A. Förster

Thermodynamic properties and plasma phase transition of xenon at high pressure and high temperature

By using recent Pade approximations for the Coulombic contribution and hard-core, Van der Waals and polarisation approximations for the atomic contribution, the thermodynamic functions for the nonideal xenon plasma are constructed. These formulae cover a large range of pressure and temperature. Single ionisation is studied by minimising the free energy with respect to the particle numbers. A second critical point is predicted. The coexistence line for the corresponding first order phase transition (weakly ionised plasma - strongly ionised plasma), the lines indicating the soft transition to a higher degree of ionisation at overcritical temperatures, as well as the border of the coexistence area are presented. The influence of the different interactions on the critical data is discussed.

Equation of state and the phase diagram of dense fluid helium in the region of partial ionization

The plasma composition, equation of state, and phase diagram of dense helium plasma were calculated for temperatures of 10 4 ...10 5 K, total atom densities of 10 15 ... 10 25 cm− 3 , and pressures up to 10 2 TPa, including the region of partial ionization and strong Coulomb coupling. The basic thermodynamic potential was chosen to be the free energy density with contributions due to Coulomb interaction, hard-core repulsion, and van der Waals-like attraction for a mixture of differently charged atoms and free electrons. For the first time, we show the potential occurrence of a sequence of plasma phase transitions. In helium, they correspond to the ionization steps He 0 →He + and He + →He ++ respectively. The properties of the coexisting phases were determined by a Maxwell construction based on the combined chemical potential.

Effective Potentials, Energies, and Pair-distribution Functions of Plasmas by Monte-Carlo Simulations

The one-component plasma (OCP), an electron gas on a positive background, and the subsystem of free charges in a two-component plasma (TCP) is investigated using a quasi-classical approach. Quantum effects such as the Heisenberg uncertainty principle and the Pauli exclusion principle are incorporated into our model by effective potentials. In case of the OCP the symmetry effects are of special interest whereas in case of TCP the Heisenberg effects play the significant role. The method of Slater sums is used to obtain effective potentials. Monte Carlo simulations for both OCP and TCP were carried out to calculate the Coulombic energy, the effective energy, and the pair-distribution functions.

Optimal fluctuations leading to transitions in bistable systems

Abstract The optimal trajectories for transitions between steady states of bistable systems under the influence of colored gaussian noise are investigated. A general procedure for estimating the transition time probability distribution in the weak noise limit is constructed.

Generalized thermodynamic functions for electrons in a mixture of hard spheres : application to partially ionized nonideal plasmas

We present an ad hoc generalization of thermodynamic functions for electrons in a mixture of hard spheres which describes the reduced-volume effect at an arbitrary degree of degeneration. The model is applied to dense low-temperature plasmas in the region of partial ionization and, therefore, has to be supplemented by expressions for the Coulomb interaction between strongly coupled ions in an electronic background. A comparison with previous treatments of the reduced-volume effect shows the significant influence of our approach on the nature of the pressure ionization what is illustrated by numerical examples for a helium-like model plasma.

Ionization kinetics in non-ideal dense plasmas: Ionization fronts

The ionization kinetics in non-ideal plasmas is described by reaction diffusion equations which follow from the non-ideal quantum kinetic theory. In the approximation of ambipolar diffusion the mathematical description reduces to only one equation which is investigated with respect to front solutions. It has been shown that in particular cases there exist two different types of travelling fronts: (i) a front connecting the homogeneous atomic phase and some ionized state and (ii) a front connecting two homogeneous ionized stable states differing in the degree of ionization. The fronts are considered with respect to their profile and their velocity. The relation of this dynamic phase transition to the plasma phase transition well known in the theory of thermodynamic equilibrium of the non-ideal plasma is discussed.

Adiabatic equation of state and ionization equilibrium of dense plasma

We consider the adiabatic equation of state for partially ionized non-ideal plasma. Plasma isentropes are calculated with the Pade technique in the chemical picture. The interplay of ionization/dissociation equilibrium and non-ideality is investigated.

Quantum wave-packets simulation of ionization processes in dense plasmas

Abstract We study the ionization of one hydrogen atom by thermal plasma electrons on the basis of quantum wave-packets simulations. By using the variational form of the Schrodinger equation, Hamilton-type equations of motion are derived. The simulation results for the density-dependent ionization rates are in good agreement with theoretical estimates and semi-empirical formulae.

Thermodynamic properties and the plasma phase transition of dense helium plasma

Abstract We complete the He phase diagram in the plasma region and show the possibility of two separate plasma phase transitionsintroducing the combined chemical potential.

SIMULATION OF IONIZATION-FRONT PROPAGATION IN DENSE PLASMA BY MARKOFF AUTOMATA

Front propagation in partially ionized dense plasma is described by reaction-diffusion equations. We introduce the new method of Markoff automata for the stochastic simulation of reaction and diffusion in dense plasma which is capable to perform simulations fast and accurate. The method is applied to the investigation of ionization fronts of different shape. A comparison with analytic results for profile and velocity of plane fronts confirms the stochastic simulation data. Furthermore, we present data for an expanding ionization spot.