Ji Paley

Magnetic energy flows during the current quench and termination of disruptions with runaway current plateau formation in JET and implications for ITER

The magnetic energy balance and magnetic energy flows for plasma disruptions in which runaway plateau plasmas are formed and terminated at JET has been analysed and compared with that of runaway-free disruptions. The analysis shows that the energy loss processes during runaway plateau plasma termination are qualitatively different from those of a runaway-free disruption because of the pre-existence of a runaway population in the first case. As a consequence, a significant fraction of the runaway plateau plasma magnetic energy is directly converted into runaway electron kinetic energy during the runaway plateau termination phase. This leads to the fluxes being deposited by runaway electrons onto in-vessel components during the termination of runaway plateaus to be significantly larger than those expected from the initial kinetic energy of the runaway electrons in the runaway plateau plasma.

Real time control of the sawtooth period using EC launchers

Tokamak plasmas operating at high performance are limited by several MHD instabilities. The sawtooth instability limits the core plasma pressure and can drive the neoclassical tearing mode unstable, but also prevents accumulation of impurities in the core. Electron cyclotron heating and current drive systems can be used to modify the local current profile and therefore tailor the sawtooth period. This paper reports on demonstrations of continuous real time feedback control of the sawtooth period by varying the EC injection angle.

Observer-based real-time control for the poloidal beta of the plasma using diamagnetic measurements in tokamak fusion reactors

The constant technological advances and progresses in tokamak research constantly show up new control challenges. In this context, the control of the poloidal beta has arisen as a relevant issue. In this paper a real-time observer based on the real-time analysis of diamagnetic measurements that has been developed for the Tokamak à Configuration Variable (TCV) is presented. The algorithm proposed combine measurements of the diamagnetic loops, flux loops and magnetic probes. Afterwards, some simulations are carried out with the purpose of testing the observer. Finally, once the observer has been developed and validated through simulations, it is implemented on the TCV reactor and applied to the real-time control of poloidal beta of the plasma. The results of the experiments show the feasibility of the observer for real-time tokamak plasma control purposes.

Architecture and commissioning of the TCV distributed feedback control system

A new modular, digital, distributed feedback control system has been developed and installed to control the TCV plasma. With many more inputs and outputs, it provides the possibility to build control algorithms using far more information on the plasma state than previously possible as well as the ability to control many more actuators, including the multi-megawatt, multi-launcher electron cyclotron heating and current drive system. This paper provides an overview of the new control system, its integration into the TCV systems and its successful application to control the TCV plasma discharge.

From profile to sawtooth control : developing feedback control using ECRH/ECCD systems on the TCV tokamak

Real time control of heating systems is essential to maximize plasma performance and avoid or neutralize instabilities under changing plasma conditions. Several feedback control algorithms have been developed on the Tokamak a Configuration Variable (TCV) tokamak that use the electron cyclotron (ECRH/ECCD) system to control a wide range of plasma properties, including the plasma current, shape, profiles as well as the sawtooth instability. Controllers have been developed to obtain sawteeth of a pre-determined period, to maximize the sawtooth period using an extremum seeking control algorithm and finally to provide simultaneous control of the plasma emission profile peak and width using multiple independent EC actuators.

Real time control of plasmas and ECRH systems on TCV

Developments in the real time control hardware on Tokamak a Configuration Variable (TCV) coupled with the flexibility of plasma shaping and electron cyclotron (EC) heating and current drive actuators are opening many opportunities to perform real time experiments and develop algorithms and methods for fusion applications. The ability to control magnetohydrodynamic instabilities is particularly important for achieving high performance fusion plasmas and EC is envisaged as a key actuator in maintaining high performance. We have successfully demonstrated control of the sawtooth instability using the EC launcher injection angle to modify the current profile around the q =1 surface. This paper presents an overview of recent real time control experiments on TCV, developments in the hardware and algorithms together with plans for the future.

Sliding mode control of a tokamak transformer

A novel inductive control system for a tokamak transformer is described. The system uses the flux change provided by the transformer primary coil to control either the electric current or the internal inductance of the transformers secondary plasma circuit load. The internal inductance control is used to regulate the slow flux penetration due to the skin effect, providing first-order control over the shape of the plasma current density profile in the highly conductive plasma. Inferred loop voltages at specific locations inside the plasma are included in a state feedback structure to improve controller performance. Experimental tests have shown that the plasma internal inductance can be controlled inductively for a whole pulse starting just 30ms after the beginning of plasma breakdown. The details of the control system design are presented, including the transformer model, observer algorithms and controller design.

Multiple electron cyclotron power deposition location tracking by break-in-slope analysis in TCV plasmas

Modulation of the amplitude of externally injected electron cyclotron (EC) power is a frequent method used to determine the radial power deposition profile in fusion plasmas. There are many tools to analyze the plasma response to the power modulations under quasi-stationary conditions. This paper focuses on the unique ability of the break-in-slope (BIS) method to retrieve a quasi-instantaneous estimate of the power deposition profile at each power step in the modulation, an outcome particularly relevant to track the power deposition location under non-stationary conditions. Here, the BIS analysis method is applied to the signals of a fast and high radial resolution wire-chamber soft x-ray camera in the Tokamak a Configuration Variable (TCV) where the plasma magnetic configuration and thus the EC resonance location are varied during the plasma discharge. As a step to validate this technique before real-time control experiments, the time-varying EC power deposition location of a single beam is successfully monitored by off-line BIS analysis. Simultaneous tracking of deposition locations of two EC beams gives promising results.