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Probing the mechanisms behind some of the universe's most violent events

A team of scientists from the University of Michigan and Lancaster University have used the Vulcan laser at STFC’s Central Laser Facility (CLF) to investigate a phenomenon known as magnetic reconnection in which magnetic fields suddenly deform, break and reconnect – accelerating charged particles within plasmas as they do so. Magnetic reconnection drives some of the most violent events in the solar system, such as coronal mass ejections, and can play havoc with our Earth-bound attempts to harness the power of the Sun through nuclear fusion.

Matter, the stuff that makes up you, me and the world around us, can exist in several states. On Earth we are most familiar with it as either a solid, liquid or a gas. Less common, on Earth at least, is the fourth state of matter: plasma.

A plasma is a hot, ionised gas in which the electrons have been knocked away from their atomic nuclei to create a soup of negatively-charged electrons and positively charged ions. Plasmas are rare on Earth, but they make up some 99 per cent of the visible universe – fuelling stars, enveloping planets, and filling the near-vacuum of space.

Because they are made up of electrically-charged particles, plasmas are strongly influenced by electric and magnetic fields in a way that neutral gases are not. In fact, plasmas can have their own set of magnetic and electric fields entrapped or generated within them. Changing magnetic fields affect the way the charged particles move and vice versa, all of which creates a complex, constantly-fluctuating system that is sensitive to even minute variations.

The magnetic fields that permeate plasmas can be visualised as field lines that act a bit like railway tracks along which the charged electrons and ions are guided. Under normal conditions, just like railway tracks, the magnetic fields line run in parallel with each other and they don’t break or merge. Sometimes however, if the field lines get too close to each other, suddenly the entire pattern can change, the lines snap, reconnect and everything realigns into a new configuration. This is called magnetic reconnection.

If this were to happen on a railway, you could expect any trains travelling along the tracks to be thrown violently from them and much the same happens with the charged particles traveling along the field lines. In a plasma, as the opposing field lines cross they form an X shape, break and reconnect to the other set of field lines coming from the opposite direction. The X turns into two U shapes that push away from each other. The amount of energy unleashed can be formidable as it taps into the stored energy of the magnetic field, converts it into heat and kinetic energy, sending particles out along the field lines and accelerating them before catapulting them away.

Magnetic reconnection is responsible for some of the most violent events on the Sun's surface, such as solar flares and coronal mass ejections, in which billions of tonnes of charged-particles are hurled into space – creating spectacular aurora and disrupting communications and electronic devices here on Earth. It is believed to be involved in some of the universe’s most powerful and energetic objects and phenomena where the particles are moving at close to the speed of light like pulsars, active galactic nuclei, or gamma ray bursts. It is also believed that particles accelerated during magnetic reconnection events at these extreme objects might be the source of the mysterious cosmic rays that bombard the Earth from deep space.

In the lab, where scientists are trying to tap into the power of nuclear fusion by recreating the Sun’s solar furnace in giant donuts of magnetically-confined plasmas called tokamaks, magnetic reconnection events can reduce their control and damage the machines. All of which makes understanding how the process works of paramount importance.

Unfortunately, magnetic fields in the depths of space, reconnecting or otherwise, are not that easy to just fly up to and measure. And while theoretical models do exist, the timescales over which the energy is released has been measured to be faster than that predicted by theory. One way to improve our understanding of magnetic reconnection is to recreate it in a laboratory where conditions can be controlled to simulate the wide range of conditions in which it occurs. This is where the Vulcan laser at the Central Laser Facility (CLF) comes in.

In their experiment, Dr Louise Willingale and her team from the University of Michigan and Lancaster University used the Vulcan laser to study reconnection between two high-intensity laser pulses. By using a very high-intensity laser like Vulcan, it is possible to recreate the conditions in which magnetic reconnection can occur within the laboratory.

By firing two laser pulses at a foil target, the team are able to create two opposing magnetic fields that form around the lasers’ focal points. A charged particle beam, produced by firing a second laser onto a nearby target, is then used to probe the evolution of the magnetic fields on the main target as they interact and determine the field strength. When the two magnetic fields are close together they begin to interact and then, by observing the interaction as the magnetic fields are driven together, the team can study processes like magnetic reconnection.

The team’s initial measurements have shown very fast formation of magnetic fields and a geometry and evolution that matched what they expected from magnetic reconnection. They hope to return to CLF’s facilities so that they can continue the experiments and look at how the fields evolve in greater detail by obtaining extra measurements, with less time between each ‘snapshot’. They also hope to be able look for the charged particles that are expected to be accelerated during magnetic reconnection.

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Last updated: 26 September 2019


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