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An attempt has been made to find an optimal quasi-helically symmetric (QHS) four period magnetic configuration with a Heliac-like structure of the magnetic surface cross-sections. This means that the cross-sections rotate in phase with the magnetic axis principal normal in contrast to the Helias-like systems where the magnetic surface cross-sections rotate half as fast as the principal normal. The equilibrium and local stability are studied with the VMEC and TERPSICHORE codes. It is shown that, in the configuration found, the equilibrium beta limit is about 9%, the Mercier beta limit is about 4% and the ballooning modes restrict the beta value to 1%. The accuracy of the fulfilment of the QHS condition is given by X approximately equal to 3 (ie the largest amplitude of the magnetic field strength Beta harmonic that violates the quasi-symmetry is a third that of of the main helical harmonic of Beta in Boozer co-ordinates).

Additional information

Authors: ISAEV M Y ET AL, Nuclear Fusion Institute, Russian Research Centre 'Kurchatov Institute', Moscow (RU);COOPER W A, Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, Lausanne (CH);MEDVEDEV S Y, Russian Academy of Sciences, Keldysh Institute of Applied Mathematics, Moscow (RU)
Bibliographic Reference: Article: Nuclear Fusion, Vol. 37 (1997) No. 10, pp. 1431-1436
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