R. T. Snider

Wall stabilization of high beta plasmas in DIII-D

Detailed analysis of recent high beta discharges in the DIII-D tokamak demonstrates that the resistive vacuum vessel can provide stabilization of low n magnetohydrodynamic (MHD) modes. The experimental beta values reaching up to {beta}{sub T} = 12.6% are more than 30% larger than the maximum stable beta calculated with no wall stabilization. Plasma rotation is essential for stabilization. When the plasma rotation slows sufficiently, unstable modes with the characteristics of the predicted {open_quotes}resistive wall{close_quotes} mode are observed. Through slowing of the plasma rotation between the q = 2 and q = 3 surfaces with the application of a non-axisymmetric field, the authors have determined that the rotation at the outer rational surfaces is most important, and that the critical rotation frequency is of the order of {Omega}/2{pi} = 1 kHz.

An optimization of beta in the DIII-D tokamak

Accurate equilibrium reconstruction and detailed stability analysis of a strongly shaped, double‐null, βT=11% discharge shows that the plasma core is in the second stable regime to ideal ballooning modes. The equilibrium reconstruction using all the available data (coil currents, poloidal magnetic loops, motional Stark effect data, the kinetic pressure profile, the magnetic axis location, and the location of the two q=1 surfaces) shows a region of negative magnetic shear near the magnetic axis, an outer positive shear region, and a low shear region connecting the two. The inner negative shear region allows a large positive shear region near the boundary, even at low q (q95=2.6), permitting a large outer region pressure gradient to be first regime stable. The inner region is in the second stable regime, consistent with the observed axial beta [βT(0)=44%]. In the low shear region p’ vanishes, consistent with Mercier stability. This is one way to extend the ballooning limit in shaped plasmas while maintainin...

Higher beta at higher elongation in the DIII-D tokamak

A theoretical and experimental evaluation of axisymmetric stability and axisymmetric control has led to a modification of the vertical position control in the DIII‐D tokamak, which now allows operation to within a few percent of the ideal magnetohydrodynamic (MHD) n=0 limit. It is found that the onset the departure from rigid shift behavior in D‐shaped plasmas limits plasma elongation to 2.5 in DIII‐D. The possibility of avoiding the vertical instability in future tokamaks with highly elongated plasmas is discussed. Recent experiments have focused on utilizing this capability for axisymmetric control to construct plasma shapes optimized to increase the achievable beta. Operation near the axisymmetric stability limit allows an increase in the achieved normalized current Ip/aBT, where Ip is the total plasma current, a is the minor radius, and BT is the toroidal field. Based on stability calculations, an equilibrium was developed to achieve marginal stability simultaneously to axisymmetric, kink, and balloon...

Investigation of physical processes limiting plasma density in high confinement mode discharges on DIII-D

A series of experiments was conducted on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mode) discharges. The typical H-mode to low confinement mode (L-mode) transition limit at high density near the empirical Greenwald density limit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevented formation of a high density, intense radiation zone (MARFE) near the X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit and increased nonlinearly with the plasma current. Magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power levels, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was ⩽3 MW. Formation of MARFEs on closed field lines was avoided with lo...

Heat pulse propagation studies on DIII-D and the Tokamak Fusion Test Reactor

Sawtooth phenomena have been studied on DIII-D and the Tokamak Fusion Test Reactor (TFTR) [D. Meade and the TFTR Group, in Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, pp. 9–24]. In the experiments the sawtooth characteristics were studied with fast electron temperature (ECE) and soft x-ray diagnostics. For the first time, measurements of a strong ballistic electron heat pulse were made in a shaped tokamak (DIII-D) [J. Luxon and DIII-D Group, in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] and the “ballistic effect” was stronger than was previously reported on TFTR. Evidence is presented in this paper that the ballistic effect is related to the fast growth phase of the sawtooth precursor. Fast, 2 ms interval, measurements on DIII-D were made of ...

Examination of energy confinement in DIII-D at small values of the plasma safety factor

The effect on H-mode energy confinement of operating at low values of the plasma safety factor q has been investigated experimentally on the DIII-D tokamak (see Plasma Physics and Controlled Nuclear Fusion Research 1986 (Poroc. 11th Int. Conf. Kyoto, 1986), Vol.1, IAEA, Vienna (1987) 159). Operationally, the linear increase of H-mode energy confinement with plasma current ceases when the ratio of the plasma current to the toroidal magnetic field is high than approximately=1 MA/T. Experimentally, it has been determined that this saturation of confinement is not the result of a ceiling imposed by saturated Ohmic confinement, by the plasma reaching a toroidal beta limit, by enhanced plasma radiation or by edge localized modes. The observed saturation of confinement is found to be quantitatively consistent with a model of global energy confinement degradation resulting primarily from the macroscopic phenomena of sawteeth. The experimental data are also qualitatively consistent with an alternative explanation for the confinement saturation which involves enhanced transport brought about by ion temperature gradient driven turbulence

PROSPECTS FOR CORE HELIUM DENSITY AND RELATED MEASUREMENTS ON ITER USING ACTIVE CHARGE EXCHANGE

The measurement of low-Z impurities, in particular He ash, in the core of International Thermonuclear Experimental Reactor (ITER) remains an outstanding diagnostic issue. The only credible candidate at present is active charge exchange recombination spectroscopy (CER) utilizing a diagnostic neutral beam (DNB) optimized for the dual requirements of beam penetration and charge exchange cross section, resulting in beam energies of 2∼100 keV/AMU. Using the existing ITER parameter profile and equilibrium data files and reasonable assumptions regarding viewing optics and DNB performance, we have employed a benchmarked multistep beam penetration code to yield signal-to-noise estimates for possible core helium concentration measurements. These studies confirm the importance of precise determination of beam intensities via accurate modeling and independent measurement, as well as the need for beam modulation, to satisfy the stated measurement precisions needed for ITER. Comparable calculations have been done for an intense pulsed neutral beam based on ion diode technology, as well as other candidate He-CER wavelengths, to assess the relative advantages of these techniques. Since any DNB-based diagnostic system actually deployed on ITER will likely be used for a variety of purposes, signal-to-noise calculations for the related active CER measurement of ion temperatures have also been performed and will be presented.

MHD phenomena in a neutral beam heated high beta, low qa disruption

A neutral beam heated, β maximizing discharge at low qa in Doublet III ending in disruption is studied and correlated with theoretical models. This discharge achieved MHD β-values close to the theoretical Troyon-Sykes-Wesson limit in its evolution. The MHD phenomena of this discharge are analysed. The sequence of events leading to the high β disruptions is hypothesized as follows: the current and pressure profiles are broad-ened continuously by neutral beam injection. A last sawtooth internal disruption initiates an (m/n = 2/1) island through current profile steepening around the q = 2 surface. The loss of plasma through stochastic field lines slows the island rotation and enhances its interaction with the limiter. The resultant enhanced island growth through island cooling or profile change enlarged the edge stochastic region. The overlapping of the edge stochastic region with the sawtooth mixing region precipitated the pressure disruption. Thus, in our hypothetical model for this discharge, β increase by neutral beam heating does not directly cause the disruption but ushers the plasma indirectly towards it through the profile broadening process and contributes to the destabilization of the 1/1 and 2/1 tearing modes.

Modification of sawtooth crash behaviour during large error field experiments on the DIII-D tokamak

Externally applied, static, resonant low m=1, n=1 magnetic field perturbations have a strong effect on the behaviour of the sawtooth crash in the DIII-D tokamak. An external coil was used to produce magnetic field perturbations large enough to slow the core plasma rotation and greatly alter the radial electric field profiles but not large enough to induce a locked mode. In these cases the global plasma parameters and the density and temperature profiles remained unchanged; however, the sawtooth oscillation, in particular the sawtooth crash, changed character from resistive in nature to ideal as the static error field increased. The m=1, n=1 resistive MHD internal kink mode normally responsible for the sawtooth crash in DIII-D appears to be stabilized by resonance detuning caused by the combination of the applied static error field and the slower plasma toroidal rotation. The stabilization of the resistive internal kink allowed the plasma to evolve until an ideal mode caused a sawtooth crash. This effect may explain the differences in sawtooth behaviour reported between different tokamaks