Effect of drift modes on currentfree plasma transport processes in a stellarator

Abstract

The authors measure the losses of charged particles and heat via the electrons in the toroidal ℓ = 3 Saturn stellarator with large shear (θmax ≲ 0.25). From an analysis of the relationships derived they conclude that the pseudoclassical nature of the energy losses observed is due to the presence of drift modes in the plasma. The particle losses for shear θ < 0.05 are determined by the turbulence of the plasma, whereas at θ > 0.05 the diffusion both in magnitude and functional dependence matches the neoclassical theory.

The effect of shear on turbulent diffusion in a microwave discharge plasma in a toroidal stellarator

“…Theoretical studies [1][2][3][4] have shown that the level of these oscillations and the plasma losses caused by them can be substantially reduced if the magnetic configuration has a sufficiently large shear or a magnetic well. The stabilizing role of both these factors has also been confirmed by numerous experiments in open [5][6][7][8] and closed [9][10][11][12][13][14][15]23] magnetic traps.…”

“…It was shown earlier on this device [11] that with increasing shear there is a decrease in the level of lowfrequency fluctuations in some parts of the frequency spectrum, 2 kHz < f < 20 kHz. This result is confirmed by the data of Fig.…”

“…[3], the observed oscillations can be stabilized by shear values between 0' o = af ei /v Te and 0 O = ^\ +TJT { pja, typically a few percent. Such a shear magnitude can be achieved in Saturn [ 16] for sufficiently large values of the helical magnetic field component.…”

“…The studies were carried out in the Saturn S. = 3 stellarator [16] with a steady-state microwave discharge plasma in argon. The microwave power (f 0 = 10 GHz) supplied to the plasma at electron-cyclotron frequency (ECR) could be varied from 2 to 25 W with ~100% absorption efficiency.…”

“…In all experiments on the influence of shear, its magnitude was changed by varying the strength of the helical component. This means that the other characteristics of the configuration were also changed [16], i.e. not only the radius and rotational transform at the separatrix, but also the radial density profiles and the strength and spectrum of the fluctuations.…”