The Shiva laser was a powerful 20-beam 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...

 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...

 (silica glass) 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...

 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 1977 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....

 (ICF) and long-scale-length laser-plasma interactions. The device was named after the multi-armed form of the Hindu

Hinduism

Hinduism is the predominant and indigenous religious tradition of the Indian Subcontinent. Hinduism is known to its followers as , amongst many other expressions...

 god Shiva

Shiva

Shiva is a major Hindu deity, and is the destroyer god or transformer among the Trimurti, the Hindu Trinity of the primary aspects of the divine. God Shiva is a yogi who has notice of everything that happens in the world and is the main aspect of life. Yet one with great power lives a life of a...

, due to the laser's multi-beamed structure. Shiva was instrumental in demonstrating a particular problem in compressing targets with lasers, leading to a major new device being constructed to address these problems, the Nova laser

Nova laser

Nova was a high-power laser built at the Lawrence Livermore National Laboratory in 1984 which conducted advanced inertial confinement fusion experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", a chain reaction of nuclear...

.

Background

The basic idea of any ICF device is to rapidly heat the outer layers of a "target", normally a small plastic sphere containing a few milligrams of fusion fuel, typically a mix of deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 and tritium

Tritium

Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

. The heat burns the plastic into a plasma

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

, which explodes off the surface. Due to Newton's Third Law, the remaining portion of the target is driven inwards, eventually collapsing into a small point of very high density. The rapid blowoff also creates a shock wave

Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 that travels towards the center of the compressed fuel. When it meets itself in the center of the fuel, the energy in the shock wave further heats and compresses the tiny volume around it. If the temperature and density of that small spot is raised high enough, fusion reactions will occur.

The fusion reactions release high-energy particles, which collide with the high density fuel around it and slow down. This heats the fuel further, and can potentially cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel – high enough density and temperature – this heating process can result in a chain reaction

Chain reaction

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....

, burning outward from the center where the shock wave started the reaction. This is a condition known as "ignition", which can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy.

To date most ICF experiments have used lasers to heat the targets. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles, as well as creating a suitable shock wave. The laser beams must also be focussed evenly across the target's outer surface in order to collapse the fuel into a symmetric core. Although other "drivers" have been suggested, lasers are currently the only devices with the right combination of features.

Description

Shiva incorporated many of the advancements achieved on the earlier Cyclops

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 ....

 and Argus laser

Argus laser

Argus was a two-beam high power infrared neodymium doped silica glass laser with a output aperture built at Lawrence Livermore National Laboratory in 1976 for the study of inertial confinement fusion...

s, notably the use of amplifiers made of Nd:glass slabs set at the brewster angle and the use of long vacuum 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 to "clean" the resulting laser beams. These features have remained a part of every ICF laser since, which leads to long "beamlines". In the case of Shiva, the beamlines were about 30 m long.

Prior to "firing", the laser glass of the Shiva was "pumped"

Laser pumping

Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

 with light from a series of Xenon flash lamps fed power from a large capacitor

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 bank. Some of this light is absorbed by the neodymium atoms in the glass, raising them to an excited state

Excited state

Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....

 and leading to a population inversion

Population inversion

In physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...

 which readies the lasing medium for amplification of a laser beam. A small amount of laser light, generated externally, was then fed into the beamlines, passing through the glass and becoming amplified through the process of stimulated emission

Stimulated emission

In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...

. This is not a particularly efficient process, only a small amount of the energy stored in the glass is dumped into the beam (about 20%) and the "pumping" wastes a considerable amount of power by generating light that the neodymium cannot absorb. In total, around ~1% of the electricity used to feed the lamps ends up amplifying the beam on most Nd:glass lasers.

After each amplifier module there was a spatial filter which was used to smooth and "clean" the beam of any nonuniformity or power anisotropy which had accumulated due to nonlinear effects of intense light passage through air and glass. The spatial filter is held under vacuum in order to eliminate the creation of plasma at the focus (pinhole).

After the light had passed through the final amplifier and spatial filter it was then used for experiments in the target chamber, lying at one end of the apparatus. Shiva's 20 beamlines delivered a ~.5 to 1 nanosecond pulse of 10.2 kJ of infrared light at 1062 nm wavelength, or smaller peak powers over longer times (3 kJ for 3 ns).

By today's standards, Shiva was fairly inexpensive. The entire device, including test equipment and buildings, cost about $25 million when it was completed in 1977 (~81 million 2005 dollars).

Shiva and ICF

Shiva was never expected to reach ignition conditions, and was primarily intended as a proof-of-concept system for a larger device that would. Even before Shiva was completed, the design of this successor, then known as Shiva/Nova, was well advanced. The Shiva target chamber utilized high-resolution, high-speed optical and X-ray instruments for the characterization of the plasmas created during implosion.

When experiments with targets started in Shiva in 1978, compression was ramped upward to about 50 to 100 times the original density of the liquid hydrogen, or about 3.5 to 7 g/mL. For comparison, lead has a density of about 11 g/mL. While impressive, this level of compression is far too low to be useful in an attempt to reach ignition. Studies of the causes of the lower than expected compression led to the realization that the laser was coupling strongly with the hot electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s (~50 KeV) in the plasma which formed when the outer layers of the target were heated, via stimulated raman scattering. John Holzrichter, director of the ICF program at the time, said:

The laser beam generates a dense plasma where it impinges on the target material. The laser light gives up its energy to the electrons in the plasma, which absorb the light. The rate at which that happens depends on the wavelength and the intensity. On Shiva, we were heating up electrons to incredible energies, but the targets were not performing well. We tried a lot of stuff to coax the electrons to transfer more of their energy to the target, with no success.

It was earlier realized that laser energy absorption on a surface scaled favorably with reduced wavelength, but it was believed at that time that the IR generated in the Shiva Nd:glass laser would be sufficient for adequately performing target implosions. Shiva proved this assumption wrong, showing that irradiating capsules with infrared light would likely never achieve ignition or gain. Thus Shiva's greatest advance was in its failure, a not entirely obvious example of the null result

Null result

In science, a null result is a result without the expected content: that is, the proposed result is absent. It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a result that does not support the hypothesis...

.

ICF research turned to using an "optical frequency multiplier" to convert the incoming IR light into the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 at about 351 nm, a technique that was well known at the time but was not efficient enough to be worthwhile. Research on the GDL laser at the Laboratory for Laser Energetics

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...

 in 1980 first achieved efficient frequency tripling techniques which were then used next (for the first time at LLNL) on Shiva's successor, the 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...

. Every laser-driven ICF system after Shiva has used this technique.

On January 24 of 1980, a 5.5 magnitude earthquake at Livermore shook the facility enough to shear fist-sized bolts off Shiva; repairs were made and the laser was subsequently put back online a month later. Many experiments including testing the "indirect mode" of compression using hohlraums continued at Shiva until its dismantling in 1981. Shiva's target chamber would be reused on the 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...

. Maximum fusion yield on Shiva was around 10¹⁰ to 10¹¹ 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.