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Non-nuclear energy

Plasma Physics R&D

Fission and radiation protection
Fusion
   

A fundamental approach to realising fusion on Earth is the understanding and control of magnetically confined, high temperature plasmas. In the past decade significant progress has been made with the successful scientific results from the largest fusion device JET (supported by other international experiments and smaller European devices). This has given the international scientific community the confidence to proceed with the next step in fusion energy research, ITER. To bring the same success to the ITER experiment and enable our researchers to design the future generations of fusion reactors it is necessary to maintain the current pace of high temperature plasma physics research in this field.

Magnetic confinement: the heart of high temperature plasma physics

Fusion reactions occur at very high temperatures when nuclei collide with sufficient energy to overcome repulsive forces due to their electrical charges. At such temperatures the gaseous atoms in the fusion fuel become completely ionised. The resulting mixture of positively charged ions and negatively charged electrons is very different from a normal gas or liquid and is given a special name – plasma.

Yellow magnetic field lines confine hot plasma

Because a plasma is a mixture of charged particles, it can be controlled and influenced by magnetic fields. The charged plasma particles can move easily along magnetic field lines, but not across them. This means that the loss of particles and energy from the plasma can be strongly reduced perpendicular to the magnetic field: with the right shape of magnetic field the plasma can be confined.

The confining magnetic fields are generated by superconducting magnets located around the reactor chamber and by electrical currents flowing in the plasma itself. By its very nature, plasma is an excellent conductor of electricity.

Early fusion experiments used linear reactor designs, usually in a cylindrical vessel with a magnetic field parallel to the main axis. These designs suffered from a loss of plasma particles and energy along the magnetic field lines and out through the ends of the vessel and could not achieve the levels of thermal insulation required for fusion.

If the cylindrical vessel and the straight magnetic field lines are bent around to join the two ends, the field lines close on themselves, the plasma particles can escape less easily to the vessel walls, and thermal insulation is improved. One of the closed configurations obtained in this way is in the shape of a doughnut or ‘torus’. Several toroidal configurations for magnetic confinement devices have been studied – the most advanced of these is called the ‘tokamak ’.

Alternative toroidal devices also being studied include the ‘stellarator’, the ‘reversed field pinch ’ and the ‘spherical tokamak ’ configurations.

The different configurations and capabilities of the various plasma devices within the European fusion programme allow studies into:

  • plasma energy confinement;
  • transport and control of impurities in the plasma.
  • control of instabilities in the plasma
  • reactor plasma operational scenarios;
  • diagnostics to measure the plasma properties;
  • the control of steady state plasmas;
  • the benefits of different magnetic confinement configurations;

Hot plasma

Hot plasma

A vigorous programme of theoretical plasma physics and modelling is pursued in the European Associations. This supports all the experimental topics mentioned above, providing tools for the analysis of existing experiments and helping in the development of new plasma operation scenarios. Also, the development of first-principle-based theories is an essential part of the programme, with the aim of providing a stronger predictive capability to the models and a deeper understanding of the complexity of the physical phenomena.

Working with plasmas

To produce controlled fusion on Earth, three conditions in the plasma are needed: high temperature >100 million °C, a particle density of ~1022 particles /m3, and both of these conditions to coincide for around one second.

To control these conditions, scientists must know a lot about the plasma, for example, how well is it heated, how are particles lost from the plasma, how stable is the plasma in the magnetic field, how are the plasma particles interacting with the magnetic field and how can unwanted particles be prevented from traveling back into the plasma. To answer these questions physicists must design specialised diagnostic tools for measurement as well as equipment to heat and control the plasma.

Tore Supra © Image: CEA

Tore Supra
© Image: CEA

A major challenge in fusion research is to maintain the temperature of the plasma for the required amount of time. The plasma is constantly being cooled by impurities picked up from the reactor vessel wall. Therefore, processes by which the structure surrounding the plasma is eroded and how these eroded particles are transported to the core of the plasma need to be understood.

If power is injected in to the plasma for heating, then this power must also come out and to control this, the plasma has to be allowed to touch specified areas called targets in the divertor .

A particular challenge with the ‘tokamak’ principle, is that the plasma can become unstable producing a current in the metal structure that surrounds it. This current reacts with the magnetic field producing high forces on the components. In addition, when the plasma touches the surrounding structure it can erode the surface which can reduce the lifetime of the components.

The instabilities in the plasma that cause these events must be understood and controlled. With increasing computing power it will be possible to model this behaviour and prevent them from happening in future reactors. Physicists are working on several methods for controlling these instabilities, for instance, the injection of microwave energy in a way that stabilises the outer cooler layers of the plasma.

An alternative approach to fusion involves ‘inertial confinement ’. Here ultra-short, high-power laser or particle beam pulses are used to heat the fusion fuels. The European fusion programme maintains a ‘keep in touch’ activity in this field and monitors developments.

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