Showing papers by "R.D. Stambaugh published in 1991"

Locked modes in DIII-D and a method for prevention of the low density mode

Abstract: Locked n = 1 tearing modes are observed over a wide range of parameter space in DIII-D and other tokamaks. Much of the difficulty with low density operation is attributed to locked modes and they are also observed as precursors of density limit disruptions. From observations of a consistent locked mode toroidal phase, it appears that the modes are locking to a small field perturbation caused by slight irregularities in the location of one or more of the vertical field coils with respect to the toroidal field coil. By intentionally producing an n = 1 field with an external coil, it was possible to influence the onset of locked modes in low q, low density plasmas. The result is a significantly expanded or reduced stable operating parameter space, depending on the polarity and magnitude of the external perturbation applied and whether or not intrinsic field errors are reduced or increased. A simple theoretical model of the non-linear stability of a tearing mode under the effect of externally applied resonant field perturbations yields a critical island width for the onset of a locked mode. The island widths, computed by field line tracing, for combinations of intrinsic field errors and 'n = 1 coil' fields are in qualitative agreement with the critical island width for instability.

H-Mode energy confinement scaling from the DIII-D and JET tokamaks

Abstract: Neutral beam heated H-mode DIII-D and JET expanded boundary divertor discharges were examined to study the parametric dependence of the thermal energy confinement on the plasma current, plasma size and neutral beam power. Single-null discharges in both machines were examined during the ELM-free phase (ELM stands for edge localized mode) to extract information about the intrinsic H-mode thermal energy confinement time τth. A power law dependence of ELM-free thermal energy confinement was assumed, with the result that for Bτ ≈ 2.2 T and κ = 1.8, τth = C Ip1.03±0.07 PL−0.46±0.06 L1.48±0.09. The size dependence of τth is described by the linear dimension L since the determination of the individual dependences on the minor and major radii is precluded by the similar aspect ratio of the two machines. For this representation of τth (units of s, MA, MW and m), when L is the plasma major radius, C = 0.106 ± 0.011, and when L is the plasma minor radius, C = 0.441 ± 0.044. A dimensionally correct version of the scaling, consistent with the constraints of a collisional high beta model, is τth∝ Ip1.06 PL−0.45 L1.40 ne0.07 Bτ0.06. These results indicate that, within the experimental error, the empirical scaling and the dimensionally correct scaling are the same.

Particle control in the DIII-D advanced divertor

Abstract: A novel, electrically biasable, semiclosed divertor was installed and operated in the DIII-D lower outside divertor location. The semiclosed divertor has yielded static gas pressure buildups in the pumping plenum in excess of 10 mtorr. Electrical bias controls the distribution of particle recycle between the inner and outer divertors by E*B drifts. Depending on sign, bias increases or decreases the plenum gas pressure. Bias greatly reduces the sensitivity of plenum pressure to separatrix position. In particular, E*B drifts in the DIII-D geometry can direct plasma across a divertor target and then optimally into the pumping aperture. Bias, even without active pumping, has also demonstrated a limited control of ELMing H-mode plasma density. >