The Compact Helical System was designed for research on transport in a low-aspect-ratio helical system. The machine parameters were chosen on the basis of a physics optimization study. Considerable...

Compact Helical System physics and engineering design

This paper intends to review recent stellarator research in various laboratories. Efforts are continuously and actively being made to improve the confinement of plasmas in stellarator fields. Advances in confinement theory are summarized. Experimental results concerning Ohmically heated plasmas as well as currentless plasmas are reviewed. Finally, stellarator reactor studies are briefly introduced.

https://iopscience.iop.org/article/10.1088/0029-5515/18/2/008/pdf

Recent stellarator research

Calculations of the neoclassical transport rates due to particles trapped in the helical ripples of a stellarator's magnetic field are carried out, based on solutions of the bounce-averaged kinetic equation. These calculations employ a model for the magnetic field strength, B, which is an accurate approximation to the actual B for a wide variety of stellarator-type devices, among which are Helical-Axis Advanced Stellarators (Helias) as well as conventional stellarators and torsatrons. Comparisons are carried out in which it is shown that the Helias concept leads to significant reductions in neoclassical transport rates throughout the entire long-mean-free-path regime, with the reduction being particularly dramatic in the nu -1 regime. These findings are confirmed by numerical simulations. Further, it is shown that the behaviour of deeply trapped particles in Helias can be fundamentally different from that in classical stellarator/torsatrons; as a consequence, the beneficial effects of a radial electric field on the transport make themselves felt at a lower collision frequency than is usual.

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Ripple transport in helical-axis advanced stellarators: a comparison with classical stellarator/torsatrons

Plasma transport simulation modeling for helical confinement systems

For a detailed prediction of the plasma parameters in the Large Helical Device (LHD), 3-D equilibrium/1-D transport simulations including empirical or drift wave turbulence models are performed which suggest that the global confinement time of the LHD is determined mainly by the electron anomalous transport in the plasma edge region rather than by the helical ripple transport in the core region. Even if the ripple loss can be eliminated, the increase in global confinement is 10%. However, the rise in the central ion temperature is more than 20%. If the anomalous loss can be reduced to half of the value used in the present scaling, as is the case in the H-mode of tokamak discharges, the neoclassical ripple loss through the ion channel becomes important even in the plasma core. The 5% radial inward shift of the plasma column with respect to the major radius improves the plasma confinement and increases the fusion product by more than 50% by reducing the neoclassical asymmetric ion transport loss and increasing the plasma radius (10%)

https://iopscience.iop.org/article/10.1088/0029-5515/32/4/I08/pdf

Plasma transport simulation modelling for helical confinement systems

A method of reducing the neoclassical transport coefficients in the l/ν regime in l = 3 and l = 2 torsatrons is presented. By appropriately modulating the pitch of the winding law of the helical conductors and applying an additional external transverse magnetic field, a reduction factor of two to four can be achieved.

http://iopscience.iop.org/0029-5515/24/9/009/pdf/0029-5515_24_9_009.pdf

Reduction of neoclassical transport in the l/ν regime in torsatrons

A study is made of the magnetic field of a solenoid formed by current-carrying loops, the plane of which is perpendicular to a helical line. Analytical expressions are derived for the scalar potential and the components of the magnetic field strength inside a straight solenoid with a dense winding. The toroidal case was investigated using a BESM-6 computer. The results are presented of an experimental study of the magnetic field of a rectilinear system with a helical magnetic axis. The paper discusses the possible design of a toroidal trap with the magnetic configuration in question.

Toroidal solenoids with a space magnetic and a plane-geometry axis

The authors study the structure of the magnetic field in an l = 1 toroidal torsatron. Experiments on the Vint-20 device and numerical calculations on a computer have shown that closed magnetic surfaces with high parameters exist in an l = 1 torsatron. The forms of the magnetic surfaces are given together with graphs showing how their parameters vary within the working volume under different operating conditions. It has been established that the magnetic surface parameters can be adjusted within wide limits by changing the vertical field. It is shown that this type of system is characterized by high inhomogeneity of the magnetic field in the working volume and that this inhomogeneity can be substantially reduced by applying a vertical field or by changing some parameters of the helical winding.

V.G. Peletminskaya

Papers

Reduction of neoclassical transport in the l/ν regime in torsatrons

Toroidal solenoids with a space magnetic and a plane-geometry axis

The structure of the magnetic field of an l = 1 toroidal torsatron