Observation of m ≥ 2 modes in plasmas with 1/2 ≤ q_a< 1

Abstract

Internal modes with m = 1 and m ≥ 2, which localize around a pitch minimum, have been studied for ultra-low-q plasmas with 1/2 ≤ qa < 1 in REPUTE-1, where m is the poloidal mode number and qa is the safety factor at the plasma surface. One-dimensional stability analysis has shown that, for finite beta, m ≥ 2 modes have a relatively large growth rate compared with those of the m = 1 modes. The internal structure of the magnetic fluctuations with m = 1 and m ≥ 2 is found to be consistent with the numerically calculated eigenfunctions.

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

“…8). The mode structure is in a good agreement with the linear normal-mode analysis (Itami et al 1988). In the quiescent phase, modes with m l 1 are not observed, because the stabilized ULQ configuration does not have m l 1 rational surface (Sec.…”

“…5.3). Localized instabilities are observed (Itami et al 1988), while the global stability of the discharge is retained for a period of time after the MHD relaxation. This observation is explained by nonlinear behaviour of the double-resonant modes (Sec.…”

Anomalous ion heating in REPUTE-1 ultra-low q and reversed field pinch plasmas

“…The fluctuations have a broad spectrum; the amplitudes of the m = 2, 3, 4 modes are larger than or almost equal to that of the m = 1 mode. Since the growth rate of the m > 2 modes localized around the pitch minimum can be relatively large [18], the observed fluctuations seem to originate in these modes.…”

“…The density pump-out seems to take place at the same time as the growth of fluctuations. The density pump-out goes together with an increase in the magnetic Reynolds number S, which may indicate that some kind of MHD instabilities that are unstable in the high S regime are triggered [18].…”

“…The linear stability of ULQ configurations has been studied theoretically and numerically [17]. Internal m > 2 modes localized around the pitch minimum have been observed in REPUTE-1 [18]. Regarding confinement, the ULQ configuration has attracted attention because of its high electron density at low toroidal field [12], although the plasma current and the electron temperature were low in such high electron density plasmas.…”