Plasma current and distributed magnetic field measurement in thermonuclear fusion reactors based on the Faraday effect in optical fibres (PhD)

The trend in the design of the next generations of thermonuclear fusion reactors like ITER and DEMO is aimed  towards the burning of thermonuclear fusion plasma in quasi steady-state conditions. The environment characterised by high levels of radiation and temperature that will be created by this steady-state operation will hamper the correct operation of the existing  inductive magnetic sensors measuring the plasma current, which is a key parameter for the control of the tokamak operation. In this context, there is a clear need to look at complementary techniques for the measurement of plasma current as well as the magnetic parameters in the future steady state thermonuclear fusion reactors. One possibility is to make use of the Faraday effect observed in an optical fibre exposed to the magnetic field of the tokamak. This effect results from an interaction between the light guided in an optical fibre and a magnetic field. The magnetic field causes changes of the refractive index of the fibre, which in turn influences the propagation properties of i.e. polarized light beams travelling inside the fibre. By measuring the changes of the optical properties the electrical current creating the magnetic field can be inferred. The sensor based on this measurment principle is referred further as Fibre-Optic Current Sensor (FOCS). Most of the R&D efforts at SCK•CEN up to now in the FOCS technology have been focused on the selection of the optical fibre able to fulfil the requirements of ITER.

The objective of the PhD work is to design and to implement an adequate optical device configuration, a data acquisition system and a signal processing scheme for high plasma current and distributed magnetic field measurements in thermonuclear fusion reactors based on the Faraday effect in optical fibres. The system to be developed must feature new capabilities leading to innovations in the field of:

• Measuring very high electrical current up to 15 MA with a relative accuracy of 1%
• Increasing the measurement robustness and the high noise rejection rate of the FOCS
• Exploiting the distributed measurement capacity of optical fibres in order to assess the magnetic field distribution. Specialists (e.g. CEA/TORE-SUPRA) confirmed the high interest of this feature for tokamaks.