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

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 i = 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 turbulence-induced 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.