Time-integrated Images Research Articles

Advanced spectroscopic analysis of 0.8-1.0-MA Mo x pinches and the influence of plasma electron beams on L-shell spectra of Mo ions.

This paper presents a detailed investigation of the temporal, spatial, and spectroscopic properties of L-shell radiation from 0.8 to 1.0 MA Mo x pinches. Time-resolved measurements of x-ray radiation and both time-gated and time-integrated spectra and pinhole images are presented and analyzed. High-current x pinches are found to have complex spatial and temporal structures. A collisional-radiative kinetic model has been developed and used to interpret L-shell Mo spectra. The model includes the ground state of every ionization stage of Mo and detailed structure for the O-, F-, Ne-, Na-, and Mg-like ionization stages. Hot electron beams generated by current-carrying electrons in the x pinch are modeled by a non-Maxwellian electron distribution function and have significant influence on L-shell spectra. The results of 20 Mo x-pinch shots with wire diameters from 24 to 62 microm have been modeled. Overall, the modeled spectra fit the experimental spectra well and indicate for time-integrated spectra electron densities between 2 x 10(21) and 2 x 10(22) cm(-3), electron temperatures between 700 and 850 eV, and hot electron fractions between 3% and 7%. Time-gated spectra exhibit wide variations in temperature and density of plasma hot spots during the same discharge.

Soft X-Ray Images of Krypton Gas-Puff Z-Pinches

A series of experiments has been carried out on Qiang-guang I generator to study the dynamics of krypton gas-puff Z-pinches. The generator was operated at a peak current of 1.5 MA with a rise-time of 80 ns. The specific linear mass of gas liner was about 20 μg/cm in these experiments. In the diagnostic system, a four-frame x-ray framing camera and a pinhole camera were employed. A novel feature of this camera is that it can give time-resolved x-ray images with four frames and energy-resolved x-ray images with two different filters and an array of 8 pinholes integrated into one compact assemble. As a typical experimental result, an averaged radial imploding velocity of 157 km/s over 14 ns near the late phase of implosion was measured from the time-resolved x-ray images. From the time-integrated x-ray image an averaged radial convergence of 0.072 times of the original size was measured. An averaged radial expansion velocity was 130 km/s and the maximum radial convergence of 0.04 times of the original size were measured from the time-resolved x-ray images. The dominant axial wavelengths of instabilities in the plasma were between 1 and 2 mm. The change in average photons energy was observed from energy spectrum- and time-resolved x-ray images.

Strategy for the formation of parametric images under conditions of low injected radioactivity applied to PET studies with the irreversible monoamine oxidase A tracers [11C]clorgyline and deuterium-substituted [11C]clorgyline.

The construction of parametric positron emission tomography images of enzyme or receptor concentration obtained using irreversibly binding radiotracers presents problems not usually encountered with reversibly binding radiotracers. Difficulties are most apparent in brain regions having low blood flow and/or high enzyme or receptor concentration and are exacerbated with noisy data. This is especially true when minimal doses of radiotracer are administered. A comparison was recently reported of the irreversible monoamine oxidase A (MAO A) radiotracers [11C]clorgyline (CLG) and deuterium-substituted [11C]clorgyline (CLG-D) in the human brain using region of interest (ROI) analysis in which the authors observed an unexpected loss of image contrast with CLG-D compared with CLG. In order to more fully investigate patterns of binding of these irreversibly binding radiotracers, a strategy was devised to reduce noise in the generation of parametric images of the model term related to enzyme or receptor concentration. The generalized linear least squares (GLLS) method of Feng et al. (1995), a rapid linear method that is unbiased, was used for image-wide parameter estimation. Since GLLS can fail in the presence of large amounts of noise, local voxels were grouped within the image to increase the signal, and the GLLS method was combined with the standard nonlinear estimation methods when necessary. Voxels were grouped together depending on their proximity and whether they fell within a specified range of the time-integrated image. It was assumed that voxels meeting both criteria are functionally related. Simulations reflecting varying enzyme concentrations were performed to assess precision and accuracy of parameter estimates in the presence of varying amounts of noise. Using this approach, images were generated of the combination parameter lambdak3 (lambda = K1/k2, where K1 and k2 are plasma-to-tissue and tissue-to-plasma transport constants, respectively) that is related to enzyme concentration as well as images of the transport constant K1 for individual subjects. Reasonably high-quality images of both K1 and lambdak3 were obtained for CLG and CLG-D for individual subjects even with low injected doses averaging 6 mCi. While there were no differences in the K1 images, the lambdak3 images revealed the loss of contrast previously reported for CLG-D using the ROI analysis. This method should be generalizable to other tracers and should facilitate the analysis of group differences.

First results from cryogenic target implosions on OMEGA

Initial results from direct-drive spherical cryogenic target implosions on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] are presented. These experiments are part of the scientific base leading to direct-drive ignition implosions planned for the National Ignition Facility (NIF) [W. J. Hogan, E. I. Moses, B. E. Warner et al., Nucl. Fusion 41, 567 (2001)]. Polymer shells (1-mm diam with walls <3 μm) are filled with up to 1000 atm of D2 to provide 100-μm-thick ice layers. The ice layers are smoothed by IR heating with 3.16-μm laser light and are characterized using shadowgraphy. The targets are imploded by a 1-ns square pulse with up to ∼24 kJ of 351-nm laser light at a beam-to-beam rms energy balance of <3% and full-beam smoothing. Results shown include neutron yield, secondary neutron and proton yields, the time of peak neutron emission, and both time-integrated and time-resolved x-ray images of the imploding core. The experimental values are compared with 1-D numerical simulations. The target with an ice-layer nonuniformity of σrms=9 μm showed 30% of the 1-D predicted neutron yield. These initial results are encouraging for future cryogenic implosions on OMEGA and the NIF.

Imaging of exploding wire phenomena

Time-integrated and laser-probing imaging diagnostics provide insight about the physics of wire explosions. Most interesting results have been obtained for refractory metal wires. In these cases, imaging diagnostics provide insight into the structure and value of the energy deposited into metal.

Electron beam inspection system based on the projection imaging electron microscope

An electron beam inspection system based on the projection imaging electron microscope was developed and the proof-of-concept system has been constructed and evaluated. The secondary electrons are projected through the projection imaging optics and imaged onto the image detection system. The projected secondary electron image is amplified by the microchannel plate and converted to an optical image by the fluorescent screen and detected by the 2048 element, eight-tap time delay and integration (TDI) image sensor. The stage is linearly moved in synchronism with the TDI signal output data rate, and then, the secondary electron image is continuously captured. The spatial resolution of around 0.1 μm has been obtained in this experiment. Several images obtained by the TDI imaging mode are also demonstrated.

Rewritable Broad-Band Color Filters for Spectral Image Analysis

We propose an optical system to implement rewritable transparent broad-band color filters. The filter consists of a linear variable filter and a liquid crystal spatial light modulator in which an expected filter function is written. A time-integrated intensity image was taken while the filter was passing the lens aperture of a CCD camera. The averaged norm error between the implemented and the expected filter functions was about 5.4#x0025;. The system was applied to spectral estimation in a two-dimensional color image.

Imaging of laser–plasma x-ray emission with charge-injection devices

This work details the method of obtaining time-integrated images of laser–plasma x-ray emission using charge-injection devices (CIDs), as has been demonstrated on the University of Rochester’s 60-beam UV OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The CID has an architecture similar to a charge-coupled device. The differences make them more resistant to radiation damage and, therefore, more appropriate for some application in laser–plasma x-ray imaging. CID-recorded images have been obtained with x-ray pinhole cameras, x-ray microscopes, x-ray spectrometers, and monochromatic x-ray imaging systems. Simultaneous images obtained on these systems with calibrated x-ray film have enabled determination of the absolute detection efficiency of the CIDs in the energy range from 2 to 8 keV.

One-dimensional x-ray imaging using a spherically bent mica crystal at 4.75 keV

One-dimensional x-ray imaging of static gold bars using a spherically bent mica crystal is presented for the first time at an x-ray energy of 4.75 keV. X rays are produced using 1-ns-square pulses on the TRIDENT laser facility driving the He-like resonance transition in solid titanium disks. Time-integrated images of square profile parallel gold bars are recorded on direct exposure film with a magnification of ∼10. Rising edge measurements of the bars demonstrate resolutions of about 6–7 μm over a 400 μm field of view.

Evidence of thermal and non-thermal mechanisms coexisting in dense plasma focus D-D nuclear reactions

Dense plasma foci are the most efficient devices in the production of fast neutrons proceeding from deuterium nuclear fusion reactions. This work deals with experiments in a small plasma focus machine and attempts to distinguish neutrons of thermal origin from non-thermal neutrons, and the search for possible sources of both. Soft x-rays emitted by bremsstrahlung in a plasma focus were experimentally studied, using a multiple pin-hole camera with different aluminium absorbers in each hole. This method allows one to obtain the time-integrated soft x-ray image, as well as an estimation of the mean electronic temperature. The time-resolved soft x-ray intensity is registered with a filtered p-intrinsic-n (PIN) diode detector, shielded with a beryllium sheet. The time-resolved hard x-ray intensity (associated with particle acceleration) was registered using a plastic scintillator coupled to a fast photomultiplier tube. With a similar system, the time-resolved neutron emission is also registered. From the soft x-ray photographic studies, bright points with temperatures two or three times higher than the bulk plasma temperature are observed. These bright points (one or two per focus) can reach temperatures of over 7 keV, and their formation seems to correlate with successive necking produced by m = 0 instabilities in the pinch column. Time-integrated and time-resolved measurements of neutron yield, performed in comparison with time-resolved measurements of soft and hard x-ray radiation, show the different influence of thermal and non-thermal mechanisms in the nuclear fusion reactions.

Imaging of fusion reaction zone in plasma focus

In a low energy (2.3 kJ) Mather-type deuterium plasma focus, neutron and charged particle emission is investigated by using time-resolved neutron detectors and time-integrated charged particle pinhole imaging camera. The time-integrated charged particle pinhole images demonstrate the varying influence of magnetohydrodynamic (MHD) instabilities vis-a-vis filling pressure. The neutron production mechanism at play strongly depends upon the pressure. At lower pressure, the plasma column is highly unstable due to MHD instabilities and the neutron emission is found to be low with fluence anisotropy exceeding 3.5. At optimum pressure (2.5 mbar for this system), an almost stable dense plasma of about 17 mm3 volume is formed about 5 mm away from the anode, with neutron emission at its highest and the fluence anisotropy lowest. At higher pressure, the plasma column is stable, although it moves away from the anode like a jet and may then be called a moving boiler. In this case, the neutron emission is lowered compared to its optimum value and fluence anisotropy is increased. The data suggest beam-target mechanism at low pressure, trapped gyrating particles at optimum pressure and a jetlike moving boiler at higher pressure.

Three-dimensional time-resolved H pellet trajectory reconstruction in RFX by position sensitive detector Hα diagnostic

Hydrogen pellets injected in reversed field pinches (RFPs) experience large poloidal and toroidal accelerations along magnetic field lines due to the rocket effect caused by asymmetric ablation processes. The 3D pellet trajectory and the global ablation rate is diagnosed in RFX by an absolutely calibrated multiple position sensitive detector (PSD) system looking at the Hα radiation coming from the pellet ablation cloud. The design of the diagnostic is presented. Then the performance is highlighted by cross-comparison between different PSD measurements and with time-integrated images from a CCD camera. Overall accuracy of the order of a few millimeters is achieved, which makes the instrument well-suited to assist transport studies (particle deposition), to estimate the radial profile of fast electron tails (derived from the modulus of pellet acceleration) and to obtain information on internal magnetic field pitch (from the direction of pellet acceleration).

Design and performance studies of an MCP based pinhole camera for laser-produced plasma diagnostics

An X-ray pinhole camera system has been developed and tested to obtain spatially resolved time-integrated X-ray images of laser-produced plasmas (Nd: YAG laser of 75mJ energy in 35 picoseconds) using a single stage micro channel plate (MCP) detector coupled to a CCD camera-frame grabber system instead of X-ray film. Thick slab targets of aluminium, copper and gold were used to produce plasmas with different X-ray yields at laser intensities ranging from 8 × 1011 to 8 × 1012 W/cm2. MCP gain was increased with the help of biasing voltages to record low intensity X-ray images, which have been analysed using an image processing software “PROMISE” developed in-house. The experimental results are presented.

Observation of the characteristics of laser-plasma expansion in a strong transverse magnetic field on the basis of x-ray emission spectra

Observations were made of expansion, in a transverse quasisteady magnetic field (B ≈ 10 T), of a Teflon plasma generated by Nd laser radiation (λ = 1.06 μm, E ≈ 6 J, and τ ≈ 1ns). Time-integrated two-dimensional images of the plasma radiance were obtained in the spectrum of continuous and line x-ray radiation and spectroholiograms were recorded of 21P1—11S0 and 23P1—11S0 lines of the He-like F VIII ion. The plasma radiation was found to be anisotropic relative to the expansion axis and the dependence of the intensity on the distance from the target changed as a result of the influence of the transverse magnetic field on the plasma dynamics.

Optical and x-ray observations of carbon and aluminium fibre Z-pinch plasmas

This paper reports investigations of a Z-pinch plasma formed from carbon and aluminium fibres by a pulsed power generator delivering 100 kA with a rise time (10 - 90%) of 55 ns. For carbon fibres the breakdown was observed to proceed from the cathode and was coincident with a pulse of hard x-rays consistent with an electron beam of about 4 kA and > 50 keV. For aluminium fibres plasma emission was observed to be uniform over the whole length of the fibre and there was no hard x-ray signal. A radial expansion with a velocity of of the plasma column was seen for both fibres. During the expansion, at about 4 ns for carbon fibres and later for thicker fibres, m = 0 perturbations distributed along the length of the column became evident with a scale length which increased with time as the column expanded. Also for carbon fibres, two groups of bright spots separated by about 10 ns emitting optical and x-ray radiation were observed; the second group was more intense, lasted longer and showed bifurcation. Coincident with the second group was an electron beam of 16 keV for and 7 keV for carbon fibres. Bright spots were also evident for aluminium fibres but these were in a single group, lasted for a much longer time and were accompanied by emission from the anode consistent with 7 keV electron beam. Analysis of time-integrated x-ray pinhole images gave temperature of the bright spots between 75 - 135 eV for carbon fibres and 80 - 100 eV for carbon fibres. The very intense bright spots observed were found to coincide with density islands which were observed later in time but only for carbon fibres. The density in these islands was measured by interferometry to be greater than . After 110 ns plasma was not observable for carbon fibres; for carbon and aluminium fibres, the plasma was observed for longer times.

レーザ光散乱法によるＳｉウエハ付着微粒子計測装置の開発

This paper describes a system used for detecting the impurity particles on a bare Si wafer surface in a short time, at a wide area, and with a high reliability. The particles are less than 100 nm in diameter. The wafer is illuminated with CW Ar-ion laser beam (diameter : 100μm, power : 600 mW) at an oblique incident angle. The laser beam scattered by the particles is collected and focused at the direction perpendicular to the wafer surface. In order to achieve a uniform illumination in the whole observation area, the laser beam is scanned over the area. The time-integrated image of the surface is taken by a long-time exposure CCD camera. As a result, 37 nm-particles are observed in the image with the signal-to-noise ratio of 2.4. According to the Rayleigh equation, the noise signal is corresponding to 32 nm-particles.

Time-resolved imaging on a realistic tissue phantom: μs′ and μa images versus time-integrated images

A method is proposed by which we construct images through turbid media, plotting directly either the transport-scattering coefficient μ(s) ' or the absorption coefficient μ(a). These optical parameters are obtained from the best fit of the time-resolved transmittance curves with a diffusion model. Measurements were performed with a time-correlated single-photon counting system on realistic tissue phantoms simulating a tumor mass within a breast. Images were obtained with an incident power of <1 mW and an acquisition time of 1 s/point. Comparison of μ(s) ' and μ(a) images with time-integrated images constructed from the same experimental data shows that the fitting method discriminates between scattering and absorption inhomogeneities and improves image quality for scattering but not for absorption inhomogeneities.

Effects of anode shape on plasma focus operation with argon

X-ray and ion beam emission from a low-energy Mather-type plasma focus energized by a single , 12 kV (2.3 kJ) capacitor is investigated employing three different anode shapes and using argon as filling gas. The radiation yield and the filling gas pressure for good focus are found to be strongly dependent on the anode shape. An appropriate tapering of the anode end enhances the emission threefold for both x-rays and ions. The intensities of the signals are also found to be correlated mutually as well as with the high-voltage probe signal intensity. Furthermore, time-integrated pinhole images reveal that the x-rays originate predominantly from the anode end surface. In other words, an appropriate shaping of the anode end switches the device to a high-emission mode for both x-rays and a charged particle beam.

Novel x-ray imaging methods at the Nova laser facility (invited)

We are pursuing several novel x-ray imaging schemes to measure plasma parameters in inertial-confinement fusion experiments. This paper will concentrate principally on two quite successful approaches, the soft x-ray moiré deflectometer, and the annular (ring) coded-aperture microscope. The first scheme uses moiré deflectometry to measure the electron density within a plasma. When a beam of collimated light travels through a plasma, it is refracted by electron-density gradients. Moiré deflectometry is a scheme to measure the refraction, and thus provides a diagnostic of the line-integrated electron-density spatial profile. To use deflectometry with a high-density laser-produced plasma, we produced a probe beam with a soft x-ray laser (λ=15.5 nm). The short wavelength of the probe radiation allows us to measure the spatial profiles at densities up to critical (e.g., 4×1021 cm−3) in long scale-length (≳1 mm) plasmas. We use finely made one-dimensional rulings (10-μm pitch), a set of synthetic multilayer mirror optics, and a soft-x-ray sensitive CCD camera to achieve a spatial resolution of 6 μm. In the second scheme, a ring coded-aperture microscope (RAM) provides a large (factor of 5–10) improvement in signal-to-noise ratio over pinhole imaging when the source size is much less than the major diameter of the annular aperture. It is therefore very useful to image the cores of imploded laser-fusion capsules. We have obtained very high-quality (SNR≊100) time-integrated images of indirectly driven targets. By coupling ring apertures with x-ray framing cameras, we have also obtained high-quality (SNR≊50) time-resolved images of directly driven capsule cores. These images can be used to observe the symmetry of the drive and the hydrodynamics of the implosion. Recently, we also obtained high-quality images of short-pulse laser (0.6 ps) irradiations of Ta targets. The spatial resolution of the RAM has been investigated both experimentally and through diffraction calculations. The resolution is at least as small as the width of the annulus, and may be smaller under particular circumstances.

Observation of plasma confinement in picosecond laser-plasma interactions.

Novel features associated with multiterawatt laser interactions with solid targets at incident irradiances in the range (1.0--12.0)\ifmmode\times\else\texttimes\fi{}${10}^{17}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ are presented. Collimation of the plasma flow normal to the target surface has been observed in time-integrated x-ray images and ion-velocity measurements. Ions emitted from the target have a characteristic energy of typically 100 keV. The ion spectrum extends to 1.5 MeV. Calculations are presented that suggest the observed collimation is due to a large magnetic field pinching the plasma.