Argus was a two-beam high power infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 neodymium

Neodymium

Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...

 doped silica glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 with a 20 cm (7.9 in) output aperture built at Lawrence Livermore National Laboratory

Lawrence Livermore National Laboratory

The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...

 in 1976 for the study of inertial confinement fusion

Inertial confinement fusion

Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....

. Argus advanced the study of laser-target interaction and paved the way for the construction of its successor, the 20 beam Shiva laser

Shiva laser

The Shiva laser was a powerful 20-beam infrared neodymium glass laser built at Lawrence Livermore National Laboratory in 1977 for the study of inertial confinement fusion and long-scale-length laser-plasma interactions. The device was named after the multi-armed form of the Hindu god Shiva, due...

.

It was known from some of the earlier experiments in ICF that when large laser systems amplified their beams beyond a certain point (typically around the gigawatt level), nonlinear optical

Nonlinear optics

Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...

 effects would begin to appear due to the very intense nature of the light. The most serious effect among these was "Kerr lensing

Kerr effect

The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...

", where, because the beam is so intense, that during its passage through either air or glass the electric field of the light actually alters the index of refraction of the material and causes the beam at the most intense points to "self focus" down to filament like structures of extremely high intensity. When a beam collapses into extremely high intensity filaments like this, it can easily exceed the optical damage threshold

Catastrophic optical damage

Catastrophic optical damage , or catastrophic optical mirror damage , is a failure mode of high-power semiconductor lasers. It occurs when the semiconductor junction is overloaded by exceeding its power density and absorbs too much of the produced light energy, leading to melting and...

 of laser glass and other optics, severely damaging them by creating pits, cracks and grey tracks through the glass. These effects became so severe after just the first few amplification stages of early lasers, that it was seen as essentially impossible to exceed the gigawatt level for ICF lasers without destroying the laser itself after just a few shots.

In order to improve the quality of the amplified beams, LLNL had started experimenting with the use of spatial filter

Spatial filter

A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of coherent light or other electromagnetic radiation. Spatial filtering is commonly used to "clean up" the output of lasers, removing aberrations in the beam due to imperfect, dirty,...

s in the single-beam Cyclops laser

Cyclops laser

Cyclops was a high-power laser built at the Lawrence Livermore National Laboratory in 1975. It was the second laser constructed in the lab's Laser program, which aimed to study inertial confinement fusion ....

, built the previous year. The basic idea was to extend the laser device into a very long "beamline", over which any imperfections that accumulated in the beam would be successively removed after every amplification stage. A series of tubes with lenses on either end would focus the light down to a point (the focal point) where it would pass through a pinhole which would reject stray unfocused light, smoothing the beam and eliminating the high intensity spots which would have otherwise been further amplified causing damage to down-beam optics. The technique was so successful on Argus it was often referred to as being "the savior of laser ICF".

After the success of Cyclops in beam smoothing, the next step was to further increase the energy and power in the resulting beams. Argus used a series of five groups of amplifiers and spatial filters arranged along the beamlines, each one boosting power until it reached a total of about 1 kilojoule and 1-2 terawatts per beam. These intensities would have been impossible to achieve without the use of spatial filtering. Argus was designed primarily to characterize large laser beamlines and laser-target interactions, there was no attempt to actually achieve the fusion ignition state in the device as this was understood to be impossible at the energies Argus was capable of delivering. Argus however, was used to further explore higher yields of the so called "exploding pusher" type targets and to develop x-ray diagnostic cameras to view the hot plasma in such targets, a technique crucial to characterization of target performance on later ICF lasers.

Argus was capable of producing a total of about 4 terawatts of power in short pulses of up to about 100 picoseconds, or about 2 terawatts of power in a longer 1 nanosecond pulse (~2 kilojoules) on a 100 micrometer

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 diameter fusion fuel capsule target. It became the first laser to perform experiments using X-rays produced by irradiating a hohlraum

Hohlraum

In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material...

. The reduced production of hard X-ray energy via the production of hot electrons while using frequency doubled and tripled laser light (as opposed to the infrared light directly produced by the laser itself) was first noticed on Argus. This technique would also be later validated in the direct drive mode (at both the LLE

Laboratory for Laser Energetics

The Laboratory for Laser Energetics is a scientific research facility which is part of the University of Rochester's south campus, located in Brighton, New York. The lab was established in 1970 and its operations since then have been funded jointly; mainly by the United States Department of...

 and Novette laser

Novette laser

Novette was a two beam neodymium glass testbed laser built at Lawrence Livermore National Laboratory in about 15 months throughout 1981 and 1982 and was completed in January 1983. Novette was made using recycled parts from the dismantled Shiva and Argus lasers and borrowed parts from the future...

) and subsequently used to enhance laser energy to target plasma coupling efficiency in experiments on nearly all subsequent laser inertial confinement devices. Argus was shutdown and dismantled in September 1981. Maximum fusion yield for target implosions on Argus was about 10⁹ neutron

Neutron

The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s per shot.

External links

• http://www.llnl.gov/50science/lasers.html
• http://www.osti.gov/bridge/servlets/purl/16710-UOC0xx/native/16710.pdf
• http://adsabs.harvard.edu/abs/1978ApOpt..17..999S