Theory Weekly Highlights for March 2001

March 30, 2001

A new FRC configuration for Magnetic Target Fusion (MTF) is proposed, which eliminates the open magnetic field coil set, an expensive component that is destroyed by the implosion, and which would therefore make MTF a more cost effective reactor concept. The conventional method attains high compression of FRC plasmas because axial contraction of the separatrix accompanies radial liner compression. In the new configuration, the FRC is formed and contained inside a right circular cylinder with no open field line region. A two-dimensional analytical solution of the Grad-Shafranov equation was found for the special pressure profile p(Ψ) = Ψ2, valid for any elongation. The solution for the O-point plasma pressure P0 obeys essentially the same scaling law as the conventional FRC compression rule, namely P0 ~ A4, with A being the radial compression ratio. This approach allows us to start with an oblate shape that should be stable to the tilt mode. We propose to form this new configuration by injecting and merging two anti-helicity spheromaks created at a standoff distance from the site of implosion, similar to the Swarthmore Spheromak Experiment (SSC).

March 23, 2001

Two new and important features were added to the time-dependent pellet ablation code P2CIP: (1) Hydrogen atomic processes, namely molecular dissociation and thermal ionization assuming LTE (valid when these rate processes are faster than the ablation flow time scale) and (2) A kinetic model of the incident Maxwellian electron heat flux deposition. Atomic processes act as a heat sink and slow the ablation rate. However as anticipated from earlier steady-state models they only extend the pellet lifetime by about 10 or 15 %. More importantly they influence cloud conductivity and thus the interaction of the ablation flow with the magnetic field (a future problem). Ionization of the neutrals takes place in the supersonic region and is nearly completed at r ~ 5 rp. It had been predicted long ago that the removal of the heat sink would cause a standing shock wave to form in close proximity, and remarkably the shock wave was indeed seen in the simulation. The 2-D simulation also shows the effect of an anisotropic ablation recoil pressure on the pellet surface. This causes the pellet to flatten in the direction of the magnetic field for ITER like plasma conditions. The simulation needs to be run longer in order to determine whether the pellet breaks up. To reduce the computation time a variable zoning technique suggested by Y. Olmelchenko will be implemented.

March 16, 2001

In collaboration with Marc Maraschek (IPP Garching) and W. Meyer (LLNL), the MHD diagnostic interpretation code SIGNALS, originally written by Kate Comer (UW Madison), has been upgraded and modified to be available as a routine working tool for MHD analysis. The code predicts MHD diagnostic signals, for example, ECE, Reflectometer, and BES, fluctuations, from perturbations computed by stability codes such as the GATO code. The code now runs on all the DIII-D workstations and is interfaced directly with the MDSplus database for the equilibrium and diagnostic data. In the near future, the stability code output will also be taken from MDSplus, thus providing a unified and consistent input to the code. The code can then be used with any database so that the source of the data is transparent to the user.

March 09, 2001

The paper 'A Study of Nonlinear Properties of Tearing Modes' is now undergoing internal review.

March 02, 2001

The new explicit gyrokinetic solver, eGYRO, is now in full production use. The code can simulate toroidal ITG turbulence with kinetic electrons and profile variation over a large portion of the plasma minor radius. Adiabatic electron flux-tube cases with no profile variation (CYCLONE benchmarks) run quickly and efficiently on the GA STELLA cluster. Currently, the effects of radial symmetry breaking (flux-tube simulations are radially periodic) as well as absorbing boundary conditions are being explored. While the domain decomposition topology from the original GYRO code has been retained, the communication algorithm has been entirely rewritten to improve parallel performance. It is expected that eGYRO in its present form will scale to more than 1000 processors. eGYRO is computationally robust and easy to use (i.e., not prone to numerical instabilities) and is complemented by a comprehensive IDL visualization package. Development of an interface to a shaped local equilibrium model package is underway.

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