- Magnetic Confinement tokomak
- Inertial Confinement Fusion
Extremely high temperature plasma is contained in a vacuum vessel. The vacuum is maintained by external pumps. The plasma is created by letting in a small puff of gas, which is then heated by driving a current through it.
The hot plasma is contained by a magnetic field which keeps it away from the machine walls. The combination of two sets of magnetic coils – known as toroidal and poloidal field coils – creates a field in both vertical and horizontal directions, acting as a magnetic bottle to hold and shape the plasma.
Large power supplies are used to generate the magnetic fields and plasma currents.
Plasma current is induced by a transformer, with the central magnetic coil acting as the primary winding and the plasma as the secondary winding. The heating provided by the plasma current (known as Ohmic heating) supplies up to a third of the 100 million degrees Celsius temperature required to make fusion occur.
Additional plasma heating is provided by neutral beam injection. In this process, neutral hydrogen atoms are injected at high speed into the plasma, ionized and trapped by the magnetic field. As they are slowed down, they transfer their energy to the plasma and heat it.
Radiofrequency heating is also used to heat the plasma. High-frequency oscillating currents are induced in the plasma by external coils or waveguides. The frequencies are chosen to match regions where the energy absorption is very high (resonances). In this way, large amounts of power may be transferred to the plasma.
Future Nuclear – do we need Generation IV or Fusion? Steven Cowley
The National Ignition Facility (NIF) at Lawrence Livermore Laboratory is the most developed ICF project using laser beams to induce fusion. In the NIF device, 192 laser beams focus on single point in a 10-meter-diameter target chamber called a hohlraum. A hohlraum is “a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity.”
At the focal point inside the target chamber, there is a pea-sized pellet of deuterium-tritium encased in a small, plastic cylinder. The power from the lasers (1.8 million joules) will heat the cylinder and generate X-rays. The heat and radiation will convert the pellet into plasma and compress it until fusion occurs. The fusion reaction will be short-lived, about one-millionth of a second, but will yield 50 to 100 times more energy than is needed to initiate the fusion reaction. A reactor of this type would have multiple targets that would be ignited in succession to generate sustained heat production. Scientists estimate that each target can be made for as little as $0.25, making the fusion power plant cost efficient.
OFFICE OF DEFENSE PROGRAMS 2015 Review of the Inertial Confinement Fusion and High Energy Density Science Portfolio: Volume I, May 2016