Plasma Confinement by Magnetic Fields

Principle

Left to itself, a plasma - like a gas - will occupy all the geometrical space available, because of the collisions between the particles. Magnetic fields can confine a plasma, because the ions and electrons of which it consists will follow helical paths around the magnetic field lines.

If a vessel containing plasma is placed in a rectilinear magnetic field, the particles of plasma cannot reach the side walls, but they will strike the ends of the vessel. To prevent the particles from coming into contact with the material walls in this way, two types of magnetic configuration have been studied :

  • Linear Configurations,
    in which the intensity of the magnetic field is increased at the ends of the container so that the particles are reflected by the ´magnetic mirrorª before they can come into contact with any material. Unfortunately particle collision effects render the system liable to high particles losses at the mirror points and such systems are no longer being considered as potential reactors;
  • Toroidal Configurations,
    in which the risk of losses is removed by curving the magnetic lines around to form a closed loop. Theoretical study of particle trajectories shows that, if the particles are to be confined, the toroidal field must have superimposed upon it a field component perpendicular to it (i.e. a poloidal field). The force lines of the total field thus become spiral (helical) paths, along and around which the plasma particles are guided.

There are several types of toroidal confinement system, each with its own method of producing helical magnetic field lines. The main types are :

In a Tokamak, the toroidal field is created by a series of coils evenly spaced around the torus, and the poloidal field is created by a strong electric current flowing through the plasma.

In a Stellarator, the helical lines of force are produced by a series of coils which may themselves be helical in shape. No current is induced in the plasma.

In a Reversed Field Pinch (RFP) device, the toroidal and poloidal components of the field are created as in a tokamak, except that the current flowing through the plasma is much stronger than in a tokamak with the same toroidal field. The magnetic fields are set up on a time scale such that they undergo a spontaneous internal reorganization,and the direction of the toroidal field within the plasma is reversed.

Plasma Heating in Toroidal Configurations

In tokamaks and RFP devices, the current flowing through the plasma and creating the poloidal component of the magnetic field also serves to heat the plasma by the Joule effect, until a temperature of about 10 million degrees is reached. Beyond that point the resistivity of the plasma is too low for there to be significant dissipation, so additional heating systems have been developed to bring the plasma to temperatures necessary for fusion. (In the case of stellarators, these heating systems have to supply all the energy needed, since in this magnetic configuration no current flows within the plasma.)

Diagram showing the various methods of heating plasma.

Three additional heating methods are used :

Neutral Beam Injection Heating :

An ion beam, created and accelerated outside the confinement device, is neutralized before entering the vacuum vessel. The neutral atoms are ionized in the plasma and confined by the magnetic field. Collisions redistribute the energy and the temperature of the plasma increases.

Rear view of a neutral beam injector for heating the plasma in the JET tokamak.
Radio Frequency Heating :

Plasma can absorb the energy of electromagnetic waves at its own characteristic frequencies (in particular the cyclotronic frequencies of the ions and electrons). Antennae fed by powerful wave generators line a part of the reactor wall. The frequency selected determines the type of particle which will be heated and the region where the wave will be absorbed and thus where heating will occur.

Heating and current-generating antenna for the FTU tokamak (Euratom-ENEA, Frascati, Italy).

Adiabatic Compression of the Plasma :

This method involves moving the plasma from a region affected by a weak magnetic field towards a region where there is a strong magnetic field. This is achieved by gradually increasing the vertical component of the magnetic field. As this involves a pulsed mode of operation, which makes considerable technical demands on the machine, it is little used. Only TFTR (Princeton, USA) has used it in recent times.


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