Optimized advanced stellarators of linked mirror type

Various different advanced stellarator configurations of the linked mirror type (LIMAS) have been investigated which differ in the magnetic mirror ratio and twist on the magnetic axis. One configuration is presented in the following. The vacuum fields are given analytically by a finite set of Dommaschk potentials. The magnetic field properties weighed in the optimization procedure are: the residue R* of two reference fixed lines (where the pressure gradient is high) which start at Z = 0 in the two symmetry planes and have the same value of the twist ι; the associated aspect ratio A; the difference ΔF of the magnetic flux F = ∫ A.dx between these two closed field lines belonging to the same rational ι-value (A is a vector potential, x is the radius vector, the line integral is performed over each of the fixed field lines); the deficit of the Hamada condition that ∫dl/B should have the same value for these two fixed lines; a condition on the magnetic well (ΔV'/V'ax < 0, V' is the specific volume); a measure 1/2 of the Pfirsch-Schlueter current density; the deviation of contours of B = const from contours of Q ∫dU/B2 = const (U is the scalar potential). This condition has the effect that the particles see small variations of the magnetic field along their poloidal rotation. This favours small values of the ambipolar diffusion coefficients in the long mean free path regime. The implementation of this constraint uses a Fourier decomposition of the magnetic field strength along a closed field line with respect to Q. The corresponding Bessel inequality is used as a quadratic measure (PR) of the 'amount of independency' of the Fourier coefficients as the poloidal coordinate. Some of the quoted conditions are also applied at a pair of fixed lines in the boundary region. (author) 4 refs., 11 figs