Magnetic mirror confinement of laser-produced LiH plasmas

Abstract

The results of measurement and analysis of the decay of mirror-confined lithium hydride plasmas are reported. The plasmas are produced by laser irradiation of a solid LiH particle suspended at the center of a mimimum-B magnetic field. In the initial state, the ratio of the Li3+ energy to H+ energy is 7 to 1, the mean energy of lithium ions is about 2000 eV, and the electrons are cold. In the subsequent decay of the mirror-confined plasmas from 3×1013 to 1012 cm−3, the H+ lifetime is sufficiently shorter than the lifetime of the Li3+ ions so that the plasma evolves to a lithium plasma. The density decay is faster than classical but quiescent for the first 250 μsec. After that time, there are sudden increases in the plasma decay rate and in the rf emission at the central cyclotron frequency of lithium ions, interpreted as evidence of the onset of the drift cyclotron loss cone instability. The quiescent behavior is correlated with the observation of optical radiation from the plasma between 70 and 250 μsec. The plasma luminosity is interpreted as evidence for the creation of cold plasma by ionization of cold neutral reflux from the walls of the baseball coil. A quasi-linear Fokker–Planck calculation of the ion distributions and the growth and damping of the drift cyclotron loss cone mode yielded results consistent with an explanation of the quiescent decay as the suppression of the instability by the cold plasma.