Plasma Physics
Almost all of the observable matter in the universe is in the plasma state. Formed at high temperatures when electrons are stripped from neutral atoms, plasmas consist of freely moving ions and free electrons. They are often called the "fourth state of matter" because their unique physical properties distinguish them from solids, liquids and gases.
Characteristics of typical plasmas Plasma densities and temperatures vary widely, from the cold gases of interstellar space to the extraordinarily hot, dense cores of stars and inside a nuclear weapon. On one end of the spectrum, plasma physicists study conditions of high vacuum, with only a few particles in a volume of one cubic centimeter – about the volume of a sugar cube. On the other end of the density range, plasmas with densities sometimes well above 1,000 times the density of a solid occur in stellar interiors and in laboratory experiments that attempt to reproduce the processes in the sun. Although we now most commonly encounter plasmas in energy-efficient light bulbs, plasmas may hold their greatest potential as a future inexhaustible source of energy (see Inertial Fusion Energy).
Two areas of plasma physics have been addressed with experiments using high-energy lasers and both are very relevant to the attempt to create inertial fusion (see How to Make a Star). First are studies of the phenomena created by the laser interacting with the plasma. Of particular importance in this area are two mechanisms of laser-plasma coupling: "stimulated Brillouin scattering" and "stimulated Raman scattering," two ways the energy of the laser beam is shared with the plasma (see Laser-Plasma Interactions). Both effects need to be minimized in order to drive the implosion of the ignition capsule as efficiently as possible.
The second area involves attempts to use the laser to emulate other phenomena occurring in nature. This research is also important to inertial fusion, but it extends well beyond that into fundamental areas of science, such as interpenetrating plasmas and plasma flow in a magnetic field.
The capabilities of NIF will allow production of hot dense plasmas that are sufficiently large and homogeneous to allow their detailed characterization, and thus to study these phenomena. NIF will allow measurement of electron and ion temperatures, charge states, electron density and plasma flow velocities, all of which are essential for understanding experiments on the two basic areas of plasma physics described above.
More Information
Basic Research Directions for User Science at the National Ignition Facility, National Nuclear Security Administration and U.S. Department of Energy Office of Science, November 2011"Taking on the Stars: Teller´s Contributions to Plasma and Space Physics," Science & Technology Review, July/August 2007
"Duplicating the Plasmas of Distant Stars," Science & Technology Review, April 1999
Perspectives on Plasmas - The Fourth State of Matter
Laser–Plasma Interactions
The coupling of high-intensity laser light to plasmas has been the subject of experimental investigations for many years. Past experiments have focused on measuring a broad range of phenomena, such as resonance and collisional absorption, filamentation, density profile and particle distribution modification, and the growth and saturation of various parametric instabilities. These phenomena depend on both the properties of the laser (such as intensity, wavelength, and pulse length coherence) and the composition of the plasma. NIF's large, uniform plasmas and laser-pulse shaping, coupled to the diagnostics possible on NIF, make it an ideal site for studying these processes with new precision, and with the hope of new insights.Laser–Plasma Instabilities
Experimental studies of laser–plasma instabilities have become particularly important in recent years as a result of the vigorous research effort in laser-driven inertial confinement fusion (ICF) (see How to Make a Star). The success of ICF depends partly on mitigating the undesirable effects of two particular parametric instabilities, stimulated Raman scattering and stimulated Brillouin scattering.These two instabilities are of particular importance because both degrade the target compression efficiency in a spherical implosion experiment. Electron Landau damping from the stimulated Raman scattering instability produces fast electrons that can preheat the core of an imploding sphere prior to the arrival of the compression shock front. The stimulated Brillouin scattering instability can scatter a substantial fraction of the incident laser light, causing an overall reduction in the laser-to-X-ray drive efficiency and modifying the X-ray drive symmetry. The 2009–2010 NIF hohlraum energetics shot campaign is yielding useful data for efforts to control and limit scattering instabilities.
Figure 1. Experiments on smaller lasers have shown that multibeam processes can reduce turbulent laser scatter. Filamentation
Another aspect of present and future research involves controlling laser-driven optical turbulence in plasmas; this would allow taming of the potential deleterious effects of multiple high-intensity laser beams interacting in an uncontrolled and unpredictable way with large hohlraum plasmas. Considering that a single laser beam can filament uncontrollably in hot plasma, it may seem hopeless to try to control multiple crossing beams. This is not necessarily the case, however, if appropriate spatial filtering and delayed feedback is introduced into the system. This finding from chaos theory can potentially be converted to the coupled, nonlinear case of high-power, interacting laser beams in plasma. Converting these theoretical insights into practical solutions in a large-scale, well-diagnosed, laser–plasma system is a principle goal in this work.Crucial pieces of the puzzle have been investigated in the past on smaller lasers and simpler systems. On NIF, these pieces are finally being merged together and a strategy for suppressing the instabilities of wide-aperture laser beams is being implemented by promoting multiple crossing beam interactions. Recent (2009–2010) shot series on NIF have demonstrated efficient heating of ignition-scale hohlraums with the radiation temperature and illumination symmetry required for inertial confinement fusion capsule implosions. The hohlraums show 85 percent coupling of the incident laser energy and scale with radiation temperature according to radiation hydrodynamic calculations and modeling. A technique called wavelength tuning was used to great effect in these hohlraum energetics shots. It makes use of the overlap of beams that occurs at each laser entrance hole as beams enter the hohlraum (Figure 2). Tuning the outer and inner beams with respect to one another controls the laser power distribution in the hohlraum by redirecting laser light in the overlapping beam area, allowing the laser to redistribute power to balance heating and produce more symmetrical target compression.
This sort of precision tuning will continue to be a topic of investigation. Very recent numerical simulations are showing that by using three tunable wavelengths between cones of beams on the NIF, rather than two, the energy transfer between beams can be more precisely tuned to redirect the light out of the target regions most prone to backscatter instabilities. It is predicted that using a third wavelength option could significantly reduce stimulated Raman scattering losses and increase the hohlraum radiation drive, while maintaining a good implosion symmetry.
Figure 2. Crossing laser beams in the plasma grating. The tuning technique has been effectively used to redistribute energy deposited on the hohlraum in 2009–2010 NIF experiments. Laser–Plasma Coupling and HED Physics
In addition to its importance for ICF, high-intensity, laser–plasma coupling presents an extraordinarily rich topic in the study of high-energy-density physics. For example, laser-produced plasmas provide a unique environment for the study of collisional and resonance absorption of laser light. Numerous experiments using various kinds of target materials and widely varying laser parameters have verified the general features of collisional absorption, such as the dependence of absorption on plasma temperature, scale length, laser wavelength, and intensity.Experiments investigating resonance absorption, which occurs at the critical density surface, show an expected dependence on the angle of incidence and polarization of the laser. These experiments, however, also show some discrepancies in the absorbed energy that may be attributed to rippling of the critical density surface. Density profile modification has been observed in experiments where resonance absorption is the dominant coupling mechanism. This profile modification can lead to harmonic generation in back-reflected laser light.
More Information
Symmetric Inertial Confinement Fusion Implosions at ultra-high laser energies, Science, Vol. 327. no. 5970, pp. 1228–1231 (2010) (subscription required)"Targets Designed for Ignition," Science & Technology Review, June 2010
Laser–Plasma Interactions
ReplyDeleteLaser–Plasma InteractionsThe coupling of high-intensity laser light to plasmas has been the subject of experimental investigations for many years. Past experiments have focused on measuring a broad range of phenomena, such as resonance and collisional absorption, filamentation, density profile and particle distribution modification, and the growth and saturation of various parametric instabilities. These phenomena depend on both the properties of the laser (such as intensity, wavelength, and pulse length coherence) and the composition of the plasma. NIF's large, uniform plasmas and laser-pulse shaping, coupled to the diagnostics possible on NIF, make it an ideal site for studying these processes with new precision, and with the hope of new insights.
Laser–Plasma Instabilities
Experimental studies of laser–plasma instabilities have become particularly important in recent years as a result of the vigorous research effort in laser-driven inertial confinement fusion (ICF) (see How to Make a Star). The success of ICF depends partly on mitigating the undesirable effects of two particular parametric instabilities, stimulated Raman scattering and stimulated Brillouin scattering.
These two instabilities are of particular importance because both degrade the target compression efficiency in a spherical implosion experiment. Electron Landau damping from the stimulated Raman scattering instability produces fast electrons that can preheat the core of an imploding sphere prior to the arrival of the compression shock front. The stimulated Brillouin scattering instability can scatter a substantial fraction of the incident laser light, causing an overall reduction in the laser-to-X-ray drive efficiency and modifying the X-ray drive symmetry. The 2009–2010 NIF hohlraum energetics shot campaign is yielding useful data for efforts to control and limit scattering instabilities.
Figure 1. Experiments on smaller lasers have shown that multibeam processes can reduce turbulent laser scatter. Filamentation
Another aspect of present and future research involves controlling laser-driven optical turbulence in plasmas; this would allow taming of the potential deleterious effects of multiple high-intensity laser beams interacting in an uncontrolled and unpredictable way with large hohlraum plasmas. Considering that a single laser beam can filament uncontrollably in hot plasma, it may seem hopeless to try to control multiple crossing beams. This is not necessarily the case, however, if appropriate spatial filtering and delayed feedback is introduced into the system. This finding from chaos theory can potentially be converted to the coupled, nonlinear case of high-power, interacting laser beams in plasma. Converting these theoretical insights into practical solutions in a large-scale, well-diagnosed, laser–plasma system is a principle goal in this work.
Crucial pieces of the puzzle have been investigated in the past on smaller lasers and simpler systems. On NIF, these pieces are finally being merged together and a strategy for suppressing the instabilities of wide-aperture laser beams is being implemented by promoting multiple crossing beam interactions. Recent (2009–2010) shot series on NIF have demonstrated efficient heating of ignition-scale hohlraums with the radiation temperature and illumination symmetry required for inertial confinement fusion capsule implosions. The hohlraums show 85 percent coupling of the incident laser energy and scale with radiation temperature according to radiation hydrodynamic calculations and modeling. A technique called wavelength tuning was used to great effect in these hohlraum energetics shots. It makes use of the overlap of beams that occurs at each laser entrance hole as beams enter the hohlraum (Figure 2). Tuning the outer and inner beams with respect to one another controls the laser power distribution in the hohlraum by redirecting laser light in the overlapping beam area, allowing the laser to redistribute power to balance heating and produce more symmetrical target compression.
ReplyDeleteThis sort of precision tuning will continue to be a topic of investigation. Very recent numerical simulations are showing that by using three tunable wavelengths between cones of beams on the NIF, rather than two, the energy transfer between beams can be more precisely tuned to redirect the light out of the target regions most prone to backscatter instabilities. It is predicted that using a third wavelength option could significantly reduce stimulated Raman scattering losses and increase the hohlraum radiation drive, while maintaining a good implosion symmetry.
Figure 2. Crossing laser beams in the plasma grating. The tuning technique has been effectively used to redistribute energy deposited on the hohlraum in 2009–2010 NIF experiments.
Laser–Plasma Coupling and HED Physics
In addition to its importance for ICF, high-intensity, laser–plasma coupling presents an extraordinarily rich topic in the study of high-energy-density physics. For example, laser-produced plasmas provide a unique environment for the study of collisional and resonance absorption of laser light. Numerous experiments using various kinds of target materials and widely varying laser parameters have verified the general features of collisional absorption, such as the dependence of absorption on plasma temperature, scale length, laser wavelength, and intensity.
Experiments investigating resonance absorption, which occurs at the critical density surface, show an expected dependence on the angle of incidence and polarization of the laser. These experiments, however, also show some discrepancies in the absorbed energy that may be attributed to rippling of the critical density surface. Density profile modification has been observed in experiments where resonance absorption is the dominant coupling mechanism. This profile modification can lead to harmonic generation in back-reflected laser light.
More Information
Symmetric Inertial Confinement Fusion Implosions at ultra-high laser energies, Science, Vol. 327. no. 5970, pp. 1228–1231 (2010) (subscription required)
"Targets Designed for Ignition," Science & Technology Review, June 2010