Phase Mixing of Nonlinear Plasma Oscillations in an Arbitrary Mass Ratio Cold Plasma

Nonlinear plasma oscillations in an arbitrary mass ratio cold plasma have been studied using 1-D particle-in-cell simulation. In contrast to earlier work for infinitely massive ion plasmas it has been found that the oscillations phase mix away at any amplitude and that the rate at which phase mixing occurs, depends on the mass ratio ($\Delta = m_{-}/m_{+}$) and the amplitude. A perturbation theoretic calculation carried upto third order predicts that the normalized phase mixing time $\omega_{p-} t_{mix}$ depends on the amplitude $A$ and the mass ratio $\Delta$ as $\sim [(A^{2}/24)(\Delta/\sqrt{1 + \Delta})]^{-1/3}$. We have confirmed this scaling in our simulations and conclude that stable non-linear oscillations which never phase mix, exist only for the ideal case with $\Delta = 0.0$ and $A < 0.5$. These cold plasma results may have direct relevance to recent experiments on superintense laser beam plasma interactions with applications to particle acceleration, fast ignitor concept etc.Comment: pp 10 and two figures in PS forma

CAN ORTHO-PARA TRANSITIONS FOR WATER BE OBSERVED ?

Author Institution: Department of Physics and Astronomy, University College LondonThe whole of the water spectrum can be considered as the juxtaposition of the spectrum of two different molecules, with different total nuclear spin: ortho-$H_{2}O$, and para-$H_{2}O$. No transitions have ever been observed between the two different nuclear-spin isotopomers, and it is widely assumed that interconversion is forbidden without some other intervention. However weak nuclear spin/rotation interaction occurs and this can drive ortho to para transitions. More than 12000 experimental vibrational rotational levels for water have been assigned so far. In this work we explore the whole of the vibrational-rotational spectrum of water, calculate ab initio values for the nuclear spin/rotational constants, and predict in which part of the spectrum the strongest transitions between ortho and para levels of water could be experimentally observed