Theory Weekly Highlights for October 2001

October 26, 2001

The enhanced effectiveness of off-midplane feedback coils on the stabilization of the resistive wall mode (RWM) has been studied using the normal mode approach. It is found that extra upper and lower bands of feedback coils will provide substantial additional stabilization effect on the RWM. Using the central band only, the strength of the RWM that can be stabilized is expected to have a strength (measured by its growth rate in units of the inverse resistive wall time) around 1; together with two off-midplane bands, the strength of the RWM that can be stabilized is up to a factor of 30 stronger. On the other hand, using the upper and lower bands only without using the central band will limit the strength of the RWM that can still be stabilized to 5.

October 19, 2001

In anticipation of the need for projections of plasma performance for Burning Plasma Experiments, a discussion paper has been prepared that outlines the Demonstration Discharge method. In this approach, discharges are developed on present devices with key non-dimensional parameter values (β, ν*, q, κ… ) identical to those of a candidate BPX discharge. Only extrapolation in rho* is needed to predict performance of the BPX . Furthermore, the rho* scaling can be established by creating Demonstration Discharges at different magnetic field strengths in present devices. Overall, the motivation is to prepare a discharge as close as possible to the foreseen BPX plasmas and to require scaling in only one nondimensional parameter. The Demonstration Discharge approach avoids relying on the controversial beta scaling of the IPB(y,2) scaling relation. By and large, these discharges lie within the operational space of DIII-D and of C-Mod (when upgraded in heating power).

October 12, 2001

A new single particle constant of motion has recently been found that allows a parallel flow to be included in the collisionless equilibrium distribution function. While ExB velocity shear is known to be a very important stabilizing mechanism for the linear microinstabilities that are thought to control heat and particle losses in tokamaks, it is also known that parallel velocity shear can destabilize these same microinstabilities. Extending the theoretical treatment of these effects to toroidal geometry has hitherto been hindered by the lack of a single particle constant of motion that would allow a parallel flow to be included in the collsionless equilibrium distribution function. The new single particle constant of motion will be implemented in existing toroidal microstability codes.

October 05, 2001

In tokamak geometry, the magnetic field is tilted at an angle to the toroidal direction at the plasma edge. Since the radiation impedance is symmetric along the B-field rather than the toroidal direction, an ICRF antenna with a toridally symmetric geometry can produce a radiative power spectrum with a strong asymmetry with respect to nφ. This effect was first noted by F.E.Jaeger et al. We have obtained similar results for parameters relevant to Alcator C-Mod ICRH experiments. The impact of this asymmetry on plasma rotation is under investigation.

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