Proton Beams Generated by Ultrashort-Pulse Lasers Will Help
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S&TR December 2003 Proton-Beam Experiments 11 Proton beams ROTONS, the positively charged, Psubatomic particles discovered by Lord Rutherford nearly 100 years ago, are still surprising scientists. Lawrence generated by Livermore researchers are discovering that proton beams created by powerful, ultrashort pulses of laser light can be used to create and even diagnose plasmas, the ultrashort-pulse superhot state of matter that exists in the cores of stars and in detonating nuclear weapons. The proton-beam experiments promise new techniques for maintaining lasers will help the nationʼs nuclear arsenal and for better understanding how stars function. The proton beams used in the Laboratoryʼs experiments are produced advance our by pulses of laser light lasting only about 100 femtoseconds (a femtosecond is 10–15 seconds, or one-quadrillionth of a second) and having a brightness, or understanding of irradiance, up to 5 × 1020 watts per square centimeter. When such fleeting pulses are focused onto thin foil targets, as many as 100 billion protons are emitted, with plasmas. energies up to 25 megaelectronvolts. The protons come from a spot on the foil about 200 micrometers in diameter, and the beamʼs duration is a few times longer than the laser pulse. The highest-energy protons diverge 1 to 2 degrees from the perpendicular, while the lowest-energy protons form a cone about 20 degrees from perpendicular. Funded by the Laboratory Directed Research and Development Program, the Livermore experiments are led by physicists Pravesh Patel and Andrew Mackinnon. Patel, who works in the Laboratoryʼs Physics and Advanced Lawrence Livermore National Laboratory 12 Proton-Beam Experiments S&TR December 2003 Technologies Directorate, is researching per gram) that exist in stars. The current Another method, direct heating with new ways to create and better understand generation of high-power lasers makes intense subpicosecond (10–12 seconds) plasmas. Mackinnon, from Livermoreʼs such studies possible because they can laser pulses, creates a highly nonuniform National Ignition Facility (NIF) Programs compress and heat matter to these heating pattern. The laser energy is Directorate, is developing new ways extreme states. absorbed within less than 100 nanometers to measure the plasmas created in Ideally, scientists want to measure of material, and the heat localization creates NIF experiments. Both physicists are plasmas in a uniform-density, single- a large temperature and density gradient. collaborating with colleagues from temperature state. As a material is heated A novel approach to isochoric heating, Queenʼs University in Belfast, Northern to several electronvolts, the pressure in discovered at Livermore, uses laser- Ireland; Heinrich-Heine-Universität in it increases to more than a million times produced proton beams to generate Düsseldorf, Germany; the LULI laser atmospheric pressure. This increased fleeting, dense plasma states at constant facility at lʼEcole Polytechnique in France; pressure causes the plasma to expand volume and density. The heating period is Rutherford Appleton Laboratory in the hydrodynamically, as in a violent explosion. shorter than the time needed for significant United Kingdom; and the University of Under these conditions, measuring plasma hydrodynamic expansion to occur, so the California (UC) at Davis. properties is extremely difficult. material is heated to a plasma state in a “Plasmas are often referred to as One way to overcome these problems few picoseconds. In effect, says Patel, the fourth state of matter,” says Patel. is to use what scientists call isochoric the proton beam dumps a huge amount “They are abundant in the universe but heating—heating at constant volume. of energy almost instantaneously and relatively uncommon on Earth. Plasmas With isochoric heating, plasmas donʼt suddenly increases a targetʼs temperature are extremely hot, highly transient objects expand during the time they are heated, to millions of degrees. and thus are difficult to control or to and their energy can be relatively uniform. accurately probe.” Established methods of isochoric heating, JanUSP Makes It Possible The team wants to develop new methods such as laser-driven shock heating, x-ray In their experiments, the researchers for creating plasmas in the laboratory, heating, and ion heating, are relatively rely on Livermoreʼs Janus ultrashort-pulse so they can study them at temperatures fast (10–6 to 10–9 seconds), but these (JanUSP) laser, one of the brightest lasers ranging from a few electronvolts to timescales are still longer than those during in the world. (See S&TR, May 2000, hundreds of electronvolts and at the high which significant hydrodynamic expansion pp. 25–27.) JanUSP produces a beam with energy densities (more than 100,000 joules can occur (10–11 to 10–12 seconds). an average intensity of 5 × 1020 watts per square centimeter that lasts about (a) Proton beam 100 femtoseconds. The laser operates at a Relativistic wavelength of 800 nanometers and delivers electron cloud Laser pulse 10 joules of energy. In one set of experiments, the laser pulse produced a proton beam from a 10-micrometer-thick sheet of aluminum Aluminum foil Film 25 MeV pack foil. The proton beam then heated a 5 MeV second 10-micrometer-thick aluminum foil (b) that was placed 250 micrometers directly behind the first. Within a few picoseconds, the heating created a 4-electronvolt plasma almost 200 micrometers in diameter—too short for much hydrodynamic expansion 6.5 8.5 10.2 11.6 13.0 14.2 16.4 18.4 19.3 to occur. Proton energy, megaelectronvolts (MeV) The discovery that intense, highly directional proton beams could be (a) Diagnostic setup for imaging proton beams produced by a 10-joule pulse of laser light that lasts generated from an ultrashort laser pulse only 100 femtoseconds. The pulse irradiates a thin aluminum foil. The generated protons are then heating a solid target was made by “pulled” from the back side of the foil by electrons traveling close to the speed of light. Protons with the Livermore researchers several years ago highest energy diverge the least from an angle perpendicular to the back foil surface. (b) Images from a while conducting experiments with the multilayer film pack taken over a broad energy range show that the number of protons decreases with Laboratoryʼs Petawatt laser. The Petawatt increasing energy. laser operated on 1 of the 10 beam lines Lawrence Livermore National Laboratory S&TR December 2003 S&TR December 2003 Proton-Beam Experiments 13 (a) (b) of Livermoreʼs Nova laser, which was 300 decommissioned in 1999. (See S&TR, Trajectory of electrons injected at 10 micrometers March 2000, pp. 4–12.) Experiments by 120 Livermore physicist Richard Snavely and others to characterize the proton 200 beams revealed a unique combination of 80 properties, including peak proton energies Laser of 55 megaelectronvolts and conversion efficiencies (of laser energy to proton 100 40 energy) up to 7 percent. Distance, micrometers Injected electrons The scientists also discovered that Electric field the protons in the beam originated 0 0 in hydrocarbons found in surface 0 20 40 60 0 50 100 contamination on the foilʼs back surface. Distance, micrometers Distance, micrometers Livermore theoretical physicists, led by Steve Hatchett and Scott Wilks, (a) Particle-in-cell code showing how a thin foil target expands after laser irradiation. (b) Hybrid code used computer simulations to study this calculation of the transport of high-energy electrons through a solid density foil. The calculation shows the behavior. They found that the pulse from complex electron trajectories and large electric fields at the rear surface of the foil that create the protons. an ultrashort laser accelerates electrons from the interaction region at the front of (a) –0.2 the target with relativistic energies; that is, (b) the electrons travel close to the speed of JanUSP laser Foil 0 200 light. The electrons emerging at the foilʼs Foil rear surface induce a large electrostatic 0.2 charge field, which in turn accelerates protons from hydrocarbon contaminants 0.4 on the rear surface. The protons accelerate from 0 to 20 megaelectronvolts at Time, nanoseconds 0.6 20 percent the speed of light and travel in a well-defined, highly directional beam 0.8 perpendicular to the target. X rays, in contrast, are emitted at random angles. Protons –0.2 Simulations by Wilks showed that by (c) (d) Aluminum foil 50 curving the laser targetʼs rear surface, the 0 320-micrometer proton beam could be focused to a far higher aluminum state of energy density. To test this design, 0.2 the team asked General Atomics in San Diego, California, to manufacture aluminum 0.4 hemispheres that are 10 micrometers thick, 320 micrometers in diameter, and almost Time, nanoseconds 0.6 perfectly smooth on the inside to ensure a high-quality proton beam. With the shaped 0.8 targets, the proton beam was almost 10 times –400 –200 0 200 400 more powerful than the beam produced from Distance, micrometers flat targets. The proton beam was focused on a 50-micrometer-diameter area of a foil (a) Layout for the proton-beam experiments using the Janus ultrashort-pulse (JanUSP) laser. When placed behind the target, which was then JanUSP irradiates an aluminum foil, it creates a proton beam that then heats a second foil. Both foils are heated to 23 electronvolts. 10 micrometers thick, and the second foil is placed 250 micrometers behind the first. (b) The plasma created “For the first time, the experiments in the second foil is almost 200 micrometers in diameter. (c) Layout for the experiments with a curved target. showed that we can focus proton beams,” (d) When the target’s rear surface is curved, the laser produces a focused proton beam that is says Patel. He notes that when the 50 micrometers in diameter and almost 10 times more powerful than the beam produced with a flat target.