Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Abstract

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through timeresolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the H α radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

The effect of biased limiter on the magnetic island width in tokamak plasma

“…tearing modes, cause the loss of confined heat and particles. On the other hand, the clear correlation between the modification of radial electric fields induced by the biasing and modification of MHD activity has been observed in several experiments; thus, investigating the effect of bias on the width of magnetic islands seems necessary (Cabral et al 1998;Nascimento et al 2007;Drevalet al 2008). Biased limiter (or electrode) creates a radial electric field which initiates shield flows that cause a transport barrier affecting turbulences and leading to the improvement of plasma confinement, as was reported in the ISTTOK (Silva et al 2005), KT-5C (Gao et al 1995), and DIII-D (Matsumoto et al 1992) tokamaks.…”

Sheared-flow induced confinement transition in a linear magnetized plasma

“…Subsequently, many experiments have used external bias to study modification of the plasma turbulence and transport. [5][6][7][8] Recent experiments [8][9][10] in the upgraded Large Plasma Device 11 (LAPD) at the University of California, Los Angeles (UCLA) have studied the effect of the sheared flow on the edge turbulence and turbulent transport by biasing a section of the vacuum chamber wall. Linear and nonlinear simulations to model the plasma instabilities based on LAPD parameters are also developed.…”

Application of Emissive Probes for Plasma Potential Measurements in Fusion Devices

“…The maximum attainable plasma density is (5-10)·10 18 m −3 and the electron temperature is in the range 80 -220 eV. In the SOL the density drops to values around (5-10)·10 16 m −3 , and T e is on the order of 10 eV [15,16]. Under the assumption of n e = 10 16 m −3 and T e =100 eV, and purely thermal electrons, Eq.…”