Plasma physics: at the heart of fusion
Research on fusion energy in Europe has focused on understanding and controlling the behaviour of high-temperature plasmas. Plasma is the 4th state of matter and is a gas of electrically-charged particles: electrons and ions. The gas is so hot its atoms have been completely ionised (the negatively charged electrons have separated from the positively charged atomic nuclei - or ions).
Europe has been at the forefront of experimental research on plasma physics through major programmes such as the Joint European Torus (JET) and other significant international and European experiments.
The success of these experiments has given the confidence to proceed with ITER, the next step in fusion energy research. But for ITER to fully succeed and lay the path to fusion power, further international investment in plasma physics research will be needed.
Magnetic confinement - controlling plasma
coil structure in W7X © IPP
Very high temperatures are needed to force fusion reactions between atomic nuclei. The nuclei need sufficient energy to overcome their mutually repulsive electrical charges. At very high temperatures the gas used as fusion fuel is completely ionised. The resulting high-energy mixture of positively charged ions and negatively charged electrons is the 4th state of matter called a plasma. It is essential that the hot plasma does not touch the walls of the reaction chamber as this will lead to rapid cooling of the fuel and possible damage to equipment.
Because plasma is a gas of charged particles it can be controlled by powerful magnetic fields. The particles in the plasma can move easily along magnetic field lines but are restricted in moving across them. This allows plasma to be held, controlled and even heated by a complex cage of magnetic fields. This is called magnetic confinement.
coils in a tokamak © EFDA
The magnetic fields are generated by superconducting magnetic field coils located around the plasma reactor chamber and by electrical currents flowing in the plasma itself.
Magnetic confinement fusion is the main approach for European fusion research. The fusion reactions take place in a reaction vessel that isolates the plasma from its surroundings and is shaped like a torus or doughnut shape - essentially a continuous tube.
Several toroidal configurations for magnetic confinement have been studied, and the most advanced of these is the tokamak. ITER, for example, is a large tokamak.
Alternative toroidal devices also being studied include the stellarator, the reversed field pinch and spherical tokamak configurations.
Advanced and improved concepts for magnetic confinement schemes that could provide potential advantage for future fusion power stations are part of longer-term R&D. This includes the construction of the advanced W7-X stellarator device in Germany.
Plasma research challenges
To actively and accurately control plasma in a fusion reactor, scientists need to understand fully:
- the properties of the plasma,
- how plasma conducts heat,
- how particles are lost from the plasma,
- plasma stability,
- how unwanted particles (impurities) can be removed from the plasma,
- how to best control the plasma and plasma instabilities.
The European research programme conducts extensive experimental, theoretical and modelling studies on plasma physics with the aim of providing a strong, comprehensive and predictive understanding of its complexities.
Achieving controlled fusion requires three conditions in the plasma:
- a high temperature (> 100 million °C),
- a particle density of at least 1022 particles per cubic metre,
- an energy confinement time of around 1 second.
The energy confinement time is a measure of the time for the energy contained in the plasma to dissipate.
Clearly one of the major challenges in fusion research is to maintain plasma temperature. The plasma is constantly being cooled by impurities picked up from the reactor wall. Scientists need to understand the processes that erode the plasma vessel wall and the mechanisms that transport the eroded impurities into the plasma. The impurities must be efficiently extracted without disrupting the plasma. In addition, the extra heating power injected into the plasma must also be extracted in a controlled manner.
Understanding and controlling instabilities in the plasma is another major challenge being tackled by both experiment and the use of powerful computer models of plasma behaviour.
An alternative approach to fusion involves inertial confinement. This technique uses ultra-fast high-power lasers or particle beams to heat and compress tiny pellets of fusion fuel. The European fusion programme maintains a watching brief on developments in this area and supports some experimental work.