A global analysis of the behaviour of the ZT-40M reversed field pinch

Experimental data from the reversed field experiment, ZT-40M, have been re-examined in an attempt to determine the scaling behaviour of the physical plasma quantities and their fluctuations. A subset of the data is defined, allowing a reduced number of independent variables to describe the behaviour. For flat-top ZT-40M discharges the independent variables are chosen as being the toroidal current, Iϕ, and the dimensionless pinch parameter, Theta , which is proportional to the ratio of the toroidal current to the toroidal magnetic flux. The amplitudes of the dependent variables, including the electron temperature, plasma resistance, toroidal flux, the ratio of Iϕ to the mean electron density and their fluctuation amplitudes, exhibit minima as functions of Θ for constant Iϕ. These minima move towards lower Theta values with increasing Iϕ. Over the range of conditions for acceptable operation, the scaling of variables with Iϕ is not unique but depends on the variation of Θ as Iϕ increases. The Θ variation is governed by the specific conditions (such as constant poloidal β, βp) chosen to set the desired RFP operational constraints. Contour plots of the dependent variables versus the two independent variables, Iϕ and Θ, allow the determination of the Iϕ-Θ trajectory that corresponds to discharges that meet the chosen condition. The analysis shows that the amplitude of the low frequency fluctuations correlates with the mean βp and energy confinement time of ZT-40M. By modifying the external circuits on ZT-40M, low frequency fluctuations were reduced. Comparing the designs of different RFP experiments and their operating behaviour, these modifications suggest design changes for present and future RFP experiments that will benefit their performance