Spherical stellarator with plasma current

Abstract

Recently proposed novel concept of a spherical stellarator (P. E. Moroz, "Spherical stellarator configuration," to appear in Phys. Rev. Lett) is enhanced by adding the plasma current to the otherwise pure stellarator system. The coil configuration of this ultra low aspect ratio system differs from that of a spherical tokamak by inclination of external parts of the toroidal field coils. It is shown that the configuration considered possesses many attractive properties, including: wide flexibility of operating regimes, compact design and coil simplicity, good access to the plasma, closed vacuum flux surfaces with large enclosed volume, significant external rotational transform, strong magnetic well, and a high plasma p [p(O) m excess of 30%] equilibrium. It is shown that the bootstrap effect in a spherical stellarator, in principle, can supply the full plasma current required for the high-p equilibrium. 0 1996 American Institute of Physics. [SlO70-664X(96)03508-2]

Low-aspect-ratio stellarators with planar coils

“…As was discussed in [3][4][5][6], the SS devices benefit from the plasma current: the rotational transform increases, higher β values can be reached, and the magnetic-field ripple can be reduced substantially; also, as was stressed there, a SS can benefit not only from the ohmic current but from the bootstrap current as well, which flows in a SS in such a direction that its rotational transform enhances the vacuum rotational transform. The LARS considered with planar coils benefit from the plasma current in a similar way (section 6).…”

“…These configurations are unique in comparison with the presently existing large-aspect-ratio stellarators for which A ≈ 5-11. This analysis is a continuation of our previous research [3][4][5][6] on a novel concept for magnetic fusion called the Spherical Stellarator (SS), which was originally proposed in [3].…”

“…This is probably a general feature of the LARS and has already been found for the SS configuration [5]. To demonstrate the positive effect of a small plasma current we suggest that a toroidal current of I p = 28 kA with a profile of (1 − s) flows in the plasma.…”

“…Moreover, all coils are the same and carry the same current, so the system looks like a toroidally symmetric one, similar to a tokamak, but has stellarator features. Such a system can benefit from the plasma current, similar to the above-mentioned SS configurations [3][4][5][6]. We consider the simplicity of the coils as a very important factor.…”

“…In the SS concept, the plasma aspect ratio is much lower, usually about, or sometimes significantly below A = 3.5; also, the use of modular coils in a SS represents a significant advantage for reactor applications where the possibility of replacing any particular part of a device has to be maintained. In [4][5][6], it was shown that combining the plasma current and the external stellarator rotational transform in a SS produces strongly positive effects, such as the possibility of high-β equilibria (β is the ratio of the thermal plasma energy to the magnetic-field energy) and the decreased magnetic-field ripple. It was also shown that at high β a relatively strong bootstrap current exists in a SS, which might be enough to obtain these advantageous characteristics.…”