REDUCTION OF THE RF DI SCHARGE PLASMA TRANSPORT IN THE U - 3M TORSATRON DUE TO E◊B SHEARED ROTATION IN T HE VIC INITY OF RATI ONAL MAGNETIC SURFACES

There are some publications (1,2,3) with indications that the formation of internal transport barriers (ITB) in toroidal devices could t ake pl ace in the vi cinity of low order rational surfaces (RS). In pr esented e xperiment the attempt to re alize the formation of such a barrier with high Er◊B velocity shear near of island chains with t=1/4 and to study its influence on a plasma confinement was undertaken on the Uragan - 3M (U-3M) torsatron. The presupposition was made that the radial electric field profile, Er(r), in this region will be determined by the increase of a transversal electroconductivity due to a longitudinal motion of electrons in stochasti c layers of magnetic field lines near RS. Therefore, the study of the case with sufficient heating power in the region of localization of RS is most inte resting for the explanation of ITB formation. 2. Experimental arrangement Experiments were carried out on the U - 3M torsatron with an open helical divertor (l=3, m=9, R0=100cm, pl=12,5cm) at magnetic field strength B0=0.7T. The measurements ma de by the triode and lumin escent rod techniques have shown that there is the possibility to realize the magnetic configuration with two chains of islands (t=1/4) which are loc ated in the region of small ma gnetic shear (4). Such a configuration takes place at the ratio of vertical magnetic field to longi tudinal one B•/B0=1.25%. The outside shift of the magnetic axis from the geometrical axis of helical coils equals to 5.5cm in this case (Fig.1). The frame type antenna was used for RF plasma production and heating in the ion cyclotron range of frequencies (f=8,8MHz, P RF 200kW) to provide a suffici ent heating power in the region of localization of isla nd ch ains. Numerical s imulations have shown that Alfven waves excited by this antenna absorb at the ext ernal part of a pl asma column