Optical Rocket with Laser-Driven Plasma Waves

In this artist’s conception of the experiment, the white orbs represent two laser pulses, with plasma waves in their wakes. The waves interfere with one another after the laser pulses cross, and electrons ride the wake field waves to higher energy. (Source: Univ. Nebraska-Lincoln)

In a recent experiment at the Univer­sity of Nebraska–Lincoln, plasma electrons in the paths of intense laser light pulses were almost instantly acce­lerated close to the speed of light. Physics professor Donald Umstadter, who led the research, said the new appli­cation might aptly be called an “optical rocket” because of the tremen­dous amount of force that light exerted in the experiment. The electrons were subjected to a force almost a trillion-trillion-times greater than that felt by an astronaut launched into space.

“This new and unique appli­cation of intense light can improve the perfor­mance of compact electron acce­lerators,” he said. “But the novel and more general scientific aspect of our results is that the appli­cation of force of light resulted in the direct acce­leration of matter.” The optical rocket is the latest example of how the forces exerted by light can be used as tools, Umstadter said. Normal intensity light exerts a tiny force whenever it reflects, scatters or is absorbed. One proposed appli­cation of this force is a “light sail” that could be used to propel spacecraft. Yet because the light force is ex­ceedingly small in this case, it would need to be exerted conti­nuously for years for the spacecraft to reach high speed.

Another type of force arises when light has an inten­sity gradient. One application of this light force is an optical tweezer that is used to mani­pulate micro­scopic objects. Here again, the force is exceedingly small. In the experiment, the laser pulses were focused in plasma. When electrons in the plasma were expelled from the paths of the light pulses by their gradient forces, plasma waves were driven in the wakes of the pulses, and electrons were allowed to catch the wake­field waves, which further accelerated the electrons to ultra-rela­tivistic energy.

The new appli­cation of intense light provides a means to control the initial phase of wakefield acce­leration and improve the perfo­rmance of a new gene­ration of compact electron acce­lerators, which are expected to pave the way for a range of appli­cations that were previously impractical because of the enormous size of conven­tional acce­lerators. The experiment was based upon numerical modeling by scientists from Shanghai Jiao Tong Univer­sity in China. Umstadter theo­retically predicted the underlying mechanism two decades ago. (Source: Univ. Nebraska-Lincoln)

Reference: G. Golovin et al.: Electron Trapping from Interactions between Laser-Driven Relativistic Plasma Waves, Phys. Rev. Lett. 121, 104801 (2018); DOI: 10.1103/PhysRevLett.121.104801

Link: Extreme Light Laboratory, Dept. of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, USA

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