Multi-MeV Photon Emission by a Laser-Driven Electron Beam

Calculated electron density (green) and magnetic field strength (yellow-orange-blue) inside a structured plastic target 300 femtoseconds after being irradiated with an intense laser pulse from the left-hand side (Source: A. V. Arefiev / U Texas)

Calculated electron density (green) and magnetic field strength (yellow-orange-blue) inside a structured plastic target 300 femtoseconds after being irradiated with an intense laser pulse from the left-hand side (Source: A. V. Arefiev / U Texas)

Intense beams of gamma rays would find a host of uses in funda­mental physics research, nuclear fusion, and medicine, but they are hard to produce. A team has now used computer simu­lations to show that a powerful laser hitting a plastic surface can generate intense gamma-ray emission. In the simulations, the laser light creates a plasma in the plastic and acce­lerates electrons enough to produce large numbers of gamma-ray photons. The researchers say that the system might work with current technology.

In extreme astrophysical environments, matter and antimatter regularly anni­hilate, producing gamma rays. Researchers would like to study the reverse process by colliding beams of gamma rays, which should create electrons and positrons, a trans­formation of light into matter. Gamma-ray beams could also enable a wide range of other funda­mental experiments and might have a role in radiation therapy and radio surgery. Previous attempts to make these beams involved the inter­action of a laser beam with an electron beam. But to produce copious gamma-ray photons with energies in the MeV range, the laser beam would need to be more intense than any current device.

Alexey Arefiev at the University of Texas at Austin and his co-workers now propose a different method that requires somewhat less laser power. It involves shining pulses of a petawatt infrared laser onto a carbon-rich, plastic target. The power density of such a pulsed laser can reach values, which are about 500 times greater than would be produced by focusing all of the sunlight reaching the Earth onto a pencil tip.

In the team’s scenario, the laser pulse heats the target, creating a plasma of electrons and ions. The electrons in the plasma are high-energy, and according to special relativity, they acquire a large effective mass, making them too sluggish to follow the oscil­lations of the laser’s electro­magnetic field. This effect renders the plasma transparent to the light, so the laser beam can penetrate tens of micrometers into the target, filling it with a dense plasma.

In the team’s simulations, the laser pulse pushes the electrons in this plasma forward, like a leaf blower propelling leaves, and this motion of charge sets up a strong magnetic field that curls around the axis of the laser beam. This field accelerates the electrons forward even more but along zigzag trajec­tories as they move through the plastic. This electron motion generates so-called synchro­tron radiation of very high energy that is emitted from the rear of the plastic target in the direction of the laser beam.

The simulations that Arefiev and colleagues conducted using the Stampede super­computer at the University of Texas showed that the method works in principle. But also that propa­gation of the laser pulse into the target may quickly become unstable and deviate from its original direction. Then the electrons are pushed in random directions, and the multi-MeV photons they emit exit in an uncoor­dinated spray. “It’s similar to what happens to a garden hose that, when left unattended, can spray water uncon­trollably everywhere,” says Arefiev.

To provide a stabilizing hand on the hose, the researchers propose adapting the structure of the target. They simulated the same process for a target containing a cylindrical channel of lower density: a plastic foam, say, surrounded by the denser material. When the laser pulse hits this channel, the resulting plasma becomes completely transparent, whereas the surrounding material is opaque. So the channel funnels the laser pulse and ensures that it stays on course, producing a narrow, intense beam of forward-moving gamma rays. “We hope that our results will motivate experimen­talists to test our predictions,” says Arefiev. The conditions they have simulated “might be within reach for existing laser facilities.”

Donald Umstadter of the University of Nebraska at Lincoln, who works on new laser technologies, says that the gamma-ray beam could be used to study nuclear weapons materials that are relevant for managing the large stockpile of obsolete warheads. However, he also foresees many potential engi­neering diffi­culties in putting the idea into practice. The expected magnetic field would be ten times stronger than that of any previous laser plasma, says Tony Bell of the University of Oxford, UK. But “the simu­lations are credible,” he says. “If it is successful, the beam of gamma rays thus generated would be extra­ordinarily intense.” (Source: APS)

Reference: D. J. Stark et al.: Enhanced Multi-MeV Photon Emission by a Laser-Driven Electron Beam in a Self-Generated Magnetic Field, Phys. Rev. Lett. 116, 185003 (2016); DOI: 10.1103/PhysRevLett.116.185003

Link: Institute for Fusion Studies (A.V. Arefiev), The University of Texas, Austin, USA


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