A bremsstrahlung gamma-ray source based on stable ionization injection of electrons into a laser wakefield accelerator

Laser wakefield acceleration permits the generation of ultra-short, high-brightness relativistic electron beams on a millimeter scale. While those features are of interest for many applications, the source remains constraint by the poor stability of the electron injection process. Here we present results on injection and acceleration of electrons in pure nitrogen and argon. We observe stable, continuous ionization-induced injection of electrons into the wakefield for laser powers exceeding a threshold of 7 TW. The beam charge scales approximately linear with the laser energy and is limited by beam loading. For 40 TW laser pulses we measure a maximum charge of almost 1 nC per shot, originating mostly from electrons of less than 10 MeV energy. The relatively low energy, the high charge and its stability make this source well-suited for applications such as non-destructive testing. Hence, we demonstrate the production of energetic radiation via bremsstrahlung conversion at 1 Hz repetition rate. In accordance with Geant4 Monte-Carlo simulations, we measure a gamma-ray source size of less than 100 microns for a 0.5 mm tantalum converter placed at 2 mm from the accelerator exit. Furthermore we present radiographs of image quality indicators

Simple and clear evidence for positive feedback limitation by bipolar behavior during scanning electrochemical microscopy of unbiased conductors

On the basis of an experimentally validated simple theoretical model, it is demonstrated unambiguously that when an unbiased conductor is probed by a scanning electrochemical tip (scanning electrochemical microscopy, SECM), it performs as a bipolar electrode. Though already envisioned in most recent SECM theories, this phenomenon is generally overlooked in SECM experimental investigations. However, as is shown here, this may alter significantly positive feedback measurements when the probed conductor is not much larger than the ti

Towards ultrahigh-contrast ultraintense laser pulses—complete characterization of a double plasma-mirror pulse cleaner

International audienceThe effects of small amounts of energy delivered at times before the peak intensity of ultrahigh-intensity ultrafast-laser pulses have been a major obstacle to the goal of studying the interaction of ultraintense light with solids for more than two decades now. We describe implementation of a practical double-plasma-mirror pulse cleaner, built into a f = 10 m null telescope and added as a standard beamline feature of a 100 TW laser system for ultraintense laser-matter interaction. Our measurements allow us to infer a pulse-height contrast of 5×10^11—the highest contrast generated to date—while preserving ∼ 50% of the laser intensity and maintaining excellent focusability of the delivered beam. We present a complete optical characterization, comparing empirical results and numerical modeling of a double-plasma-mirror system

Effect of the Laser Wave Front in a Laser-Plasma Accelerator

A high-repetition rate electron source is generated by tightly focusing kHz, few-mJ laser pulses into an underdense plasma. This high-intensity laser-plasma interaction leads to stable electron beams over several hours but with strikingly complex transverse distributions even for good quality laser focal spots. We find that the electron beam distribution is sensitive to the laser wave front via the laser midfield distribution rather than the laser focal spot itself. We are able to measure the laser wave front around the focus and include it in realistic particle-in-cell simulations demonstrating the role of the laser wave front on the acceleration of electrons. Distortions of the laser wave front cause spatial inhomogeneities in the midfield laser intensity and, consequently, the laser pulse drives an inhomogeneous transverse wakefield whose focusing and defocusing properties affect the electron distribution. These findings explain the experimental results and suggest the possibility of controlling the electron spatial distribution in laser-plasma accelerators by tailoring the laser wave front

Observation of spectral gain narrowing in a high-order harmonic seeded soft-x-ray amplifier

International audienceWe report an observation of spectral gain narrowing of a high-order harmonic amplified by a soft-x-ray optical-field-ionized plasma. The temporal coherence and spectral linewidth of both the seeded and unseeded soft-x-ray lasers were experimentally measured using a varying-path-difference interferometer. The results showed that the high-order harmonic is subject to a strong spectral narrowing during its propagation in the plasma amplifier without rebroadening at saturation. This is in good agreement with a radiative transfer calculation including gain narrowing and saturation rebroadening

Aberration-free laser beam in the soft x-ray range

International audienceBy seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e^2) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as λ/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture

Second generation X-ray lasers

International audienceHigh harmonic amplification is shown in an optical-field ionized X-ray laser. The 4d–4p X-ray line of Kr^8+ at 32.8 nm, which has a close match to the 25th harmonic of the infrared laser, was amplified up to 200 times. The energy-extraction regime was also achieved, which depended on the level of seeding. This second-generation X-ray laser is fully polarized, has low divergence and shows a high degree of coherence. The duration is also expected to be short, fulfilling the requirements for an ultra-intense tabletop X-ray laser. Conditions for higher energy output are also suggested