Theory Weekly Highlights for February 2013

February 22, 2013

While the TGLF turbulent transport model has been extensively validated for conventional sawtoothing H-mode and hybrid discharges with normalized beta < 2, the model has not been validated for medium to high normalized beta discharges with elevated minimum safety-factor values. As a result, its predictive capabilities for AT scenarios of next-generation burning-plasma facilities or fusion reactors such as FDF have largely been unknown. To make progress in this area and to examine where the model may have issues predicting the transport, we have begun assembling a database of high minimum safety-factor values DIII-D discharges. To date, a database of 22 discharges has been constructed and the temperature profiles have been predicted using TGLF. We find that the RMS errors in the ion and electron temperatures for the 22 discharges are noticeably higher than what has been previously obtained for 40 DIII-D sawtoothing H-modes discharges. In particular, the RMS errors are 5% higher for ion temperature and 10% higher for electron temperature.

February 15, 2013

Spallation or disintegration of a large solid deuterium pellet into daughter fragments upon impact with a rigid surface is under investigation as a method of producing a collimated stream of small pellets for disruption mitigation in ITER. We propose that the fracture of the pellet is caused by tensile loading brought about during the initial impact and generation of stress waves that interact upon reflection from the free surface of the “brittle” pellet. The concept of (reversible) surface free energy was used to calculate the total surface area increase due to the many fragments produced after the impact. This together with the conservation of momentum, energy and volume before and after impact allows us to estimate the (average) size and number of daughter fragments produced as a function of initial pellet size, impact velocity, and trajectory obliquity. Strain energy dissipation, an extra irreversible energy cost process, was neglected in this analysis but will be included in future work.

February 08, 2013

A study of ballooning criticality near the separatrix in model equilibria based on DIII-D, JET, Alcator C-Mod and ITER has been conducted to explore possible connections to the observed Scrape-off-layer (SOL) width. Empirical scaling of SOL width observations suggests the possibility that the ITER SOL width may be very narrow, possibly 1 mm or less, leading to concerns about divertor heat loads on ITER. An initial study of near-separatrix (normalized poloidal flux = 0.995) ballooning critical pressure scale lengths finds that observed SOL widths on existing devices (DIII-D, JET, C-Mod) are approximately consistent with ballooning criticality, but that ~1mm SOL widths for ITER would significantly exceed the ballooning critical width, suggesting that ballooning physics might lead to wider SOL widths on ITER. Further more detailed study is underway to pursue this possibility.

February 01, 2013

A small code ALPHA has been programmed to project the ITER profiles for the fusion alpha classical slowing down density, equivalent Maxwellian alpha temperature, and cross-over energy from the EPED1 predicted pedestal beta and the plasma ion temperature and density peaking factors (over the pedestal values). The simplicity of this approach follows from the TGLF ITER performance projections [J.E. Kinsey, G.M. Staebler, R.E. Waltz, and R.V. Budny, Nucl. Fusion 51, 083001 (2011)], which showed the ITER core to be “stiff” so that there should be relative little variation of peaking factors with injected power. ALPHA will be used for the forthcoming GYRO energetic article Alfven mode ITER stability studies (DoE 2014 Milestone). The predicted instability threshold will be with respect to the classical slowing down alpha density profile. New GYRO coding for classical slowing down in place of the Maxwellian alpha distributions is nearly complete.

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