Geodesic acoustic mode observations in the Globus-M spherical tokamak

Abstract

The results of geodesic acoustic mode (GAM) studies in the spherical torus Globus-M via Doppler reflectometry are presented. The intermittent character of the GAM evolution is similar to the limit-cycle oscillation behavior of zonal flows. The evident correlation between the GAM rotational velocity and both Dα emission and plasma density oscillations is exhibited and discussed. The obtained experimental results are compared with those from tokamaks with large aspect ratios.

Long-Distance Correlations of Geodesic Acoustic Modes in T-10

“…На большинстве установок ГАМ исследуются с помощью корреляционной рефлектометрии (KP), зондов Ленгмюра и доплеровского рассеяния, но эти методы позволяют получать информацию только из периферийной плазмы [13][14][15][16]. Самый прямой и надёжный метод исследования ГАМ в горячей зоне плазмы -это зондирование пучком тяжёлых ионов (ЗПТИ, или HIBP в английской транскрипции), который много лет используется на токамаке T-10 для измерения профилей как средних значений потенциала, так и его колебаний [17].…”

Review of Advanced Implementation of Doppler Backscattering Method in Globus-M

“…The measured Doppler frequency shift was obtained either from the spectrum shift of the IQ detector signal or as the phase derivative of the complex signal. GAMs manifest themselves as coherent oscillations in the frequency domain of 23-28 kHz in the case of deuterium plasma and around 36 kHz in hydrogen plasma [38,39]. These frequencies are within the large orbit drift width limit when compared with theoretical predictions [40].…”

“…The GAMs velocity amplitude V ⊥ (t) was recalculated to the radial electric field E r assuming that the phase velocity V phase (t) induced by GAM oscillations is relatively small. The amplitude of the electric field oscillations reached up to 3 kVm −1 , which sometimes exceeded the mean value of the radial electric field causing a velocity reversal at a certain phase of the GAMs oscillations [39].…”

“…The study of the temporal evolution of GAMs showed that they do not exist continually; rather, an intermittent increase and decrease of GAM amplitude was observed. There were two characteristic times for the GAM level changes [39]. The first one is the duration of a quasi-coherent GAM burst which was about 0.2 ms with an interval of 0.4 ms between bursts.…”

“…It was shown that an SMBI pulse can reduce the Doppler shift frequency, which corresponds to the flattening of the electric field. In a series of experiments on the EAST tokamak using two separate DBS systems, one for the Q-band (33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) and the other for the V-band (50-75 GHz), Doppler shifted signals were obtained and radial profiles of perpendicular velocities were computed for the L-and H-mode [15]. In the Globus-M2 tokamak, the first results of the Doppler backscattering diagnostics during a discharge with the L-H transition show that the transition process is linked to the deformation of the poloidal rotation velocity profile [16].…”