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    My last post was about computer modeling of currents in tokamak plasmas. This post will be addressing computer modeling of the effects of laser beams on plasmas in a different approach to nuclear fusion.
    Researchers at the University of Rochester have recently published a report of their work in Nature Physics on ways to increase the accuracy of computer models that simulate the effects of laser-driven implosions to produce nuclear fusion.
    At the University of Rochester's Laboratory for Laser Energetics (LLE), researchers study laser-driven inertial confinement fusion (ICF). In these experiments, intense pulses of laser light lasting for billionths of a second are used to deliver heat to and to compress a pellet of frozen hydrogen. If this technology can be perfected, the system will generate more energy that it consumes and could hopefully be used for commercial power generation.
    The laser-driven ICF systems require many laser beams to propagate through a plasma in order to reach their target. However, as they pass through the plasma, they interact with it in many different ways including some which can complicate the process.
    David Trumbull is an LLE scientist and the first author of the paper published in Nature Physics. He said, “ICF necessarily generates environments in which many laser beams overlap in a hot plasma surrounding the target, and it has been recognized for many years that the laser beams can interact and exchange energy.”
    In order to properly simulate this interaction, researchers need to find out exactly how the energy from the laser beams interacts with the plasma. Scientists have developed theories about how they think that laser beams alter plasma but none of these theories has been subjected to experimental verification.
    Researchers at the LLE in collaboration with scientists at Lawrence Livermore National Laboratory in California and the Centre National de la Recherche Scientifique in France have now directly demonstrated for the first time exactly how laser beams change the conditions in the plasma that they are passing through. These changes affect the transfer of energy in ICL fusion research.
    Michael Campbell is the director of the LLE. He said, “The results are a great demonstration of the innovation at the Laboratory and the importance of building a solid understanding of laser-plasma instabilities for the national fusion program.”
    Scientists often use powerful, sophisticated computers to study implosions in ICL experiments. Obviously, it is very important that these computer simulations be as accurate as possible in modeling the physical processes involved. This must include exchanges of energy from the laser beams to the plasma and ultimately the pellet that is the target.
    Over the past decade, researchers have used computer models that describe the laser beam interaction involved in ICL experiments. These previous models made the assumption that the energy from the laser beams interacted in a type of equilibrium referred to as “Maxwellian distribution”. This is an equilibrium which would be expected in the energy exchanges if there were no lasers shining through the plasma.
     Dustin Froula is a senior scientist at the LLE. She says, “But, of course, lasers are present.” She has pointed out that almost forty years ago scientists predicted that lasers would alter the conditions of the plasma in significant ways. In 1980, a theory was proposed that predicted that non-Maxwellian distribution functions in laser-plasma interactions. The theory suggested that these effects would be due to the preferential heating of slow electrons in the plasma by the lasers. Following that proposal, Rochester graduate Bedros Afeyan predicted that one of the effects of these non-Maxwellian electron distribution functions would result in changes in how the energy of the lasers is exchanged between the different beams. However, because there was no observation of such effects in physical experiments, the researchers did not include these effects in their simulations.
    Turnbull, Froula, and physics and astronomy graduate student Avram Milder carried out experiments at the Omega Laser Facility at the LLE in order to make very detailed measurements of the laser-heated plasmas. The experimental results displayed for the first time that the distribution of electron energies in a plasma is affected by their interaction with the laser radiation. This means that laser-plasma interactions cannot be precisely described by the currently prevailing models.
    Turnbell said, “New inline models that better account for the underlying plasma conditions are currently under development, which should improve the predictive capability of integrated implosion simulations.”