The high-n Toroidal Alfven Eigenmode (TAE) has now been identified in gyrokinetic calculations using the GYRO code. The mode is driven solely by a fast particle beta gradient and appears to be unstable only within the Fu-Cheng predicted Alfven frequency gap. Other (non-perturbative MHD) Energetic Particle Modes appear to exist at higher beta. Work is in progress to characterize and distinguish these modes from the general gyrokinetic drift-Alfven-ballooning modes driven by the background plasma gradients.
A 1-D Fast Current Quench (FCQ) code was developed previously for studying massive density increases using deuterium liquid jet/pellet train injection as a technique for mitigating the severity of plasma disruptions (see Highlight for November 16 2007 at http://fusion.gat.com/theory/Weekly1107). In that study, it was found that after the initial phase where dilution cooling dominates, the densified discharge enters the strongly collisional Pfirsch-Schluter regime in which neoclassical heat transport dominates all other heat transport mechanisms and tends to prevent temperature and current density profile contraction near the magnetic axis. It is found that while the Pfirsch-Schluter thermal diffusion removes temperature and current density profile spikes on the magnetic axis, it cannot completely prevent the strong near-axis current contraction. The Kadomtsev magnetic reconnection model has now been implemented to address the issue of the on-axis safety factor q0 dropping below unity and the associated n = 1, m = 1 kink instability. The reconnection is found to effectively prevent current lock-up near axis, and to promote faster plasma current decay. The FCQ code is tested for DIII-D and ITER operating conditions with the existence of certain impurities (C in DIII-D and Be, Ar in ITER). Results suggest that fast and smooth plasma shutdown is in principle achievable for the desired plasma cooling time (10-15ms for DIII-D and 100-150ms for ITER), under 100 times or higher densification. To achieve this goal, liquid jet or doped pellet injection may be required.
Dr. Junya Shiraishi successfully completed a 10 month ORISE post-doctoral research assignment at GA during which he worked with Ming Chu on extending the MARG2D code by developing a new package for the stability of the resistive wall mode. The new code has been benchmarked with the NMA code developed at GA for studying the resistive wall mode stability. Dr. Shiraishi will be joining the theory group at JAEA (Japan Atomic Energy Agency).
A new predictive model for the pedestal height, EPED1, has been developed and tested, with encouraging results. The pedestal height has a large impact on core confinement and overall fusion performance, and its prediction is an important element of performance projection and optimization in ITER. Calculations of peeling-ballooning stability of the pedestal using the ELITE code, give a constraint on the maximum pedestal height as a function of the pedestal width, with the height scaling roughly with the width to the 3/4 power. Accurately calculating this constraint, and performing dedicated experiments on DIII-D and comparisons with other tokamaks, has allowed well-constrained testing of physics models for the pedestal width, and has led to a working model of the pedestal width. Combining this width model with direct peeling-ballooning stability calculations using TOQ and ELITE yields a new predictive model for the pedestal height, called EPED1. EPED1 has been tested both against systematic scans and randomly selected points in the DIII-D pedestal database, with good agreement found thus far. EPED1 was used to make pedestal height predictions before a recent DIII-D experiment, with results presently being analyzed.
These highlights are reports of research work in progress and are accordingly subject to change or modification