Researchers Analyze 20-Million-Degree Plasma Using Femtosecond X-Ray Pulses

  • Researchers used an X-ray laser to examine how a plasma produced by high-intensity laser expands in a few femtoseconds after it is formed. 
  • The method can be used to study fusion energy, high-energy cosmic rays, and novel particle accelerators. 

If you shine an ultrahigh-intensity laser pulse at a metal piece, it would generate a plasma – an ionized gas containing nearly equal numbers of negatively charged electrons and positively charged ions.

Since plasmas are very different from those of conventional neutral gases, they are considered a ‘fourth state of matter’. They have a collective behavior, meaning they can either flow like a liquid or contain regions that are like groups of atoms attached to each other.

Studying plasmas help researchers understand how ultrashort lasers interact with solids, what happens inside stars, and could accelerate development of advanced particle accelerators to treat serious diseases like cancer.

For the first time, scientists at SLAC National Accelerator Laboratory have used an X-ray laser from a free-electron laser to examine how a plasma produced by an ultrahigh-intensity laser expands in a few femtoseconds (1015 seconds) after it is formed. Ultimately, this method can uncover minor instabilities in plasma.

Scientists have been working to exploit plasma characteristics to develop a new kind of particle accelerator for proton therapy – a cancer treatment in which charged particle is used (instead of X-rays) to kill tumor cells. The technique doesn’t harm nearby tissues and much safer than conventional radiation therapy.

The interaction between the solid material and ultra-high intensity laser produces a plasma, and throughout the process, a continuous stream of protons is ejected from the back side of the material.

The aim is to use these streams to eradicate tumor cells. However, forming these proton streams in an effective and reliable manner requires a detailed insight into how plasma alters as it grows. Different modes of plasma instabilities could occur due to ions traveling back and forth and a complex stream of electrons within the plasma.

How Did They Probe Plasma Changes?

Since plasmas are too small and they occur on astonishingly fast time scales, it’s very difficult to analyze plasma changes. To accurately perform this task, the authors used ultrashort, high-power optical laser pulse to form plasma and X-ray free-electron laser to investigate it.

At the MEC (matter in extreme conditions) equipment, they created immensely dense and hot matter that mimics the harsh conditions of planets and matters. According to the simulations, authors reached a record temperature for matter subjected to the laser: 20,000,000°C. To put this into context, Sun’s core temperature is nearly 5,500°C.

Reference: Physical Review X | doi:10.1103/PhysRevX.8.031068 | SLAC National Accelerator Lab

Million-Degree Plasma using femtosecond X-raySchematic of the experimental setup | Courtesy of researchers 

The authors made materials that had raised silicon bars, like knuckles sticking out from the fist. They shined optical pulses on the material and discovered that small amount of plasma stocked up between the knuckles, within hundreds of femtoseconds. Then they used a unique type of scattering of X-ray pluses to observe what’s going inside the plasma and trace its evolution.

How It Is Useful?

The method will allow researchers to better understand the instabilities of plasma, which will further help them to generate reliable sources of protons for cancer treatment with tiny footprints. It can also be used to study fusion energy, lab astrophysics (including high-energy cosmic rays) and other kinds of particle accelerators.

Read: New Electro-Optic Laser Emits 30 Billion Pulses Per Second

In the future, they will try to produce jets of cosmic ray particles (similar to active galactic nucleus jet – the largest known particle accelerator) in a lab to examine the process of formation of instabilities and answer how it actually happens.

Written by
Varun Kumar

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