Introduction to fusion physics
Two English physicists, Tonks and Langmuir, are credited with coining the term ‘plasma’ to designate an ionised gas, when they were working on the study of discharges in gases in the 1920s. Since then, interest in this discipline has grown considerably as the many applications of plasmas have been discovered, both in fundamental research (astrophysics) and in industry (surface treatment, welding, flat screens, etc.). Plasma physics then developed by incorporating all the advances of modern physics. It is a complex science, rooted in many of the concepts used to describe solids, liquids or gases, but drawing on virtually every area of physics (electrodynamics, statistical mechanics, quantum mechanics, collision theory, atomic and molecular physics, nuclear physics, kinetic theory, transport equations, thermodynamics, wave propagation, radiation, spectroscopy, etc.), all of which generally lead to coupled non-linear equations that are difficult to solve, even with today’s numerical techniques.
In addition to numerous technological challenges (components capable of withstanding high heat fluxes, superconducting magnets, remote handling, etc.), nuclear fusion poses vast theoretical problems, and has given rise to a particularly active branch of plasma physics.
The aim of nuclear fusion research is to produce energy by confining a sufficiently hot and dense plasma efficiently. So start by discovering the Lawson criterion, which sets out the conditions under which energy can be produced from a fusion plasma. The resulting questions can then be summarised as follows:
- How can plasma particles be effectively confined ?
This is the whole problem of magnetic confinement and the transport of heat and particles. - How can the temperatures required for the future reactor be reached ?
This is the problem with heating the plasma, which also generates current in the machine. - How do you protect the components in the vacuum chamber from the plasma and, in turn, the plasma from impurities emitted by the surrounding walls ?
This is the whole problem of plasma/wall interactions and the extraction of particles and heat, with WEST’s original response testing ITER’s tungsten divertor and its effects on the plasma.
Finally, it is impossible to answer all these questions without the appropriate measuring equipment to analyse what is happening at the heart of the tokamak : these are the diagnostics.
WEST, one of the few machines of significant size capable of long pulse operation thanks to its superconducting magnets, offers physicists a unique opportunity to tackle these problems with a view to steady state operation, which is essential for the future reactor. This is WEST’s speciality: long discharges.