Title
Control of sawteeth and neo-classical tearing modes in tokamaks using electron cyclotron waves
Author
de Baar, M.R.
Bongers, W.A.
van Berkel, M.
Doelman, N.J.
Hennen, B.A.
Nuij, P.
Oosterbeek, J.W.
Steinbuch, M.
Westerhof, E.
Witvoet, G.
Publication year
2011
Abstract
Resistive magneto-hydrodynamic instabilities are expected to limit the performance of nuclear fusion reactors. Prime examples are the sawtooth instability and the neoclassical tearing modes. The sawtooth instability will affect the refueling of the plasma core and the fast particle concentration. In reactor relevant conditions, the sawtooth can also trigger secondary instabilities. These are the neo-classical tearing modes, which can deteriorate the plasma performance or even disrupt the discharge. These modes (sawteeth and tearing modes) appear at specific locations in the tokamak plasma, associated with the distribution of the current density and the toroidal magnetic field. Localized current drive from electron cyclotron waves is foreseen as a possible actuator for the width of the tearing modes and the period of the sawteeth. Magnetic pick-up coils, electron cyclotron emission (ECE) and soft X-ray emission are the most likely sensors. A system approach for real-time detection, localisation and control of resistive magneto-hydrodynamic modes in tokamaks is presented. The system combines an ECE diagnostic for sensing of the instabilities in the same sight-line with a steerable Electron Cyclotron Resonant Heating and Current Drive (ECRH/CD) antenna. A model for the sawtooth is used to derive the linearized input-output relations (transfer functions) from the varying deposition location of high power Electron Cyclotron waves to the sawtooth period. The transfer functions are derived around a large collection of operating points. Proportional-integral-integral (PII) action can be applied to achieve fast and perfect tracking, while satisfying robustness constraints. The launcher dynamics seriously affects the closed loop performance in present-day experiments. Special emphasis is put on the issue of real-time sawtooth period detection. An algorithm based on time-scale wavelet theory and edge detection for accurate real-time sawtooth period estimation has been developed. The period is estimated by detecting subsequent crashes. The realized accuracy of the detection algorithm is well below the uncertainty of the crash period for most crashes. Multi-resolution analysis enables distinction between different sizes of sawtooth crashes due to the different sizes of wavelets (scales), resulting in an algorithm, which is robust and accurate. A methodology for fast detection of q m/n 2/1 tearing modes and retrieval of their location, rotation frequency and phase is presented. Set-points to establish alignment of the ECRH/CD deposition location with the centre of the tearing mode are generated in real-time and forwarded in closed-loop to the steerable launcher and as a modulation pulse train to the gyrotron. © 2011 IEEE.
Subject
Mechatronics, Mechanics & Materials
OM - Opto-Mechatronics
TS - Technical Sciences
Electronics
Closed-loop performance
Current drives
Detection algorithm
Electron cyclotron emission
Electron cyclotron waves
Electron cyclotrons
Fast particle
Input-output relations
Localisation
Localized current drive
Magnetohydrodynamic instabilities
Multi-resolutions
Neoclassical tearing modes
Operating points
Pick-up coils
Plasma core
Plasma performance
Pulse train
Real-time detection
Resonant heating
Rotation frequencies
Sawtooth crashes
Sawtooth instability
Sawtooth periods
Secondary instability
Setpoints
Soft x-ray emissions
Steerable launcher
System approach
Tearing modes
Time-scales
Tokamak plasmas
Toroidal magnetic fields
Wavelet theory
Algorithms
Cyclotron radiation
Edge detection
Electric discharges
Electron beams
Fluid dynamics
Fusion reactors
Magnetic fields
Magnetohydrodynamics
Magnetoplasma
Plasma diagnostics
Plasma stability
Plasma waves
Signal detection
Steerable antennas
Transfer functions
Cyclotrons
To reference this document use:
http://resolver.tudelft.nl/uuid:316ca106-0be9-4c00-9936-2fec49edc22d
TNO identifier
442953
ISBN
9781457710629
Source
2011 20th IEEE International Conference on Control Applications, CCA 2011, 28-30 September 2011, Denver, CO, USA. Conference code: 87149, 294-300
Article number
No.: 6044376
Document type
conference paper