Internal disruptions and RHF in IR-T1 tokamak

Abstract

Summary form only given, as follows. Internal disruptions or sawtooth oscillations have been observed and investigated with and without a weak resonant helical field (RHF). IR-T1 is a small air core transformer tokamak with circular cross-section (R=45 cm, a=12.5 cm, Bt<1 T, Ip=20-45 kA). The horizontal X-ray imaging system on IR-T1 is an array of 23 silicon surface-barrier detectors, with a 1 cm spatial resolution. Sawtooth oscillations are observed on IR-T1 Tokamak during low ql (ql=5a/sup 2/Bt/RIp<4.75) discharge with a disruption time of about 30-60 /spl mu/s. The q=1 singular surface occurs at radius 3-3.5 cm and inverted. Sawtooth from chords outside this radius. The superimposed (m=1) oscillation with a frequency of about 19 kHz/spl ap/25 kHz, according to the tokamak discharges parameters, preceding the sawtooth oscillation. One major effect the sawtooth oscillation is to flatten the temperature and density profiles approximately out to a mixing radius rm=/spl radic/2 rs, where rs/spl lsim/rm/spl lsim/a, and rs is the q=1 radius. Furthermore, by applying RHFs (L=2 and L=3), the sawtooth behavior is modified. The magnitude of the weak RHFs used in our experiments did not exceed 1% of Bp. Results showed that the weak RHFs magnetic perturbation would change the MHD instabilities and the sawtooth behavior, as well as plasma confinement. On the other hand, the effectiveness of the RHF depended on the discharge conditions. The RHFs was not effective in high Zeff discharges. The RHFs could reduce energy and particle losses. The RHFs produced large sawtooth characterized by their amplitudes, periods and slopes.