Theory Weekly Highlights for March 2012

March 30, 2012

The Remote Control Room (RCR) was utilized this week to enable DIII-D scientists to have a virtual presence in the EAST control room. Data transfer capabilities, EFIT display, real time scope display, and video teleconferencing have all been improved and enhanced to enable easier and more capable remote sessions to EAST. This in turn, not only allows for productive contributions to EAST experiments, but also allows for DIII-D scientists to lead EAST experiments. Data Acquisition and Analysis Group member G. Abla was in Hefei this week and was able to, among his other duties, directly assist and confirm connections to the EAST control room.

March 23, 2012

The hypothesis that partial magnetic helicity between low order rational surfaces (or ‘annular helicities’) should be conserved was tested by calculating the helicities in simulations and found to be approximately valid, but with some caveats. Conservation of partial magnetic helicity (or ‘annular helicities’) has been proposed as a constraint that could be imposed in a 3-D equilibrium calculation of the plasma response to a non-axisymmetric field (See Highlight Theory Weekly Highlights for September 2011 for September 02). One simulation using NIMROD was for a non-axisymmetric field arising from a linear instability in DIII-D discharge #133221, and the other, using M3D-C1, from an imposed I-coil field in discharge #126006. The results show the expected behavior of jumps at the low order rational surfaces, but suggest the annular helicities between rationals may not be strictly constant as hypothesized, as some additional change in helicity is evident away from island region. Possible reasons are that additional islands may be contributing, in the final state some flux tubes escape, whereas the helicity calculations assume they all remain inside the volume, and also that resistive diffusion, which changes the fluxes but not the linking number, may be contributing to the helicity evolution.

March 16, 2012

A new model was developed for the interior region of a massive gas jet interacting with a low temperature current quench plasma. The gaseous medium of the jet behaves like a perfect insulator initially, and interrupts the plasma current density J flowing to its broadside surfaces. This model supposes that the interrupted current is replaced by a displacement current, which leads to electrical gas break down that runs to completion in a short time, allowing for a transition from neutral gas to a quasi-steady discharge plasma characterized by a low degree of ionization f ~ 6 x 10-4. Being partially conductive, the jet allows passage of the plasma current. In the jet discharge, J, ne, and f are found to be 104 to 105 times higher than those in typical glow discharge plasmas. Hence, unlike a glow discharge plasma where the electron distribution function is far from Maxwellian, the electrons in the jet discharge Maxwellianize, permitting local thermodynamic equilibrium ionization calculations. It was found that Te/Ti is large, and furthermore, ud = J/(ene) > Cs (Cs is the sound speed), both necessary conditions for the current driven ion-acoustic instability. The non-linear saturated state can lead to anomalous resistivity 10-100 times the classical one, thus improving the Putvinski repetitive gas injection concept.

Dr. Orso Meneghini has joined the GA Theory Group as an ORISE post-doctoral fellow to work on the development of integrated modeling capability for analysis and experimental planning.

GA hosted the US-Japan Workshop on Integrated Modeling from March 12 to 14. Fourteen talks were presented by US and Japanese attendees.

March 09, 2012

A new gyrokinetic code is being developed for microinstability studies of plasmas in highly collisional regimes, such as the plasma edge. The new code is being built as a gyrokinetic extension of NEO. The code uses pitch angle and normalized energy as the velocity-space coordinates with the collision term is implemented using the same spectral expansion in velocity-space as NEO to ensure accurate evaluation of the collision operator even for species with disparate masses. It presently solves the linear, multi-species, electrostatic gyrokinetic equation with a model collision operator (Connor or zeroth-order Hirshman-Sigmar) that includes full cross-species collisional coupling. It will eventually implement the full linearized gyro-averaged Fokker-Planck collision operator. The code has been benchmarked with GYRO for linear ITG physics and collisional zonal flow damping. While high resolution in the poloidal angle theta and pitch angle are expected to be needed at low collisionality due to the discontinuity in the collisionless distribution function across the trapped/passing particle boundary, it is found that even in the highly-collisional limit many grid points in theta are needed to fully resolve the structure of the gyrokinetic eigenmodes, usually more than twice as many for the equivalent neoclassical simulation.

March 02, 2012

The work described in “Gyrokinetic Simulations with External Resonant Magnetic Perturbations: Island Torque and Nonambipolar Transport with Rotation” by R.E. Waltz and F.L. Waelbroeck (to appear in Physics of Plasmas) is being extended to apply the (Fitzpatrick 1993) generalized Rutherford island width formula to the screening of external RMP fields. The generalized formula relates the ratio of the screened island width to the vacuum width w/wvac, combined with the island locking angle α induced by the toroidal rotation, the usual “delta prime” driving the intrinsic tearing mode, Δ'mode, and Δ'coil associated with the external vacuum field, to the “internal island” Δ'island. The real Δ'mode and Δ'coil are calculated by “external” ideal MHD solutions. The complex Δ'island (usually denoted by just “Δ”) is associated with the induced screening current internal to the island. Δ is simulated with the GYRO gyrokinetic turbulence code. The locking angle α scales like Im(Δ) which is proportional to the island torque (Maxwell stress), and which scales like β w I1/2Er, where I is the intensity of the turbulence. Er is the radial electric field associated with the toroidal rotation, and β is the plasma “beta”. (Re(Δ) presumably has a similar scaling.) From the GYRO simulations we hope to make a TGLF -like model for the “turbulent island” Δ. From the generalized Rutherford formula, we can then calculate the screened island width w and locking angle α. RMP (and error field) screening and mode locking in relation to toroidal rotation is an important but complex and poorly understood problem. The novel feature here is the relation to high-n turbulence.

These highlights are reports of research work in progress and are accordingly subject to change or modification