Edge Probe Measurements of the Dynamo in the MST RFP

Abstract

Summary form only given. The Madison Symmetric Torus (MST) is a toroidal reversed-field pinch (RFP) device for plasma physics and fusion research. Standard discharges in the MST RFP are characterized by cyclical rapid relaxation events (sawtooth oscillations), when the substantial toroidal flux (and edge poloidal current) is generated due to the RFP dynamo. Previous work indicated that the MHD dynamo <V tilde times B tilde> is operating at the plasma edge, but is substantially reduced at the reversal surface. Present dynamo measurements with a newly developed insertable optical probe (which has improved design and resolution) and magnetic probes are aimed to understand the cause of this reduction. It is observed that B tilde <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> changes phase by pi near reversal surface, while phases of all the other fluctuating quantities remain unchanged. Thus, < V tilde <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> B tilde <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> > component of the MHD dynamo changes sign and cancels <V tilde <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> B tilde <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> > . The Hall dynamo <j tilde times B tilde> is believed to be important for balancing the parallel component of the generalized Ohm's law near the reversal surface. A new magnetic probe, which combines 6 magnetic coil triplets is presently under construction and will be employed to study this effect. We have also recently developed a Mach probe with embedded magnetic coils, which will be used to measure other components of the MHD dynamo. This set of tools, combined with other MST diagnostics, will allow study of the relative importance of different terms in the generalized Ohm's law across the plasma minor radius. The magnetic probe will also be utilized to measure the Maxwell stress tensor in the edge of MST, and to characterize the degree of the charge transport non-ambipolarity. Both of these effects might play an important role in providing torques, which cause intrinsic (without external momentum input) plasma rotation.