Plasma equilibrium and stability in the Scyllac toroidal sector experiments

Abstract

Experiments are reported on helical plasma equilibrium and stability in the Scyllac toroidal θ-pinch sectors (120°) which have major radii of 2.375 and 4.0 m with coil arc lengths of 5.0 and 8.4 m, respectively. In these experiments the outward toroidal drift force was compensated by a combination of ℓ = 1 helical and ℓ = 0 bumpy fields which are generated by shaping the inner surface of the compression coil or by driven ℓ = 1 windings. Time-resolved measurements were made of the gross plasma-column motion, the plasma radius, the magnetic flux excluded by the plasma, the external magnetic field, the plasma density, the electron and ion temperatures, and the plasma β at axial locations of minimum and maximum plasma radius. These data are used to study the approach to the theoretically predicted toroidal equilibrium (including axial pressure equilibrium). The plasma column remained in stable equilibrium for 7 – 10 μs in the 8-m sector compared with 4 – 7 μs in the 5-m experiment, at which times the onset of a terminating m = 1, k ≈ 0 sideways motion occurred. The results show that the plasma achieved axial pressure equilibrium (nkT = const) in 4 – 6 μs, while maintaining equilibrium in the toroidal plane for 10 μs or longer. The measurements of the plasma radius, β and magnetic field in the various experiments have confirmed in detail the stable toroidal equilibrium observed in the streak photographs during the first 4-10 μs of the discharge. The observed toroidal equilibria of the high-β, θ-pinch plasma are in quantitative agreement with MHD sharp-boundary theory and confirm the theoretical scaling of the equilibrium field between the 5-m and the 8-m sector experiments.

Feedback stabilization of an l = 0, 1, 2 high-beta stellarator

“…The growth rate for motion in the vertical plane from Eq. 3is y v = 0.17 fis' 1 . The corresponding values from sharp-boundary theory are y H = 0.38 (i&- 1 and y v = 0.19 /is 1 .…”

“…17is to have stable solutions. * 1 The range over which the gains tn and T, may be varied and still have stable motion shrinks to 0 as ••• increases toward 1.…”

Imploding to equilibrium of helically symmetric theta pinches

“…The m,= 1 unstable mode was observed and it s growth rate was in approximate agreement with theory6. The next set of experiments were performed on a toroidal sector, where the forces F 12 or F 13 were found to 1,2 0,1 was about 50% discrepancy with the theory in the ISAR Tl experiments4.…”

LTPF: a linear theta-pinch neutron source

“…The value of 1.0 kV/cm may be compared with 0.51 kV/cni measured in the Scyilac 5-meter sector experiment [21], 0.70 kV/cm measured in the Scylla I-B theta pinch [19], and 1.1 kV/cm measured in the Scylla iV-1 device [20]; in each of these cases EQ was produced by the primary capacitor bank (no separate shock heating). Experiments with separate shock heating have traditionally produced higher fields (2-3 kV/cm) [13,22,23].…”