Compact Stellarator Experiments
Author: Stephen Knowlton
Submitted: 2005-12-20 14:03:15
206 Allison Laboratory
Auburn, AL 36849
Because the helical magnetic field of the stellarator is typically generated by currents in external coils, the stellarator concept has long offered the promise of passive steady-state operation of fusion reactors. Moreover, stellarators that operate nearly free of driven or bootstrap current should largely be immune from disruptions and other virulent current-driven instabilities to which other toroidal systems are susceptible. New compact stellarators (CS) such as NCSX and QPS will nonetheless operate with modest fractions of rotational transform produced by plasma current because (1) pressure-driven currents cannot be readily minimized by appropriate design of the magnetic spectrum in low-aspect ratio configurations as they may be in larger aspect ratio devices, and (2) the additional transform from plasma current may reduce the requirements of twisted modular coil design that become particularly challenging in compact geometry. A major goal of CS research is to verify that current-driven instabilities from the low level of plasma current may be rendered relatively benign by external control of the shear, transform, and other shaping properties afforded by control of the external helical field. The other key thrust is the demonstration of tokamak-like neoclassical transport and viscosity through symmetrization of the magnetic field, currently being pursued by pioneering studies on HSX. Moreover, the CS program is currently addressing the need to provide accurate equilibrium reconstructions of fully 3-D plasmas for equilibrium and stability analysis, and also investigates the potential consequences of the break-up of outermost flux surfaces due to error fields and finite beta.
Exploration of flux surface fragility and control of current-driven instabilities, and the validation of a new 3-D reconstruction code, are being carried out on the Compact Toroidal Hybrid (CTH) torsatron (R0 = 0.75 m, aVessel= 0.3 m, B ≤ 0.5 T) that has recently begun operating. Studies to identify and correct multiple sets of magnetic islands at rational surfaces in the vacuum field are taking place, and will be extended to experiments in plasmas in which the response to applied 3-D fields may be significantly modified by the plasma, as in tokamak locked-mode and ELM-suppression studies.
This talk will provide an overview of the physics issues facing low aspect ratio operation in current-carrying stellarator plasmas, and describe ongoing experimental efforts to attain and modify low aspect ratio magnetic equilibria in helical systems. The status of the V3FIT 3-D equilibrium reconstruction procedure will also be reviewed.
This work supported in part by US DoE Grant DE-FG02-00ER54610
If accepted, please place after invited abstract by D. T. Anderson of Univ. of Wisconsin