Unconverted Light Research Articles

Developing time-resolved x-ray diffraction diagnostics at the National Ignition Facility (invited).

As part of a program to measure phase transition timescales in materials under dynamic compression, we have designed new x-ray imaging diagnostics to record multiple x-ray diffraction measurements during a single laser-driven experiment. Our design places several ns-gated hybrid CMOS (hCMOS) sensors within a few cm of a laser-driven target. The sensors must be protected from an extremely harsh environment, including debris, electromagnetic pulses, and unconverted laser light. Another key challenge is reducing the x-ray background relative to the faint diffraction signal. Building on the success of our predecessor (Target Diffraction In Situ), we implemented a staged approach to platform development. First, we built a demonstration diagnostic (Gated Diffraction Development Diagnostic) with two hCMOS sensors to confirm we could adequately protect them from the harsh environment and also acquire acceptable diffraction data. This allowed the team to quickly assess the risks and address the most significant challenges. We also collected scientifically useful data during development. Leveraging what we learned, we recently developed a much more ambitious instrument (Flexible Imaging Diffraction Diagnostic for Laser Experiments) that can field up to eight hCMOS sensors in a flexible geometry and participate in back-to-back shots at the National Ignition Facility (NIF). The design also allows for future iterations, such as faster hCMOS sensors and an embedded x-ray streak camera. The enhanced capabilities of the new instrument required a much more complex design, and the unexpected issues encountered on the first few shots at NIF remind us that complexity has consequences. Our progress in addressing these challenges is described herein, as is our current focus on improving data quality by reducing x-ray background and quantifying the uncertainties of our diffraction measurements.

Second harmonic generation of focused beams on the LFEX laser facility.

There is a strong demand for efficient second harmonic generation (SHG) in ultra-intense short-pulse lasers. This paper demonstrates the generation of an unconverted fundamental (1ω)+second harmonics (2ω) mixed laser on the LFEX laser system. The experimental setup utilizes 0.5 mm-thick LBO crystal plates in a focusing beams implemented after an off-axis parabola, the design reduces the size and cost of the SHG system. The LFEX laser beams with four-beams combined energy of 222 J and a pulse duration of 1.5 ps, is successfully converted to 102 J of 2ω light and 100 J of unconverted 1ω light, 20 J is lost through surface reflections, and they are mixed at the focal point. Verification of successful SHG is confirmed through X-ray pinhole imaging and electron spectrometry. This novel technique is not limited to LFEX lasers and holds applicability for various ultra-intense lasers. Consequently, this accomplishment significantly contributes to expanding the capability for high-energy density laser-plasma experiments.

Etendue maintained white solid-state light generation by light emitting diodes pumped green aluminate green-yellow phosphor

We propose a technique to generate etendue maintained white light. The proposed technique uses three high-power blue light emitting diodes (LEDs) with identical characteristics to pump green aluminate (GAL) green-yellow phosphor coated on the inner channel of a waveguide. The technique offers two methods of generation of white light. Standalone partial conversion of blue to green and yellow light by GAL phosphor, combined with unconverted blue light, produces white light homogenized within the light guide. Complete conversion of blue to green and yellow light with the further addition of blue and red wavelengths from additional LED sources multiplexed with the output is also possible to further increase total white light output. Following design and optical modeling to prove the concept, the proposed technique is demonstrated, which shows white light emissions from the waveguide with no increase in aperture size while the original LED etendue is maintained. The proposed technique offers an alternative to low-lifetime, low-efficiency xenon lamps for light sources in etendue critical applications such as endoscopy.

Demonstration of a kilowatt average power, 1 J, green laser.

A λ=515nm laser generating joule-level pulses at 1 kHz repetition rate was demonstrated by frequency doubling 1.2 J, 2 ns temporally shaped square pulses from a cryogenically cooled Yb:YAG laser in an LBO crystal. A doubling efficiency of 78% resulted in 0.94 J second-harmonic pulses at 1 kHz. The unconverted light interacted with a second LBO crystal to generate >100mJ second-harmonic pulses to reach a total green average power of 1.04 kW. A conversion efficiency of 89% was achieved for 0.58 J green pulses at 1 kHz. These results open the possibility to pump high energy femtosecond lasers at kilohertz repetition rates.

Revisiting beam filamentation formation conditions in high power lasers

The Bespalov-Talanov gain (BT-gain) and IL-rule (i.e., the product of input intensity and self-focusing length is constant) expressions are examined and generalized for filamentation under realistic conditions associated with high power lasers: filamentation seeded by both amplitude and phase perturbations on a large, flat-top beam, and the impact of cross-phase modulation from unconverted light in UV frequency-converted lasers. The validity of these models is examined with NLSE numerical calculations, which show that there are parameters beyond the commonly-used IL rule, such as the perturbation amplitude and period content. The BT-gain model presents a fair description of the tendency of spatial periods to filament, but not of the quantitative self-focusing length. Spatial filtering of short periods is shown to suppress filamentation, due to both, the removal of the more prone to filament periods, as well as the reduction of the spatial intensity amplitude root-mean-square. At the edge of a top hat beam we find that the IL product reduces in the roll-off regions, even though the self-focusing length increases. When adding a co-propagating harmonic, we find that the cross-phase modulation (XPM) could enhance or inhibit the filamentation formation, depending on the perturbation period.

Assessment of the intricate nickel-based anodic reactions mechanism within a methanol fed solid oxide fuel cell based on a co-ionic conducting composite electrolyte

We propose here a comprehensive modelling approach to predict the intricate anodic electrochemical reactions within a direct methanol fed solid oxide fuel cell based on a H+/O2− dual conducting composite electrolyte. We assess the effect of the ionic transference number (tn) on open circuit voltage of the cell operating at 650 °C, 700 °C and 750 °C to determine an optimum tn. The modelling proposes three electrochemical reactions scenarios associated to direct methanol fed solid oxide fuel cell i.e. (1) full methanol oxidation (2) total methanol reforming and subsequent light gases oxidations (3) mixed oxidation of unconverted methanol and light gases. Simulation results prove the capability of scenario 2 to predict adequately the anodic reactions mechanism indicating the total reforming of methanol over the Nickel-samarium doped ceria anode at 650 °C and the electrochemical oxidation of around 1% and 7% of methanol molecules at 700 °C and 750 °C, respectively. Furthermore, scenario 1 exhibits the highest efficiency ∼50% in the temperature range between 650 °C and 750 °C with limited fuel utilization ∼39%. Nevertheless, the fuel utilization increases to 30% and 70% at 650 °C and 750 °C respectively, when using scenario 3.

Wide-Range Correlated Color Temperature Light Generation From Resonant Cavity Hybrid Quantum Dot Light-Emitting Diodes

This study presents extremely uniform colloidal quantum dot white light-emitting diodes (QD-WLEDs) that demonstrate a high color rendering index (CRI) and correlated color temperatures (CCTs) ranging from 2500 to 4500 K. Experimental results indicate that the structure of the distributed Bragg reflector (DBR) containing a stopband in the UV region enhances the intensity output of both monochromatic QD-LEDs and QD-WLEDs by reflecting the unconverted UV light back onto the package to excite the QDs further. Furthermore, the angular CCT uniformity of the QD-WLEDs also improved considerably because of the dependence of the DBR structure on the incident angle. The angular CCT deviation in the range of –70° to 70° decreased to 39 K and the CRI of the WLED is higher than 90. The high-CRI and uniform angular CCT QD-WLED containing the DBR demonstrates potential applicability as the lighting source of next-generation display devices and solid-state lighting.

Development of a Big Area BackLighter for high energy density experiments.

A very large area (7.5 mm(2)) laser-driven x-ray backlighter, termed the Big Area BackLighter (BABL) has been developed for the National Ignition Facility (NIF) to support high energy density experiments. The BABL provides an alternative to Pinhole-Apertured point-projection Backlighting (PABL) for a large field of view. This bypasses the challenges for PABL in the equatorial plane of the NIF target chamber where space is limited because of the unconverted laser light that threatens the diagnostic aperture, the backlighter foil, and the pinhole substrate. A transmission experiment using 132 kJ of NIF laser energy at a maximum intensity of 8.52 × 10(14) W/cm(2) illuminating the BABL demonstrated good conversion efficiency of >3.5% into K-shell emission producing ~4.6 kJ of high energy x rays, while yielding high contrast images with a highly uniform background that agree well with 2D simulated spectra and spatial profiles.

The influence of laser clipped by the laser entrance hole on hohlraum radiation measurement on Shenguang-III prototype

Measuring the x-ray flux exiting the target's laser entrance hole (LEH) is the most common diagnostic that quantifies the x-ray intensity inside the laser-driven hohlraum. However, this signal accounts for only a small portion of the incident laser power and thus is likely to be affected by unwanted x-ray background from non-target area, leading to an overestimation of the hohlraum drive. Unwanted emission might be produced when the laser light is clipped by the LEH (LEH clipping) because of a lack of clearance for laser spot, or with a laser spot comprising of discrete structure, or even with a poor pointing accuracy. Its influence on the hohlraum radiation diagnostic is investigated on Shenguang-III prototype laser facility with the typical 1 ns square pulse. The experiment employed three types of targets to excite the unwanted x-ray background from LEH clipping, unconverted light, and both effects, respectively. This work gives an absolute evaluation of x-ray produced by the LEH clipping, which is measured by flat-response x-ray detectors (FXRD) at multiple view angles. The result indicates that there is little variation in measured emission to various view angles, because the unwanted x-rays are mainly generated at the side face of the LEH lip when laser is obliquely incident. Therefore, the LEH clipping brings more overestimation in hohlraum radiation measurement at larger view angle since the hohlraum LEH as an emitting source viewed by FXRD is decreased as the view angle increases. In our condition, the LEH clipping contributes 2%-3.5% overestimation to the hohlraum flux measurement.

Demonstration of Color Separation with 2ω KDP Wedge in High Power Laser Facilities

A 2ω wedge design is proposed with KDP crystal to disperse the unconverted light away from the target in a high power laser facility for inertial confinement fusion. The ultraviolet B-integral problem is released, and about 1.2 times in color separation angle is achieved according to both theoretical and experimental investigations when compared with conventional 3ω wedge. The frequency conversion efficiency is unaffected when the wedge is along the non-sensitive axis of the tripler.

Observation of two output light pulses from a partial wavelength converter preserving phase of an input light at a single-photon level

We experimentally demonstrate that both of the two output light pulses of different wavelengths from a wavelength converter with various branching ratios preserve phase information of an input light at a single-photon level. In our experiment, we converted temporally-separated two coherent light pulses with average photon numbers of ∼ 0.1 at 780 nm to light pulses at 1522 nm by using difference-frequency generation in a periodically-poled lithium niobate waveguide. We observed an interference between temporally-separated two modes for both the converted and the unconverted light pulses at various values of the conversion efficiency. We observed interference visibilities greater than 0.88 without suppressing the background noises for any value of the conversion efficiency the wavelength converter achieves. At a conversion efficiency of ∼ 0.5, the observed visibilities are 0.98 for the unconverted light and 0.99 for the converted light. Such a phase-preserving wavelength converter with high visibilities will be useful for manipulating quantum states encoded in the frequency degrees of freedom.

Fabrication Improvements of the Aluminum Unconverted Light Shields for the National Ignition Campaign

The National Ignition Campaign (NIC) target consists of precisely machined and assembled components. A subset of the components of this target is the aluminum shielding around the silicon support and cooling arms, which is designed to alleviate harmful unconverted light reflecting from the arms into the laser optics. This NIC target shielding consists of two external shields and four inner shields located between the arms. Recently, we have developed a process to add a plastic coating to the shields with precisely defined edges that can survive pressing the part into a three-dimensional shape. After this process was demonstrated on prototypes, it was further refined to improve yield and is currently being used to fabricate and deliver parts for NIC experiments on a regular basis. The final process that we developed consists of seven steps to fabricate these shields: (1) applying a photolithographed plastic layer to electrically isolate the shields from the electrical traces on the cooling arms, (2) plasma etching to improve adhesion during the subsequent aluminum coating, (3) large-batch electron-beam aluminum coating, (4) laser cutting and custom die cutting to various shapes and specified patterns, (5) utilizing heat pressing techniques that soften the plastic coating enough to be coined into a three-dimensional shape, (6) accurate positioning and bonding of Mylar liners under the tops of the shields, and (7) final characterization. Through these process steps high process yields were achieved against the rigorous NIC requirements.

Collection of solid and gaseous samples to diagnose inertial confinement fusion implosions

Collection of representative samples of debris following inertial confinement fusion implosions in order to diagnose implosion conditions and efficacy is a challenging endeavor because of the unique conditions within the target chamber such as unconverted laser light, intense pulse of x-rays, physical chunks of debris, and other ablative effects. We present collection of gas samples following an implosion for the first time. High collection fractions for noble gases were achieved. We also present collection of solid debris samples on flat plate collectors. Geometrical collection efficiencies for Au hohlraum material were achieved and collection of capsule debris (Be and Cu) was also observed. Asymmetric debris distributions were observed for Au and Be samples. Collection of Be capsule debris was higher for solid collectors viewing the capsule through the laser entrance hole in the hohlraum than for solid collectors viewing the capsule around the waist of the hohlraum. Collection of Au hohlraum material showed the opposite pattern: more Au debris was collected around the waist than through the laser entrance hole. The solid debris collectors were not optimized for minimal Cu backgrounds, which limited the conclusions about the symmetry of the Cu debris. The quality of the data limited conclusions on chemical fractionation effects within the burning, expanding, and then cooling plasma.

NIF unconverted light and its influence on DANTE measurements

NIF laser facility produces 1053 nm light and a fundamental requirement for NIF is to give up to 1.8 MJ of 351 nm light for target physics experiments. The 351 nm light is provided by frequency tripling the 1053 nm light in nonlinear crystals in the final optics assembly, just before the laser light enters the target chamber. Since this tripling process is not 100% efficient, unconverted light from the conversion process also enters the chamber. This unconverted light does not directly hit the target but it can strike target support structures at average intensities of few TW/cm2 where it can generate unwanted, background soft x-rays that are measured by the soft x-ray diagnostic DANTE installed on the NIF target chamber. This diagnostic quantifies the x-radiation intensity inside the hohlraum by measuring the x-ray flux coming from the target's laser entrance hole. Due to its centimeter wide field of view, it integrates x-ray emission from both the flux exiting a hohlraum laser entrance hole and from the target support structure irradiated by residual 1omega and 2omega unconverted light. This work gives quantitative evaluations of the unconverted light for the first time and the effects on DANTE measurements for the future NIF tuning experiment called "Shock timing." Emission spectra are significantly modified leading to an overestimation of radiative temperature during the foot of the laser pulse since background x-rays are predominant in first two DANTE channel measurements. Mitigations of these effects by coating silicon paddle with plastic, using a smaller collimator to reduce DANTE field of view or eliminating DANTE channels in the analysis have been investigated.

Continuous depth-of-interaction encoding using phosphor-coated scintillators

We investigate a novel detector using a lutetium oxyorthosilicate (LSO) scintillator and YGG (yttrium–aluminum–gallium oxide:cerium, Y3(Al,Ga)5O12:Ce) phosphor to construct a detector with continuous depth-of-interaction (DOI) information. The far end of the LSO scintillator is coated with a thin layer of YGG phosphor powder which absorbs some fraction of the LSO scintillation light and emits wavelength-shifted photons with a characteristic decay time of ∼50 ns. The near end of the LSO scintillator is directly coupled to a photodetector. The photodetector detects a mixture of the LSO light and the light emitted by YGG. With appropriate placement of the coating, the ratio of the light converted from the YGG coating with respect to the unconverted LSO light can be made to depend on the interaction depth. DOI information can then be estimated by inspecting the overall light pulse decay time. Experiments were conducted to optimize the coating method. 19 ns decay time differences across the length of the detector were achieved experimentally when reading out a 1.5 × 1.5 × 20 mm3 LSO crystal with unpolished surfaces and half-coated with YGG phosphor. The same coating scheme was applied to a 4 × 4 LSO array. Pulse shape discrimination (PSD) methods were studied to extract DOI information from the pulse shape changes. The DOI full-width-half-maximum (FWHM) resolution was found to be ∼8 mm for this 2 cm thick array.

Predicting the Performance of System for the Co-production of Fischer-Tropsch Synthetic Liquid and Power from Coal

A co-production system based on Fischer-Tropsch (FT) synthesis reactor and gas turbine was simulated and analyzed. Syngas from entrained bed coal gasification was used as feedstock of the low-temperature slurry phase Fischer-Tropsch reactor. Raw synthetic liquid produced was fractioned and upgraded to diesel, gasoline, and liquid petrol gas (LPG). Tail gas composed of unconverted syngas and FT light components was fed to the gas turbine. Supplemental fuel (NG, or refinery mine gas) might be necessary, which was dependent on gas turbine capacity, expander through flow capacity, etc. FT yield information was important to the simulation of this co-production system. A correlation model based on Mobil’s two step pilot plant was applied. This model proposed triple chain-length-dependent chain growth factors and set up correlations among reaction temperatures with wax yield, methane yield, and C2–C22 paraffin and olefin yields. Oxygenates in the hydrocarbon, water, and vapor phases were also correlated with methane yield. It was suitable for syngas, iron catalyst, and slurry bed. We can show the effect of temperature on the products’ selectivity and distribution. User models that can predict product yields and cooperate with other units were embedded into Aspen plus simulation. Performance prediction of syngas fired gas turbine was the other key of this system. The increase in mass flow through the turbine affects the match between compressor and turbine operating conditions. The calculation was carried out by GS software developed by Politecnico Di Milano and Princeton University. The simulated performance assumed that the expander operates under choked conditions and turbine inlet temperature equals the NG fired gas turbine. A “F” technology gas turbine was selected to generate power. Various cases were investigated to match the FT synthesis island, power island, and gasification island in co-production systems. Effects of CO2 removal/LPG recovery, co-firing, and CH4 content variation were studied. Simulation results indicated that more than 50% of input energy was converted to electricity and FT products. Total yield of gasoline, diesel, and LPG was 136–155g∕Nm3(CO+H2). At coal feed of 21.9kg∕s, net electricity exported to the grid was higher than 100MW. Total production of diesel and gasoline (and LPG) was 118,000t(134,000t)∕year. Under the economic analysis conditions assumed in this paper, the co-production system was economically feasible. The after tax profits can research 17 million euro. Payback times ranged from 6 to 7 years.

Dante soft x-ray power diagnostic for National Ignition Facility

Soft x-ray power diagnostics are essential for measuring the total x-ray flux, radiation temperature, conversion efficiency, and albedo that define the energetics in indirect and direct drive, as well as other types of high temperature laser plasma experiments. A key diagnostic for absolute radiation flux and radiation temperature in hohlraum experiments is the Dante broadband soft x-ray spectrometer. For the extended range of x-ray fluxes predicted for National Ignition Facility (NIF) compared to Omega or Nova hohlraums, the Dante spectrometer for NIF will include more high energy (<2 keV) edge filter band-pass channels and access to an increased dynamic range using grids and signal division. This will allow measurements of radiation fluxes of between 0.01 to 100 TW/sr, for hohlraum radiation temperatures between 50 eV and 1 keV. The NIF Dante will include a central four-channel imaging line-of-sight to verify the source size, alignment as well as checking for any radiation contributions from unconverted laser light plasmas.

Compact vacuum ultraviolet source for photoelectron spectroscopy

A simple source chamber is described which enables the introduction of laser-generated, vacuum ultraviolet (VUV) radiation below 102 nm into most existing photoelectron and photoion spectrometers. A long capillary light guide provides a conductance barrier between the VUV source and experimental chambers, and also provides a small, well-collimated VUV beam at the interaction region. A platinum photodiode is found to provide a useful measure of the VUV intensity with minimal interference from the unconverted UV light. The performance and capabilities of the source are demonstrated by several applications using photoelectron spectroscopy.