Drake - High-Energy-Density Physics: Fundamentals, Inertial Fusion, and Experimental Astrophysics

Springer International Publishing AG 2018

R Paul Drake 1

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Abstract

This chapter begins by introducing and defining the field of high-energy-density physics. It goes on to compare its domain with those of other areas of science and to survey the historical developments that led to the emergence of this field as a discipline. The chapter then identifies and discusses the regions within which various physical effects, such as Fermi degeneracy, become important. Brief introductions to inertial confinement fusion and to laboratory astrophysics at high energy density follow. The chapter concludes with a discussion of the connections between the present book and various other books, and some discussion of variables and notation.

This book concerns itself with the physics of matter within which the density of energy is high. The first formal definition of the field, early in the twenty-first century, was in a report of the National Research Council of the United States, entitled Frontiers in High-Energy-Density Physics: the X-games of Contemporary Science . It suggested a definition of high-energy-density systems as those having a pressure above one million atmospheres. The units of this pressure can be designated by 1 Mbar, 1011 Pa, 1011 J/m3, 1012 dynes/cm2, or 1012 ergs/cm3. We will tend to express the pressure in Mbars, as this is the most common of these units found in the relevant literature. This definition reflected several observations. For example, one learned in school that solids and liquids are incompressible, but this is not strictly true. If one applies a pressure exceeding 1 Mbar to ordinary solid matter, it compresses. Another way to make this point is to say that the internal energy density of a hydrogen molecule is 1 Mbar.

Thinking further, one might realize that once the energy that holds a collection of particles together, whether as applied pressure or as binding energy , becomes of the order of the internal energy of the molecules and atoms, their behavior will change. Beyond this point, the system will behave more as ions and electrons than as neutral particles. In the decade since the definition suggested above, it has become clear that solid matter exhibits novel behavior once the medium begins to ionize, creating free electrons. This can occur at a pressure as low as about 0.1 Mbar. This has led some researchers to suggest revising the above definition by lowering the threshold pressure. But in the absence of newly freed electrons, one does not get any novel behavior. In addition, the field as it functions includes the study of plasmas produced using devices that can create high-energy-density conditions, even when their density of energy is somewhat smaller. So the present author suggests the following revised definition of high-energy-density physics:

Figure . Finally, quark-gluon plasmas are a form of high-energy-density matter produced by colliding relativistic heavy ions. They have a temperature of order 1012 K, and a density far to the right of those shown here.

We will discuss some further aspects of the transition into the high-energy-density regime in Sect. discuss how radiation affects it.

Chapter are essential to presenting a comprehensive and comprehensible discussion. For example, one might launch a shock wave that converts ordinary matter into high-energy-density matter. Such shock waves have velocities above 10 km/s. At constant pressure, shock velocities increase as density decreases, so that shock waves above 100 km/s (>360,000 km per hour) are common in high-energy-density physics. Alternatively, one might produce an intense beam of photons, electrons, or ions that can penetrate the matter and directly heat it, and particle beams themselves may reach this regime.

High-energy-density physics encompasses more than the regime of dense plasma, in the sense just described. It also includes conditions in which pressures >0.1 Mbar result from very high temperature at very low density. For example, air at a density of 1 mg/cm3, of the order of atmospheric density, reaches a pressure of 0.1 Mbar at a temperature above 1 keV. (Throughout this text we express temperature in the energy unit of an electron Volt, so that the Boltzmann constant is 1.6 1012 ergs/eV or 1.6 1019 J/eV.) A temperature of 1 keV is roughly 10 million Kelvin, so that temperatures of millions of Kelvin or more are common in high-energy-density physics. As the density decreases further, conditions in which the pressure remains above one Mbar soon become relativistic, and thus also outside the realm of traditional plasma theory. Overall, what high-energy-density systems have in common with traditional plasmas and with condensed-matter systems is that collective effects are an essential aspect of the behavior. The difference from traditional plasma physics is that the particles are more correlated, and/or relativistic, and/or essentially radiative. The difference from traditional condensed-matter physics is that ionization and Coulomb interactions are essential. Various chapters that follow are focused on one or more aspects of these differences.

The present text was the first book to be written as a textbook in high-energy-density physics. (We place it in the context of prior work later in this chapter.) This reflects the fact that high-energy-density physics is in some sense a new field. One can see that the regimes just discussed offer some challenges beyond established areas of physics, but one might wonder both in what sense this is new and why. The material discussed here, as in condensed-matter physics and other areas, is entirely built on the foundations of classical and modern physics as established from the mid-nineteenth to the mid-twentieth century. In addition, much of the material discussed herein is discussed in more depth in one of a dozen or so more-advanced books. The fundamental sense in which this is a new area is that there are new tools and that new tools beget new areas of science. It is now practical for scientists in an academic or laboratory setting to perform experiments to study the fundamental behavior of high-energy-density systems over a significant range of parameters. This creates a need for the treatment of this material as an integrated subject, moving from fundamentals to their applications, for the presentation of the material in a common voice suitable for graduate courses and as a first working reference, and for a discussion that spans the range of conditions now (or soon to be) available for study. Hence the emergence of high-energy-density physics as a distinct field and hence this text.