Laser-heated Plasma Research Articles

Energy Deposition in Laser-Heated Plasmas

A set of relations obtained by Brueckner between a bremsstrahlung spectrum and the underlying electron distribution do not hold if the energy dependence of the Gaunt factor is retained. A different set of relations is then derived. Their practical usefulness for interpretation of laser-heated plasmas appears to be limited.

Line emission in AI XI as an optical diagnostic in laser-heated plasmas

An extension of an earlier model of a laser-heated aluminum plasma is made to describe line emission in these plasmas from the lithiumlike Al XI ion. The model includes a satellite to the Heα-like line and the six most prominent lines originating from single excitations in Al XI. The core of the plasma is heated directly and cooled by thermal conduction losses to an equal volume of outer plasma, by a free hydrodynamic expansion, and by radiative losses principally from the hydrogen, heliumlike, and lithiumlike ionization stages. A study is made of the emission from plasmas initially at ion densities that, for a fully ionized plasma, would have critical electron densities for absorption of CO2, neodymium, and frequency-tripled neodymium laser radiation. Curves showing the integrated emission from different lines in Al XI are presented and correlated to the different plasma densities, peak average temperatures, and conduction loss rates.

Energy Deposition in Laser-Heated Plasmas

The number and energy distribution of suprathermal electrons produced in a laser-heated plasma can be quantitatively obtained directly from the experimental x-ray spectrum with only the assumption that the fast electrons lose energy by bremsstrahlung and electron-electron collisions. The result is independent of the spatial and temporal distribution of electron density and temperature.

Kinetic processes in a laser-heated helium-nitrogen plasma for use as a uv laser medium

Experimental and theoretical studies of an optically pumped electric discharge laser in helium-nitrogen mixtures are described. While the inherent efficiency of this system is rather low, these studies illustrate the basic parameters controlling the dynamics and efficiency of such a laser. In the experiments, intense diffuse discharges were obtained at pressures up to 12 atm and amplified spontaneous emission along the axis of the discharge was observed. The numerical modeling studies showed qualitative agreement with the experimental data and gave an optimized value of the kinetic coupling efficiency (absorbed 10.6-μ power to uv laser power) of 1.6% and of the total efficiency (energy stored in the TEA laser supply to uv output energy) of 0.1–0.2%.

Diffraction losses in a laser-heated solenoid plasma

A solution for diffraction loss of a laser beam propagating in a plasma light pipe is found by analogy to the tunnel effect of quantum mechanics. It is shown that diffraction tends to dissipate the beam layer by layer, working from the outside inward. A characteristic dissipation length can be defined for each layer of the beam. Diffraction is estimated for a very long narrow plasma relevant to the fusion reactor concept and is found not to be a significant loss mechanism.

Long-pulse generation in rhodamine-6G pumped by a CO2-laser-heated plasma

Results obtained using a laser-heated plasma as the pumping source for a dye laser are described. A small dye laser using a cylindrical close-coupled pumping arrangement was operated at various xenon pressures. Dye-laser pulses with 20-ns rise times and 800-ns pulsewidths were obtained.

Measurement of Thermal Conductivity in a Laser-Heated Plasma

The plasma thermal conductivity has been measured in a C${\mathrm{O}}_{2}$-laser-heated $Z$ pinch at a number density of 8 \ifmmode\times\else\texttimes\fi{} ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Ruby-laser scattering was used to monitor the heating and rarefaction. Comparison with a numerical simulation indicates that the thermal conductivity is about a factor of 2 below that predicted by classical theory.

Optimal laser plasma heating in a solenoidal magnetic field

Laser heating of a plasma column confined by a solenoidal magnetic field is studied via modern optimal control techniques. Plasma density and temperature at a given axial position along the plasma column are assumed to be uniform in the radial direction but time varying; the (local) laser intensity as a function of time is taken as the control. Classical absorption of laser energy by the electrons through the mechanism of inverse bremsstrahlung is assumed, with ion heating resulting from collisional energy transfer. It is also assumed that electrical conductivity is very large, so that particle diffusion and Ohmic heating are negligible. The theory for determining optimal laser pulse shapes in time, I (t), to achieve specified performance criteria is then developed and applied to the problem of raising the ion temperature from an initial equilibrium value to a prescribed level in minimum time. Constraints on the maximum laser intensity and energy available are imposed and typical numerical results presented.

Hot-spot model of K-line emission from laser-heated plasmas

A nonhydrodynamic model is developed to describe x-ray emission from laser-heated plasmas on a subnanosecond time scale. The model is first correlated with the experiment and then is used to study the conversion efficiency of laser energy to K-line x rays in carbon, fluorine, and aluminum plasmas as a function of incident laser intensity. It is found that these conversion efficiencies peak at well-defined values of input energy per area for laser pulse widths smaller than the pumping times for K-line emission. In this case, the degree and not the rate of plasma heating is important in determining the peak conversion efficiency. It is also found that peak conversion to K radiation in excess of roughly 10 or 20% occurs only when the C, F, and Al plasmas are optically thick to the Ka lines.

Some calculations of the heating of small pellets by laser radiation using the similarity model

The validity of the similarity or self-similar model of laser plasma heating is examined and some criteria relating to the maximum radium of pellets to which it may be applied, are derived. Numerical studies of the parametric variation of plasma parameters with laser variation are presented. It is found that substantially increased energies may be expected with CO2 lasers, compared with ND:glass.

Control of density correlations in laser-heated plasma

The excitation of plasma density perturbations of selected wavenumber spectra by two oppositely directed short-wavelength laser beams and the simultaneous interaction with these perturbations of a long-wavelength laser beam whose frequency is comparable to the electron Langmuir frequency is analyzed. Perturbations of the plasma density having a spatial period equal to one-half the wavelength of the short-wavelength beams are produced at zero frequency, and at the ion-acoustic frequency. The third beam excites the oscillating two-stream instability and the ion-acoustic decay instability, so that these initial perturbations will grow rapidly and dominate the density fluctuation spectrum. The present scheme may be useful in avoiding superthermal electrons that are typically produced when radiation from a single beam is absorbed and long-wavelength density fluctuations become significant.

Conversion efficiencies of laser to x-ray energy in C, F, and Al plasmas

A simple, but illustrative, nonhydrodynamic model is described which predicts potentially large conversion efficiencies of laser to x-ray energy in laser-heated plasmas. It is found that, when laser energy is deposited in 25 psec into the electron gas, a substantial conversion to x rays is possible in less than 1 nsec, provided the energy deposited per unit volume is not excessive. Moreover, while maximum K-line conversion increases with ion density, for a fixed electron critical density it decreases with increasing plasma Z.

Laser plasma heating in a strong magnetic field

The heating of a deuterium plasma created from solid granule (∼ 100 μ diam; ∼ 1017 particles) confined by strong magnetic fields B0 up to 1 G is studied. Taking into account the inverse bremsstrahlung absorption process, an inhomogeneous hydrodynamic model is used to describe the plasma dynamics. The skin depth due to the diffusion of the magnetic field inside the plasma is calculated. Ohmic heating is taken into account. The magnetic body force for a diffusion plasma is given by J × B while for the infinite conductivity model the force is calculated from the magnetic pressure B02/2μ0 on the plasma boundary. The efficiencies for CO2 lasers and neodymium glass lasers are compared. It is important to feed energy slowly to obtain a high efficiency of incident laser energy to the plasma thermal energy.

Occurrence of high-energy electrons and surface expansion in laser-heated target plasmas

It is shown that limitations on collisionless electron thermal conduction place a lower limit on the range of energies of electrons in the surface of target plasmas which are heated by incident electromagnetic radiation, and that in cases of contemporary experimental interest these energies can be hundreds of keV. It is then shown that these high-energy electrons can cause a much faster expansion of plasma from the heated surface than would be predicted by a single fluid theory. Numerical simulations demonstrate both effects.

Absorption of Light at Oblique Incidence on a Plasma Layer

Two effects are examined which may lead to enhanced absorption of light by sharply bounded plasma layers at oblique incidence. The first is a resonance absorption caused by interference of light reflected from the front and rear boundaries of the layer. The second is the linear coupling of the incident transverse wave into longitudinal plasma waves. It is thought that these effects may be useful in obtaining increased efficiency in laser heating of plasmas.

Production of Net Fusion Energy from Laser-Heated Target Plasmas

The heating of a small particle of deuterium-tritium by laser radiation is considered and conditions for energy gain determined. The size of the D-T particle for a balance of energy input and neutron energy output is computed as a function of initial plasma temperature. The electrical energy required for this breakeven condition is shown and increases as the inverse fourth power of laser efficiency. Because electrons are first heated by the laser radiation and then must transfer their energy to ions by collisions, a computation of electron-ion thermalization is presented. These results show that electron-ion thermalization through collisions applies a severe limitation on most cases for producing net fusion energy and must be considered in a consistent manner for smaller particles.

Temperature of a Laser-Heated Carbon Plasma

The temperature of a laser-heated carbon plasma, formed by the interaction between a Q-switched ruby laser and a pyrolytic graphite slab, has been measured using interferometric, spectroscopic, and energy conservation techniques. Results indicate that the maximum plasma temperature is less than 10 eV. A method of determining whether the plasma is in thermodynamic equilibrium is employed and indicates that at laser fluxes of 109 W/cm2 the plasma is not in LTE with the electron temperature nearly an order-of-magnitude larger than the plasma temperature. For fluxes of 1010 W/cm2, however, the plasma appears to approach a state of thermodynamic equilibrium with a temperature of 2eV.

Neutrons generation in laser-heated plasma

Neutrons generation in laser-heated plasma

Radiation from Laser-Heated Plasmas in Nitrogen and Argon*

The time-integrated radiant energy from laser-produced and laser-heated breakdown plasmas was spectroscopically examined. The spectra consisted of a strong continuum with several lines. Band spectra characteristic of molecular species could not be detected in pure diatomic gases. The lines were very broad especially in the same spatial region as that from which the continuum was emitted. Line spectra emitted from outside this region were predominantly from un-ionized atoms. The spectral lines from singly ionized atoms were emitted in a volume only slightly larger than that for the continuum. Photomultiplier–interference-filter measurements of the afterglow indicated that in nitrogen the atom lines last five times as long as do the ion lines. When the peak laser power exceeded 600 MW the spectrum of doubly ionized nitrogen atoms was detected. The absence of molecular spectra and the threshold for the appearance of doubly ionized nitrogen indicate that the plasma may be in a state of total equilibrium at a high temperature.