Chirped pulse Raman amplification in warm plasma: towards controlling saturation

Stimulated Raman backscattering in plasma is potentially an efficient method of amplifying laser pulses to reach exawatt powers because plasma is fully broken down and withstands extremely high electric fields. Plasma also has unique nonlinear optical properties that allow simultaneous compression of optical pulses to ultra-short durations. However, current measured efficiencies are limited to several percent. Here we investigate Raman amplification of short duration seed pulses with different chirp rates using a chirped pump pulse in a preformed plasma waveguide. We identify electron trapping and wavebreaking as the main saturation mechanisms, which lead to spectral broadening and gain saturation when the seed reaches several millijoules for durations of 10's - 100's fs for 250 ps, 800 nm chirped pump pulses. We show that this prevents access to the nonlinear regime and limits the efficiency, and interpret the experimental results using slowly-varying-amplitude, current-averaged particle-in-cell simulations. We also propose methods for achieving higher efficiencies.close0

Compression of X-ray Free Electron Laser pulses to attosecond duration

State of the art X-ray Free Electron Laser facilities currently provide the brightest X-ray pulses available, typically with mJ energy and several hundred femtosecond duration. Here we present one- and two-dimensional Particle-in-Cell simulations, utilising the process of stimulated Raman amplification, showing that these pulses are compressed to a temporally coherent, sub-femtosecond pulse at 8% efficiency. Pulses of this type may pave the way for routine time resolution of electrons in nm size potentials. Furthermore, evidence is presented that significant Landau damping and wave-breaking may be beneficial in distorting the rear of the interaction and further reducing the final pulse duration

Nonlinear ion-acoustic (IA) waves driven in a cylindrically symmetric flow

By employing a self-similar, two-fluid MHD model in a cylindrical geometry, we study the features of nonlinear ion-acoustic (IA) waves which propagate in the direction of external magnetic field lines in space plasmas. Numerical calculations not only expose the well-known three shapes of nonlinear structures (sinusoidal, sawtooth, and spiky or bipolar) which are observed by numerous satellites and simulated by models in a Cartesian geometry, but also illustrate new results, such as, two reversely propagating nonlinear waves, density dips and humps, diverging and converging electric shocks, etc. A case study on Cluster satellite data is also introduced.Comment: accepted by AS

Prevalence and Prognostic Impact of Pathogenic Variants in Patients With Dilated Cardiomyopathy Referred for Ventricular Tachycardia Ablation

OBJECTIVES This study aimed to assess the frequency of (likely) pathogenic variants (LP/Pv) among dilated cardiomyopathy (DCM) ventricular tachycardia (VT) patients referred for CA and their impact on procedural outcome and long-term prognosis. BACKGROUND The prevalence of genetic variants associated with monomorphic VT among DCM is unknown. METHODS Ninety-eight consecutive patients (age 56 +/- 15 years; 84% men, left ventricular ejection fraction [LVEF] 39 12%) referred for DCM-VT ablation were included. Patients underwent electroanatomical mapping and testing of >= 55 cardiomyopathy-related genes. Mapping data were analyzed for low-voltage areas and abnormal potentials. LP/Pv-positive (LP/Pv+) patients were compared with LP/Pv-negative (LP/Pv-) patients and followed for VT recurrence and mortality. RESULTS In 37 (38%) patients, LP/Pv were identified, most frequently LMNA (n = 11 of 37, [30%]), 17N (n = 6 of 37, [16%]), PLN (n = 6 of 37, [16%]), SCN5A (n = 3 of 37, [8%]), RBM20 (n = 2 of 37, [5%]) and DSP (n = 2 of 37, [5%]). LP/Pv+ carriers had tower LVEF (35 + 13% vs. LP/Pv-: 42 11%; p 0.005) and were less often men (n 27 [73%] vs. n 55 [90%] p 0.03). After a median follow-up of 2.4 years (interquartile range: 0.9 to 4.4 years), 63 (64%) patients had VT recurrence (LP/Pv+: 30 of 37 [81%] vs. LP/Pv-: 33 of 61 [54%]; p = 0.007). Twenty-eight patients (29%) died (LP/Pv +: 19 of 37 [51%] vs. LP/Pv-: 9 of 61 [15%]; p <0.001). The cumulative 2-year VT-free survival was 41% in the total cohort (LP/Pv+: 16% vs. LP/Pv-: 54%; p 0.001). The presence of LP/Pv (hazard ratio: 1.9; 95% confidence interval: 1.1 to 3.4; p = 0.02) and unipolar low-voltage area size/cm(2) increase (hazard ratio: 2.5; 95% confidence interval: 1.6 to 4.0; p <0.001) were associated with a decreased 2-year VT-free survival. CONCLUSIONS In patients with DCM-VT, a genetic cause is frequently identified. LP/Pv+ patients have a tower LVEF and more extensive VT substrates, which, in combination with disease progression, may contribute to the poor prognosis. Genetic testing in patients with DCM-VT should therefore be recommended. (C) 2020 by the American College of Cardiology Foundation

Advantages to a diverging Raman amplifier

The plasma Raman instability can efficiently compress a nanosecond long high-power laser pulse to sub-picosecond duration. Although, many authors envisaged a converging beam geometry for Raman amplification, here we propose the exact opposite geometry; the amplification should start at the intense focus of the seed. We generalise the coupled laser envelope equations to include this non-collimated case. The new geometry completely eradicates the usual trailing secondary peaks of the output pulse, which typically lower the efficiency by half. It also reduces, by orders of magnitude, the initial seed pulse energy required for efficient operation. As in the collimated case, the evolution is self similar, although the temporal pulse envelope is different. A two-dimensional particle-in-cell simulation demonstrates efficient amplification of a diverging seed with only 0.3 mJ energy. The pulse has no secondary peaks and almost constant intensity as it amplifies and diverges

Attosecond-scale absorption at extreme intensities

A novel non-ponderomotive absorption mechanism, originally presented by Baeva et al. in one dimension, is extended into higher dimensions for the first time. This absorption mechanism, the Zero Vector Potential (ZVP), is expected to dominate the interactions of ultra-intense laser pulses with critically over-dense plasmas such as those that are expected with the Extreme Light Infrastructure laser systems. It is shown that the mathematical form of the ZVP mechanism and its key scaling relations found by Baeva et al. in 1D are identically reproduced in higher dimensions. The two dimensional particle-in-cell simulations are then used to validate both the qualitative and quantitative predictions of the theory

Optimization of plasma amplifiers

Plasma amplifiers offer a route to side-step limitations on chirped pulse amplification and generate laser pulses at the power frontier. They compress long pulses by transferring energy to a shorter pulse via the Raman or Brillouin instabilities. We present an extensive kinetic numerical study of the three-dimensional parameter space for the Raman case. Further particle-in-cell simulations find the optimal seed pulse parameters for experimentally relevant constraints. The high-efficiency self-similar behavior is observed only for seeds shorter than the linear Raman growth time. A test case similar to an upcoming experiment at the Laboratory for Laser Energetics is found to maintain good transverse coherence and high-energy efficiency. Effective compression of a 10 kJ , nanosecond-long driver pulse is also demonstrated in a 15-cm-long amplifier

Demonstration of laser pulse amplification by stimulated Brillouin scattering

The energy transfer by stimulated Brillouin backscatter from a long pump pulse (15 ps) to a short seed pulse (1 ps) has been investigated in a proof-of-principle demonstration experiment. The two pulses were both amplified in different beamlines of a Nd:glass laser system, had a central wavelength of 1054 nm and a spectral bandwidth of 2 nm, and crossed each other in an underdense plasma in a counter-propagating geometry, off-set by 10∘. It is shown that the energy transfer and the wavelength of the generated Brillouin peak depend on the plasma density, the intensity of the laser pulses, and the competition between two-plasmon decay and stimulated Raman scatter instabilities. The highest obtained energy transfer from pump to probe pulse is 2.5%, at a plasma density of 0.17ncr, and this energy transfer increases significantly with plasma density. Therefore, our results suggest that much higher efficiencies can be obtained when higher densities (above 0.25ncr) are used