Slashing exposure time for plasma imaging1/11/2013
Laser-generated plasmas evolve too quickly and create forces too powerful to allow for easy observation, but electrons surfing on light waves can reveal their magnetic fields 1000 times faster than ever before. By watching how plasmas grow and develop, researchers could discover how to better control them for purposes such as producing particle beams for nuclear medicine and igniting nuclear fusion.
“Our experiment is the first to exploit a relatively new electron acceleration technique to probe magnetic fields on a timescale that conventional accelerators and previous laser methods cannot reach,” said William Schumaker, a graduate student of nuclear engineering and radiological sciences (NERS) and first author on the paper, published last week in Physical Review Letters.
Electrons that pass through a magnetic field reveal its structure in the way they are deflected – because they are moving electric charges, the field bends their paths. But the plasmas that Schumaker and his colleagues generated would eat ordinary electrons for breakfast.
In their experiments, the team used the most intense laser in the world, the Hercules laser at the U-M Center for Ultrafast Optical Science, to shoot a pulse of light at a thin sheet of plastic or aluminum foil. The light heated a point on the surface, just one hundredth of a millimeter across, to such an extreme that the electrons separated from their atoms, producing a gaseous soup of loose electrons. Those electrons then fled the laser beam at near the speed of light, spreading as a disc of plasma that grew to half a millimeter in diameter within a trillionth of a second.
A set of mirrors, transparent to the naked eye yet reflective to infrared light, guides the laser light.
Many mirrors are needed to split and guide the laser pulse, timing it so that the electrons arrive at the plasma while it is growing.
Schumaker adjusts the golden focusing mirror. It reflects the light much more efficiently than your bathroom mirror, preserving the intensity of the laser pulse.
The laser pulse travels through a tunnel to accommodate long focusing lengths.
Schumaker uses a crosshair and a test beam to find the laser's route through the optical system.
The best method for measuring a small, strong magnetic field produced by a growing plasma is to send charged particles through it, but this approach faces two problems when a plasma expands this fast. The high-speed electrons set up an extraordinarily powerful field, 100 million times stronger than the earth’s magnetic field. Charged particles heading into such fields need a lot of momentum to avoid getting trapped.
“An electron travelling near the speed of light is going to punch through,” said Alec Thomas, an assistant professor of NERS. “There’s not much else that can make an image of electric and magnetic fields that are so strong and fast-moving.”
Exposure time is another matter – it depends on the length of the charged particle pulse. Previously, these plasmas hadn’t been measured with a pulse shorter than one trillionth of a second, which meant that the expansion appeared as a blur.
Hercules was the key to reducing exposure times by a factor of a thousand. “The laser emits a very, very short pulse, so it’s kind of like a pancake of light, one hundredth of a millimeter thick,” said Thomas.
The team realized that they could use a known method for turning a short light pulse into an even shorter burst of electrons. They split the laser pulse and sent part of it through a puff of helium and nitrogen gas. The pulse cut through the gas like a speedboat moving through water, creating a wake of plasma, Thomas explained. Electrons from the gas hitched rides on those waves like surfers, accelerating to almost the speed of light.
Meanwhile, the other half of the pulse generated plasma on the aluminum foil or plastic. When the electrons passed through the plasma, they deflected into a circular or ring-shaped formation. The team had only enough time with the laser to measure seven plasmas at seven different points in their evolutions, but with more time, they could reveal the plasma growth in 150 frames.
“The entire movie happens in right around a trillionth of a second. Before, that would be a single frame. That’s the power of having a very fast pulse,” said Schumaker. “We could see the plasma field expand at almost the speed of light.”
The method could help fusion scientists find ways to control magnetic fields that arise when using lasers to ignite nuclear fusion.
Alternatively, hospitals could use lasers to generate short-lived versions of elements on-site for nuclear medicine. High-intensity laser pulses focused onto solid materials produce particle beams, which in turn can be used to make radioactive atoms for imaging and treatments. As these lasers become smaller and less expensive, researchers anticipate that this approach to particle acceleration could become more practical than conventional methods. With the new measurement technique, the plasma’s magnetic field could be manipulated to steer particle beams or serve as a diagnostic for tuning and optimizing them.
This work is described in an article titled "Ultrafast Electron Radiography of Magnetic Fields in High-Intensity Laser-Solid Interactions", DOI: 10.1103/PhysRevLett.110.015003.
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