Fusion energy is generated when the nuclei of two light elements in the periodic table combine to form a single nucleus of a heavier element.  Energy is released in this process depending on the particular elements that are interacting.  However, nuclei which are positively charged repel each other with an electrostatic force of repulsion which increases the closer they get together.  In order to get close enough together for the fusing into a single nucleus to occur the initial particles must have very high energies to overcome the electrostatic repulsion, requiring very high temperatures for the reacting species.  At these high temperatures the electrons are completely stripped off of the nuclei, a state which is called an ionized plasma.  This requires some mechanism for heating the reacting species.   In order to have a high enough reaction rate the density of the species must also be sufficient that net energy is generated in the time scale of the heating and confinement of the plasma.

These requirements can be embodied in a triple product of density, temperature and time [reference – triple product pop up], which defines the minimum combination of density, time and temperature required to have net energy gain.  The basic requirement is to achieve conditions even more extreme than in the centre of the sun and stars where fusion is the primary source of most of the radiated energy generated in the Universe.  However, a minimum temperature of the order of 50-million degrees Celsius is required in order for fusion reactions to occur sufficiently and rapidly for fusion power systems.

There are a number of possible approaches to fusion following several different technology paths.  There are two broad approaches to fusion being pursued currently.  These are

  • Magnetic Confinement Fusion (MCF)
  • Inertial Confinement Fusion (ICF)

The Magnetic Confinement Fusion (MCF) approach uses very powerful large scale magnetic fields generated by field coils to confine a low density hot plasma for long durations of seconds to hours in order to obtain net energy gain.

The Inertial Confinement Fusion (ICF) approach uses very short and intense laser or ion beams to heat up small fuel capsules to the hot dense conditions, well beyond those in the sun, in order for fusion reactions to occur in a tiny fraction of a second releasing the energy in a burst after each heating pulse, repeated over and over again at many times a second, like an internal combustion engine.

The Magnetized Target Fusion (MTF) approach uses a magnetic field generated in the interaction process itself to assist in confining the plasma, employing electrical, particle or laser heater beams in order to create and heat the plasma.

Most approaches to fusion are pursuing the use of isotopes of hydrogen, namely deuterium and tritium, which have one and two extra neutrons in the nucleus, respectively.  These are the two nuclei with lowest threshold to achieve energy gain and thus the lowest triple product requirement.  Deuterium is extremely abundant in the world making up 1 part in 6000 of all natural hydrogen, such as in seawater.  Tritium must be fabricated in the reactor itself by bombarding lithium with the accelerated neutrons produced in the fusion reactions.  The amount of fuel required for one year`s operation of a gigawatt  (GW) electrically generating plant is of the order of a few hundred kilograms each which can easily be supplied by extracting deuterium and lithium from seawater.

There are also a number of privately funded groups pursuing alternative approaches to fusion which are either related to the above mainstream approaches or take a different approach.   Some of these involve much higher magnetic fields, the use of intense electron beams or extreme current pulses to heat or control the plasma, extreme laser intensities to enhance the heating of the plasma or the use of alternative fuels such as hydrogen and boron which have thresholds or triple products in order to reach energy breakeven conditions than deuterium and tritium.