Experimental evidence of early-time saturation of the ion-Weibel instability in counterstreaming plasmas of CH, Al, and Cu.

The collisionless ion-Weibel instability is a leading candidate mechanism for the formation of collisionless shocks in many astrophysical systems, where the typical distance between particle collisions is much larger than the system size. Multiple laboratory experiments aimed at studying this process utilize laser-driven (I≳10^{15} W/cm^{2}), counterstreaming plasma flows (V≲2000 km/s) to create conditions unstable to Weibel-filamentation and growth. This technique intrinsically produces temporally varying plasma conditions at the midplane of the interaction where Weibel-driven B fields are generated and studied. Experiments discussed herein demonstrate robust formation of Weibel-driven B fields under multiple plasma conditions using CH, Al, and Cu plasmas. Linear theory based on benchmarked radiation-hydrodynamic FLASH calculations is compared with Fourier analyses of proton images taken ∼5-6 linear growth times into the evolution. The new analyses presented here indicate that the low-density, high-velocity plasma-conditions present during the first linear-growth time (∼300-500 ps) sets the spectral characteristics of Weibel filaments during the entire evolution. It is shown that the dominant wavelength (∼300µm) at saturation persists well into the nonlinear phase, consistent with theory under these experimental conditions. However, estimates of B-field strength, while difficult to determine accurately due to the path-integrated nature of proton imaging, are shown to be in the ∼10-30 T range, an order of magnitude above the expected saturation limit in homogenous plamas but consistent with enhanced B fields in the midplane due to temporally varying plasma conditions in experiments.

Measurement of Kinetic-Scale Current Filamentation Dynamics and Associated Magnetic Fields in Interpenetrating Plasmas.

We present the first local, quantitative measurements of ion current filamentation and magnetic field amplification in interpenetrating plasmas, characterizing the dynamics of the ion Weibel instability. The interaction of a pair of laser-generated, counterpropagating, collisionless, supersonic plasma flows is probed using optical Thomson scattering (TS). Analysis of the TS ion-feature revealed anticorrelated modulations in the density of the two ion streams at the spatial scale of the ion skin depth c/ω_{pi}=120 µm, and a correlated modulation in the plasma current. The inferred current profile implies a magnetic field amplitude ∼30±6 T, corresponding to ∼1% of the flow kinetic energy, indicating that magnetic trapping is the dominant saturation mechanism.

Extreme Hardening of Pb at High Pressure and Strain Rate.

We study the high-pressure strength of Pb and Pb-4wt%Sb at the National Ignition Facility. We measure Rayleigh-Taylor growth of preformed ripples ramp compressed to ∼400 GPa peak pressure, among the highest-pressure strength measurements ever reported on any platform. We find agreement with 2D simulations using the Improved Steinberg-Guinan strength model for body-centered-cubic Pb; the Pb-4wt%Sb alloy behaves similarly within the error bars. The combination of high-rate, pressure-induced hardening and polymorphism yield an average inferred flow stress of ∼3.8 GPa at high pressure, a ∼250-fold increase, changing Pb from soft to extremely strong.

Bremsstrahlung x-ray generation for high optical depth radiography applications on the National Ignition Facility.

We have tested a set of x-ray sources for use as probes of highly attenuating, laser-driven experiments on the National Ignition Facility (NIF). Unlike traditional x-ray sources that optimize for a characteristic atomic transition (often the n = 2 → n = 1 transition in ionized, He-like atoms), the design presented here maximizes the total photon flux by optimizing for intense, broadband Bremsstrahlung radiation. Three experiments were performed with identical targets, including a uranium x-ray source foil and a tungsten substrate with a narrow (25 µm wide) collimating slit to produce a quasi-1D x-ray source. Two experiments were performed using 12 beams from the NIF laser, each delivering approximately 46 kJ of laser energy but with different laser spatial profiles. This pair yielded similar temporal x-ray emission profiles, spatial resolution, and inferred hot electron temperature. A third experiment with only 6 beams delivering approximately 25 kJ produced a lower hot electron temperature and significantly lower x-ray flux, as well as poorer spatial resolution. The data suggest that laser pointing jitter may have affected the location and intensity of the emitting plasma, producing an emission volume that was not well centered behind the collimating slit and lower intensity than designed. However, the 12-beam design permits x-ray radiography through highly attenuating samples, where lower energy line-emission x-ray sources would be nearly completely attenuated.

Characterizing the modulation transfer function for X-ray radiography in high energy density experiments.

The Modulation Transfer Function (MTF) is an established means for characterizing imaging performance of X-ray radiography systems. We report on experiments using high energy, laser-driven X-ray radiography systems that assess performance using MTF values measured with the knife-edge projection method. The broadband, hard X-ray systems under study use line-projection imaging produced by narrowing the laser-generated X-ray source with a slit. We find that good contrast resolution can be achieved (the MTF = 0.5 at 75 µm wavelength) and that this performance is reproduced on different laser facilities. We also find that the MTF is sensitive both to the thickness of the line-projection slit and to the backing material thickness under the knife-edge. Both these sensitivities are due to a common mechanism, namely induced changes in the spectrally-averaged spatial widths of the X-ray source. The same line-projection system is also used on experimental campaigns measuring Rayleigh-Taylor instability growth by dynamically imaging sinusoidal, high Z micro-targets with wavelengths of 100 µm or less. By applying the measured MTF values to correct the ripple target contrast measurements, we can predict ripple growth to approximately 10% accuracy.

A platform for x-ray Thomson scattering measurements of radiation hydrodynamics experiments on the NIF.

We present an experimental design for a radiation hydrodynamics experiment at the National Ignition Facility that measures the electron temperature of a shocked region using the x-ray Thomson scattering technique. Previous National Ignition Facility experiments indicate a reduction in Rayleigh-Taylor instability growth due to high energy fluxes, compared to the shocked energy flux, from radiation and electron heat conduction. In order to better quantify the effects of these energy fluxes, we modified the previous experiment to allow for non-collective x-ray Thomson scattering to measure the electron temperature. Photometric calculations combined with synthetic scattering spectra demonstrate an estimated noise.

How high energy fluxes may affect Rayleigh-Taylor instability growth in young supernova remnants.

Energy-transport effects can alter the structure that develops as a supernova evolves into a supernova remnant. The Rayleigh-Taylor instability is thought to produce structure at the interface between the stellar ejecta and the circumstellar matter, based on simple models and hydrodynamic simulations. Here we report experimental results from the National Ignition Facility to explore how large energy fluxes, which are present in supernovae, affect this structure. We observed a reduction in Rayleigh-Taylor growth. In analyzing the comparison with supernova SN1993J, a Type II supernova, we found that the energy fluxes produced by heat conduction appear to be larger than the radiative energy fluxes, and large enough to have dramatic consequences. No reported astrophysical simulations have included radiation and heat conduction self-consistently in modeling supernova remnants and these dynamics should be noted in the understanding of young supernova remnants.

Lessons learned on recruitment and retention in hard-to-reach families in a phase III randomised controlled trial of preparatory information for children undergoing general anaesthesia.

BACKGROUND: Recruitment and retention are documented as two of the most difficult elements of conducting clinical trials. These issues are even more challenging in paediatric trials, particularly when the families being recruited and retained are deemed 'hard to reach'. METHODS: Through the authors' own reflection on the conduct of the trial this paper examines recruitment and retention with hard to reach families from the perspective of a recently completed clinical trial on preparatory information for children undergoing general anaesthesia for tooth extractions in which approximately 83% of those approached and eligible agreed to participate. RESULTS: The lessons learned for recruitment include: the importance of children's assent; maximising limited resources when screening and approaching potential participants; valuing families' time; and developing effective professional relationships. The retention rate was 83-85.5% at follow up time points up to 3.5 weeks following recruitment, insights into how this was accomplished include: ensuring continuity of care; determination to connect via telephone; valuing families' time; and close monitoring of appointment date changes. CONCLUSIONS: Implications for future paediatric trials with hard to reach families are discussed. TRIAL REGISTRATION: ISRCTN18265148 ; NIHR Portfolio 10,006. Date of Registration: 29 November 2013. The trial was registered after commencement but before completion of data collection.

Transition from Collisional to Collisionless Regimes in Interpenetrating Plasma Flows on the National Ignition Facility.

A study of the transition from collisional to collisionless plasma flows has been carried out at the National Ignition Facility using high Mach number (M>4) counterstreaming plasmas. In these experiments, CD-CD and CD-CH planar foils separated by 6-10 mm are irradiated with laser energies of 250 kJ per foil, generating ∼1000 km/s plasma flows. Varying the foil separation distance scales the ion density and average bulk velocity and, therefore, the ion-ion Coulomb mean free path, at the interaction region at the midplane. The characteristics of the flow interaction have been inferred from the neutrons and protons generated by deuteron-deuteron interactions and by x-ray emission from the hot, interpenetrating, and interacting plasmas. A localized burst of neutrons and bright x-ray emission near the midpoint of the counterstreaming flows was observed, suggesting strong heating and the initial stages of shock formation. As the separation of the CD-CH foils increases we observe enhanced neutron production compared to particle-in-cell simulations that include Coulomb collisions, but do not include collective collisionless plasma instabilities. The observed plasma heating and enhanced neutron production is consistent with the initial stages of collisionless shock formation, mediated by the Weibel filamentation instability.

Quality of life and outcomes for femoral hernia repair: does laparoscopy have an advantage?

BACKGROUND: Due to their relative scarcity and to limit single-center bias, multi-center data are needed to study femoral hernias. The aim of this study was to evaluate outcomes and quality of life (QOL) following laparoscopic vs. open repair of femoral hernias. METHODS: The International Hernia Mesh Registry was queried for femoral hernia repairs. Laparoscopic vs. open techniques were assessed for outcomes and QOL, as quantified by the Carolinas Comfort Scale (CCS), preoperatively and at 1, 6, 12, and 24 months postoperatively. Outcomes were evaluated using the standard statistical analysis. RESULTS: A total of 80 femoral hernia repairs were performed in 73 patients: 37 laparoscopic and 43 open. There was no difference in mean age (54.7 ± 14.6 years), body mass index (24.2 ± 3.8 kg/m2), gender (60.3 % female), or comorbidities (p > 0.05). The hernias were recurrent in 21 % of the cases with an average of 1.23 ± 0.6 prior repairs (p > 0.1). Preoperative CCS scores were similar for both groups and indicated that 59.7 % of patients reported pain and 46.4 % had movement limitations (p > 0.05). Operative time was equivalent (47.2 ± 21.2 vs. 45.9 ± 14.8 min, p = 0.82). There was no difference in postoperative complications, with an overall 8.2 % abdominal wall complications rate (p > 0.05). The length of stay was shorter in the laparoscopic group (0.5 ± 0.6 vs. 1.3 ± 1.6 days, p = 0.02). Follow-up was somewhat longer in the open group (23.8 ± 10.2 vs. 17.3 ± 10.9 months, p = 0.02). There was one recurrence, which was in the laparoscopic group (3.1 vs. 0 %, p = 0.4). QOL outcomes at all time points demonstrated no difference for pain, movement limitation, or mesh sensation. Postoperative QOL scores improved for both groups when compared to preoperative scores. CONCLUSION: In this prospective international multi-institution study of 80 femoral hernia repairs, no difference was found for operative times, long-term outcomes, or QOL in the treatment of femoral hernias when comparing laparoscopic vs. open techniques. After repair, QOL at all time-points postoperatively improved compared to QOL scores preoperatively for laparoscopic and open femoral hernia repair. While international data supports improved outcomes with laparoscopic approach for femoral hernia repair, no data had existed prior to this study on the difference of approach impacting QOL. In the setting where recurrence and complication rates are equal after femoral hernia repair for either approach, surgeons should perform the technique with which they are most confident, as the operative approach does not appear to change QOL outcomes after femoral hernia repair.

A novel method to recover DD fusion proton CR-39 data corrupted by fast ablator ions at OMEGA and the National Ignition Facility.

CR-39 detectors are used routinely in inertial confinement fusion (ICF) experiments as a part of nuclear diagnostics. CR-39 is filtered to stop fast ablator ions which have been accelerated from an ICF implosion due to electric fields caused by laser-plasma interactions. In some experiments, the filtering is insufficient to block these ions and the fusion-product signal tracks are lost in the large background of accelerated ion tracks. A technique for recovering signal in these scenarios has been developed, tested, and implemented successfully. The technique involves removing material from the surface of the CR-39 to a depth beyond the endpoint of the ablator ion tracks. The technique preserves signal magnitude (yield) as well as structure in radiograph images. The technique is effective when signal particle range is at least 10 µm deeper than the necessary bulk material removal.

Late-Time Mixing Sensitivity to Initial Broadband Surface Roughness in High-Energy-Density Shear Layers.

Using a large volume high-energy-density fluid shear experiment (8.5 cm^{3}) at the National Ignition Facility, we have demonstrated for the first time the ability to significantly alter the evolution of a supersonic sheared mixing layer by controlling the initial conditions of that layer. By altering the initial surface roughness of the tracer foil, we demonstrate the ability to transition the shear mixing layer from a highly ordered system of coherent structures to a randomly ordered system with a faster growing mix layer, indicative of strong mixing in the layer at a temperature of several tens of electron volts and at near solid density. Simulations using a turbulent-mix model show good agreement with the experimental results and poor agreement without turbulent mix.

Proton pinhole imaging on the National Ignition Facility.

Pinhole imaging of large (mm scale) carbon-deuterium (CD) plasmas by proton self-emission has been used for the first time to study the microphysics of shock formation, which is of astrophysical relevance. The 3 MeV deuterium-deuterium (DD) fusion proton self-emission from these plasmas is imaged using a novel pinhole imaging system, with up to five different 1 mm diameter pinholes positioned 25 cm from target-chamber center. CR39 is used as the detector medium, positioned at 100 cm distance from the pinhole for a magnification of 4 ×. A Wiener deconvolution algorithm is numerically demonstrated and used to interpret the images. When the spatial morphology is known, this algorithm accurately reproduces the size of features larger than about half the pinhole diameter. For these astrophysical plasma experiments on the National Ignition Facility, this provides a strong constraint on simulation modeling of the experiment.

Laparoscopic appendectomy and cholecystectomy versus open: a study in 1999 pregnant patients.

BACKGROUND: When pregnant patients require surgery, whether to perform an operation open or laparoscopic is often debated. We evaluated the impact of laparoscopy for common general surgical problems in pregnancy to determine safety and trends in operative approach over time. METHODS: Pregnant patients undergoing appendectomy or cholecystectomy were identified using the National Surgical Quality Improvement Program (NSQIP) database. We analyzed demographics, operative characteristics, and outcomes. Univariate comparison and multivariate regression analysis (MVA) were performed adjusting for confounding factors: age, body mass index (BMI), diabetes, and smoking, and an additional MVA was performed for perforated cases. RESULTS: A total of 1999 pregnant patients between 2005 and 2012 were evaluated. Of 1335 appendectomies, 894 were performed laparoscopically (LA) and 441 open (OA). For 664 cholecystectomies, 606 were laparoscopic (LC) and 58 open (OC). There were no deaths. For LA versus OA, patient characteristics were not different {age: 27.7 vs. 28.2 years, p = 0.19; diabetes: 1.8 vs. 0.9%, p = 0.24; smoking: 19 vs. 16.1%, p = 0.2} except for BMI (27.9 vs. 28.4 kg/m(2); p = 0.03). LA had shorter operative times (ORT), length of stay (LOS), and fewer postoperative complications compared to OA. In MVA, difference between approaches remained statistically significant for ORT (<0.0001), LOS (<0.01), and wound complications (<0.01). MVA was performed for perforated cases alone: LA had equal ORT (p = 0.19) yet shorter LOS (p = <0.001). The majority of LA were performed in the last 4 years versus the first 4 years (61 vs. 39%, p < 0.001). For LC versus OC, patient characteristics were not different: age (28.3 vs. 28.7 years; p = 0.33), BMI (31.4 vs. 33.2 kg/m(2), p = 0.25), diabetes (2.8 vs. 3.5%, p = 0.68), and smoking (21.1 vs. 25.9%, p = 0.4). LC had a shorter ORT, LOS, and fewer postoperative complications than OC. In MVA, the difference between approaches remained statistically significant for ORT (<0.0001), LOS (<0.0001), and minor complications (<0.01). In MVA for cholecystitis with perforation, no difference was seen for LOS, ORT, or postoperative complications (p > 0.05). The percentage of LC cases appeared to increase over time (89 vs. 93%, p = 0.06). CONCLUSION: While fetal events are unknown, LA and LC in pregnant patients demonstrated shorter ORT, LOS, and reduced complications and were performed more frequently over time. Even in perforated cases, laparoscopy appears safe in pregnant patients.

Risk factors for postoperative sepsis in laparoscopic gastric bypass.

INTRODUCTION: Postoperative sepsis is a rare but serious complication following elective surgery. The purpose of this study was to identify the rate of postoperative sepsis following elective laparoscopic gastric bypass (LGBP) and to identify patients' modifiable, preoperative risk factors. METHODS: The American College of Surgeons National Surgical Quality Improvement Program database was queried from 2005 to 2013 for factors associated with the development of postoperative sepsis following elective LGBP. Patients who developed sepsis were compared to those who did not. Results were analyzed using the Chi-square test for categorical variables and Wilcoxon two-sample test for continuous variables. A multivariate logistic regression analysis was utilized to calculate adjusted odds ratios for factors contributing to sepsis. RESULTS: During the study period, 66,838 patients underwent LGBP. Of those, 546 patients developed postoperative sepsis (0.82%). The development of sepsis was associated with increased operative time (161 ± 77.8 vs. 135.10 ± 56.5 min; p < 0.0001) and a greater number of preoperative comorbidities, including diabetes (39.6 vs. 30.6%; p < 0.0001), hypertension requiring medication (65.2 vs. 54%; p < 0.0001), current tobacco use (16.7 vs. 11.5%; p = 0.0002), and increased pack-year history of smoking (8.6 ± 18.3 vs. 5.6 ± 14.2; p = 0.0006), and the Charlson Comorbidity Index (0.51 ± 0.74 vs. 0.35 ± 0.57, p < 0.0001). Sepsis resulted in an increased length of stay (10.1 ± 14.4 vs. 2.4 ± 4.8 days; p < 0.0001) and a 30 times greater chance of 30-day mortality (4.03 vs. 0.11%, p < 0.0001). Multivariate logistic regression analysis showed that current smokers had a 63% greater chance of developing sepsis compared to non-smokers, controlling for age, race, gender, BMI, and CCI score (OR 1.63, 95% CI 1.23-2.14; p = 0.0006). CONCLUSIONS: Laparoscopic gastric bypass is uncommonly associated with postoperative sepsis. When it occurs, it portends a 30 times increased risk of death. A patient history of diabetes, hypertension, and increasing pack-years of smoking portend an increased risk of sepsis. Current smoking status, a preoperative modifiable risk factor, is independently associated with the chance of postoperative sepsis. Preoperative patient optimization and risk reduction should be a priority for elective surgery, and patients should be encouraged to stop smoking prior to gastric bypass.