Physicists at the University of Nevada, Reno are settling the debate about heating mechanisms with ultrashort-pulse lasers for high-energy-density plasma research.
Using an intense laser based on the technique invented by the 2018 Nobel Prize in Physics, at the University’s Nevada Terawatt Facility, Hiroshi Sawada, associate professor of physics, and Yasuhiko Sentoku, a former professor of physics at the University, have defined the heating mechanism, which can be a new plasma heating technique to mimic the conditions of the Sun for basic science and nuclear fusion research.
Lasers are one of the indispensable tools in our daily lives. For example, low-power lasers serve as barcode scanners, DVD readers and laser printers, while high-power lasers can even cut thick plates of steel.
“In high-energy-density plasma physics, high-power lasers have been a main driver to compress and transform a material into an ionized material or plasma under extremely high pressures that are comparable with those found at the centers of planets and stars,” Sawada said.
By re-creating such plasmas in the laboratory, scientists have been studying various properties of the plasma and test theoretical models for fundamental sciences and applications including inertial confinement laser fusion for unlimited source of clean energy.
As featured in the 2018 Nobel Prize in Physics, the invention of a pulse compression technique called the chirped pulse amplification has boosted conventional lasers to ultrahigh-intensity regimes. High-intensity, short-pulse lasers based on the CPA technology could offer an alternative route to heating plasma. Because of the short duration of a laser pulse less than one trillionth of a second, a material can be rapidly heated to above one million degrees Kelvin before it starts cooling. This new heating technique allows researchers to access plasmas in a different regime than those created with the conventional lasers.
Despite extensive research on the ultrashort-pulse laser-target interaction in the last two decades, details of the heating mechanism have been under debate.
Recently, a research team led by Sawada has revealed that the heating of a thin metal foil with a high-intensity, short-pulse laser is a two-step process; heating induced by laser-accelerated electrons during the laser pulse, followed by significant thermal diffusion after the pulse ends. The team developed a new X-ray imaging diagnostic to measure the volume of strongly heated area in the foil. A numerical simulation comparing to the data shows that the localized area of the titanium foil is heated to three million degrees Kelvin within 1.5 trillionth of a second.
“The importance of our work is not only to identify the dominant heating mechanism and have solved the puzzle of the heating mechanism for the first time, but also to determine the condition and volume of the heated plasma,” Sawada said. “Three million degrees Kelvin sounds like a very high temperature, but our experiment was carried out using only a 15 Joule laser pulse energy.
“There are much more powerful lasers available within the U.S. and overseas. We believe that the result is promising to reach 10 million degrees Kelvin with existing high-energy, short-pulse lasers. Furthermore, once laser parameters are optimized for efficient heating, such lasers could heat plasmas to even higher temperatures and assist to ignite a compressed fusion fuel in fast ignition laser fusion, analogous to a spark plug in an internal combustion engine.”
This research, entitled “Monochromatic 2D Kα Emission Images Revealing Short-Pulse Laser Isochoric Heating Mechanism,” was published in the science journal Physical Review Letters.
Source: University of Nevada, Reno