MHD Database for Design of Quasi-axisymmetric Stellarator

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MHD stability of quasi-axisymmetric stellarator CHS-qa is estimated by referring to CHS experimental data and by using computer codes. Although magnetic configurations of CHS-qa and CHS (low aspect ratio heliotron / torsatron) are different, stability criteria should be available for both configurations. Mercier criterion on the local interchange mode has been widely used as a guideline for evaluating MHD stability of CHS and LHD, because both machines are of a magnetic hill even in the finite J3 configuration, which makes the configurations possibly unstable against ideal interchange modes. Experimental results of CHS showed that finite <β> plasmas, where instabilities are expected from Mercier criterion, did not show any deleterious effect on the stored energy. Recent CHS experiments again confirmed this result for more strongly inward shift of vacuum magnetic axis than before, where the magnetic hill becomes stronger. This was also confirmed recently in LHD experiments. The important conclusion is that it is not necessary to take serious account of Mercier criterion in evaluating MHD stability of CHS-qa. In CHS-qa plasmas with finite <β>, the bootstrap current flows in the direction to augment the vacuum rotational transform. The current might excite tearing modes and external kink modes, where m=2/n=l mode is the most dangerous one. By inducing Ohmic and beam driven currents in CHS, MHD modes with m=2/n=l have been studied. Again there is no deleterious effect on the plasma performance. Because the resonant surface of m=2/n=l can not exist in vacuum in CHS, the external kink mode can not be simulated in the experiment. The mode has been analyzed by using Terpsichore and CAS3D codes in CHS-qa. The plasma is stable up to <β> of 2.5 - 3%. The ballooning mode has been analyzed with Terpsichore WBAL and Cobra codes, giving stability <β> of up to about 3%.