A compact, tabletop terawatt forsterite laser (1.24 μm /100 mJ/ 80 fs) is used for generation of fast hard x-ray radiation from laser-produced plasmas. Plasmas are created on massive solid Fe and Cu targets. X-ray radiation is studied using a focusing crystal von Hamos spectrometer with a CCD linear array as x-ray detector. High efficiency of the spectrometer in a wide spectral range allows us to record x-ray spectra by one laser shot. Intense Kα radiation is studied with high spectral resolution (λ/δλ~1000) at various focusing conditions: main laser radiation and second harmonic radiation. With a copper target the Kα radiation yield was equal to 4×109 photon/pulse in 4π steradian, that corresponds to conversion efficiency of 0.02%. Processes responsible for ultrafast hard x-ray radiation are discussed.
Absolute x-ray calibration of laser-produced plasmas was performed using a focusing crystal von Hamos spectrometer. The plasmas were created by an Nd-YAG laser (0.53 μm/200 mJ/3 ns/10 Hz) on massive solid targets (Mg, Cu, Zn, Sn, Mo, Ta, Ti, Steel). Cylindrical mica crystal (radius of curvature R=20 mm) and a CCD linear array detector (Toshiba model TCD 1304AP) were used in the spectrometer. Both the mica crystal and CCD linear array were absolutely calibrated in the spectral range of λ=7-15 Å. The spectrometer was used for absolute spectral measurements and the determination of the plasma parameters. The unique target design allowed for multiple instruments to observe the plasma simultaneously which improved analysis. High spectrometer efficiency allows for the monitoring of absolute x-ray spectra, x-ray yield and plasma parameters in each laser shot. This spectrometer is promising for absolute spectral measurements and for monitoring laser-plasma sources intended for proximity print lithography.
A set of spectral analytic instruments has been developed for absolute intensity measurements in a spectral range of 1 - 600 Å: (1) several modifications of grazing incidence spectrographs; (2) EUV monochromator- spectrometer with a constant angle of deviation; (3) focusing crystal von Hamos spectrometer using cylindrical mica and pyrolytic graphite crystals and a CCD linear array as a detector. These instruments are useful for plasma diagnostics, x-ray and EUV spectroscopy of laser-generated plasmas and capillary discharge plasmas, x-ray and EUV reflectometry, radiometry and x-ray fluorescence application.
This paper describes a simple method for formation of quasi-monochromatic soft x-ray radiation from laser-produced plasmas. The method uses a special combination of laser target materials and x-ray filters. Targets with middle atomic numbers Za~10-25 are chosen, so hydrogen- and helium-like ions ([H]- and [He]-like ions) are excited in the laser-produced plasmas. The x-ray K-absorption filters isolate only a few lines in the spectrum: resonance lines and corresponding satellites of [H]- and [He]-like ions. These lines occupy a narrow spectral band λ/δλ=10-100. The contribution of continuum radiation out of the separated spectral band is studied theoretically and experimentally. It is shown that the continuum radiation contribution does not exceed 20% of the line radiation intensity. The method of formation of quasi-monochromatic soft x-ray radiation is used for reflectometry (measurements of crystal reflectivity, calibration of crystal spectrometers and x-ray detectors) and for radiometry (absolute radiatin yield measurements) of laser-produced plasmas.
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 project because it could be used for future Mars missions for analysis of rocks. The instrument uses a CCD to capture both the energy and the position 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 instrument's critical features, the x-ray source, collimation optics and x-ray transparent windows need to be optimized in the size and power to allow the instrument to be portable. In this paper the instrument's design parameters as well as the properties of both the CCD as x-ray detector and the low-power consumption tube are investigated.
Intense soft x-ray radiation is observed when a high temperature laser produced plasma collides with a solid surface (wall). The laser plasma is produced by Nd:YAG laser radiation (0.53 micrometers ; 200 mJ; 3 ns; 5x1012 w/cm2 focused onto a Mg target. The wall is placed in the path of the plasma expansion. Two experimental techniques are used. 1) One-dimensional x-ray images of the plasma-wall gap formed by a slit are observed using an absolutely calibrated CCD linear array. 2) X-ray spectra of [H]- and [He]-like Mg ions are recorded using a crystal focusing x-ray von Hamos spectrometer. At a measured electron temperature of Te equals 195 eV, emission of [He]-like ions Mg XI gives the main contribution to the observed x-ray intensity. The spatial structure and the intensity of the radiation are studied at various laser plasma-wall distances (r0 equals 0.3 - 3.0 mm). Intense radiation near the wall is caused by an electron density jump in a shockwave formed in front of the wall. At small wall distances r0<<v(tau) (v is the expansion velocity, tau is the laser pulse duration) the radiation is caused by excitation processes, and at distances r0<0.3 mm the near wall intensity exceeds the emission from the laser plasma itself. At distances r0$GTR$GTRv(tau) the radiation causes recombination processes. These results are important for developing collision and recombination schemes for x-ray lasers in colliding plasmas.
Absolute x-ray calibration of laser-produced plasmas was performed using a CCD linear array and a focusing crystal spectrometer. The plasmas were created by a Nd glass laser and a high repetition rate Nd-YAG laser. A commercial CCD was used for x-ray detection. The CCD linear array has 3724 pixels giving a total length of approximately 30 mm. First the CCD detector was absolutely calibrated using laser- produced plasmas and an FE55 isotope source. The sensitivity of the detector was about two orders of magnitude higher than x-ray photographic film. The CCD detector was used for monitoring the absolute x-ray yield, for finding the optical focal position, for estimating the flux density on the target and for measuring the electron temperature of the plasma. In combination with a focusing von Hamos spectrometer the detector was used for absolute spectral measurements and determination of the plasma parameters. This spectrometer is promising for absolute spectral measurements of x-ray radiation of low-intensity sources and for numerous practical applications.
A compact device, based on fast capillary discharge plasmas, is an intense EUV and soft x-ray source of radiation. Th plasma is created by a discharge of low-inductance capacitors through a gas-filled ceramic capillary. Parameters of the discharge are: maximum current of 25 kA at applied voltage 40 kV, a pulse duration of 20-30 ns at FWHM, and a rise time of 1.5 ns. The soft x-ray and EUV emission of multiply charged ions is investigated using a compact 1 meter grazing incidence spectrometer-monochromator with a constant angle of deviation. The use of various gases allows the observation of XUV spectra in a wide spectral range (4- 45 nm). A Xe-filled capillary discharge shows intense radiation near 13.5 nm - the region of interest for EUV lithography applications. A reflectometer is used for testing grazing incidence gratings.
A compact device, based on fast capillary discharge plasmas, is used as an intense EUV and soft x-ray source of radiation. The plasma is created by a discharge of low-inductance capacitors (30 nF) through a capillary (2 mm diameter, 20 mm length). Two types of capillary are used: a polyacetal (empty) and a gas-filled (Ar, Xe). All components are assembled in a coaxial geometry. The total size of the device including capacitors, switch and capillary is 30 cm long and 25 cm in diameter. Discharge current is monitored using a shunt circuit.
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.
We propose to develop a time-gated x-ray imaging technique for screening mamography. If successful, it will greatly improve the accuracy of current mammography with about 50% reduction of current patient radiation. The technique can be applied to other x-ray imaging procedures also. We are now working to determine the feasibility of the key technologies- an x-ray source that can be pulsed on and off at a subnanosecond rate and a detector that can be gated at the same rate- that represent potential roadblocks to achieving this goal. A field emission x-ray tube will be used as the source. The field emission cathode consists of a substrate with sharp points and a metallic grid. A voltage on the grid creates an electric field that produces tunnel emission from the array of tips. The geometry of such a tube make electron optics easier, high currents are possible and the tube can be turned on and off rapidly. It also appears that synchronous electronic gating of the detector is possible.
The observation of EUV spectra of multiply charged ions excited by an ultrafast capillary discharge suggest the possibility of developing an intense compact source of soft x- ray and EUV radiation for practical applications (EUV lithography, microscopy, etc.). The discharge is driven by a compact water transmission line having a very fast current pulse (12 ns FHWM, rise time approximately 1 ns). Spectra of OV-OVI ions from a polyacetal capillary and ArVII-ArVIII ions from an Ar filled capillary were observed and investigated in a spectral range of 100 - 300 Angstrom using a 1-m grazing incidence spectrograph. The resulting capillary discharge plasmas have an electron temperature of approximately 25 - 35 eV, which could be increased to approximately 100 eV using a more powerful transmission line.
Our earlier papers showed that laser gain is possible on the innershell 2s-2p transitions of Ne-like ions. This result was confirmed by more comprehensive calculations. In more recent works it was shown that laser gain is also possible on transitions between highly excited states. We summarize these results and present atomic-kinetic calculations for gain values along the Ne-like sequence for argon, iron, krypton, silver and xenon. For these ions the optimum plasma conditions are found for all possible laser transitions. We have also found some unknown laser transition in Ni-like tantalum and the optimum plasma parameters for laser action on these transitions.
Within accurate atomic kinetic model it is shown that the inversion state of heavy Ne-like ions in plasma is possible at electron densities ne greater than 1023 cm-3 if electron temperature is high enough (Te greater than Eioniz/2). Near optimum plasma conditions are found for Ne-like silver lasers. New effective lasing transitions in Ne- like silver are found. Large ASE effect (gL greater than 100) is predicted for Ne-like silver plasma produced by a powerful short-pulse drive laser.
Energy level populations and spectral line intensities in plasmas are studied using an advanced uniform approach for the calculation of ion energy levels and elementary processes rate coefficients in plasmas. The spectrum of Ne-like argon for the uniform steady-state plasma is calculated for a wide variation of electron temperature and density. Hence the most efficient lasing at 5 3p - 3s transitions with wavelengths 697.6, 860.6, 727.5, 725.7, 670.7 angstroms should be expected at 1019 < ne < 1020 cm-3 and Te approximately 60 eV. It is shown that at electron densities of 1015 < ne < 1021 cm-3 ratios of the strongest 3p - 3s line intensities are sensitive to electron density; their dependence on electron temperature is much less pronounced.
Our group is studying the structure and interfaces of soft x-ray multilayers by various techniques including x-ray diffraction and Raman spectroscopy. Raman spectroscopy is particularly useful since it is sensitive to the identity of individual bonds and thus can potentially characterize the abruptness of interfaces in multilayers. Blocking interfacial mixing is very important in achieving and maintaining high reflectivity. We report our studies of the as-deposited and postannealed structure of Mo/Si and W/C multilayers. A high normal- incidence, peak reflectance is mandatory for imaging applications that involve many reflections. The reported theoretical and achieved reflectances of the Mo/Si system are 80% and 65%, respectively. This loss of 15% can bring about a six-fold loss in system throughput in the eight-reflection system contemplated. The interfaces in the Mo/Si system are thought to play a significant role in the degrading reflectance so characterization techniques which have interfacial sensitivity are particularly important. The Mo/Si multilayer system is susceptible to Raman characterization since both the a-Si spacer layer and the MoSi2 compound which forms at the interface have Raman active modes. In this paper we report the first Raman studies, to the best of our knowledge, of the a-Si layers and their crystallization and the crystallization of the Mo/Si interface of the multilayer brought about by a one-hour 1000 degree(s)C anneal. These changes are apparent in the Raman spectra before they can be unambiguously detected by x-ray diffraction.
X-ray phase diffraction gratings can be designed to behave in a fashion similar to blazed gratings, directing the majority of the energy into certain desired orders. They should be easy to fabricate using conventional semiconductor production technology, and offer advantages in design flexibility and efficiency over conventional amplitude grating or blazed grating structures. As a multilayered structure, a phase grating has structure in depth as well as across the surface. Most theoretical analyses in the literature treat the embedded structure through simplifying approximations or assumptions. We will discuss a model which treats the embedded structure explicitly using the Fresnel-Kirchhoff integral in the Fraunhofer diffraction limit. This approach produces a set of equations which are identical to the result for an amplitude diffraction grating except for an additional factor which depends on the phase relationships of the various surfaces in the multilayer stack.
We have produced arrays of 10,000 sharp p-type silicon points using an etch plus oxidation method. These points were used as electron emitters. No high vacuum caseation or high temperature cleaning was needed to observe the electron emission. These are seen to be photosensitive sources of electrons at 200 K and 300 K. They were also used to produce AlK(alpha ) x rays. This constitutes the first use of etched, point arrays for generating electrons for x-ray sources.
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