Startup of reversed field pinches and current ramping using dynamo action

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Abstract
The different startup modes of a reversed-field pinch (RFP) are examined and compared. The RFP startup is not the same as startup of other toroidal devices such as the tokamak because of the spatial and temporal variations of the toroidal field and the reversed toroidal field in the outer region of the pinch near the wall. In matched mode startup, used in many RFP experiments today, the toroidal flux is held constant during the current rise, with the field reversal occurring before the peak current. This mode, with its short rise time, has a low volt-second (V⋅s) input but requires a high toroidal voltage to reach a specific current in a relatively short time. In a ramped current mode, alow current RFP discharge is ramped to its final peak value. The toroidal flux needed as the current rises is generated by a dynamo action. This slower startup mode can be driven by a lower voltage but requires more V⋅s input than the matched mode. Different startup modes in the ZT-40M experiment at Los Alamos are compared and an analytic expression is given for characterizing the V⋅s contributions. The resistive component of the toroidal loop voltage during the current rise in ramped discharges is found to depend on theta (Θ = Bpol(wall)/Bave)- At a theta of 1.45, the resistive voltage has a minimum and it has been possible to reduce the V⋅s input by as much as 40% in ramp discharges by keeping theta close to this value.