The operating conditions for obtaining a quasi-single helicity (QSH) state with a good reproducibility are found in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi, Fusion Eng. Des. 45, 421 (1999)]. In these conditions, the reversal of toroidal magnetic field (Bta) is maintained at a very shallow value (∼-0.2 mT) after the setting up phase and the following fast current rising phase. After a certain period at this shallow reversal (∼15-25 ms), the mn=16 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) rapidly grows and saturates before the termination of discharge. The growth of this mode dominates the other modes and the QSH state with mn=16 is finally achieved. This QSH state can be sustained for a long period (up to ∼45 ms) almost until the end of discharge by applying a delayed reversal of Bta with appropriate trigger timing and magnitude. The initial setup of the QSH states shows a reproducibility of almost 100%, but its sustainment for a long period shows a slightly reduced reproducibility (∼85%). The initial rapid growth of the single dominant mode is compared with the numerical results of linear stability and nonlinear three-dimensional (3D) calculations by assuming the experimental magnetic field profile estimated with a standard model. Linear calculations show that the mn=16 mode has the maximum growth rate to the ideal magnetohydrodynamic instability and can explain the dominant growth of this mode. The 3D calculations also show a qualitative agreement with the experiment, where under some conditions the mn=16 mode becomes dominant after an initial relaxation and continues to the end of the simulation. These results indicate that the present QSH state is the combined result of the linear growth and nonlinear saturation of a particular mode. © 2005 American Institute of Physics.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
Hirano, Y., Paccagnella, R., Koguchi, H., Frassinetti, L., Sakakita, H., Kiyama, S., & Yagi, Y. (2005). Quasi-single helicity state at shallow reversal in TPE-RX reversed-field pinch experiment. Physics of Plasmas, 12(11), 1 - 11. . https://doi.org/10.1063/1.2118728