* Astronomy

Jets of charged particles have been caught in the act of forming for the first time in a new laboratory experiment.
The research may help astrophysicists understand how magnetic fields confine jets over hundreds or thousands of light years in length - similar to those that spew from stars and galaxies.

Astrophysical jets are among the largest and most energetic objects in the universe. The matter inside them travels at nearly the speed of light from colossal black holes at the centres of galaxies. Smaller jets spew at lower speeds from young stars and even brown dwarfs.

Theorists believe they arise when material falls onto a massive spinning object from a surrounding disc, before being funnelled outwards from the object's poles by magnetic fields generated by the object and disc.
The exact mechanism is unclear, but a couple of research groups in the world are using magnetic fields to build small-scale jets in the laboratory.

One of those groups, led by plasma physicist Paul Bellan of the California Institute of Technology in Pasadena, US, created a jet in the laboratory about three years ago. But the formation process was so fleeting - lasting just 20 millionths of a second - it was difficult to understand the underlying physics.



"People think magnetic fields accelerate the jet, but I don't think anyone had any great models on why the jets are skinny" - Paul Bellan.

Now, the same team has captured a jet in the process of forming by improving its setup, including the addition of a super-fast camera. The pictures that emerged show a much more structured evolution than researchers predicted and reveal why the jets are so streamlined.

"Originally we thought a big cloud of plasma would form - we didn't expect all the detail we saw" - Paul Bellan.

In the experiment, a high voltage was applied between a metal disc and a surrounding metal ring. A magnetic coil behind the disc generated magnetic fields that looped through the ring-disc system.
So when gas was pumped through holes in the disc, it became ionised and travelled along the curved magnetic field lines to meet ionised gas pumped through holes in the ring - forming "spider legs".
The formation soon straightened into a jet because of a simple law of physics - currents flowing in the same direction attract each other, while currents flowing in opposite directions repel each other.

So the streams of charged particles in the plasma flowing out of holes in the central disc - lines of parallel current - drew together.
This strong central beam repelled the spider's "feet", where current flowed back towards the disc.

The process eventually ripped the spider legs apart, forming a jet.

An alternative view of the process involves magnetic fields that curl around the streams of plasma, as they would around any flowing electrical current. As the streams draw together, the magnetic fields that surround them strengthen, like rubber bands tightening.

"The rubber bands squeeze down on the plasma and give it a shape" - Paul Bellan.

As the plasma shoots along, the band-like magnetic fields move with it.

"It's as if you were sliding rubber bands along the axis of a paper tube. So many magnetic field lines are squeezing it down, it gets long and skinny" - Paul Bellan.
Operating in a vacuum chamber, Bellan's group was able to make jets up to a third of a metre long that travelled at between 10 and 50 kilometres per second. Bigger jets would require a bigger power supply.

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