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The behaviour of magnetic ripple wells along field lines in current heliotrons and stellarators: comparison with a classical heliotron

For the stellarator-type devices of today, which are very different from the quasi-symmetric ones, the analytical conditions for the existence of two kinds of ripple wells along field lines are derived taking into account the multiple helicity and multiple toroidicity of the magnetic field. It is shown that a typical feature of the above-mentioned devices, in contrast to a classical heliotron, is a considerable deformation of magnetic wells, both ordinary and secondary, by additional ripple fields. When this deformation exceeds a critical level, one kind of magnetic ripple well can be suppressed in some toroidal region. This happens if the amplitudes of the helical and toroidal magnetic fields and the amplitudes of the ripple fields are correlated with each other through the relations known as conditions for parametric resonance. When the parametric resonance conditions are fulfilled, charged particle confinement is improved because of a suppression of the toroidal drift.