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 :
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.
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.)
Three additional heating methods are used :
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.
|Back to CD-ROM Homepage|
|Back to Fusion Homepage|
© FUSION EXPO 2003