C. Rapson

The effect of boundaries on the ion acoustic beam-plasma instability in experiment and simulation

The ion acoustic beam-plasma instability is known to excite strong solitary waves near the Earths bow shock. Using a double plasma experiment, tightly coupled with a 1-dimensional particle-in-cell simulation, the results presented here show that this instability is critically sensitive to the experimental conditions. Boundary effects, which do not have any counterpart in space or in most simulations, unavoidably excite parasitic instabilities. Potential fluctuations from these instabilities lead to an increase of the beam temperature which reduces the growth rate such that non-linear effects leading to solitary waves are less likely to be observed. Furthermore, the increased temperature modifies the range of beam velocities for which an ion acoustic beam plasma instability is observed.

Disruption mitigation by injection of small quantities of noble gas in ASDEX Upgrade

The most recent experiments of disruption mitigation by massive gas injection in ASDEX Upgrade have concentrated on small-relatively to the past-quantities of noble gas injected, and on the search for the minimum amount of gas necessary for the mitigation of the thermal loads on the divertor and for a significant reduction of the vertical force during the current quench. A scenario for the generation of a long-lived runaway electron beam has been established; this allows the study of runaway current dissipation by moderate quantities of argon injected. This paper presents these recent results and discusses them in the more general context of physical models and extrapolation, and of the open questions, relevant for the realization of the ITER disruption mitigation system.

Development environments for Tokamak plasma control

We describe a software system known as the Plasma Control System Simulation Platform (PCSSP) that is being constructed to support development of the ITER plasma control system. When mature, PCSSP will provide support for industry standard practices such as model-based controller design, simulation testing of controllers, auto-generation of controller code, hardware- and software-in-the-loop testing, use of source code management tools, and open-source methods in development of the software and control algorithms for the ITER Plasma Control System (PCS). It will also contribute to fusion-specific objectives such as validation of ITER pulse schedules prior to their use in experimental operation. We also describe a more mature but less sophisticated software suite known as TokSys, which was developed at General Atomics to support plasma control development and is expected to eventually merge with PCSSP. Plasma control development activities that use or have used these software systems are also described. We will discuss the ongoing ITER PCS control algorithm development for initial plasma operation, which takes advantage of the existing PCSSP infrastructure and library of modules. Plasma control development and initial plasma operation of the EAST and KSTAR tokamaks relied heavily on the TokSys suite, which is now in routine use at EAST, KSTAR, DIII-D, and NSTX-U.

Real time control of plasma performance on ASDEX upgrade and its implications for ITER

Sustained burning plasma operation is the primary target of thermonuclear fusion research and development. To generate economically viable electricity from nuclear fusion requires a high reaction rate over long times with high reliability. Plasma performance must continue to improve above the current levels to reach this target. Non-linear stability limits and internal couplings make optimization of plasma performance a complex, convoluted task, which can be mastered only by assistance of automated control tools. The ASDEX Upgrade fusion experiment has a long tradition in the exploration of physics relations and the subsequent derivation of control strategies. Based on examples from current projects, radiative cooling, NTM stabilization and ELM mitigation, we show typical facets of performance control and how they are implemented by the ASDEX Upgrade Discharge Control System (DCS). Finally, we explain how the methods could be further developed and which additional features would be necessary in the ITER context.