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This work focuses on a set of experiments designed to clarify the impact of the rotational transform and pellets on confinement quality at the TJ-II stellarator. For this purpose, the net plasma current, Ip, is controlled using external coils, resulting in the modification of the rotational transform profile and hence the radial location of specific rational surfaces. In the configuration studied here, the main rationals affecting confinement are 3/2, 8/5 and 5/3.
Significant and systematic variations of the edge electron density gradients (up to 50 %−60 %) and the plasma energy content (20 %−30 %) are achieved, depending on Ip and hence the location of specific rationals. The explanation of this behaviour relies on the placement of low-order rational surfaces in relation to the edge gradient region, which affect local turbulence fluctuation levels, facilitating the formation of zonal flows and the concomitant transport barriers. This hypothesis is confirmed experimentally on the basis of a broad array of diagnostic measurements [1].
The injection of pellets leads to the suppression of MHD turbulence, as is clear from Mirnov coil measurements. Local measurements in the plasma core region made by the Heavy Ion Beam probe allow studying the intermittence (degree of fractality) in the plasma gradient region. It is shown that the plasma responds differently to the injection of pellets, depending on the net plasma current. Turbulence suppression is more effective with positive plasma current, which is interpreted in terms of the radial profile of the rotational transform and the corresponding density of low order rationals.
Calculations based on a resistive magnetohydrodynamic turbulence model provide some support for these observations, clarifying the impact on confinement of specific rational surfaces and highlighting the complex nature of magnetically confined fusion plasmas.
[1] B.Ph. van Milligen, et al. The rotational transform and enhanced confinement in the TJ-II stellarator. J. Plasma Phys., 91(4):E98, 2025.