J. Kallman

High density Langmuir probe array for NSTX scrape-off layer measurements under lithiated divertor conditions

A high density Langmuir probe array has been developed for measurements of scrape-off layer parameters in NSTX. Relevant scale lengths for heat and particle fluxes are 1-5 cm. Transient edge plasma events can occur on a time scale of several milliseconds, and the duration of a typical plasma discharge is ∼1 s. The array consists of 99 individual electrodes arranged in three parallel radial rows to allow both swept and triple-probe operation and is mounted in a carbon tile located in the lower outer divertor of NSTX between two segments of the newly installed liquid lithium divertor. Initial swept probe results tracking the outer strike point through probe flux measurements are presented.

Strike point control for the National Spherical Torus Experiment (NSTX)

This paper presents thefirst control algorithm for the inner- and outer-strike point position for a Spherical Torus (ST) fusionexperimentandtheperformanceanalysisofthecontroller. Aliquidlithiumdivertor(LLD)willbeinstalledon NSTX which is believed to provide better pumping than lithium coatings on carbon PFCs. The shape of the plasma dictates the pumping rate of the lithium by channelling the plasma to LLD, where the strike point location is the most important shape parameter. Simulations show that the density reduction depends on the proximity of the strike point to LLD. Experiments were performed to study the dynamics of the strike point, design a new controller to change the location of the strike point to the desired location and stabilize it. The most effective poloidal field (PF) coils in changing inner- and outer-strike points were identified using equilibrium code. The PF coil inputs were changed in a step fashion between various set points and the step response of the strike point position was obtained. From the analysis of the step responses, proportional‐integral‐derivative controllers for the strike points were obtained and the controller was tuned experimentally for better performance. The strike controller was extended to include the outer-strike point on the inner plate to accommodate the desired low outer-strike points for the experiment with the aim of achieving ‘snowflake’ divertor configuration in NSTX. (Some figures in this article are in colour only in the electronic version)

Bishop-Taylor equilibria for calibration equilibrium and equilibrium reconstruction codes

The properties of Bishop-Taylor equilibria and the algorithm of their calculations, implemented into numerical code Cbbsh, are described. These equilibria are unique in having the shape of magnetic surfaces not dependent on the poloidal flux distribution, which, in its turn, determines both the current and pressure profiles in this case. These equilibria can be calculated with any precision, using only 3 ordinary differential equations. Possessing a free profile, they can be used for calibrating equilibrium and stability codes. They are also especially valuable for equilibrium reconstruction as a precise example of a configuration where the external magnetic field does not depend on the current density.