Theory Weekly Highlights for August 2005

August 26, 2005

A new analytical model was developed for the interaction of an expanding plasma fireball with a cusp magnetic field used to protect an IFE target chamber wall from the energetic ions, and safely divert the charged particle debris out of the target chamber. The lifetime and survivability of the wall is a serious concern for IFE power plants since high-yield targets release bursts of neutrons, X-rays and energetic ions with instantaneous heat loading on the wall a thousand times larger than wall loadings in an MFE power reactor. The physics-based model describes the braking of a quasi-spherical plasma expansion against the magnetic “cushion” and calculates the equilibrium/stopping radius of the plasma cavity. The model indicates that magnetic wall protection, an idea that emerged from the High-Average-Power-Laser (HAPL) program, looks promising and deserves further study. The conventional approach of filling the target chamber with a Xe buffer gas to absorb the ion energy faces the problem of target heating by hot buffer gas which can potentially ruin the frozen DT fuel layer.

Professor Jaiqi Dong from the SWIP in China completed a successful three-week visit to GA to learn how to use several of the GA MHD tools, notably the EFIT, TOQ, GATO, and BALOO codes. An equilibrium for the HL-2A tokamak was successfully constructed and tested for stability with and without the HL-2A vacuum vessel, which was modeled by a suitably smoothed harmonic expansion in poloidal angle.

August 19, 2005

Recent nonlinear GYRO simulations including kinetic electrons show that the ion and electron energy transport follows a linear q scaling for q values ranging from 1 to 4. The simulations were performed around several reference cases in an annulus with flat profiles and assuming shifted circle geometry. The results are equally valid for positive and negative shear and for cases where the spectrum is dominated by either ITG or TEM modes. The particle transport also follows a linear scaling if the diffusivity is positive (outward). If a particle pinch is predicted, then the passing electron contribution can change sign as q is varied within a scan, resulting in little or no observed q dependence over the entire range of interest. The passing electron contribution to the diffusivity is relatively small compared to the trapped electron contribution, but it is large enough to impact the scaling with q.

August 12, 2005

A general expression was derived, valid in an arbitrary coordinate system for the existence of magnetic surfaces in 3D. The expression is coordinate independent, which is important when axisymmetry is violated. Applied to a torus, the fact surfaces are guaranteed to exist with any special symmetry, drops out trivially from this formulation. In the general case, surfaces exist if and only if a certain simple equation has solutions. An analogous expression is also obtained for the existence of current surfaces. An effort is underway to derive simplified conditions to yield useful criteria for existence of proper nested flux surfaces.

August 05, 2005

The NIMROD code has been successfully ported to the GA Opteron/Infiniband cluster DROP (see Highlight for April 22 2005 at Theory Weekly Highlights for April 2005). So far, performance on most NIMROD routines is up to a factor of four faster than on the NERSC SEABORG machine. Production studies of edge-localized modes with flow shear and current drive from resistive instabilities in DIII-D have begun on the machine. However, the NIMROD iteration stage is currently a bottleneck, limiting overall performance to about twice that of SEABORG. The port to the DROP cluster required a special build of the MVAPICH library to prevent communication lock-up on parallel computations, where an offending optimization was removed and this may be responsible for the performance lag. Curiously, this lock-up problem has also been observed in attempts to port NIMROD to the new NERSC Opteron/Infiniband cluster, JACQUARD. We are working with the MVAPICH team to identify the root cause of the problem.

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