Jets of charged particles 鈥� similar to those that spew from stars and galaxies 鈥� 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 of thousands of light years in length.
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鈥檚 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.
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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 鈥� lasting just 20 millionths of a second 鈥� it was difficult to understand the underlying physics.
鈥淧eople think magnetic fields accelerate the jet, but I don鈥檛 think anyone had any great models on why the jets are skinny,鈥� says Bellan.
Spider legs
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.
鈥淥riginally we thought a big cloud of plasma would form 鈥� we didn鈥檛 expect all the detail we saw,鈥� Bellan told New 杏吧原创.
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 鈥� forming 鈥渟pider legs鈥�.
The formation soon straightened into a jet because of a simple law of physics 鈥� currents flowing in the same direction attract each other, while currents flowing in opposite directions repel each other.
Parallel current
So the streams of charged particles in the plasma flowing out of holes in the central disc 鈥� lines of parallel current 鈥� drew together. This strong central beam repelled the spider鈥檚 鈥渇eet鈥�, where current flowed back towards the disc.
The process eventually ripped the spider legs apart, forming a jet. You can view Bellan鈥檚 presentation describing the experiment and illustrating the apparatus .
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. 鈥淭he rubber bands squeeze down on the plasma and give it a shape,鈥� says Bellan.
As the plasma shoots along, the band-like magnetic fields move with it. 鈥淚t鈥檚 as if you were sliding rubber bands along the axis of a paper tube,鈥� he explains. 鈥淪o many magnetic field lines are squeezing it down, it gets long and skinny.鈥�
Operating in a vacuum chamber, Bellan鈥檚 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.
A paper on the results will appear shortly in Physical Review Letters.
