THE HELICALLY SYMMETRICAL EXPERIMENT, (HSX) GOALS, DESIGN AND STATUS

HSX is a quasi-helically symmetric (QHS) stellarator currently under c onstruction at the Torsatron-Stellarator Laboratory of the University of Wisconsin Madison. This device is unique in its magnetic design in that the magnetic field spectrum possesses only a single dominant (hel ical) component. This design avoids the large direct orbit losses and the low-collisionality neoclassical losses associated with conventiona l stellarators. The restoration of symmetry to the confining magnetic field makes the neoclassical confinement in this device analogous to a n axisymmetric q = 1/3 tokamak. The magnet coil design has been attain ed through the application of the HELIAS(1) approach developed at IPP Garching. The 48 modular twisted coils produce a magnetic field with R (0) = 1.2 m, <r(p)> = 15 m, (sic)(0) = 1.04; (sic)(a) = 1.11, V'' simi lar to -.6% (well), and B < 1.4 T. Plasma production and heating will be accomplished with the application of up to 200 kW of 28 GHz Electro n Cyclotron Resonant Heating (ECRH). The HSX device has been designed with a clear set of primary physics goals; demonstrate the feasibility of construction of a QHS device, examine single particle confinement of injected ions with regard to magnetic field symmetry breaking, comp are density and temperature profiles in this helically symmetric syste m to those for axisymmetric tokamaks and conventional stellarators, ex amine electric fields and plasma rotation with edge biasing in relatio n to L-H transitions in symmetric versus non-symmetric stellarator sys tems, investigate QHS effects on (1)/(nu) regime electron confinement, and examine how greatly-reduced neoclassical electron thermal conduct ivity compares to the experimental chi(e) profile. The HSX magnet coil fabrication has just commenced, and ancillary components are either u nder fabrication or have been designed and are ready for fabrication. A support structure has been designed to allow independent, accurate c oil alignment coupled with good coil support for the magnetic and ther mal loads. The vacuum vessel is helical in shape, following the magnet ic separatrix with 3 cm clearance, and is to be explosively fabricated from stainless steel. Magnetic flexibility has been incorporated into the design through the inclusion of a set of independently powered au xiliary coils. These coils permit rotational transform control, the ad dition of magnetic mirror and symmetry breaking magnetic field perturb ations, and variation of the magnetic well depth. Initial assembly and coil alignment will occur as the components are fabricated, with comp leted final assembly planned for August 1996. First plasma production is planned for the end of 1996.