Degradation of temporal contrast from post-pedestal interference with a chirped pulse in an optical parametric amplifier

Pre-pedestal generation is observed in a 0.35-PW laser front end coming from a post-pedestal via instantaneous gain and pump depletion in an optical parametric amplifier during chirped-pulse amplification. Generalized simulations show how this effect arises from gain nonlinearity and applies to all optical parametric chirped-pulse-amplification systems with a post-pedestal. An experiment minimizing the effect of B-integral is used to isolate and study the newly observed conversion of a continuous post-pedestal into a continuous pre-pedestal. Matching numerical simulations confirm experimental results and additionally reveal how third-order dispersion largely controls the slope of the generated pre-pedestal.

Final amplifier of an ultra-intense all-OPCPA system with 13-J output signal energy and 41% pump-to-signal conversion efficiency

Optical parametric chirped-pulse amplification (OPCPA) using high-energy Nd:glass lasers has the potential to produce ultra-intense pulses (>1023 W/cm2). We report on the performance of the final high-efficiency amplifier in an OPCPA system based on large-aperture (63 × 63-mm2) partially deuterated potassium dihydrogen phosphate (DKDP) crystals. The seed beam (180-nm bandwidth, 110 mJ) was provided by the preceding OPCPA stages. A maximum pump-to-signal conversion efficiency of 41% and signal energy up to 13 J were achieved with a 52-mm-long DKDP crystal due to the flattop super-Gaussian pump beam profile and flat-in-time pulse shape.

Achieving 100 GW idler pulses from an existing petawatt optical parametric chirped pulse amplifier

Optical parametric chirped-pulse-amplification produces two broadband pulses, a signal and an idler, that can both provide peak powers >100 GW. In most cases the signal is used, but compressing the longer-wavelength idler opens up opportunities for experiments where the driving laser wavelength is a key parameter. This paper will describe several subsystems that were added to a petawatt class, Multi-Terawatt optical parametric amplifier line (MTW-OPAL) at the Laboratory for Laser Energetics to address two long-standing issues introduced by the use of the idler, angular dispersion, and spectral phase reversal. To the best of our knowledge, this is the first time that compensation of angular dispersion and phase reversal has been achieved in a single system and results in a 100 GW, 120-fs duration, pulse at 1170 nm.

Design and characterization of "flow-cell" integrated-flow active cooling for high-average-power ceramic optics

We used COMSOL Multiphysics to design a prototype actively cooled "flow-cell" substrate targeted at high-average-power applications, fabricated the prototype from cordierite ceramic, and tested the substrate under load in our thermal loading test stand. Sub-aperture testing revealed average-power handling up to 3.88-W/cm² absorbed power density, in excellent agreement with model predictions. Gratings fabricated on 2-in.-diam cordierite coupons were subjected to laser-damage testing and showed a damage threshold of 250 mJ/cm².

Effect of the pump beam profile and wavefront on the amplified signal wavefront in optical parametric amplifiers

We present a theoretical and experimental analysis of the signal phase introduced by the pump-beam wavefront and spatial profile during optical parametric amplification (OPA) process. The theory predicts the appearance of an additional wavefront in the amplified signal beam that is proportional to the spatial derivative of the pump-beam wavefront. The effect of the pump-beam profile on the signal-beam wavefront is also investigated. Our experiments tested these theoretical predictions by comparing the wavefront of the signal beam before and after amplification in a multi-joule broadband OPA. The measured signal wavefront was shown to have the expected dependence on the pump-beam profile and wavefront. These results can be considered when designing petawatt-scale ultrabroadband optical parametric chirped-pulse-amplification systems.

Picosecond Thermodynamics in Underdense Plasmas Measured with Thomson Scattering

The rapid evolutions of the electron density and temperature in a laser-produced plasma were measured using collective Thomson scattering. Unprecedented picosecond time resolution, enabled by a pulse-front-tilt compensated spectrometer, revealed a transition in the plasma-wave dynamics from an initially cold, collisional state to a quasistationary, collisionless state. The Thomson-scattering spectra were compared with theoretical calculations of the fluctuation spectrum using either a conventional Bhatnagar-Gross-Krook (BGK) collision operator or the rigorous Landau collision terms: the BGK model overestimates the electron temperature by 50% in the most-collisional conditions.

Simulation of grating compressor misalignment tolerances and mitigation strategies for chirped-pulse-amplification systems of varying bandwidths and beam sizes

The effects of pulse compressor grating misalignment on pulse duration and focusability are simulated for chirped-pulse-amplification systems of varying bandwidths, beam sizes, groove densities, and incident angles. Tilt-alignment tolerances are specified based on a 2 drop in focused intensity, illustrating how tolerances scale with bandwidth and compressor beam size, which scales with energy when transformed via known grating damage thresholds. Grating-alignment tolerance scaling with grating groove density and incident/diffracted angles is investigated and applied to compressor design. A correlation between grating tip and in-plane rotation error sensitivity is defined and used to compensate residual out-of-plane angular dispersion, even for ultra-broadband pulses. Simulation of dispersion compensation methods after grating misalignment is shown to mitigate pulse lengthening, limited by temporal contrast degradation and higher-order effects for ultrabroad bandwidths.

Raman Amplification with a Flying Focus

We propose a new laser amplifier scheme utilizing stimulated Raman scattering in plasma in conjunction with a "flying focus"-a chromatic focusing system combined with a chirped pump beam that provides spatiotemporal control over the pump's focal spot. Pump intensity isosurfaces are made to propagate at v=-c so as to be in sync with the injected counterpropagating seed pulse. By setting the pump intensity in the interaction region to be just above the ionization threshold of the background gas, an ionization wave is produced that travels at a fixed distance ahead of the seed. Simulations show that this will make it possible to optimize the plasma temperature and mitigate many of the issues that are known to have impacted previous Raman amplification experiments, in particular, the growth of precursors.

Minimized weight gain between hemodialysis contributes to a reduced risk of death

The risk of death is higher in dialysis patients compared to age matched healthy subjects, the main reason being cardiovascular. This prospective study investigated if the extent of ultrafiltration was of importance for the outcome.

MATERIAL AND METHODS

88 hemodialysis patients were included and followed prospectively. The outcome was registered in regard to death, acute myocardial infarction or coronary vascular intervention. The extent of ultrafiltration needed at dialysis was calculated as a mean during the observation period as were other variables. The mean extent of ultrafiltration was compared for patients who had survived without end-points (group 1, n=53) versus those who reached any end-point during the period (group 2, n=35).

RESULTS

In total, 40% of the patients reached end-point during the observation period. There was no difference at baseline between the groups in regard to age, prevalence of diabetes mellitus or history of previous cardiovascular disease, KT/V, residual renal function ultrafiltration need, C-reactive protein, s-albumin, cholesterol, LDL-cholesterol, HDL-cholesterol, appetite or wellbeing, while triglyceride was lower in group 2 (p=0.035). The observation period for group 1 was at a mean 24.7 months (SD13.1) and for those in group 2 at a mean 13.8 (+/-11.7 months, p<0.001). Patients representing group 1 at 24 and 30 months had less need of ultrafiltration than those in group 2. Thus, the need of ultrafiltration was about 27% lower at 24 months (for 29 persons in group 1: 3.63+/-1.93 weight% versus 4.97+/-1.70 weight% for 9 patients from group 2, p=0.046) and 46% at 30 months (for 18 from group 1: 3.48+/-1.95 versus 6.45+/-1.55 for 3 from group 2, p=0.030). C-reactive protein did not differ significantly between the groups during the period.

CONCLUSION

After a prolonged period of 24 months the extent of ultrafiltration need seems to be important for the outcome of the patients. Thereby those with higher need of ultrafiltration had worse prognosis. It seems important to motivate patients to reduce the extent of fluid intake between dialysis to prolong survival.