A stopping model that explains the lack of direct penetration of broad supersonic gas jets during MGI experiments in current tokamak experiments has been developed with the stopping force provided by the plasma ablation pressure. The plume pressure from the ablation surrounding the frontal cap of the neutral gas jet is communicated to the supersonic neutral interior region, untouched by incident plasma electrons, by means of a backward propagating shock wave. The plume pressure itself is regulated by the parallel expansion dynamics. The model has been specialized to argon jets with radiation cooling rates, and ionization potential energy changes derived from the CRETIN radiation code. From the point of view of the Maxwell stress tensor, the forward momentum lost by the jet is ultimately taken up by the toroidal magnetic field because the low beta ablation plume digs a weak localized magnetic well.

Dr. Christian Konz is visiting GA from IPP Garching, Germany, for a month to work with Phil Snyder on edge MHD stability studies, including studies of the impact of near-X-point geometry, and analysis of the recent DIII-D/ASDEX-U beta scan experiments.

Dr. Morrell Chance is visiting GA from PPPL to work with Ming Chu on the development of the Normal Mode Approach (NMA) for resistive wall stabilization by active feedback.

The Third post APS Error/Non-axisymmetric Magnetic Field Workshop was successfully held at the Rosen Centre Hotel in Orlando, FL on November 16, 2007 with more than 40 participants. The PDF files of the Workshop presentations can be found at http://web.gat.com/conferences/ef/efws2007/

The toroidal angular momentum diagnostic in GYRO has been reformulated in terms of the moments of the 6-D kinetic equation in Cartesian velocity space. The spontaneous toroidal rotation found in C-Mod and DIII-D in the absence of any obvious external source of toroidal angular momentum requires the presence of some form of momentum "pinch" effect. There are two sources of momentum pinch effects: The Dominguez-Staebler 1993 E×B shear effect in slab geometry and the Peeters-Angioni-Strinzi 2006 finite parallel velocity curvature drift effect in toroidal geometry. From an analysis of over 100 GYRO simulations these pinch effects can now be clearly demonstrated. Correctly modeling the spontaneous toroidal rotation will require these pinch effects to be incorporated in the new TGLF transport model. This work has now been accepted for publication in Physics of Plasmas.

Jon Kinsey presented an invited talk on “First Transport Code Simulations using the TGLF Model” at the 49th APS DPP Meeting in Orlando on November 12. Valerie Izzo presented an invited talk on “MHD simulations of disruption mitigation on DIII-D and Alcator C-Mod” at the 49th APS DPP Meeting in Orlando on November 15.

A 1-D code was developed for studying massive ~ 100 fold density increases using deuterium liquid jet/pellet train injection as a technique for mitigating the severity of runaway electrons during emergency plasma termination/plasma disruptions. The model includes toroidal electric field and current evolution, ohmic heating, dilution cooling, thermal diffusion, radiation losses, neutral jet ionization, and afterglow recombination. In calculations for DIII-D or ITER the predicted current density profile is practically unaltered during the jet propagation and ablation phase and the pressure profile is only slightly modified because dilution cooling dominates. This indicates that an MHD induced thermal collapse seems unlikely during this initial phase, where it is of prime importance to have complete particle assimilation without “shine through.” With further radiation cooling, the afterglow becomes so strongly collisional that Pfirsch-Schluter ion heat transport dominates, with an extremely high thermal diffusivity (around 100 m²/s in DIII-D). This tends to prevent temperature and current density profile contraction near the magnetic axis, which helps prevent the m/n = 1 kink instability. No runaways were found in any cases examined. A Kadomtsev mixing model is being implemented to study the effect of m/n = 1 reconnections on the current quench rate in ITER.

Extensive benchmarking of the NIMROD/KPRAD combined MHD/atomic physics code has been performed for Alcator C-Mod plasmas with both neon and helium gas jet injection. The time between the initial edge cooling and the core temperature collapse is found to be strongly dependent on the perpendicular thermal transport coefficient. With the coefficient set close to experimentally measure values, neon simulations predict the observed thermal quench onset time very well and the temperature profile evolution also matches the experimental results quite well. However, the edge density is much larger in the simulations than the experiments. This could be due to differences in transport or ionization fraction. The addition of boron to the neon simulations reduced the edge temperature and the neon ionization fraction, which partially solves the primary discrepancy between the code and the experiments. A previously reported result (see October 12 highlight at http://fusion.gat.com/theory/Weekly1007) that adding boron to a dense helium gas jet can reduce the thermal quench onset time will be highlighted in an invited talk by Valerie Izzo at the upcoming APS meeting on November 15.

The first validation of the TGLF transport model against a large database of tokamak discharges is nearly complete. The results will be presented in an invited talk by Jon Kinsey at the APS-DPP meeting in Orlando Fl. Compared to its predecessor GLF23, TGLF has much better fidelity to gyro-kinetic linear stability and non-linear turbulence driven fluxes computed with the GYRO code. Both GLF23 and TGLF are quasilinear models but the improved physics of TGLF is resulting in better agreement between predicted and measured temperature and density profiles. The lower level of electron thermal transport due to ITG/TEM modes in TGLF compared to GLF23 translates into a larger impact of high-k ETG modes on the electron thermal transport. This is particularly found to be true in DIII-D hybrid discharges. The weaker impact of electron-ion collisions on the transport in TGLF compared to GLF23 also plays a significant role in reducing the over-prediction of the temperatures in L-modes. The density profiles predicted by both codes are similar, although the wall source needed to match the line average density can be very different indicating different particle confinement times.