Effect of low magnetic shear induced by lower hybrid current drive on high performance internal transport barriers in the Joint European Torus (JET)

A low/negative magnetic shear profile is maintained in the Joint European Torus in a plasma target with plasma current of 2.4 MA using 2.2 MW of lower hybrid (LH) power combined with neutral beam injection and ion cyclotron radiofrequency heating. In this scenario, an internal transport barrier (ITB) with time duration up to about 4 s is produced. The fraction of LH driven current is about 25% the total plasma current. During LH power application, the layer with reversed shear q-profile can be maintained in a suitable radial position to inhibit the onset of turbulence, which might otherwise drive the ITB to collapse. LH power could be used as a tool to drive moderate amounts of noninductive off-axis current and sustain high performance ITBs at high plasma current.