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In Europe, the different configurations and capabilities of the various plasma devices have allowed researchers to develop their understanding of high temperature plasma physics through experiments on a wide variety of magnetically confined fusion devices . In fusion research there are crucial areas of investigation and development that brings together both disciplines of plasma physics and engineering.
Perfecting the engineering technology required to monitor, control a sustained plasma ‘burn’ and realise commercial fusion power is a major challenge which involves the close collaboration of engineers and physicists in the fields of heating systems, diagnostic development and plasma wall interactions studies.
Plasma heating systems are an essential part of high temperature plasma engineering research as the fusion reaction would not continue if the plasma was not heated by an external source. In addition, the heating systems have become an essential tool for the control of instabilities in the plasma that can cause it to touch the surrounding structures cooling the plasma and damaging the surfaces.
Three main types of heating systems are required in fusion experiments:
ICRH- Ion Cyclotron Resonance Heating – in this system ions in the plasma are heated by a high energy beam with a resonance frequency of 30 to 50MHz. Here the main issues are how to couple the high energy beam to the plasma and what effect the beam has on the performance of the plasma.
ECRH- Electron Cyclotron Resonance Heating – here the electrons in the plasma are heated by a high energy beam with a resonance frequency of 100 to 200MHz. This system is also being used to heat the outer surface of the plasma as a control mechanism for the build up of certain instabilities that lead to cooling of the plasma. ECRH has the advantage that the beam can be transmitted through air which simplifies the design and allows the source to be far from the plasma making maintenance simpler.
NNBI- Negative ion Neutral Beam Injection – in this system a small amount of plasma is generated outside the fusion device. The plasma is then accelerated to a very high speed (equivalent to 1MeV energy) and neutralised so that the high energy particles can pass through the magnetic field and enter the fusion plasma. As a result plasma is heated by a transfer of the kinetic energy.
All these heating systems are capable of driving a current in the plasma which will ultimately be used in a true steady state fusion reactor.
In a fusion reactor, instruments that measure a variety of parameters are needed to control the plasma performance including: temperature, density and the type of impurities present. Diagnostics must be developed to monitor every aspect of the machine. Some plasma parameters are measured by multiple instruments using different methods.
One method of measuring the level of impurities is to take measurements of the ultra violet radiation from the particles. Different size particles will radiate different wavelengths of ultraviolet since they have different excitation energies. Knowing the ultraviolet spectrum of the plasma therefore reveals the nature and amount of impurities present.
These are only two examples of many diagnostic techniques developed through fusion research and now used in other fields .
Plasma wall interaction studies
In order to remove the heat, waste fusion products (helium) and impurities from the plasma, the plasma is allowed to touch the wall of the surrounding structure in a controlled manner. This is achieved by shaping the magnetic field lines in such a way that they strike the divertor system. The divertor consists of two targets designed to withstand heat loads of up to 20MW/m2.
Elsewhere, when the plasma makes contact with the vessel wall the high energy of the particles will erode the surface. Studying and controlling this eroded material is a very important subject as any impurities entering the core of the plasma will cool it and stop the fusion reaction. It is also very important to limit this erosion to maximise the lifetime of reactor components.
The material currently used as the target for the divertor is carbon reinforced with carbon fibre. In addition to this critical part of the divertor design, it is also important to design components that can withstand the high mechanical loads experienced in the reactor chamber, allow high vacuum pumping to remove the helium ‘ash’ from the plasma and still perform after long exposure to neutron radiation which makes materials brittle and reduces their ability to conduct heat.