Quasi-Periodic Growth of a Single Helical Instability in a Low-Aspect Ratio RFP

Abstract

Quasi-periodic growth of a single core-resonant instability has been observed in a low-aspect ratio (low-A) reversed field pinch experiment in RELAX. In round-topped current discharges, the innermost core resonant m = 1/n = 4 mode grows with suppression of the neighboring m = 1/n = 5, 6 modes. The resultant toroidal mode spectrum of the m = 1 modes is similar to that of the quasi-single helicity (QSH) state with higher amplitudes. These phenomena are discussed along with the low-A characteristic of RELAX, indicating simpler MHD mode dynamics and easier access to the QSH state. The reversed field pinch (RFP) is a magnetic confinement concept for compact high-beta plasmas. Recent progress in confinement improvement in RFP results from realizing the tearing-mode-stable current profile with pulsed poloidal current drive (PPCD) [1,2]. The quasisingle helicity (QSH) RFP state, where the internally resonant single tearing mode grows significantly larger than the others, is another candidate for confinement improvement [3].The aspect ratio A is an important parameters for characterizing the RFP properties. Recent analysis has shown that when A is lowered to 2, a neoclassical equilibrium with a bootstrap current fraction higher than 90% exists with reactor regime plasma parameters [4]. An additional advantage of a low-A RFP configuration is the possibility of simpler MHD mode dynamics because the radial separation of the mode rational surfaces increases in the core region.We have started experiments to study low-A RFP properties in Reversed field pinch of Low-Aspect ratio eXperiment (RELAX) machine, which has the world's lowest aspect ratio of 2 (R = 0.5 m/a = 0.25 m) [5]. RELAX uses a 4-mm thick vacuum vessel with a field penetration time of ∼1.5 ms. It is operated without any other shell structure. The edge magnetic fluctuations are measured with two toroidal arrays, each consisting of 14 pick-up coils set at the top and bottom ports, which are equally spaced in the toroidal direction except at the two poloidal gaps.Signals from these coils are sampled with a 2 MHz sampling frequency. After integrating numerically, we use a high-pass filter to obtain a fluctuating component with f > 0.5 kHz. The difference of the top and bottom signals author's e-mail: ikezoe1t@nuclear.dj.kit.ac.jp provides the odd (mostly m = 1) component, while the sum, the even (both m = 0 and m = 2) components. The signals are then Fourier transformed spatially to obtain a time evolution of the amplitude and phase. Figure 1 shows the time evolution of the plasma current I p , loop voltage V loop , edge toroidal field B tw and average toroidal field B t of a round-topped discharge. The Fig. 1 Time evolution, from top to bottom, of the plasma current I p , toroidal loop voltage V loop , edge toroidal field B tw , and average toroidal field B t in a round-topped discharge of RELAX.

Modeling of Formation of Helical Structures in Reversed-Field Pinch

“…This dominant growth of the m = 1 / n = 4 component for a wide range of parameters and profiles agrees qualitatively with experimental observations in RELAX, in which QSH states of the 1/4 mode appear periodically with suppression of the other modes [8].…”

Design and Initial Result of Time-Resolved Vertical Soft X-Ray Imaging System in Low-<i>A </i>RFP

“…In the low-A RFP machine RELAX, a quasi-periodic transition to a QSH state in which the internally resonant single tearing mode grows significantly larger than other modes has been observed [4]. During the QSH state, the fluctuation power is concentrated in the single dominant m = 1 mode.…”

“…This may allow easier access to the quasisingle helicity (QSH) state, in which the internally resonant single tearing mode grows significantly larger than other modes. In the low-A RFP machine RELAX [2,3], (R = 0.51 m, a = 0.25 m (A = 2)), a quasi-periodic transition to the QSH state has been observed [4]. During the QSH state, the fluctuation power is concentrated in the dominant single m = 1 mode.…”

Tangential Image of Helical SXR Emissivity Structure in Low-Aspect-Ratio RFP

“…Moreover, equilibrium analyses have shown that the innermost mode rational surface can be located away from the axis in a lower aspect ratio (A = R/a) of the RFP configuration, where R(a) is the major (minor) radius of the plasma Manuscript column. Therefore, the growth of a single mode to a higher amplitude can be expected in a low-A RFP than in medium-and high-A RFPs, which may allow easier access to quasi-singlehelicity (QSH) state, in which the internally resonant single tearing mode grows significantly larger than other modes.In a low-A RFP machine REversed field pinch of Low Aspect eXperiment (RELAX) [2] (R = 0.51 m/a = 0.25 m (A = 2)), a quasi-periodic transition to QSH state has been observed [3]. During the QSH state, the fluctuation power is concentrated to the dominant m = 1/n = 4 mode.…”

“…In a low-A RFP machine REversed field pinch of Low Aspect eXperiment (RELAX) [2] (R = 0.51 m/a = 0.25 m (A = 2)), a quasi-periodic transition to QSH state has been observed [3]. During the QSH state, the fluctuation power is concentrated to the dominant m = 1/n = 4 mode.…”