C. M. Greenfield

The beam emission spectroscopy diagnostic on the DIII-D tokamak

A beam emission spectroscopy system has been installed on DIII-D to provide localized density fluctuation measurements for long-wavelength turbulent modes with k⩽3 cm−1 which are typically associated with anomalous radial transport. High signal-to-noise fluctuations measurements are accomplished through use of high speed electronics to maintain a frequency response of over 500 KHz and cryogenically cooled amplifiers and detectors to reduce electronic noise. The optics and neutral beam-sightline geometry have been optimized to allow for spatial resolution of Δr⩽1 cm. In addition, a half-scale two-dimensional (2D) fiber array to measure the 2D turbulent density field, necessary to measure the full S(kr,kθ) wavenumber spectra, has been implemented and initial results obtained.

Improved core fueling with high field side pellet injection in the DIII-D tokamak

The capability to inject deuterium pellets from the magnetic high field side (HFS) has been added to the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. It is observed that pellets injected from the HFS lead to deeper mass deposition than identical pellets injected from the outside midplane, in spite of a factor of 4 lower pellet speed. HFS injected pellets have been used to generate peaked density profile plasmas [peaking factor (ne(0)/〈ne〉) in excess of 3] that develop internal transport barriers when centrally heated with neutral beam injection. The transport barriers are formed in conditions where Te∼Ti and q(0) is above unity. The peaked density profiles, characteristic of the internal transport barrier, persist for several energy confinement times. The pellets are also used to investigate transport barrier physics and modify plasma edge conditions. Transitions from L- to H-mode have been triggered by pellets, effectively lowering the H-mode threshold power by 2.4 MW. Pel...

QUIESCENT H-MODE PLASMAS IN THE DIII-D TOKAMAK

H-mode operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge-localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the β limit and reduced core transport regions needed for advanced tokamak operation. Recent experimental results from DIII-D have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation which is ELM-free and yet has good density control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or >25 energy confinement times τE). By forming the core barrier and then stepping up the input power, we have achieved βNH89 = 7 for up to 10 times the τE of 160 ms. The βNH89 values of 7 substantially exceed the value of 4 routinely achieved in standard ELMing \mbox{H-mode.} The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density control in the quiescent H-mode is possible because of the presence of an edge MHD oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.

Nondimensional transport scaling in DIII‐D: Bohm versus gyro‐Bohm resolved

The scaling of cross‐field heat transport with relative gyroradius ρ* was measured in low (L) and high (H) mode tokamak plasmas using the technique of dimensionally similar discharges. The relative gyroradius scalings of the electron and ion thermal diffusivities were determined separately using a two‐fluid transport analysis. For L‐mode plasmas, the electron diffusivity scaled as χe∝χBρ1.1±0.3* (gyro‐Bohm‐like) while the ion diffusivity scaled as χi∝χBρ−0.5±0.3* (worse than Bohm‐like). The results were independent of the method of auxiliary heating (radio frequency or neutral beam). Since the electron and ion fluids had different gyroradius scalings, the effective diffusivity and global confinement time scalings were found to vary from gyro‐Bohm‐like to Bohm‐like depending upon whether the electron or ion channel dominated the heat flux. This last property can explain the previously disparate results with dimensionally similar discharges on different fusion experiments that have been published. Experimen...

Progress toward fully noninductive, high beta conditions in DIII-D

The DIII-D Advanced Tokamak (AT) program in the DIII-D tokamak [J. L. Luxon, Plasma Physics and Controlled Fusion Research, 1986, Vol. I (International Atomic Energy Agency, Vienna, 1987), p. 159] is aimed at developing a scientific basis for steady-state, high-performance operation in future devices. This requires simultaneously achieving 100% noninductive operation with high self-driven bootstrap current fraction and toroidal beta. Recent progress in this area includes demonstration of 100% noninductive conditions with toroidal beta, βT=3.6%, normalized beta, βN=3.5, and confinement factor, H89=2.4 with the plasma current driven completely by bootstrap, neutral beam current drive, and electron cyclotron current drive (ECCD). The equilibrium reconstructions indicate that the noninductive current profile is well aligned, with little inductively driven current remaining anywhere in the plasma. The current balance calculation improved with beam ion redistribution that was supported by recent fast ion diagno...

Advances in understanding quiescent H-mode plasmas in DIII-D

Recent QH-mode research on DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1996 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] has used the peeling-ballooning modes model of edge magnetohydrodynamic stability as a working hypothesis to organize the data; several predictions of this theory are consistent with the experimental results. Current ramping results indicate that QH modes operate near the edge current limit set by peeling modes. This operating point explains why QH mode is easier to get at lower plasma currents. Power scans have shown a saturation of edge pressure with increasing power input. This allows QH-mode plasmas to remain stable to edge localized modes (ELMs) to the highest powers used in DIII-D. At present, the mechanism for this saturation is unknown; if the edge harmonic oscillation (EHO) is playing a role here, the physics is not a simple amplitude dependence. The increase in edge stability with plasma triangularity predicted by th...

Boronization in DIII-D

A thin boron film has been applied to the DIII-D tokamak plasma facing surfaces to reduce impurity influx, particularly oxygen and carbon. A direct result of this surface modification was the observation of a regime of very high energy confinement, VH-mode, with confinement times from 1.5 to 2 times greater than predicted by H-mode scaling relation for the same set of parameters. VH-mode discharges are characterized by low ohmic target densities, low edge neutral pressure, and reduced cycling. These conditions have reduced the collisionality, {nu}*, in the edge region producing a higher edge pressure gradient and a significant bootstrap current, up to 30% of the total current. We will describe the edge plasma properties after boronization including reductions in recycling inferred from measurements of {tau}{sup p}*. In particular we will discuss the edge plasma conditions necessary for access to VH-mode including the boronization process and properties of the deposited film.

Transport and performance in DIII-D discharges with weak or negative central magnetic shear

Discharges exhibiting the highest plasma energy and fusion reactivity yet realized in the DIII-D tokamak have been produced by combining the benefits of a hollow or weakly sheared central current profile with a high confinement (H-mode) edge. In these discharges, low power neutral beam injection heats the electrons during the initial current ramp, and {open_quotes}freezes in{close_quotes} a hollow or flat central current profile. When the neutral beam power is increased, formation of a region of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a transition is triggered from the low (L-mode) to high (H-mode) confinement regimes, thereby broadening the pressure profile and avoiding the disruption. These plasmas continue to evolve until the high performance phase is terminated nondisruptively at much higher {beta}{sub T} (ratio of plasma pressure to toroidal magnetic field pressure) than would be attainable with peaked profiles and an L-mode edge. Transport analysis indicates that in this phase, the ion diffusivity is equivalent to that predicted by Chang-Hinton neoclassical theory over the entire plasma volume. This result is consistent with suppression of turbulence by locally enhanced E x B flow shear, and is supported by observations of reduced fluctuations in the plasma. Calculations of performance in these discharges extrapolated to a deuterium-tritium fuel mixture indicates that such plasmas could produce a DT fusion gain Q{sub DT} = 0.32.

Very high confinement discharges in DIII‐D after boronization

A regime of very high confinement (VH mode) has recently been observed in DIII‐D with global energy confinement times up to a factor of 3.5 above the ITER89‐P L‐mode scaling [Nucl. Fusion 30, 1999 (1990)] and 1.5 to 2 times greater than the DIII‐D/JET edge‐localized‐mode‐free H‐mode scaling relation [Nucl. Fusion 31, 73 (1991)]. These discharges were obtained after boronization in DIII‐D and are characterized by low radiated power and Zeff, increasing confinement time during the VH phase of the discharge and low Ohmic target density. The low radiated power and Zeff are a consequence of the boronization. During the VH phase these discharges exhibit an inward shift in the region of highest electric field shear and a large calculated edge bootstrap current. The outer region (ρ≳0.85) is calculated to be in the second stable regime to ideal ballooning modes.

Toroidal rotation in neutral beam heated discharges in DIII-D

It is known that the toroidal angular momentum and the ion thermal energy are correlated in tokamak discharges heated by neutral beam injection. Here, data from ten years of measurements on DIII-D are considered, for representative discharges from all types and all conditions. The ratio of simple replacement times for momentum and energy is found to order this correlation indicating that these times are approximately equal, across the minor radius. Representative discharges of several types are discussed in more detail, as well as transport analysis results for the momentum and thermal ion diffusivities.