INTRODUCTION TO PLASMA PHYSICS AND CONTROLLED FUSION SECOND EDITION Volume 1: Plasma Physics

It has often been said that 99% of the matter in the universe is in the plasma state; that is, in the form of an electrified gas with the atoms dissociated into positive ions and negative electrons. This estimate may not be very accurate, but it is certainly a reasonable one in view of the fact that stellar interiors and atmospheres, gaseous nebulae, and much of the interstellar hydrogen are plasmas. In our own neighborhood, as soon as one leaves the earth's atmosphere, one encounters the plasma comprising the Van Allen radiation belts and the solar wind. On the other hand, in our everyday lives encounters with plasmas are limited to a few examples: the flash of a lightning bolt, the soft glow of the Aurora Borealis, the conducting gas inside a fluorescent tube or neon sign, and the slight amount of ionization in a rocket exhaust. It would seem that we live in the 1% of the universe in which plasmas do not occur naturally. The reason for this can be seen from the Saha equation, which tells us the amount of ionization to be expected in a gas in thermal equilibrium: Here tzi and n, are, respectively, the density (number per m3) of ionized atoms and of neutral atoms, T is the gas temperature in °K, K is Boltzmann's constant, and U; is the ionization energy of the gas-that 2 Chapter One is, the number of ergs required to remove the outermost electron from an atom. (The mks or International System of units will be used in this book.) For ordinary air at room temperature, we may take n,, = 3 x 1O25 mm3 (see Problem 1-1), T 25: 300°K, and Vi = 14.5 eV (for nitrogen), where 1 eV = 1.6 x 10-l' J. The fractional ionization ni/(n, + ni) z ni /n, predicted by Eq. [1-1] is ridiculously low: ni-z 10-122 n, As the temperature is raised, the degree of ionization remains low until Ui is only a few times kT. Then ni/n, rises abruptly, and the gas is in a plasma state. Further increase in temperature makes n, less than ni, and the plasma eventually becomes fully ionized. This is the reason plasmas exist in astronomical bodies with temperatures of millions of degrees, but not on the earth. Life could not easily coexist with a plasma-at least, plasma of the type we are talking about. …