Magnetic field alignment studies for the URAGAN-3 torsatron

Abstract

If the major axes of the helical and vertical field coils of a torsatron plasma confinement device are not aligned to within ∼1% of their major radii, the resulting error field can break up closed magnetic flux surfaces and reduce the effective plasma confinement volume. A novel technique for accurately locating the magnetic symmetry axes of torsatron helical and vertical coil sets to within ∼1 mm using magnetic field measurements near the device major axis (i.e., away from the confinement volume) is described and applied to the URAGAN-3 torsatron. The axis of the vertical field coil set was found to be shifted by ∼1 cm relative to the axis of the helical coil set; this shift could account for the reduced confinement volume observed in previous experiments. The magnetic measurements were repeated after coil repositioning to verify correct alignment.

Error fields in the Wendelstein 7-X stellarator

“…The source of this field was attributed to coil bus-work and was corrected [3]. The URAGAN-3 stellarator built on the ATF results to measure and correct error fields associated with misalignment of their vertical field coil pair [4]. The W7-AS stellarator performed extensive flux surface mapping of its configuration space finding both low and high order islands.…”

Optimum Fourier representations for stellarator magnetic flux surfaces

“…For the inverse representation of MHD equilibria [12], the magnetic flux co-ordinates (p,0,f) are considered the independent variables and the cylindrical co-ordinates (R,0,Z) are the dependent variables. This is clearly the interpretation ascribed to Eqs (3) and ( 4), with the substitution </ > = c (13) All equilibrium quantities can be determined in terms of Rmn> Z m n , ±(p) and $(p). These include the magnetic field B*(p,0,f) and the stream function X(p,0,f).…”

“…In this section we develop flux surface representations for a number of different stellarator configurations: ATF (operating) [9], Uragan-3 (operating) [13,14] and TJ-II (under design) [15]. The first two of these configurations have fi = 2 torsatron-type windings; essentially planar, circular magnetic axes; and relatively low rotational transform per helical field period, * /M <, 0.1.…”

“…The puncture points (solid circles) were computed using a 20-filament model for the helical coils described in Ref. [13], and the flux surfaces (solid lines) were evaluated using 23 modes. The low central transform (^0) of this £ =3 system, together with toroidal effects, results in a large axisymmetric elongation…”