Fusion Concepts

At the high temperatures needed to produce significant fusion power some or all of the electrons in the atoms of the fuel become detached. This creates an electrically conducting fluid comprising electrons and partially or fully ionised ions, which is called a plasma.  As explained on the Lawson Criterion page, for net power to be produced by a fusion system the fuel needs to be contained for sufficient time at sufficiently high temperature and density.  This is not possible unless measures are taken to make the energy and particle diffusion of the electrons and ions acceptably small.  There are three approaches to achieving the required conditions:

Magnetic Confinement

If a magnetic field is applied to a plasma, the ions and electrons are forced into helical orbits around the magnetic field lines.  This enormously reduces the diffusion of energy and particles perpendicular to the field lines and makes it easier to achieve the required temperature and density.  However, particle and energy transport are still high parallel to the magnetic field. The high losses from the ends of linear magnetic systems (e.g. Z pinches) may be acceptable if the plasma duration is short but these are really examples of magneto-inertial concepts discussed below. In magnetic confinement systems the losses are reduced in one of two ways by:

• making the field lines form closed loops so there are no ends. This is typically achieved by making the magnetic confinement system toroidally symmetric (as in tokamaks and toroidal pinches) or helically symmetric (as in stellarators).
• increasing the magnetic field strength at the ends of an open system, which results in many of the particles being reflected by the magnetic ‘mirror’ effect.

However, some particle and energy losses still occur because collisions between particles results in them moving perpendicular to the field lines. In magnetic mirrors, collisions also result in initially confined particles being lost from the open ends.

Inertial Confinement

Inertial confinement concepts use fast compression of a target containing the fusion fuel to ~1000 times solid density to reach very high temperatures of ~100 millions of degrees kelvin, (100MK) for a short time.  The very high pressure needed is produced by the outer surface of the target material being very rapidly ablated, compressing the core by its rocket effect. After reaching peak compression, and hopefully igniting, the core expands on a short timescale determined by its inertia, which is the origin of the term inertial confinement.  The energy used to compress the target is normally from a very short (~1ns), very high power (~1PW) pulse from a set of laser beams but light and heavy ion beams have also been proposed.  This energy can either be used to ablate the target surface directly or indirectly by heating the interior of a metal shell surrounding the target to very high temperatures to produce x-ray radiation that then ablates the target to compress it.  The direct approach minimises the amount of energy needed but requires very uniform illumination of the target by the laser beams, which is difficult to achieve due to diffractive effects and the finite number of laser beams.  The indirect approach requires greater energy but has the advantage that the black body x-ray radiation in the metal shell can produce more uniform illumination of the target. High power efficiency lasers are desirable to minimise power consumption and repetition rates of a few Hz are typically needed.

An alternative inertial fusion concept utilises an electromagnetically accelerated projectile hitting a target similar to that for inertial systems, wherein the process uses shockwaves to compress the target to create fusion.

Magneto-inertial Confinement

As the name implies, magneto-inertial confinement is a hybrid of the two approaches.  The idea is that the application of a magnetic field can help improve the confinement of the target plasma and hence achieve better performance.  The term now covers a very wide range of concepts including:

• applying a magnetic field to a primarily inertial fusion concept.
• using the pinch effect to compress a linear plasma to high density on ~µs timescales.
• compressing a magnetically confined plasma on ~ms timescales to achieve magnetic fields and plasma densities that otherwise could not be possible.