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Quasisymmetric (QS) configurations are defined as the fully 3D systems in which the guiding centre motion equations conserve an additional integral just as in symmetric systems. It means that the charged particle drift motion in QS systems is similar to that in fully symmetric configurations.
It was shown by Boozer [1] that for systems to have an additional conserved integral of the drift motion equations, it is enough that the modulus of the magnetic field B= |B| (mod-B) be independent on one of the angular variables of Boozer flux coordinates.
The discovery of the possibility of quasisymmetry was made by Nuhrenberg and Zille [2]. They showed numerically that the QS condition can be satisfied with high enough accuracy on the boundary magnetic surface. The results of [2] demonstrate that the effective control of the behaviour of mod-B on the magnetic surfaces is possible through the appropriate choice of the boundary magnetic surface.
The fulfilment of the QS condition is very desirable for neoclassical transport improvement in stellarators. This is the main reason of the great interest in QS systems. Many articles are devoted to the numerical investigations of plasma equilibrium, stability and transport in near-QS configurations. Moreover, some projects of QS systems are currently in development: the Helically Symmetric Experiment (HSX) [3] at the University of Wisconsin-Madison, and the National Compact Stellarator Experiment (NCSX) -a quasiaxi-symmetric stellerator with toroidal plasma current at the Princeton Plasma Physics Laboratory [4].
Unfortunately, the QS condition too strong to be fulfilled in the entire plasma column [5]. Furthermore, the attempt to fulfil the QS condition with maximal possible accuracy can be incompatible with the condition of stability [2]. On the other hand, the optimized stellarator WVII-X [6] with enhanced neoclassical plasma confinement does not satisfy QS. Therefore, it seems natural to try to formulate less restrictive c e c

Additional information

Authors: COOPER A, Centre de Recherches en Physique des Plasmas Ecole Polytechnique Federale (CH);MIKHAILOV M I, Nuclear Fusion Institute, Russian Research Centre, Moscow (RU);izaEV M YU, Nuclear Fusion Institute, Russian Research Centre, Moscow (RU);SHAFRANOV V D, Nuclear Fusion Institute, Russian Research Centre, Moscow (RU);SKOVORODA A A, Nuclear Fusion Institute, Russian Research Centre, Moscow (RU);SUBBOTIN A A, Nuclear Fusion Institute, Russian Research Centre, Moscow (RU)
Bibliographic Reference: Article: Papers on Theory of Fusion Plasmas, (1998)
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