Theory Weekly Highlights for March 2005

March 25, 2005

Atlas, the primary MDSplus data server for DIII-D data, was upgraded to most recent Red Hat Enterprise operating system and the disk space was expanded to add capacity. The additional disk space is expected to provide sufficient capacity through the end of operations and into the next fiscal year. The operating system upgrade was needed to keep Atlas up-to-date with the latest security patches, to keep it in synch with our other Linux machines, thus simplifying maintenance, and is a requisite for adopting MDSplus access restrictions. Options for solving the long-term problem of keeping up with increasing storage demands through some sort of mass storage solution are being considered.

March 18, 2005

A new “momentum model” has been constructed to describe jet penetration in disruption mitigation experiments using massive gas injection from a collimated supersonic argon jet. In the DIII-D experiments, camera images indicate that the jets do not penetrate much past the separatrix. In the model, the jet forms a thin ionized boundary layer as it enters the plasma and the plasma and magnetic field are pushed aside. The net magnetic field inside is slightly less than outside the jet. The net magnetic force opposing the jet motion then balances the neutral pressure piling up as a shock behind the tip of the jet. Force balance yields an equation for the actual jet “tip speed” in terms of the “jet speed” in the absence of the shock. The jet cannot penetrate significantly unless the ratio of tip speed to jet speed, U > Ucrit = 1/4, since otherwise the backward propagating shock wave reaches the rear surface. For DIII-D at B ~ 2 T, the model suggests that U is near Ucrit so that penetration may be possible by lowering the field.

March 11, 2005

Professor Shaojie Wang completed a three-month visit to GA and returned to the Hefei Institute of Plasma Physics in China. During the visit, Professor Wang completed a series of ideal MHD stability calculations with a double transport barrier (DTB). This work extended a previous study of plasmas with an internal transport barrier (ITB), which suggested that beta limit improves with increasing ITB width or moving the ITB location to the edge. The DTB stability calculations assumed a conducting wall at 1.5 times the minor plasma radius. Model equilibria for the study are computed using the TOQ code with a hyperbolic tangent pressure profile for both the ITB and edge transport barrier (ETB). By varying the ratio of ETB to ITB height with the location and width of the ETB fixed, the study suggests that modest ETBs can improve plasma stability. However, strong ETBs drive ideal MHD instabilities, and there is a window in the pedestal pressure, above which the ETBs drive instabilities with a substantial edge component.

March 04, 2005

A new certificate-based Secure MDSplus system, developed by MIT researchers as part of the National Fusion Collaboratory, was implemented for DIII-D. Regular MDSplus uses host-based authentication, which requires access through a list of known hosts. The new secure system allows users to be authenticated based on their certificate regardless of their host; this means that scientists can access MDSplus while on travel from anywhere offsite, whether from a DHCP-assigned address from home or another laboratory, or through wireless Internet from a coffee shop, library, or airport. For more information see

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