Year

2018

Degree Name

Doctor of Philosophy

Department

School of Physics

Abstract

At present, the design of the International Thermonuclear Experimental Reactor (ITER) project involves a superconducting magnet system that is based on NbTi and Nb3Sn. The system consists of 18 Nb3Sn Toroidal Field (TF) Coils, a 6 module Nb3Sn Central Solenoid (CS), 6 NbTi Poloidal Field (PF) Coils, and 9 NbTi pairs of Correction Coils (CCS). In the current ITER plan, 112 tonnes of Nb3Sn superconducting magnets will be used. Despite outstanding high magnetic field properties, the current superconductor solution, Nb3Sn, is not ideal and has several drawbacks.

First and foremost, Nb based superconductors must be cooled down to 4 K using very expensive and not-readily-available liquid helium. The world shortage of helium supplies is an inherent issue, as helium resources are limited and recovery following use is unsustainable and unreliable. Secondly, if the 10 mSv/h requirement for the remote handling of recycling, set by ITER, is to be satisfied, there will be an impact on the maintenance and radioactive waste treatment schedules. Therefore, it is critical to consider the radio activation of components such as superconducting magnets over time within the fusion reactor from the viewpoints of radiological and environmental pollution. This implies that thicker shielding is necessary in front of the Nb based superconducting magnets if long-term operation is considered. Therefore, it is essential that all of the components within the fusion reactor should be composed of low-activation materials. Lastly, a practical test using the SULTAN facility (PSI, Switzerland) on Nb3Sn based cables demonstrated that they have limited reliability. In this test, the deterioration of superconducting properties was observed at magnetic field/ current cycling that was 10 times lower than the required target, and there was mechanical degradation. Mechanical degradation has been proved to be the reason why the mechanical properties need to be improved. Therefore, alternative solutions for decreasing these drawbacks are in demand.

The aim of this thesis was to study and demonstrate the possibility of using Mg11B2 superconducting material for fusion reactor application because of its ideal properties. MgB2 has a higher critical temperature (Tc = 39 K), which means that the cryogenic system does not necessarily require a cryogenic agent. Indeed, MgB2 superconductors have acceptable performance at a temperature as high as 20 K, while the conventional superconductors require 4.2 K.

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