Resonant phOtopumping is an attractive and potentially very efficient technique for achieving x-ray lasing in plasmas. At the Naval Research Laboratory we are pursuing the realization of one of the most promising photopumped schemes wherein heliumlike sodium pumps heliumlike neon. The theoretical side of this research has revealed the conditions required for the sodium and neon plasmas to maximize the possibility of photopumped fluorescence and lasing. On the experimental side, a powerful (25 GW) z-pinch source of sodium line pumping radiation has been developed and characterized. A separate neon plasma, driven by part of the return current from the sodium-bearing plasma, has been deployed side-by-side with respect to the sodium line source at a distance of 5 cm. The presence of photopumping has been indicated in fluorescing spectra. The remaining steps toward achievement of an x-ray laser are reducing the spatial separation of the pumped and pumping plasmas and increasing the pump line power of the pumping, sodium-bearing plasma.
New target geometries for collisional excitation x-ray laser experiments (in nickel) were proposed, analyzed and experimentally studied on the GDL laser. Experiments using a short line focus lens with new target geometries showed general agreement with predictions. The new geometries are designed to yield a higher gain and reduced refraction due to: (i) a higher plasma density, (ii) a wider lateral density profile, and (iii) a concave lateral density profile. These new geometries were: (i) two parallel exploding (thin) foils, irradiated from one side only; (ii) two ablating (thick) foils, one of which is irradiated on its inner face, and (iii) an exploding foil in front of an ablating foil, irradiated by a single laser beam incident on the thin foil. New experiments with a long line focus are in progress. The intensity ratio of Ne-like and F-like Ni lines is used to deduce the temperature, and these results together with the absolute intensity yields the density profile. The results show improvement achieved with the new target geometries: the density is higher (leading to a higher gain), and the concave density profile results in collimation rather than divergence of the x-ray laser beam. Theoretical developments included: (i) prediction of high gain on new type transitions in neon like ions, involving the excitation of an inner (2s) electron, and (ii) development of a ray tracing code for an amplifying medium of varying (e.g., collimating) lateral density profile. Results using this code are shown for convex as well as concave lateral density profiles.
We present recent experiments on soft x-ray amplification in lithium-like ions in a CO2 laser-produced recombining plasma confined in a magnetic field. The maximum gain-length products observed are GL = 3 - 4 for the 154 A, 4f - 3d transition in Al XI and GL = 1 - 2 for the 129 A, 4f-3d transition in Si XII, respectively. A one-dimensional hydrodynamic code with a collisional-radiative atomic model was used to model the plasma and comparisons of theoretical results with experimental observation are discussed. Descriptions of both hydrodynamic and atomic physics code are given.
A scheme employing neonlike krypton ions is under intensive theoretical and experimental investigation to determine the feasibility of developing a pulsed-power driven laboratory x-ray laser. The scheme depends on discharging 100's of kilojoules of electrical energy through co-axial cylindrical krypton gas puffs generating a dense, hot, uniform, homogeneous, and highly ionized krypton plasma. The dynamics of energy absorption are such that self-generated magnetic fields compress and accelerate radially inward theiouter plasma with speeds approaching 5 x 10 cm/sec. When the outer plasma impinges and stagnates on the inner plasma, shock waves are sent through the system as the plasma reverberates and bounces outward. Near the interface between the two interacting plasmas, and along the axis, conditions appear to be conducive to the establishment of a population inversion with the subsequent emission of coherent soft x-rays with measurable gain. The theory, analysis, and numerical simulations are based on a fully coupled self-consistent one-dimensional non-LTE radiation hydrodynamics model including the effects of opacity and radiation transport. The multi-level ionization dynamics is evaluated in the collisional radiative equilibrium (CRE) approximation for the manifold of both ground and excited states distributed throughout the various stages of ionization. In addition, particular emphasis is placed on the atomic structure of the neonlike ionization stage which in our model consists of 48 excited levels in j-j coupling. The evolution of the level populations as functions of the various atomic processes provides information on the conditions necessary to establish population inversions and the emission of coherent radiation in the lasing transitions. The spectral line profiles are represented by Voigt functions. A complete history of the implosion and radiation dynamics will be provided for the cases under investigation.
The DNA/PITHON pulsed power generator was used to produce hot, dense, aluminum plasmas in a Z-pinch configuration. An annulus of neon gas was radially imploded on to a parylene capillary on axis, which wa$ coatqd with aluminum. The pinch produced 3 cm and 5 cm long Al plasmas with densities - 1020 cm' and temperatures - 500 eV, with a 25 ns duration. After the implosion, the Al plasma expanded and cooled for the next 50 ns, producing Li-like Al XI ions by recombination from He-like Al XII ions. A time-gated, grazing incidence spectrograph (MCPIGS) was used to image the expanding plasma and to capture three consecutive 25 ns duration spectra. The Al XI, 4f-3d and 4d-3p lines at 154.6 and 150.6Å were observed to be.anomalously bright compared to the Al XI, 4d-2p line. Also, the Ne X, 4-3 line at 187.3A was brighter than the Ne X, 4-2 Balmer-p line. Spatial and temporal variations of these lines suggest an inversion of the 4d and 4f levels in Al XI and of the n = 4 level in Ne X, driven by three-body recombination. A rough estimate of the gain-length product for these lines is between 3 and 4. From measured K-line yields in neon, using branching ratios and detector response, it was estimated that the total energy output of these laser lines ::: 60 mJ/pulse. Further confirmation of lasing awaits experiments in which on-axis and off-axis XUV measurements, as well as systematic length scaling, are planned. However, these early results signify a major milestone for pulsed power driven X-ray lasers. The power in these lasers is ~ 2 MW. These power levels are comparable to those,produced in the glass laser driven X-ray lasers at Lawrence Livermore National Labs (LLNL).1" If these recombination laser. The 4f-3d transition has an oscillator strength of unity, and recombination followed by cascades will feed the 4f level very effectively. Collisional mixing of the n = 4 levels will also pump the 4d level.
We describe the construction and operation of a 109 nm, photoionization-pumped, single-pass laser in Xe III. The laser is pumped by soft x-rays emitted from a laser-produced plasma in a traveling-wave geometry. Using a 3.5 J, 300 psec , 1064 nm laser pump pulse, we measure a small-signal gain coefficient of 4.4 cm-1 and a total small signal gain of exp(40). The laser is fully saturated and produces an output energy of 20µJ in a beam with 10 mrad divergence.
We report the observation of the wavelength spectrum of the double ionization of Ba from - 280 nm to nearly 700 run, using laser pulses 5ns long of peak intensity - 1010 W/cm2. All the strong resonances can be assigned to Ba+ transitions, and many of the assignments have been verified by pump-probe techniques. Thus, the Be+ observed is due to sequential ionization. The shortest wavelengths used are not as effective as λ ~ 500 nm.
For overall modeling of radiative properties of plasmas in various conditions, a variety of atomic and thermodynamic data are required as input. In this paper, we will briefly outline our work on calculation of photoionization cross-sections of atoms and ions and the modeling of Equation-of-state properties of plasmas.
Stimulated bremsstrahlung in an undulating electric field in the lasing beam direction (electric wiggler) was shown to be possible from the quantum-mechanical viewpoint. Herein, this possibility is scrutinized from the viewpoint of classical electrodynamics. It is found that if stimulated bremsstrahlung in a transverse undulating magnetic field (magnetic wiggler) occurs, stimulated bremsstrahlung in the electric wiggler must also occur. We further show that a free electron laser (FEL) using a magnetic wiggler to provide a catalyzer field for stimulated bremsstrahlung cannot serve as a practical FEL operating in the soft x-ray region from both theoretical and experimental viewpoints. On the other hand, we demonstrate that the FEL using a traveling wake field in a two-beam elliptical pill-box cavity is well suited as a source of coherent radiation in the soft x-ray region.
Recent work on the spectra of laser-produced plasmas and tokamaks has led to the observation of long sequences of isoelectronic ions extending to very high ionic charge states. The measurements of the wavelengths and energy levels provide data that are important for the development of x-ray lasers. In addition to contributing to a knowledge of the energy levels and transitions of possible lasing media, the data provide reference lines for wavelength calibration of x-ray laser experiments and reference data for testing theoretical methods used for predicting the properties of lasing ions.
The principal focus in the gamma-ray laser program lies upon the feasibility of the various schemes for pumping at the laboratory scale. The nuclear analog of the ruby laser embodies the simplest of the concepts and not surprisingly, the greatest rate of advance has occurred in this direction. Several major milestones have been reached, and this presentation reviews the current status and remaining issues affecting the course of continuing research.
A method of switching from a nuclear isomeric state to a lasing state is examined. A semi-classical model of laser-electron-nuclear coupling is developed. In it the electrons are treated as free in the external field of the laser, but with initial conditions corresponding to their atomic orbits. Application is made to testing this model in 235U and to the design criteria of a gamma-ray laser.
In the two-step pumping scheme for a gamma-ray laser, an essential step is that of exciting the nucleus from a long-lived storage isomer to a nearby short-lived state that then decays to the upper lasing level. An experiment is in progress to induce this transfer by first exciting the atomic electrons with UV photons. The incident photons couple well to the electrons, wOich then couple via a virtual photon to the nucleus. As a test case, excitation of the 2.3U nucleus is being sought, using a high-brightness UV laser. The excited nuclear state, having a 26-minute half-life, decays by internal conversion, result in emission of an atomic electron. A pulsed infrared laser produces an atomic beam of 4-317 which is then bombarded by the UV laser beam. Ions are collected, and conversion electrons are detected py a channel electron multiplier. In preliminary eriments, an upper limit of 7 x 10- has been obtained for the probality 0 exciting a 4J'U atom in the UV beam for one picosecond at an intensity of about 10 W/cm4. Experiments with higher sensitivities and at higher UV beam intensities are underway.
A numerical, time-dependent quantum mechanical model is used to describe the interaction of an isolated ion with an intense applied laser field, including both electron and nuclear degrees of freedom. Calculated results are presented. We find that the model ion radiates in low odd harmonics of the laser frequency, in qualitative agreement with experimental observations. In addition, it radiates strongly in the x-ray region, at frequencies comparable with the electron Rydberg frequency. Such radiation should be possible to observe in future experiments. If it exists, it could provide a basis for a reasonably coherent x-ray source. We find that the probability of induced nuclear excitation is small for higher electric multipoles, although observable probabilities are obtained under appropriate circumstances for L-1.
A reliable estimate of the properties of isomers that may be viable candidates for a gamma-ray laser requires the use of the most accurate wave functions possible. The majority of models that have been used to estimate the properties of isomers are applicable to only selected regions of the nuclear mass table. In particular, the Bohr-Mottelson model of odd-A and odd-odd nuclei will fail if the even-even core is not strongly deformed or if the deformations are changing strongly as a function of mass. The problem is overcome in a new core-quasiparticle model for odd-odd nuclei; it is a generalization of work by Donau and Frauendorf-'2 for odd-A nuclei. The model introduces the pairing interaction ab initio; the odd-A states are mixtures of particle and hole states. The core may be soft towards deformation or axial asymmetry and may change rapidly as a function of mass. Thus, the model is ideally suited for application to the region of transitional nuclei such as the Te, La, and Os regions.
We present results of theoretical nuclear structure model calculations for the gamma-ray laser candidate nucleus 186Re proposed by Collins. Our calculations of this odd-odd transitional nucleus are based on an axially-asymmetric (particle plus triaxial rotor) model for constructing the orbitals of the odd nucleons that couple under the influence of the residual neutron-proton interaction. We include pairing correlations in the determina-tion of these orbitals by using the BCS ap-proximation with newly determined pairing strengths. The matrix elements of the resid-ual neutron-proton interaction are obtained using phenomenological spin-dependent 8 function potentials of both surface and volume forms. We examine the sensitivity of the calculated low-excitation level structure of 186Re to the strength of these potentials. Calculated energy levels of 186Re will be presented and compared with experiment. The impact of our results on the proposed use of 186Re as a gamma-ray laser will be discussed. In addition, based upon these and other model calculations to be described, we assess the level of effort necessary in a full-scale the-oretical search for a viable candidate nucleus for a gamma-ray laser.
Three Dimension Nuclear Hartree-Fock calculations have been performed for isotopes of Osmium, Mercury, and Tungsten. Many of the calculated potential energy surfaces show a second minimum at large prolate deformation indicating the possibility of shape isomeric states. Several are at sufficiently low energy to be possible candidates as storage states for the gamma ray laser.
Results from a data base search of computerized nuclear structure libraries have been extended and augmented so as to expand the in-formation available for nuclei suitable as gamma-ray laser candidates. The spectrum of nuclear levels occurring in deformed rotational nuclei have been calculated and have been used in conjunction with isomeric state data for odd-A systems. The results of this augmentation effort are presented with particular emphasis on results obtained for 177Lu, 177Hf, and 179Hf. For these cases some possibly interesting cases were identified that meet energy spacing criteria. However, significant hindrance factors exist for them which negate their interest for gamma-ray laser applications.
Conversion electrons from the decay of low-lying levels of 237Np have been measured to detect the population of these levels by 2i/ gamma-ray decay of the Np shape isomer. Analysis of the 208-keV transition L conversion-electron peak _gives an upper limi3J?f about 17 pb for the population of the 3/2 267-keV level in Np from the shape isomer decay. Model calculations are compared with the measured limit. Improvements are suggested for this experiment.
Gamma-ray experiments involving the Mossbauer Effect and multibeam Borrmann Effect in single crystal silver are reported. Calculations for multibeam radiation modes for electronic scattering in a (111)-oriented crystal of iron are detailed.
All efforts to obtain short wavelength (< 1 gm) chemical lasers (SWCL's) have failed to date. The reasons for failure and possible ways to overcome the difficulties are discussed. In particular, a novel approach to obtaining SWCL's based on a premixed fuel-lasant capable of producing high energy pulses, is described. Electronically-excited nitrogen molecules are produced by detonation of metal azides. Lasing is expected via energy transfer from the nitrogen molecules to metal atoms or within the nitrogen molecules. Preliminary results obtained by detonation of lead azide are presented. Calculations based on a coupled hydrodynamic-kinetic model point out that the detonation should result in population inversion between potential laser levels of lead atoms.
Photochemical production of electronically excited NO(A2:?) was studied in mixtures of ozone(03) and hydrogen azide(HN3). Rate constants for kinetic processes forming electronically excited NO(A) following a 248 nm laser initiation pulse were calculated from the temporally and spectrally resolved NO(A-X) emission. NO(A) is formed by the following two-step process: 0(1D) + HN3 --> OH + N3, k = 3.2+1.0 x 10-1° cm3 s-1; and 0(1D) + N3 --> NO(A) N3; k = 1.0+.3 x 10-1° cm3 s-1. Emission from vibrationally excited NO(A) up to v' = 2 was observed in the region of 200 nm - 250 nm.
A short-wavelength chemical laser concept producing the well-known excimer lasant XeF* has been developed and is under study. By photoinitiating the reaction of pre-mixed hydrogen azide (HN3) and xenon difluoride (KeF2 )' the scheme has been demonstrated to produce XeF*, as well as other excited species. The production of various excited states has been studied as functions of reactant concentration and initiation conditions.
The suitability of 1,2-dioxetanes, specifically the tetramethylderivative (TMD), as a fuel for a Short Wavelength Chemical Laser is investigated experimentally. Experiments in the pure solid state show that the explosive decomposition of. TMD is accompanied by intense blue emission. Due to the high densities involved, a novel type of chain reaction is operative in the system that has the characteristics of a branched chain. The primary quantum yield for singlet acetone generation has been measured in the gas phase (1.7%) using emission and a lower limit of 12% for the primary triplet acetone yield has been found using transient absorption at 300 nm. Energy transfer experiments involving triplet acetone and IF have been undertaken with the goal of observing emission from IF (B31?(e)).
The potential of isocyanic acid as a fuel for a short-wavelength chemical laser has been investigated. The experiments were performed by carrying out reactions of F atoms with HNCO to obtain NCO radicals and with coreagents of the type RHn.to obtain R atoms. The NCO + R reactions occurred subsequently. The resulting chemiluminescence was observed to detect electronically excited NR* and other significantly emitting species. Eight coreagents, namely CH4, H20, PH3, NH3, HC1, HBr, SiH4 and B2H6, have been studied in this manner. The results indicate that the NCO radicals react with C, 0 and P atoms to generate the corresponding NR* species. Photon yields from these species have been determined. The CN(B X) photon yield was found to be the highest of all of these, however, the absolute value of the CN(B X) photon yield is low which may be due to the inefficiency of the C-atom production from the F + CH4 reaction. Research is being carried out to develop an efficient C-atom source.