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A magnetic trap captures elusive ultracold plasma

Researchers discovered a new way to keep cold plasma in place for a short time.

Trapping a plasma is on par with capturing a cloud and pinning it down, or holding a moonbeam in your hand in terms of difficulty. Physicists, on the other hand, have developed a new method for magnetically bottleing ultracold plasma in the lab.

Researchers report that using magnetism to trap frigid plasmas in which particles move slowly could enable physicists to study plasma activity in slow motion. This could reveal new information about how plasmas behave in even hotter, more chaotic conditions, such as the roiling interiors of fusion reactors or the whirling cores of stars.

“It takes a lot of tricks” to stopper a cold plasma, says physicist Thomas Killian of Rice University in Houston.

He and his colleagues began by boiling a lump of strontium metal and passing the vapor through a drain. The atoms were effectively cooled to three-thousandths of a degree above absolute zero (–273° Celsius) by light from a laser beam.

The researchers used a second laser to knock an electron off each atom, resulting in a plasma of negatively charged electrons and positively charged strontium ions. “We have to completely isolate this plasma,” Killian says. “If it bumps into a wall, [the particles] will just stick to the wall … or the wall will heat it up,” because even room-temperature equipment is much warmer than the plasma.
This ionized gas could not be held in a regular bottle.

The plasma will dissipate in tens of microseconds if left out in the open. As a result, Killian’s team developed plasma by sandwiching two coils of electric current between opposing magnetic fields. For up to 500 microseconds, the charged particles’ equal and opposite magnetic forces held the plasma together.

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