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
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]
The detailed analysis of the vacuum magnetic field structure produced by the inclined toroidal field (TF) coils is presented. This configuration has a potential for adding stellarator properties to the tokamak configuration while maintaining the simplicity of planar coils. Parameters of the system are identified that result in significant stellarator-like effects: large vacuum flux surfaces and appreciable rotational transform. Two sets of closed flux surfaces with opposite helicity are studied: the internal one and the external one. It is found that the external set of flux surfaces possesses a magnetic well and, hence, is favorable for the magnetohydrodynamic (MHD) stability. Also, it has larger enclosed volume and rotational transform. It is, hence, preferential in our studies, in comparison with the internal set that usually features a magnetic hill. Analysis of the flux surface structure and the helical harmonic spectrum yields optimization rules required for the configuration to be of practical interest for possible fusion applications. In a few examples it is demonstrated what occurs if the parameters are set differently than optimal. It is found that toroidal inhomogeneity is a key factor and vacuum flux surfaces disappear in the limit of a very high number of TF coils. The important role of the poloidal field (PF) coil system is stressed, and the possibility of the compensated PF system (with zero total current) is found.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.