Almost all of the observable matter in the universe is in the plasma state. Formed at high temperatures, plasmas consist of freely moving ions and free electrons. They are often called the “fourth state of matter” because their unique physical properties distinguish them from solids, liquids and gases.
Plasma densities and temperatures vary widely, from the cold gases of interstellar space to the extraordinarily hot, dense cores of stars and inside a detonating nuclear weapon. Plasma densities range from those in a high vacuum with only a few particles inside a volume of 1 cubic centimeter to 1,000 times the density of a solid.
NIF experiments are addressing two areas of plasma physics. First are studies of the phenomena created by laser beams interacting with plasma. Of particular importance are “stimulated Brillouin scattering” and “stimulated Raman scattering.” Both effects must be minimized to efficiently drive the implosion of the NIF fuel capsule in order to achieve ignition.
The second area involves using NIF to emulate other plasma phenomena occurring in nature, such as interpenetrating plasmas. For example, one of the mysteries of astrophysics is how highly organized structures such as magnetic fields stretching millions of light years can emerge from the frenetic motion of plasmas. A team of Lawrence Livermore researchers has discovered that supersonic counter-streaming (directed at each other) plasmas created by powerful lasers give rise to “self-organized” electromagnetic fields similar to those found throughout the universe.
As revealed in proton radiography images, these electromagnetic structures are oriented perpendicular to the direction of the two plasma flows, have detailed features, and are much larger and persist much longer than would be predicted from the chaotic motions of the plasma ions and electrons.
Researchers plan to use NIF to investigate whether intersecting plasmas created by lasers are capable of forming collisionless shocks (where plasma constituents pass by largely without colliding with each other) and whether these shocks could produce large-scale electromagnetic fields. Collisionless shocks appear in a wide array of exotic astronomical settings such as violent solar flares, outbursts from galaxies, and supernova remnants. Astrophysical collisionless shocks can’t be directly measured, so scientists look to laboratory experiments to better understand these objects.
NIF is the only facility capable of creating plasmas with sufficiently high density (greater than 1020 particles per cubic centimeter), high flow velocity (greater than 2000 kilometers per second), and high temperature (greater than 1 keV) to approximate the astrophysical conditions observed in space.
NIF experiments in the planning stage are designed to produce antimatter plasma at nearly the speed of light. These experiments will use the Advanced Radiographic Capability, currently under construction, which will produce more penetrating, higher energy x rays than is possible with conventional radiographic techniques. Powerful lasers are proving to be the first tools to access the antimatter plasma phenomena observed in nature.