The main purpose of this PhD project is the development of Fiber Optics Current Sensor (FOCS) prototype suitable for performing measurements in future thermonuclear fusion reactors, considering WEST and ITER tokamaks as milestones on this path. Solution of this task requires innovations in fiber-optics current sensing combined with the adaptation of the FOCS engineering design, making is suitable for the use during Burning Plasma eXperiments (BPX). Based on experimental work and numerical simulations, an innovative FOCS design will be proposed. The design must take into account and mitigate the impact of the harsh environment on the sensors performance characteristics. Perturbations related with high temperatures, vibrations, and radiation combined with the need to measure extreme currents, far beyond currents measured in any other application, make this task extremely challenging. The proposed design will be validated by comparing the modelling results with measurements on the FOCS test bed. The final step of the validation will be performing current measurements on tokamaks with an ITER-representative environment.
In magnetic fusion reactors the plasma current is a fundamental parameter required for the control of the reactor operation. It should typically be measured with an accuracy of 1%. Nowadays, the plasma current is mainly measured by using inductive coils (ex. Pick-up coils and/or Rogowski coils) placed around the reactor . Their functioning principle requires an integration step, which makes the sensor insensitive to stationary currents. The usage of this type of sensor in future BPX is therefore extremely challenging. Already for ITER, which will operate still in a quasi-stationary mode (plasma pulses of typically 3000 s) development of dedicated integrators with a very low drift appears to be an extremely difficult task.
An alternative to the traditional inductive sensors consists in exploiting the Faraday effect in an optical fibre when it is subject to a magnetic field: the presence of a magnetic field aligned with the fibre axis induces a circular birefringence. As a consequence of this circular birefringence, an input state of polarization rotates by an angle θ proportional to the integral of the magnetic field component parallel to the fibre along the fibre length. It follows from the Ampere theorem, that if the fibre forms a loop around a current I, the rotation θ is proportional to I. Thus, this approach provides a direct measurement of the current without requiring any integration step , which is of high interest in the context of ITER.
Commercial FOCSs are successfully used in power industry for decades. Unfortunately, those sensors could not be immediately installed in fusion devices. For example, one of the FOCS issue is the unavoidable presence of linear birefringence in the fibre, which competes with the useful current-induced circular birefringence. Solutions of this problem implemented in commercial devices are not suitable for fusion installations due to restrictions imposed by the presence of radiation, high temperatures, and vacuum compatibility. Another problem is the need to measure currents in the order of 20 MA, while the operation of commercial devices is limited by 800 kA. The high currents makes the available detection schemes unsuitable and can also bring into play new effects. This list can be extended. This list can be extended. However, advantages which FOCS brings for ITER warrant search of their solution.
In that context, a collaboration work between the University of Mons, the SCK•CEN and the IRFM (CEA Cadarache) has started in the frame of a PhD thesis that will end in September 2015. One of the main result of it is the determination of the best type of fibre that should be used in order to minimize the linear birefringence detrimental effect in the context of ITER monitoring. Existing studies have shown that the radiation effects on fibre attenuation are sufficiently limited in the ITER environment . Within this collaboration framework, it has been demonstrated that a FOCS located around the tokamak vacuum vessel are suitable to measure the plasma current .
The proposed new PhD work can be considered as a continuation of the previous work. For the fulfilment of the PhD the collaboration with the IRFM is extremely beneficial since it allows participating in the WEST project. WEST (Tungsten (W) Environment in Steady-state Tokamak) project consists of a major modification of the Tore Supra tokamak. With WEST, Tore Supra will become a test platform for a key component of ITER: the tungsten divertor. To achieve this goal, Tore Supra must be re-configured: a tungsten divertor will be added and poloïdal field coils will be installed in the vacuum chamber. The measurement of the current plasma and the quantification of the current circulating in the conductive elements installed in the chamber are required in order to ensure a precise measurement of the shape and the position of the plasma. In the context of WEST, fibre-optics sensors are also of high interest since the size of optical fibres is such that it could fit in the small space available for the sensor.
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