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The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself—the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of4×1020W/cm2). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.

The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ϵ>500MeV) with an intense laser pulse (a0>10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γrays), consistent with a quantum description of radiation reaction. The generatedγrays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energyϵcrit>30MeV.

Laser wakefield accelerators promise to revolutionize many areas of accelerator science. However, one of the greatest challenges to their widespread adoption is the difficulty in control and optimization of the accelerator outputs due to coupling between input parameters and the dynamic evolution of the accelerating structure. Here, we use machine learning techniques to automate a 100 MeV-scale accelerator, which optimized its outputs by simultaneously varying up to six parameters including the spectral and spatial phase of the laser and the plasma density and length. Most notably, the model built by the algorithm enabled optimization of the laser evolution that might otherwise have been missed in single-variable scans. Subtle tuning of the laser pulse shape caused an 80% increase in electron beam charge, despite the pulse length changing by just 1%. Laser wakefield accelerators are compact sources of ultra-relativistic electrons which are highly sensitive to many control parameters. Here the authors present an automated machine learning based method for the efficient multi-dimensional optimization of these plasma-based particle accelerators.

Two signatures of quantum effects on radiation reaction in the collision of a ${\\sim}$ GeV electron beam with a high intensity ( ${>}3\\times 10^{20}~\\text{W}~\\text{cm}^{-2}$ ) laser pulse have been considered. We show that the decrease in the average energy of the electron beam may be used to measure the Gaunt factor $g$ for synchrotron emission. We derive an equation for the evolution of the variance in the energy of the electron beam in the quantum regime, i.e. quantum efficiency parameter $\\unicode[STIX]{x1D702}\\not \\ll 1$ . We show that the evolution of the variance may be used as a direct measure of the quantum stochasticity of the radiation reaction and determine the parameter regime where this is observable. For example, stochastic emission results in a 25 % increase in the standard deviation of the energy spectrum of a GeV electron beam, 1 fs after it collides with a laser pulse of intensity $10^{21}~\\text{W}~\\text{cm}^{-2}$ . This effect should therefore be measurable using current high-intensity laser systems.

Modern laser technology is now sufficiently advanced that collisions between high-intensity laser pulses and laser-wakefield-accelerated (LWFA) electron beams can reach the strong-field regime, so that it is possible to measure the transition between the classical and quantum regimes of light-matter interactions. However, the energy spectrum of LWFA electron beams can fluctuate significantly from shot to shot, making it difficult to clearly discern quantum effects in radiation reaction (RR), for example. Here we show how this can be accomplished in only a single laser shot. A millimetre-scale pre-collision drift allows the electron beam to expand to a size larger than the laser focal spot and develop a correlation between transverse position and angular divergence. In contrast to previous studies, this means that a measurement of the beam's energy-divergence spectrum automatically distinguishes components of the beam that hit or miss the laser focal spot and therefore do and do not experience RR.

We describe the use of a genetic algorithm to apply active feedback to a laser wakefield accelerator at a higher power (10 TW) and a lower repetition rate (5 Hz) than previous work. The temporal shape of the drive laser pulse was adjusted automatically to optimize the properties of the electron beam. By changing the software configuration, different properties could be improved. This included the total accelerated charge per bunch, which was doubled, and the average electron energy, which was increased from 22 to 27 MeV. Using experimental measurements directly to provide feedback allows the system to work even when the underlying acceleration mechanisms are not fully understood, and, in fact, studying the optimized pulse shape might reveal new insights into the physical processes responsible. Our work suggests that this technique, which has already been applied with low-power lasers, can be extended to work with petawatt-class laser systems.

The spatial-intensity profile of light reflected during the interaction of an intense laser pulse with a microstructured target is investigated experimentally and the potential to apply this as a diagnostic of the interaction physics is explored numerically. Diffraction and speckle patterns are measured in the specularly reflected light in the cases of targets with regular groove and needle-like structures, respectively, highlighting the potential to use this as a diagnostic of the evolving plasma surface. It is shown, via ray-tracing and numerical modelling, that for a laser focal spot diameter smaller than the periodicity of the target structure, the reflected light patterns can potentially be used to diagnose the degree of plasma expansion, and by extension the local plasma temperature, at the focus of the intense laser light. The reflected patterns could also be used to diagnose the size of the laser focal spot during a high-intensity interaction when using a regular structure with known spacing.

Flea bites can cause irritating symptoms, secondary infections, and may potentiate the spread of vector-borne disease. Flea infestation and bites may also cause significant psychological distress, and can reduce the morale and fighting fitness of deployed military personnel. The problem of flea infestation was highlighted during Op HERRICK 12 in two 'front line' Check Points (CPs) where the entire population of soldiers suffered from multiple symptoms due to flea infestation and bites. Several attempts at infestation control initially succeeded but later lead to recurrence. Such failure was due to the incomplete killing of all stages of the flea life cycle, and due to constraints on education, training, communication, and resupply in the isolated and austere environment of the CPs. A dedicated operation (designated Op Insecticide) was put into action in order to eradicate the problem and return the affected troops back to full fighting fitness. Op Insecticide was thorough, systematic and sustainable, and lead to an eradication of the flea infestation problem at the affected CPs.

The use of targets with surface structures for laser-driven particle acceleration has potential to significantly boost the particle and radiation energies because of enhanced laser absorption. We investigate, via experiment and particle-in-cell simulations, the impact of micron-scale surface-structured targets on the spectrum of electrons and protons accelerated by a picosecond laser pulse at relativistic intensity. Our results show that, compared with flat-surfaced targets, structures on this scale give rise to a significant enhancement in particle and radiation emission over a wide range of laser–target interaction parameters. This is due to the longer plasma scale length when using micro-structures on the target front surface. We do not observe an increase in the proton cutoff energy with our microstructured targets, and this is due to the large volume of the relief.

Single-stranded DNA(ssDNA) viral life cycles must balance double-stranded DNA (dsDNA) and ssDNA biosynthesis. Previously published in vitro results suggest that microvirus C and host cell SSB proteins play antagonistic roles to achieve this balance. To investigate this in vivo, microvirus DNA replication was characterized in cells expressing cloned C or ssb genes, which would presumably alter the C:SSB protein ratios. Representatives of each microvirus clade (ϕX174, G4, and α3) were used in these studies. α3 DNA replication was significantly more complex. Results suggested that the recognized α3 C gene (CS: small) is one of two C genes. A larger 5′ extended gene could be translated from an upstream GTG start codon (CB: big). Wild-type α3 acquired resistance to elevated SSB levels by mutations that exclusively frameshifted the CB reading frame, whereas mutations in the origin of replication conferred resistance to elevated C protein levels. Expression of either the cloned CB or CS gene complemented am(C) mutants, demonstrating functional redundancy. When the CS start codon was eliminated, strains were only viable if an additional amber mutation was placed in gene C and propagated in an informational suppressing host. Thus, CB protein likely reaches toxic levels in the absence of CS translation. This phenomenon may have driven the evolution of the CS gene within the larger CB gene and could constitute a unique mechanism of regulation. Furthermore, cross-complementation data suggested that interactions between the α3 C and other viral proteins have evolved enough specificity to biochemically isolate its DNA replication from G4 and ϕX174.