Magnetohydrodynamic instabilities in a current-carrying stellarator

Numerical studies on the stability of kink and resistive tearing modes in a current-carrying linear stellarator are presented for various current profiles and helical fields. In the case of an l=2 helical field, the magnetic shear vanishes and the stability diagram is given by the straight lines with iotasup(sigma)+iotasup(delta)=const., where iotasup(sigma) is the rotational transform due to the plasma current and iotasup(delta) is that due to the helical field. For the l=2 stellarator with iotasup(delta)=iotasup(delta)/2π>0.5, the magnetohydrodynamic stability against kink and tearing modes is improved compared to that in tokamaks. When an l=3 helical component exists, magnetic shear plays an important role in the stability properties. The stability diagrams become fairly complex; they are, however, understandable from properties of the Euler equation. It should be noted that the internal kink modes are made more unstable than in a tokamak, because of the l=3 helical field. (author)