Processes arising in the edge and diverted plasmas during ITB formation in the U-3M torsatron

Abstract

Spontaneous changes in confined plasma parameters have been observed recently in the l = 3/m = 9 Uragan-3M torsatron with an RF produced and heated plasma, these being interpreted as transition to an improved confinement mode due to ITB formation near the ι = 1/4 rational magnetic surface. In the work presented joint studies are carried out of changes in some edge and diverted plasma characteristics that accompany ITB formation. It is shown that ITB formation induces a hard E r bifurcation at the boundary presumably driven by the ion orbit loss. As a result, E r becomes more negative, and an E r shear layer occurs, where the low-frequency microturbulence and the turbulenceinduced anomalous transport are suppressed, i.e. an ETB is formed. At the pre-bifurcation phase of transition a reduction of fast ion loss takes place. The bifurcation results in an improvement of electron confinement, while the ion loss increases.

Recognition of stable distribution with Lévy indexαclose to 2

“…Changes in fluctuation behavior are good indicators of transition to improved confinement modes. In particular, investigations of spontaneous LH-like transition (hereinafter, transition) in the U-3M torsatron reveal a distinct decrease of fluctuation level and radial turbulent particle flux [37]. In this section we analyze the three data sets.…”

“…[14,36]. One can observe obvious change of the fluctuation amplitude in the middle position of every data source, which indicates the transition [37]. Therefore, we split each data set into two parts: before (data1) and after the transition point (data2).…”

U-3M ion energy distribution measurements during frame antenna plasma production and heating in the ICRF range

“…The typical plasma parameters in the discharges under consideration are given in [10,11,[13][14][15]. The evolution of the fixed energy neutral flux in the U-3M was reported in [10,14,15]. A previously observed fast neutral flux rise in the plasma build-up stage based on 1 ms NPA signal integration time was confirmed using the 10 µs integration time and an 8 mega samples/second analog-to-digital converter board.…”

“…The typical plasma parameters in the discharges under consideration are given in [10,11,[13][14][15]. The evolution of the fixed energy neutral flux in the U-3M was reported in [10,14,15].…”

“…The electron-ion energy exchange time estimation [12] τ ei E ≈ (4m i T 3/2 e )/( 8√ 2πm e n ln e 2 ) shows that it is longer than the U-3M discharge duration (30-50 ms), where m i , m e are the ion and electron masses, e is the electron charge, T e ≈ 0.5-1 keV is the electron temperature, n is the plasma density and ln is the Coulomb logarithm. In the case under consideration the ion temperature should be very low (≈10 eV); however, a significant ion temperature T i = 200-400 eV is reported in [13][14][15]. Measurements of the ion energy distribution characterize a direct RF energy deposition onto the ions.…”