Larry V. Knight

Laser-induced thermal lens effect: a new theoretical model

A theoretical model for the laser-induced thermal lens effect in weakly absorbing media is derived. The model predicts the intensity variation in the far field of the laser beam in the presence of the lensing medium and takes into account the aberrant nature of the thermal lens. Some experimental results which support the validity of this approach are presented.

Compact focusing von Hamos spectrometer for quantitative x-ray spectroscopy

A compact focusing crystal spectrometer based on the von Hamos scheme is described. Cylindrically curved mica and graphite crystals with a radius of curvature of R=20 mm are used in the spectrometer. A front illuminated charge-coupled device (CCD) linear array detector makes this spectrometer useful for real-time spectroscopy of laser-produced plasma x-ray sources within the wavelength range of λ=1.8–10 A. Calibration of crystals and the CCD linear array makes it possible to measure absolute photon fluxes. X-ray spectra in an absolute intensity scale were obtained from Mg, Ti, and Fe laser-produced plasmas, with a spectral resolution λ/δλ=800–2000 for the mica and λ/δλ=200–300 for graphite crystal spectrometers. The spectrometer has high efficiency in a wide spectral range, it is compact (40 mm diam, 150 mm length), easy to align, and flexible. The spectrometer is promising for absolute spectral measurements of x-ray radiation of low-intensity sources (femtosecond laser-produced plasmas, micropinches, ele...

Transverse modes of a laser resonator with Gaussian mirrors

Analytical methods are presented for transverse mode analysis of a laser resonator having spherical mirrors with a Gaussian reflectivity profile. The modes of this type of resonator have a form similar to that of the conventional Gaussian modes, but it is necessary to define an additional beam parameter to meet the self-consistency requirement for resonator modes. Stability of both the conventional complex beam parameter and the additional parameter is discussed. It is predicted mathematically that small perturbations in the new beam parameter will cause the intensity profile of the higher-order modes to evolve into that of the fundamental mode. Mode losses and discrimination are also discussed. The results may be useful in the design of regenerative laser amplifiers.

EUV spectroscopy of plasmas created in the final anode-cathode gap of the Z-Machine high-current pulsed generator (SNL)

The effect of short-circuit across the final anode-cathode gap of powerful pulsed current generators could hamper efficient power delivery to the Z-pinch plasma. To study this effect, a novel EUV diagnostics of plasmas created in the final section of the transmission line (the anode-cathode gap near the main load) of the Z-Machine high-current generator (Sandia National Laboratories, United States) was developed. The work included developing spectroscopic instruments, theoretical and experimental studies of EUV spectra of iron ions in well-diagnosed laser-produced plasmas, and a comparison of these spectra with those of plasmas created in the final anode-cathode gap of the transmission line. The EUV spectra of highly charged Fe ions in the spectral range λ ∼ 20–800 Å were investigated. In experiments performed at Sandia National Laboratories, spectra of FeXIII-FeXVII ions were observed. A comparison of the measured and calculated spectra shows that the electron plasma temperature in the anode-cathode gap is Te ∼ 200 eV.

Focusing crystal von Hamos spectrometer for x-ray spectroscopy and x-ray fluorescence applications

The use of bent crystals with high integrated reflectivity in focusing crystal spectrometers (Johann and von Hamos schemes) is considered. It is shown that in a von Hamos scheme mosaic focusing takes place. Thus a mosaic crystal simultaneously provides high spectral resolution and high efficiency. Expressions for the mosaic focusing are obtained. Focusing mica and graphite crystal von Hamos spectrometers (radius of crystal curvature is 20 mm) are investigated: spectral and spatial resolution and absolute efficiency are measured in a spectral range of 2 - 2.6 angstroms using laser-produced plasma and iron isotope x-ray sources. The mica crystal spectrometer showed high spatial (up to 10 micrometers ) and spectral ((lambda) /(delta) (lambda) approximately 1000) resolution, whereas the graphite spectrometer showed very high efficiency (30 - 70 times higher than the mica crystal) and moderate spectral resolution ((lambda) /(delta) (lambda) approximately 500 - 750). In the latter case mosaic focusing is observed: spectral resolution is 10 - 15 times higher than spectral resolution determined by the mosaic spread of the crystal ((lambda) /(delta) (lambda) approximately 50). The results allow one to estimate a maximum efficiency for focusing crystal spectrometers. Prospects for using the von Hamos spectrometers for x-ray spectroscopy and x-ray fluorescence are considered.

Emitted Current Instability from Silicon Field Emission Emitters Due to Sputtering by Residual Gas Ions

We have fabricated arrays of silicon field emitters using semiconductor lithography techniques. The density of the tips was 105/cm2. The maximum current that can be extracted from each emitter is limited by resistive heating. We have investigated how the electron current emitted changes under constant applied voltage. We found that the current is very sensitive to the vacuum conditions. We attribute this to sputtering of the emitters due to ionized residual gas molecules. The poorer the vacuum, the higher the instability in the current. We studied this phenomenon at 10−6 and 10−8 Torr. The model of two concentric spherical shells is used to obtain the ion energy distribution. This is then used to calculate the rate of ion bombardment and the rate of atoms sputtered. A lifetime of the tip can be deduced from these calculations.

Using thin film stress to produce precision, figured X-ray optics

We are studying the possibility of producing precision, aspherical mirrors for X-rays and visible light. Our study examines the use of ultrastructure processing to replace mechanical methods of material removal. The method starts with a chemically-mechanically polished, flat silicon wafer. The aim is to preserve atomic scale smoothness of the surface wafer while the wafer is bent to a desired figure. We report measurements of the mechanical properties of various stressing layers. This involves measuring the deformation of several thin silicon wafers coated with chemically vapor deposited nickel and boron films of known thickness. We have found that, under normal conditions, the film does not add to the microroughness of the substrate on either the front or the back surfaces. Film and substrate thicknesses, however, vary by as much as 10%. This is the present limit on figure accuracy. We have developed a model that describes bending of B/Si and Ni/Si structures. The model relates stress and Youngs modulus to the measured thickness of the film, and the thickness and curvature of the substrate. This approach is used to measure the stress and Youngs modulus for boron and nickel films. The Youngs modulus Ef was 3.05 x 1012 Pa for the boron films and 1.4 x 1010 Pa for the nickel films. From the relationship developed and verified for predicting the radii of curvature of the substrate, if may be possible to define a film thickness pattern which would provide a desired optical figure.

Image Quality Of Figured Multilayered Optics

The reflectivity and resolution of a multilayer structure is strongly affected by the roughness at the interfaces between two successive layers and by the amount that the constituent materials will diffuse into one another at the interfaces. Performance is also affected by the variations in individual layer thicknesses and by inhomogeneities in the materials. These deviations from the ideal multilayer will also affect the quality of the image from a figured multilayer optical element. The theory used to model the effects of non-ideal multilayers on the image quality of figured optics will be discussed. The relationship between image quality and multilayer structure quality will be illustrated with several examples.

An XRD/XRF instrument for the microanalysis of rocks and minerals

A breadboard setup constructed at MOXTEK, Inc., is capable of capturing both x-ray diffraction (XRD) and x-ray fluorescence (XRF) information simultaneously using a charge-coupled device (CCD) as the x-ray detector. This preliminary setup will lead to a prototype simultaneous XRD/XRF instrument. NASA is funding the instruments construction because of its capabilities and small size; it could be used for future Mars missions for analysis of rocks. The instrument uses a CCD to capture both the energy and the spatial information of an incoming x-ray. This is possible because each pixel acts as a spatially addressable energy-dispersive detector. A powdered sample of material is placed in front of the CCD, which in turn is bombarded by a collimated x-ray beam. The instruments critical features - namely the x-ray source, collimation optics and x-ray transparent windows - allow for the first time, to the best of our knowledge, mounting the sample outside the CCD camera. In this paper the instruments design parameters as well as the properties of both a front-side-illuminated (FSI) CCD and back-side-illuminated (BSI) CCD as x-ray detectors are investigated.

SPECTRUM OF PLASMA CONTAINING NE- AND NA-LIKE IONS : CONSISTENT ACCOUNT FOR RYDBERG AND AUTOIONIZING RYDBERG SERIES IN BALANCE EQUATIONS

This is a theoretical study of argon plasma under conditions, where Ne- and Na-like ionization stages are dominant. Balance equations are generalized to treat 37 3l states of Ne-like ion and 37 adjacent series of Rydberg states of Na-like ion simultaneously. This allows us to include in the kinetics a diffusion-like motion of the state of the system Ne-like ion plus one electron through the multitude of excited levels spread over an energy region of 50 eV. The populations Ni - i (i = (1, ..., 37)) of the Ne-like ion states are introduced explicitly; those of adjacent 37 Rydberg series are accounted for through continuous functions Ni(e). These functions describe the population distribution within each Rydberg series dependent on the Rydberg electron enegy e. The elementary processes of the collisionalradiative model connecting all Ne- and Na-like states, as well as processes of redistribution of populations inside each Rydberg series, are accounted for. The rate coefficients for all processes within the Ne-like residue have been calculated previously, using a detailed many-body relativistic theory. The dielectric capture cross sections and autoionization probabilities are presented as analytical continuation of the collisional excitation cross sections. The excited-excited states transitions are included. The rest of the processes are treated in a simple semiclassical approximation. The Lotz formula is generalized by unambiguous analytical continuation to cover the case of bound-bound transitions between Rydberg states of Na-like ions. The radiation reabsorption in a long plasma cylinder is included through Biberman-Holstein coefficients for all transitions. The inclusion of Na-like states, accounting for diffusion-like processes, increases the population inversion for the lasing candidates by at least a factor of two for a wide range of plasma conditions. This is important for the ionization equilibrium too. Besides, the functions Ni(e) bear diagnostic information. Detailed calculations have been done for the homogeneous steady-state Maxwellian plasma. The role of transient processes in the population inversion creation is under discussion.