Evolving Landscapes in High-Energy Density Physics: From Petawatt Lasers to Multifaceted Experiments
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It wasn’t so long ago that I was in graduate school, participating in my first high-intensity laser-plasma experiment. About once every hour, the high-powered laser would unleash one petawatt of energy in a burst less than one trillionth of a second long, focused into a spot one tenth the diameter of a human hair, on a tiny metal foil target. The intensity was such that we would generate incredibly hot and incredibly dense plasmas—matter so hot it’s a gas of ions and free electrons—for the study of what we call high-energy density physics (HEDP). Depending on the experiment, the precise heating and compression of the target sample could generate tiny explosions which replicate, on a much smaller scale, what happens inside supernovae…In some cases, the extreme crushing of material using enormous light pressure even resulted in entirely new states of matter, never before generated on earth, by completely rearranging the atomic and molecular structures.
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In that tiny fraction of a second, our suite of neutron, charged-particle, x-ray and optical diagnostics would have captured the instantaneous interaction of the laser with the small target and the plasma it generated. All of us would then scamper into the target area to retrieve our data, collecting film and saving images from the, sometimes, thirty or more instruments. That was 2006. Fast forward to 2020, and yes, the field of HEDP has evolved. Facilities have become more versatile, combining multiple lasers, or lasers with x-ray free electron lasers (XFELs), or with pulsed power machines. Experimentalists have developed a multitude of new measurement technologies, capable of greater accuracy at ultrashort time and length scales. Targets have become more complex and advanced; they may consist of metal solids or foams, or peppercorn-sized beads of hollow plastic containing gases. All this new technology has led to enormous advances in HEDP, produceding new knowledge relevant to planetary science, astrophysics, materials physics and fusion… ! Advances in laser architecture and advanced cooling schemes allow the lasers to fire many times per second without the heat build-up that leads to thermal distortions.
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HED experiments can be run at high-rep-rate, leading to an increase in the amount of data acquired, thus reducing the error bars and making our knowledge more precise. Plasmas are the fourth state of matter and the most ubiquitous form of matter in the universe; the phase space of plasmas to consider is enormous, so more experimental throughput in service of that exploration is certainly welcome.
