Spherical Stellarator Configuration

Abstract

A novel ultralow aspect ratio stellarator system that can be called a spherical stellarator (SS), in analogy with the spherical tokamak concept, is considered. The coil configuration of a simplest SS differs from that of a spherical tokamak by inclination of the external parts of the toroidal field coils. This system possesses many attractive properties including 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 simple divertor configuration. [S0031-9007(96)00759-4] PACS numbers: 52.55.Hc, 52.55.Fa Tokamaks and stellarators are two leading systems in the controlled fusion program via magnetic plasma confinement. The best plasma parameters have been obtained in the largest tokamaks: JET [1], TFTR [2], JT-60 [3], and some others. Tokamak operations with tritium plasmas carried out in TFTR [2] and JET [1] demonstrated significant fusion energy output. Stellarators are presently somewhat behind in their development. Nevertheless, the large stellarator devices are presently under construction in Japan [Large Helical Device (LHD) [4]] and in Germany [Wendelstein 7-X (W7-X) [5]].Recently, strong interest has appeared in a compact tokamak design, where a single central post replaces the central parts of all toroidal field (TF) coils. This low aspect ratio (LAR) tokamaks with A 1.5 2.5, (where A is the aspect ratio, i.e., the ratio of the average major radius to the average minor radius for the last closed flux surface), or ultralow aspect ratio (ULAR) tokamaks, with A 1.05 1.5, are promising for obtaining plasmas with the high number density and high b (b is the ratio of thermal plasma energy to the magnetic field energy) in a device of moderate size and relatively low magnetic field. Good plasma access is another advantage of these configurations. The LAR or ULAR tokamaks are often called the spherical tokamaks (ST). The results reported from the spherical tokamaks START at Culham [6] and CDX-U [7] were very promising. In addition to the relatively high b obtained, low plasma disruptivity has been reported. Because of this initial success, the program on spherical tokamaks is quickly extending [8].Some difficulties of the ST program are related to the fact that, because of tight space for the Ohmic current transformer, it cannot support the inductive current for a long time. Other, noninductive current drive (CD) methods [such as radio-frequency (RFCD) or neutral beam injection current drive (NBCD)] have to be used to maintain the plasma current beyond the initial Ohmic start-up. Another set of potential problems of the ST approach, as a next-step device or especially as a prototype for the fusion

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).…”

“…One of the main new ideas behind the Spherical Stellarator concept [3][4][5][6] is that the plasma current is an important factor for improving many characteristics of such devices. In this paper it has been shown that LARS devices with planar coils benefit from the plasma current as well.…”

“…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].…”

“…In figure 3(a), the total rotational transform, ι, is shown together with its external ι ex , and internal ι in , components, calculated respectively for the outboard and inboard halves of flux surfaces [3] …”

“…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.…”