Plasma control system upgrade and increased plasma stability in NSTX

A real-time velocity diagnostic for NSTX

A new system for fast measurements of the plasma toroidal velocity has been installed on the National Spherical Torus Experiment, NSTX [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. The diagnostic, based on active charge-exchange recombination spectroscopy, can measure at up to six radial locations with maximum sampling rate of 5 kHz. The system is interfaced in real time with the NSTX plasma control system, in order to feed back on plasma velocity by means of actuators such as neutral beams and external coils. The paper describes the design criteria and implementation of the diagnostic. Examples from the initial tests of the system during neon glows are also discussed.

Fringe-jump corrected far infrared tangential interferometer/polarimeter for a real-time density feedback control system of NSTX plasmas

The far infrared tangential interferometer/polarimeter (FIReTIP) of the National Spherical Torus Experiment (NSTX) has been set up to provide reliable electron density signals for a real-time density feedback control system. This work consists of two main parts: suppression of the fringe jumps that have been prohibiting the plasma density from use in the direct feedback to actuators and the conceptual design of a density feedback control system including the FIReTIP, control hardware, and software that takes advantage of the NSTX plasma control system (PCS). By investigating numerous shot data after July 2009 when the new electronics were installed, fringe jumps in the FIReTIP are well characterized, and consequently the suppressing algorithms are working properly as shown in comparisons with the Thomson scattering diagnostic. This approach is also applicable to signals taken at a 5 kHz sampling rate, which is a fundamental constraint imposed by the digitizers providing inputs to the PCS. The fringe jump correction algorithm, as well as safety and feedback modules, will be included as submodules either in the gas injection system category or a new category of density in the PCS.