magnetic Z-pinch machines. With the aid of such experiments, and through new analytical, computational, and technical breakthroughs, it may soon be possible to gain improved understanding of the physics underlying some of the universe’s most extreme phenomena and to answer some of the fundamental questions outlined in the following sections. Indeed, over the past decade a new genre of laboratory astrophysics has emerged, made possible by the new high energy density (HED) experimental facilities, such as large lasers and Z-pinch generators. On these facilities, macroscopic collections of matter can be created in astrophysically relevant conditions, and their collective properties measured. Examples of processes and issues that can be experimentally addressed include compressible hydrodynamic mixing, strong-shock phenomena, radiative shocks, radiation flow, high-Mach-number jets, complex opacities, photoionized plasmas, equations of state of highly compressed matter, and relativistic plasmas. These processes are relevant to a wide range of astrophysical phenomena, such as supernovae and supernova remnants (see Figure 2.1), astrophysical jets (see Figure 2.2), radiatively driven molecular clouds, accreting black holes, planetary interiors, and gamma-ray bursts. In this chapter these phenomena are discussed in the context of laboratory astrophysics experiments possible on existing and future HED facilities. Key questions in each area will be raised, with the hope and expectation that future experiments on HED facilities will play some role in their resolution.

HIGH ENERGY DENSITY DEFINITIONS FOR ASTROPHYSICS

Stars are plasma. This state requires energy in excess of the binding energy of molecular or solid matter—which for the most abundant element, hydrogen, corresponds to 4.4 electronvolts or to a gas temperature of about 23,000 K.

More extreme conditions abound. They may be classified by equating a thermal kinetic energy (a temperature), or a quantum degeneracy energy (a Fermi energy) to the specified energy. For example, the temperature corresponding to the rest-mass energy of an electron is 6 billion K, and the density at which the electron Fermi energy equals its rest-mass energy is 1 million times that of water.

Another set of extremes can be constructed from velocities. Relativistic conditions are energetically extreme: as the velocity of light is approached, the energy of a particle exceeds the rest-mass energy. For typical conditions in the interstellar medium, the sound velocity is about 10 kilometers per second (km/s), while gas motions often exceed this by factors of 10 to 100. Under these conditions, strong shocks, with Mach numbers of 10 to 100, are generated.



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