Abstract. Compact optimized stellarators offer novel solutions for confining high-beta plasmas and developing magnetic confinement fusion. The 3D plasma shape can be designed to enhance the MHD stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the low-aspect ratio, high beta-limit, and good confinement of advanced tokamaks. Quasi-axisymmetric equilibria have been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4 -4.4 and average elongation ~1.8. Even with bootstrap-current consistent profiles, they are passively stable to the ballooning, kink, vertical, Mercier, and neoclassical-tearing modes for β > 4%, without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at β=4% (the rest is from the coils), thus the equilibrium is much less non-linear and is more controllable than similar advanced tokamaks. The enhanced stability is a result of 'reversed' global shear, the spatial distribution of local shear, and the large fraction of externally generated transform. Transport simulations show adequate fast-ion confinement and thermal neoclassical transport similar to equivalent tokamaks. Modular coils have been designed which reproduce the physics properties, provide good flux surfaces, and allow flexible variation of the plasma shape to control the predicted MHD stability and transport properties.3
A quantitative comparison is made between the tearing-type modes observed in TFTR supershot plasmas and the nonlinear, neoclassical p: ssure gradient (Vp) driven tearing mode theory. Good agreement is found on the nonlinear magnetic island evolution of a single helicity mode {mjn = 3/2, 4/3 or 5/4, where m and n are the poloidal and toroidal mode numbers, respectively). Statistical data on the island width and growth rate are also found to be consistent with this theory. The results imply that the supershot plasmas are stable to the classical current-driven tearing modes.
PRINCETON Pl,.AIIMA I_-IYBICB LABORATORY NOTICEThis reportwas preparedas an account of work sponsoredby an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or impl;ed,or assumes any legal liability or responsibilityfor the accuracy, completeness, or usefulness of any information,apparatus, product,or processdisclosed,or r.,Jpresents that its use would not infringe privately owned rights. Reference herein to any specific commercial produce, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarilyconstituteor imply its endorsement,recommendation,or favoring by the United States Govemment or any agency thereof. The views and opinionsof authorsexpressed hereindo not necessarilystate or reflect those of the United States Government or any agency thereof. AbstractRecent studies on TFTR of high density disruptions have made significant advances in closing the gap between theoretical models of disruptions and the experimental data. For the first time, an (ro,n) = (1,1) 'cold bubble' precur-* sor to the high density disruptions has been experimentally observed. The precursor resembles the 'x-acuum bubble' model of disruptions first proposed , ,.,ca ._,, by Kadomtsev and Pogutse. ", .',
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