Recent results of ultra low q experiments in TORIUT-6 AND REPUTE-1

Abstract

In REPUTE-1, the radiation barrier has been overcome and ion and electron temperatures of ⪅ 0.6 keV (Ti) and ⪅0.3 keV (Te) have been achieved at an electron density of (0.2–0.7) × 1020 m−3 and toroidal beta values of 12-18%. A resistivity anomaly due to m = l kink activity has been observed which may be connected with the peaked temperature profile and the rather broad current density profile. The plasma resistance is lower than that of reversed field pinch discharges in REPUTE-1 and decreases with increasing plasma current. In TORIUT-6, the effects of current ramp-up and carbonization of the first wall have been studied. A hollow current density profile, typical of ultra low q equilibria, is preserved which may be due to the skin current effect. The difference between current ramp-up discharges and standard discharges is the extended duration of the quiescent phase and the preservation of the position of the pitch minimum away from the axis in the ramp-up discharges. After carbonization, the duration of the quiescent phase increases and the plasma resistivity is reduced by 50%.

Current problems in ball lightning science

“…In particular, a regime of improved global stability is found for l x < 1 (hollow current distribution), ]8 p < 1 (strong paramagnetism) and B z (a) < B^a) (low q(a)). These results are consistent with recent experimental observations in the ULQ discharges reported in Refs [8][9][10], as well as with earlier stability studies of screw pinches with particular profiles [21,22]. The influence of a non-circular shape of the plasma cross-section, as for instance in the Extrap configuration [7], is negligible for relatively flat or peaked current profiles.…”

“…Configurations that (more or less) belong to this category are, for instance, the 'stabilized pinches', the reversed field pinches, the Extrap configuration, the high density Z-pinches and the plasma focus experiments summarized in Refs [4][5][6][7]. In particular, the ultra low q (ULQ) toroidal and linear discharges recently reported by Kamada et al [8] and Brunsell et al [9,10] clearly indicate the existence of parameter regions of improved global stability that are not consistent with the Kruskal-Shafranov condition. The ULQ plasmas are characterized by a significant level of MHD activity similar to the relaxation processes occurring in reversed field pinches (and in other configurations as well) and thus should, to some extent, be 'paramagnetic' (i.e.…”

“…the internal axial magnetic field is larger than the applied field), which is a typical feature of the relaxed Taylor states [11]. Furthermore, hollow current profiles have been recognized as an important signature of the ULQ equilibria [8].…”

Self-organization, anomalous resistance and anomalous heating in magnetized plasmas

“…Each step of the transition is an MHD relaxation process where a rapid change of the pitch occurs. From the statistics of the resistance of ULQ discharges in the parameter space of q (Kamada et al 1989), we observe gaps at major rational numbers 1\n (n l 1, 2, …) on the q axis. This implies that a state violating the condition (8) is forbidden.…”

Investigations of low q a discharges in the SINP tokamak

“…This range includes the normal q a (NQ), very low q a (VLQ) and ultra low q a (ULQ) discharges. The low q a (LQ) discharges [5][6][7][8], especially the ULQ discharges [9][10][11] have been obtained in a very small number of machines throughout the world. Moreover, the low q a discharges are considered to be beneficial and interesting from both technological and physical point of view [4].…”