Compatibility between high energy particle confinement and magnetohydrodynamic stability in the inward-shifted plasmas of the Large Helical Device

摘要

The experimentally optimized magnetic field configuration of the Large Helical Device @A. Iiyoshiet al., Nucl. Fusion 39, 1245 ~1999!#, where the magnetic axis is shifted inward by 15 cm from theearly theoretical prediction, reveals 50% better global energy confinement than the prediction of thescaling law. This configuration has been investigated further from the viewpoints of high energyparticle confinement and magnetohydrodynamic ~MHD! stability. The confinement of high energyions is improved as expected. The minority heating of ion cyclotron range of frequency wassuccessful and the heating efficiency was improved by the inward shift. The confinement of passingparticles by neutral beam injection was also improved under low magnetic field strength, and therecould be obtained an almost steady high beta discharge up to 3% in volume average. This was asurprising result because the observed pressure gradient exceeded the Mercier unstable limit. Theobserved MHD activities became as high as beta but they did not grow enough to deteriorate theconfinement of high energy ions or the performance of the bulk plasma, which was still 50% betterthan the scaling. According to these favorable results, better performance would be expected byincreasing the heating power because the neoclassical transport can also be improved there.