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Some of the most awe-inspiring scientific images are of astrophysical objects – just think of all the beautiful pictures of galaxies and star clusters taken by various telescopes over the years. Astrophysicists harvest huge amounts of high-quality observational data with such telescopes with the aim of comprehending how and why the universe was formed and how it has changed since its creation.
But observing and studying the phenomena responsible for images of such interest and importance is not enough. The time-scale over which the observations are made in incredibly short in comparison to the lifetime of the astrophysical object and such inspections do not allow for exhaustive testing of models to help create a complete picture of how the system has and will evolve over time.
Material in a high-energy density (HED) state is commonplace in astrophysics, but directly measuring such systems is impossible and involves temperatures of millions of degrees Celsius, pressures exceeding millions of bars and densities up to hundreds of grams per cubic centimeter.
Astrophysicists have turned to numerical modeling and computer simulations to help fill in some of the missing pieces in the story. While successful, they still don’t help to paint a complete picture. But what if scientists could perform experimental simulations in a controlled laboratory? Materials in HED conditions enter a plasma regime where atoms collide with enough energy to strip electrons from an atom’s nucleus to form a gas of charged ions and free electrons. Such regimes are complex, but the advent of high-power lasers has made laboratory experiments possible and offer many advantages over simple observation and computer modeling.
However, laser energy is not directly useful for HED experiments - it must be converted to other forms:
- To kinetic energy by irradiating a foil, which evaporates material and produces a reactive force that drives a high-pressure shock wave into the bulk material.
- To X-rays by irradiating the inside of a high-atomic-number cavity: the light is absorbed at the wall where the material is heated enough to radiate much of the absorbed energy as X-rays. This heats other wall areas which reradiate similarly. The X-ray field is partly trapped and smoothed and can be used to drive an experimental package mounted from the wall.
- To quicken energetic particles that warm a package directly or through induced electrical return currents.
A team from the University of York, UK, have used high-intensity lasers and wire-array z-pinches to produce energy densities of magnitudes only ever seen before in astrophysical phenomenon. They carefully designed laboratory experiments that, if interpreted correctly, could deliver valued insight into some of the unsettled problems in astrophysics. A combination of the three – direct observation, numerical simulation, and experimental work – will lead to a greater understanding of the universe.
The York team undertook experiments to study the physics of Young Stellar Object (YSO) jets; their aim was to produce plasma jets that may be scalable to YSO jets, by irradiating a target with a laser to yield a high-velocity, low-intensity plasma. They used two Vulcan beams – one for each V-foil target; two thin foils were placed at an angle to the central axis, and the laser incident from the outside of the V with a separation between the focal spots. The plasma expanded from the rear of the foil in a direction normal to the target surface and as it meets the axis there is a sluggishness of the flows in the direction perpendicular to the axis, causing heating. The axial factor of the momentum is preserved, and the result is a bulk flow along the central axis, moving away from the collision region. Such a design means a large number of variables can be changed easily - foil material/thickness, angle between the foils, and the separation between the two focal spots - with the intention of finding the optimum condition for jet production
Measurements taken were used to determine plasma expansion speed, jet shape and to infer the electron density. Results showed a similarity between the laboratory and astrophysical jets that were consistent with previous computational works and indicates interstellar medium could play a role in collimation of YSO jets.
The same team, in 2006, also reproduced a young supernova remnant in the laboratory under the conditions of temperature and pressure that are met in extreme stellar environments. They employed two millimeter‐sized counter‐streaming laser to produce plasma in a strong transverse magnetic field to achieve scaling.
These are just two examples of how lasers are revolutionizing astrophysics, by allowing scientists to carry out experiments in the laboratory to help improve the understanding of our universe. There are many more, and the number is likely to grow as improvements are made in laser capabilities.
Sources and Further Reading
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