Hydrogen-helium Plasma Research Articles

Investigation of the characteristics of the active medium of a helium hydride exciplex laser

A model is developed of plasma-chemical reactions in a helium–hydrogen plasma formed by a hard ionizer. It is shown that, in principle, lasing is possible as a result of a photodissociation transition in the HeH* exciplex (λ = 460 nm, Δλ = 310 nm). The characteristics of the active medium subjected to pulsed and pulse-periodic pumping are analyzed analytically in the steady-state approximation and also numerically. A considerable gain amounting to ≈2×10−2cm−1 and an average efficiency of the active medium ηav ≈ 3% can be achieved by pumping a dense medium ([He] ≈ 1021cm−3) with a small amount of hydrogen ([H2] ≈ 2×1017cm−3) by ∼ 100 nsec pulses with a peak current density je ≈ 1 kA/cm2 in the case of an electron beam and jp ≈ 2 A/cm2 in the case of a proton beam (the particle energy is assumed to be 300 keV). In the case of pulse-periodic pumping by a beam modulated at the microwave frequency the pulse repetition frequency should be in the range f ≲ 50 MHz.

Transport phenomena and abundance anomalies in cosmic plasma

Hydrodynamical equations for a fully ionized hydrogen-helium plasma are derived by the Chapman-Enskog method. The electron and ion transport coefficients are found as the functions of electron and ion temperatures and number densities as well as of the magnetic field strength. The presented equations are needed for describing transport phenomena in laboratory and cosmic plasmas. It is shown that transport phenomena can produce abundance anomalies; e. g., a sound wave propagating through a homogeneous plasma may be accompanied by the oscillations of chemical composition. Various astrophysical consequences of the theory are discussed.

Resistivity Phenomena in Multi-Ion Plasma with Ion Acoustic Turbulence

The effect of different species of ions on the resistivity of a plasma in the presence of ion acoustic turbulence has been studied. Results have been applied to the case of hydrogen-helium plasma mixture. The variation of drift velocity with Te/Ti for quasi stationary fluctuation levels for different percentages of light and heavy ions available in the plasma is also investigated. It is found that the variation is quite significant even for small percentages of light ion contamination.

Ion-acoustic resistivity of a multicomponent plasma

The expression for a general dispersion relation in the presence of ion-acoustic turbulence has been derived for a two-ion plasma and is used to study its effect on the turbulent resistivity. Results have been applied to the case of a hydrogen-helium plasma mixture. The effect of the addition of light ions on the variation of the drift velocity and resistivity of the plasma with T e T i is investigated for quasi-stationary fluctuation levels.

Line intensity fluctuations from dense plasmas

The problem is considered of relating small “shot-to-shot” variations in spectral line intensity radiated by a given dense plasma, to the corresponding variations in electron density and temperature. For the specific example of hydrogen lines from a helium-hydrogen plasma, generated in a device such as an electromagnetically driven shock tube, a clear distinction is shown between intensity fluctuations due to density changes, and those due to temperature changes. Thereby, the possibility is indicated of reducing the scatter in experimental data on spectral line shapes by simple first-order corrections.

Stepwise Excitation and Local Thermodynamic Equilibrium on Excited Hydrogen Levels in Helium-Hydrogen Plasma behind Shock Front

The calculation is carried out on the ionization relaxation as well as on the time developments of population densities of excited hydrogen levels for helium-atomic hydrogen mixtures (H: 0∼3 atomic %) behind the reflected shock front under the conditions of the experiment with an electromagnetic T-tube conducted by the present authors. The result gives the shortening of ionization relaxation time depending on mixing ratio of hydrogen and the characteristic initial hump in time development of population density, which is remarkable for low-lying excited levels of hydrogen in a mixture of low hydrogen mixing ratio. The temporal changes of the population and depopulation rates in the relaxation zone are presented for several excited hydrogen levels so that the existence of stepwise excitation between the coronal equilibrium and LTE is illustrated and the validity criterion is proposed for the establishment of LTE.

Thermodynamic properties of hydrogen-helium plasmas.

The thermodynamic properties of an atomic hydrogen-helium plasma are calculated and tabulated for temperatures from 10,000 to 100,000 K as a function of the mass fraction ratio of atomic hydrogen. The tabulation is for densities from 10 to the minus 10th power to 10 to the minus 6th power gm/cu cm and for hydrogen mass fraction ratios of 0, 0.333, 0.600, 0.800, and 1.0, which correspond to pure helium, 50 percent hydrogen per unit volume, 75 percent hydrogen per unit volume, 89 percent hydrogen per unit volume, and pure hydrogen plasmas, respectively. From an appended computer program, calculations can be made at other densities and mass fractions. The program output agrees well with previous thermodynamic property calculations for limiting cases of pure hydrogen and pure helium plasmas.

Boltzmann equation for a photon gas interacting with a plasma

The dynamics of a photon gas is described in a Boltzmann-equation formulation for the rate of change of the photon occupation number. The interaction with the electron component of a hydrogen-helium plasma by Compton scattering is introduced as a Fokker-Planck operator. This operator, as well as the total energy exchange rate by Compton scattering, is derived by simple physical arguments from the limiting formula for the case of a zero-temperature nondegenerate electron gas; the effects of stimulated scattering are discussed. Photon production by various atomic processes (bremsstrahlung, direct radiative recombination, line emission, and 2 s-1 s two-photon continuum emission) is described as source terms in the Boltzmann equation. The spontaneous emission by these atomic processes is conveniently described in terms of the basic bremsstrahlung term which dominates the other atomic processes for kT e ≳ 1 Ry; at lower temperature the other processes, which involve bound atomic states, dominate. Absorption and stimulated emission by the atomic processes are discussed briefly. The process of radiative Compton scattering (double Compton scattering) is evaluated in the limit of low photon energies for interaction with a zero-temperature electron gas; this process is shown to be negligible compared with bremsstrahlung for the important application to the problem of the primordial plasma and photon gas.

Deviation from a Maxwellian Velocity Distribution in Low-Density Plasmas

The problem of the electron velocity distribution for a plasma in a steady state is investigated. The principal deviation from Maxwellian results from inelastic collisions of electrons with atoms and ions wherein the excited level depopulates predominantly by radiating a photon. A simple analytic solution to the Boltzmann equation is obtained for the case where we are in the tail of the distribution (this allows simplification of the Fokker-Planck Coulomb operator) and for the case where the inelastic cross section varies as 1/E, where E is the kinetic energy of the incident electron. A generalization is made to the case where many atomic levels can be excited, if the cross section for each varies as 1/E. In applying the results to astrophysical plasmas it is found that the non-Maxwellian corrections are always small. For example, the relative corrections to inelastic collision rates for the primordial hydrogen-helium plasma are less than 10−3. In gaseous nebulae the typical corrections are even smaller (about 10−6). For some cosmic x-ray sources and for stellar coronae the corrections are also of this order (10−6).

Atomic processes in a low-density hydrogen-helium plasma

For a partially ionized hydrogen-helium plasma at 10 4–10 6 °K the processes bremsstrahlung, radiative and dielectronic recombination, excitation of discrete levels, and collisional ionization due to electron collisions with various species are discussed. The steady-state ionization conditions are computed assuming a balance between collisional ionization and radiative and dielectronic recombination. This is done for the cases where the plasma is optically thin and optically thick to photons resulting from radiative captures to the ground states of the recombined species. The cooling rates for the plasma are computed for all of the above processes. A very brief discussion is given of the effects of deviations from a Maxwellian electron velocity distribution. Possible applications of the results in astrophysics are mentioned, the principal application being to the primordial gas after decoupling of the matter from the radiation.

Equilibrium radiation from isothermal hydrogen-helium plasmas

The composition and radiation of high temperature hydrogen-helium plasmas have been calculated for thicknesses of 0·1 to 10·0 cm for shock velocities of 140 000 and 190 000 ft/sec and for ambient densities from 10 -7 to 10 -4 g/cm 3. Results were obtained from 50/50 volumetric mixtures as well as for the pure He and the pure H cases. These plasmas are plane parallel isothermal layers in local thermodynamic equilibrium; their calculated radiation includes both line and continuum processes. Thirty-four helium lines are considered with Stark broadening as the principal broadening mechanism. Lyman, Balmer, Paschen and Brackett transitions below the thirteenth level are considered for atomic hydrogen and singly ionized helium. Doppler, natural and Stark broadening are considered for all transitions below the seventh level, while the lines involving the seventh through the twelfth levels are assumed to be broadened by electron impact. The emission results indicate that the layers are optically thin at low ambient densities and become optically thick as the ambient density increases. The line radiation is usually more important than the continuum at low ambient densities. As the ambient density increases, the continuum radiation becomes the major portion of the emitted radiation, not only because of the expected line saturation, but also because the plasma microfields reduce the ionization potential and thus eliminate many of the upper levels from which line transitions would have taken place. The drop of ultraviolet emissivity at high temperature is brought out, as well as the respective roles of hydrogen and helium in the energy exchanges of H 2-He mixtures.