A helically symmetric stellarator (HSX)

Abstract

Fig. 1. An isometric view of the HSX stellarator device showing the key features of the device beginning with the plasma boundary, the vacuum chamber, and the main and auxiliary magnet coils along with the support structure.Abstract-The Helically Symmetric Experiment (HSX) is a quasi-helically symmetric (QHS) stellarator currently under construction 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 (helical) component. This design avoids the large direct orbit losses associated with conventional stellarators. The restoration of an ignorable coordinate in the confining magnetic field makes this device analogous to an axisymmetric q = 1=3 tokamak with respect to neoclassical confinement. A XISYMMETRIC toroidal fusion research devices (e.g., tokamaks) possess magnetic field symmetries which ensure good single-particle confinement and are conceptually simple in magnet coil design. However, the pulsed nature of inductive current drive, the large source of free energy in the plasma current which can drive instabilities (disruptions), the requirements for auxiliary and divertor magnet coil sets for plasma shaping, and for stability and position control, force increased engineering costs and complexities for such devices. Stellarators, however, have long been considered attractive fusion reactor candidates from the point of view of no required ohmic current, permitting true steady-state operation. The magnet coil sets for such devices, although complicated, are no more so than present-day tokamak devices. In conventional axisymmetric toroidal systems such as tokamaks, two magnetic field components produce the confining fields. They are 1) a conventional toroidal magnetic field and 2) a "poloidal" magnetic field produced by an ohmic current induced in the toroidal direction. The axis of symmetry therefore, is the toroidal direction. Classical stellarators, however, use helical coils to produce both the poloidal and toroidal magnetic fields which have magnetic field variations in the toroidal direction and therefore a breaking of the symmetry as compared to a tokamak. This gives rise to significant deviation of the particle drift surfaces off the magnetic flux surfaces 0093-3813/99$10.00

Common Features of Core Electron-Root Confinement in Helical Devices

“…Often in this case, only a single solution for E i e T T ≈ r exists, usually negative, which is referred to as the ion root (solutions of the non-linear ambipolarity condition traditionally being determined using iterative root-finding techniques). To a good approximation ε , either through quasi-symmetries with a very small fraction of localized particles [7,8] or by drift optimization of the magnetic field topology [9]. Reduction of eff ε is an optimization for ion-root operation with small 1/ν electron losses responsible for good neoclassical confinement of the entire plasma.…”

“…Circumvention of this unfavorable 1/ν-regime, for which the heat flux density scales with , is one of the major goals for stellarator optimisation [6]. This is usually treated as a matter of optimising the magnetic configuration, either by reducing the fraction of ripple-trapped particles (concept of quasisymmetry; in particular, the quasi-helically symmetric HSX [7] and the quasi-axisymmetric NCSX [8]) or by drift optimisation (concept of quasi-isodynamicity; see e.g. [9] for W7-X).…”

Simulation Study of Neutral Beam Injection Heating in the HSX Plasma

“…The Helically Symmetric Experiment (HSX) [1] is a modular coil stellarator of quasi-helical symmetry located at the University of Wisconsin-Madison. The magnetic field in HSX is generated by coils arranged in four field periods.…”

Optimization of energy confinement in the 1/ν regime for stellarators

“…For new optimized stellarators (e.g. HSX [9], NCSX [10,11], QPS [12,13]) conditions corresponding to low 1/m transport have been a design goal. For existing stellarators a certain decrease of this transport can be achieved by changes in coil currents (as it is done, e.g.…”