantities

[en] Confinement enhancement of the reversed magnetic shear plasmas in Tokamak Fusion Test Reactor (TFTR) and DIII-D is discussed in the context of the hypothesis of E x B flow shear suppression of turbulence. Our recent nonlinear theory on flow-shear-induced fluctuation suppression in an arbitrary shape finite aspect ratio tokamak plasma (Phys. Plasmas 2, 1648 (1995)) indicates that the relevant E x B shearing rate in toroidal geometry is proportional to the radial shear of E_r⁽⁰⁾/RB_θ. Recent results from both TFTR Enhanced Reversed Shear (ERS)-mode plasma and DIII-D Negative Central Shear (NCS)-mode plasmas show that the shearing rate is indeed significant and exceeds the maximum growth rate of the prominent trapped-electron-ion-temperature-gradient modes in the region where fluctuation is suppressed and transport barrier is formed. (author)

[en] In plasmas equipped with neutral beam injection, excitation of atomic spectral lines via charge-exchange with neutral atoms is the basis of one of the standard plasma diagnostic techniques for ion density, temperature, and velocity. In order to properly interpret the spectroscopic results, one must consider the effects of the energy dependence of the charge-exchange cross-section as well as the motion of the ion after charge-exchange during the period when it is still in the excited state. This motion is affected by the electric and magnetic fields in the plasma. The present paper gives results for the velocity distribution function of the excited state ions and considers in detail the cross-section and ion motion effects on the post charge-exchange velocity. The expression for this velocity in terms of the charge-exchange cross-section and the pre charge-exchange velocity allows that latter velocity to be determined. The present paper is the first to consider the effect of the electric as well as the magnetic field and demonstrates that electric field and diamagnetic terms appear in the expression for the inferred velocity. The present formulation also leads to a novel technique for assessing the effect of the energy dependence of the charge-exchange cross-section on the inferred ion temperature.

[en] In recent DIII-D experiments, we concentrated on extending the operating range and improving the overall performance of quiescent H-mode (QH) plasmas. The QH-mode offers an attractive, high-performance operating mode for burning plasmas due to the absence of pulsed edge-localized-mode-driven losses to the divertor (ELMs). Using counter neutral-beam injection (NBI), we achieve steady plasma conditions with the presence of an edge harmonic oscillation (EHO) replacing the ELMs and providing control of the edge pedestal density. These conditions have been maintained for greater than 4s (∼30 energy confinement times, τ_E, and 2 current relaxation times, τ_R [1]), and often limited only by the duration of auxiliary heating. We discuss results of these recent experiments where we use triangularity ramping to increase the density, neutral beam power ramps to increase the stored energy, injection of rf power at the electron cyclotron (EC) frequency to control density profile peaking in the core, and control of startup conditions to completely eliminate the transient ELMing phase