X-ray framing cameras based on proximity-focused micro-channel plates (MCP) have been playing an important role as diagnostics of inertial confinement fusion experiments . Most of the current x-ray framing cameras consist of a single MCP, a phosphor, and a recording device (e.g. CCD or photographic films). This configuration is successful for imaging x-rays with energies below 20 keV, but detective quantum efficiency (DQE) above 20 keV is severely reduced due to the large gain differential between the top and the bottom of the plate for these volumetrically absorbed photons . Recently developed diagnostic techniques at LLNL require recording backlit images of extremely dense imploded plasmas using hard x-rays, and demand the detector to be sensitive to photons with energies higher than 40 keV . To increase the sensitivity in the high-energy region, we propose to use a combination of two MCPs. The first MCP is operated in low gain and works as a thick photocathode, and the second MCP works as a high gain electron multiplier [4,5]. We assembled a proof-of-principle test module by using this dual MCP configuration and demonstrated 4.5% DQE at 60 keV x-rays.
The requirements for beam and target alignment for successful ignition experiments on the National Ignition Facility
(NIF) are stringent: the average of beams to the target must be within 25 μm. Beam and target alignment are achieved
with the Target Alignment Sensor (TAS). The TAS is a precision optical device that is inserted into target chamber
center to facilitate both beam and target alignment. It incorporates two camera views (upper/lower and side) mounted on
each of two stage assemblies (jaws) to view and align the target. It also incorporates a large mirror on each of the two
assemblies to reflect the alignment beams onto the upper/lower cameras for beam alignment. The TAS is located in the
chamber using reference features by viewing it with two external telescope views. The two jaws are adjusted in elevation
to match the desired beam and target alignment locations. For some shot setups, a sequence of TAS positions is required
to achieve the full setup and alignment. In this paper we describe the TAS, the characterization of the TAS coordinates
for beam and target alignment, and summarize pointing shots that demonstrate the accuracy of beam-target alignment.
X-ray imaging is integral to the measurement of the properties of hot plasmas. To this end, a suite of gated x-ray imagers
have been developed for use in a wide range of experiments at the National Ignition Facility (NIF). These instruments
are sensitive to x-rays over the range of 0.7-90keV and can acquire images at 20ps intervals for source intensities
ranging over several orders of magnitude. We review the design, technology, and construction of these instruments and
present recent results obtained from NIF experiments in which gated x-ray imagers have played a key role.
The radiation environment associated with Inertial Confinement Fusion (ICF) experiments presents unique challenges
for x-ray imaging. We report on the performance of gated imagers that have been optimized for this harsh environment
and describe diagnostics to be deployed in the near future that will provide x-ray images of imploding ICF capsules in
the presence of backgrounds associated with neutron yields above 1016. Such images will provide crucial data that will
enable even higher neutron yields and successful ignition.
X-ray imaging instruments will operate in a harsh ionizing radiation background environment on implosion experiments
at the National Ignition Facility. These backgrounds consist of mostly neutrons and gamma rays produced by inelastic
scattering of neutrons. Imaging systems based on x-ray framing cameras with film and CCD's have been designed to
operate in such harsh neutron-induced background environments. Some imaging components were placed inside a
shielded enclosure that reduced exposures to neutrons and gamma rays. Modeling of the signal and noise of the x-ray
imaging system is presented.
A method for the determination of the deposited layer thickness distribution through the stack has been presented in a
previous article . We illustrate the validity of this model by considering the deposition of a Mo/B4C design. This
method is further improved to allow for the additional determination of roughness/diffusion through the multilayer stack.
We show results of the analysis for a deposited small d-spacing W/B4C design (davg=1.5nm) which give compelling
evidence for the existence and determination of a minimum value of thickness that can be allowed in the design of the
structure. From this analysis the multilayer was redesigned with a constraint on the minimum thickness allowed in the
stack. We show successful results of the deposited redesigned structure. Finally, we show the influence of random
layer thickness error on the resultant reflectivity.
We present recent results using multi-monochromatic X-ray imaging of direct drive implosions at the OMEGA laser facility. An array of pinholes coupled to a flat multilayer mirror provides multi-spectral images distributed over a wide spectral range. Using Argon as a dopant in the DD-filled plastic shells produces emission images in the Ar He-β and Ly-β spectral regions. When used in conjunction with gated imaging detectors the instrument provides images with spatial resolution of ~10μm and temporal resolution of ~50ps. A special algorithm has been developed to reconstruct narrow-band images, which will allow the retrieval of temperature, and density maps of the core as it evolves through peak compression.
We propose to generate few-fs or as X-ray laser pulses by beating of two or more X-ray laser lines with appropriate frequency separation. X-ray lasers operating on transitions in neon- or nickel-like ions typically have gain on several lines with difference frequencies of around 1015 Hz. Moreover, it is found in specific cases that a few almost equidistant lines may exhibit gain. Beating of these lines results in a series of pulses with durations down to the attosecond range. It is shown that phase locking can be achieved by means of a Langmuir wave in the X-ray laser medium itself, which is resonant with the difference frequency.
X-ray pulses from a fs-laser plasma were focused by an X-ray capillary lens, generating a spot smaller than 100 μm. Fe Kα radiation (λ = 0.194 nm) is produced by focusing 200 mJ/130 fs pulses from the ATLAS titanium-sapphire laser at 10 Hz onto a moving iron tape. The capillary lens enhanced the intensity by a factor of about 1600. Diffraction from samples of small size is demonstrated by producing diffractograms from a Si (111) crystal in only about 10 seconds. The model of a novel ultrafast streak camera which takes advantage of the different path lengths of rays propagating through the lens is demonstrated. Preliminary experiments using a semi-lens for collimating X-rays are also reported.
A novel laser-pumped X-ray source is used to investigate generation of shock waves in a semiconductor and conformational changes in a molecular crystal. Ultrashort Cu-Kα pulses are generated by focusing 130 fs laser pulses from the ATLAS titanium-sapphire laser of our institute on a slowly moving copper tape. Irradiating Si(111) surfaces with a few 100 mJ/cm2 pulses at 800 nm we observe an increase in the integrated reflection on a relatively slow time scale of several 100 ps. This observation is explained by the increased geometrical structure factor generated by the shock wave propagating into a mosaic crystal. The work on conformational changes was performed with DMABN (dimethylaminobenzonitrile, sum formula C9H10N2). A pump-probe experiment using the third harmonic of the titanium-sapphire laser (λ = 265 nm) as the pump yields indications of an increase of the 004 reflection in a time shorter than 10 ps. Such an increase is expected owing to photo-induced rotation of the two methyl groups around the major axis of the molecule.
We present measurements of electron densities of plasmas with fs resolution. The plasmas are generated by laser pulses with different intensities at different time delays. Such plasmas are of great interest as preplasmas for transient, collisionally excited X-ray lasers. The prepulse is generated by stretching part of a 130-fs laser pulse of the ATLAS titanium-sapphire laser of our institute. Focusing this radiation to a line on molybdenum and silver targets generates preplasmas highly interesting to research directed towards a 10 Hz sub-Joule soft X-ray laser. The electron density is measured as a function of distance from the target by interferometry using a Wollaston prism. The ultrashort probe pulse allows one to obtain data as close as 10 - 20 μm from the target surface. Experimental data are compared with simulations using the MULTI hydrocode. The results allow optimization of prepulse-main pulse delay times and compare ablation from a hard (Mo) and a soft (Ag) material.
Experimental investigations on the conditions to achieve transient gain in neon-like Ti and nickel-like molybdenum XUV laser pumped by a 10-Hz sub-Joule femtosecond laser are presented. The 4d-4p (J equals 0-1) (lambda) equals 18.9 nm and 4f-4d (J equals 1-1) (lambda) equals 22.6 nm lines in Ni-like Mo as well as the 3p-3s (J equals 0-1) (lambda) equals 32.6-nm line in neon-like titanium have been observed. The Ni-like laser lines show a threshold behavior with respect to the pump irradiance as they appear only above 1015 W/cm2. Simulation for the fs-laser pumped Ni-like Mo XUV laser are also presented.
We report on the experimental demonstration of saturated x- ray laser output from collisionally pumped Ne-like Fe at 25.5 nm as well as Ni-like Ag and Pd at 14.0 nm and 14.7 nm, respectively, using a 100-ps drive pulse irradiation. A double-prepulse scheme and a 3-m radius-of-curvature target resulted in a gain-length product of gL equals 16.5 in the case of Fe. With a single prepulse and flat slab targets, gain- length products of 15.3 and 15.8 were obtained for Ag and Pd, respectively. Saturation was also confirmed by the observed reduction in beam divergence with increasing target length. The required drive energy used was only 30 J in a 100-ps pulse, corresponding to an irradiance of 12 TW/cm2. A key role in the achievement of these results was played by the reduction in the roughness of the target surface in the case of the Pd x-ray laser.