Through dedicated research, European scientists have increased our understanding of the properties of plasma and how it behaves under the conditions necessary for fusion.
In fusion research, physics and engineering work very closely together, perfecting the engineering technologies that will be required to monitor and control a sustained plasma 'burn' to realise the dream of commercial fusion power. These technologies, such as heating systems, diagnostic developments and plasma-wall interactions, represent significant engineering challenges.
Plasma heating systems are essential for obtaining and maintaining high-temperature plasma. Without external heating the fusion reactions would not be able to continue. Heating systems may also play an essential role in controlling other plasma properties: in particular controlling instabilities.
Three main types of heating systems are currently being developed:
Ion Cyclotron Resonance Heating (ICRH) - rather like a massive microwave, this system is used to heat the positive ions in the plasma using electromagnetic waves with a resonance frequency of 30 to 50 megahertz. The engineering issues for this system include how to couple this intense radiation to the plasma and how the power input affects the behaviour of the plasma.
Electron Cyclotron Resonance Heating (ECRH) - this is similar to the ICRH but heats the electrons in the plasma with a resonance frequency between 100 and 200 megahertz. This system can also be used to heat the outer surface of the plasma to control certain instability modes. ECRH beams can be transmitted through air so the source can be relatively remote with respect to the reactor.
Neutral Beam Injection - this system heats the plasma using kinetic energy by accelerating charged fusion fuel particles to a very high degree (equivalent to a kinetic energy of 1 mega electron volt). The particles are then neutralised (the charge is removed) so that the high-energy neutral particles can pass through the magnetic field and enter the plasma.
In a fusion reactor many instruments to measure a variety of parameters are needed to monitor and control plasma performance. These include instruments to measure temperature, density and the type of impurities present.
Diagnostics are needed to measure every aspect of the machine, and fall into three main categories:
- Machine protection or basic control.
- Advanced control.
- Physics studies.
Some 45 different diagnostic systems will be deployed around the ITER tokamak.
For example, the most reliable way of measuring plasma temperature in the reactor is to use a laser. The photons in the laser light scatter off the plasma electrons and the Doppler shift of the scattered light is measured. This gives a direct measurement of the speed of the hot electrons and their temperature. The intensity of the scattered light is also related to the density of the plasma.
One method of measuring the impurity level in the plasma is to take measurements of the ultraviolet (UV) radiation from the plasma. Different impurities will radiate at varying UV wavelengths due to different excitation energies. By examining the UV spectrum of the plasma, the nature and number of impurities present can be determined.
Plasma wall interactions
in TOSKA at KIT © IPP/Garching
Removing heat, impurities and reaction products from the plasma is a significant engineering challenge that requires the plasma to come in contact with the reactor chamber in a controlled manner.
This is achieved by shaping the magnetic field line so that the plasma comes into contact with the divertor system. The divertor system is effectively an exhaust technology for fusion reactors and must be able to withstand extreme heat. The two main targets in the divertor can stand heat loads up to 20 megawatts per square metre. The material currently used for the divertor is carbon reinforced with carbon fibre.
Elsewhere in the reactor the plasma is kept away from the walls. Contact with the wall produces eroded material. This erosion material must be limited to maximise the efficiency of the reactor operation and the lifetime of reactor components.
All wall components must be resilient to high mechanical loads, allow for high vacuum pumping to remove helium 'ash' from the plasma and be able to withstand long exposure to neutron radiation. Neutron radiation is a particular advanced materials challenge as it tends to make materials brittle and reduces their ability to conduct heat.