Piping hot! The Astra laser was used create ‘fibre optic cables’ of sun-surface temperature plasma.
A team of scientists from Oxford, Liverpool, Maryland and the Central Laser Facility (CLF) have used the Astra Laser to develop an innovative way to keep laser beams focused over long distances. By firing a laser into hydrogen gas, they have created ‘fibre-optic cables’ made of plasma hotter than the surface of the Sun.
Diagram of the experiment showing how the plasma waveguide keeps the laser focused for longer.
Since lasers were first invented in 1960, their ability to deliver highly-focused beams of intense light has meant that they have become a byword for precision. But even lasers have their limits when it comes to really extreme focusing. To generate the extreme intensities that are needed for some scientific laser applications, such as plasma experiments, lasers must be focused down to incredibly tiny spots – often far less than the width of a human hair. When you do this, however, thanks to a process called diffraction, the laser light immediately starts to spread out again, which means the time (and distance) over which it is focused is very small indeed.
Finding a way to create highly focused beams of laser light that remain focused over long distances has been the subject of research for almost 30 years. It is a problem that the team using CLF’s Astra laser has come one step closer to solving.
The team aimed to focus a laser pulse down to a tiny spot and then, before it could spread out again, capture that highly-intense pulse and guide it over a very long distance. To do this, they split a laser into two beams. The first of these beams was fired into a container of hydrogen gas to create an incredibly hot plasma (ionised gas in which the electrons are knocked off the hydrogen atoms to create a soup of negatively-charged electrons and positively-charged hydrogen ions) with a temperature of over 100,000 degrees Celsius, which is hotter than the surface of the Sun.
This plasma exploded outwards at speeds approaching 30 kilometres per second – driving a blast wave into the still-cold hydrogen gas surrounding it to create a cylinder-like structure of plasma. Once this plasma tunnel, or ‘waveguide’, had formed, which took only a few billionths of a second, the team fired the second focused laser pulse into it. The second laser pulse was able to travel along the tunnel – maintaining its focus all the way along it.
The ‘tunnel’ only lasted a fraction of a second, but it was long enough to show that a highly-focused laser pulse could indeed be made to travel long distances. All that is needed to transport a second pulse is to zap the hydrogen with another laser pulse to cause the tunnel to reform.
Recalling the moment they saw the first results, Dr. Rob Shalloo, from Imperial College London, said: “It was an amazing feeling sitting in the control room and seeing that first guided laser pulse emerge out of the end of the waveguide. It was an almost perfect copy of the input pulse – demonstrating the high-quality of these new waveguides."
The team’s research could have implications for many fields of laser science and could lead to the development of the next generation of laser-driven particle accelerators.
Unlike their colossal, synchrotron brethren – such as the Large Hadron Collider (LHC), which propel particles to close to the speed of light by flinging them around massive accelerator rings – laser-driven particle accelerators use lasers to accelerate particles across extremely short distances. So, instead of requiring hundreds of square kilometres of land, such laser-driven devices could, in principle, reduce the size of large scale accelerators by as much as a factor of 1000.
Compact particle accelerators have huge potential applications in scientific research, medicine and industry. Such small-scale accelerators could find a use in hospitals in cancer therapy devices, and in industry where they could be used to take ultrafast images of components to inspect their performance and structure.
The team, which was led by Prof. Simon Hooker at the University of Oxford, hopes to continue developing this low-density plasma technique in future experiments at the CLF using the more powerful Gemini laser.
Last updated: 02 May 2019