Picosecond laser filament-guided electrical discharges in air at 1 kHz repetition rate.

Laser-induced filaments have been shown to reduce the voltage necessary to initiate electrical discharges in atmospheric air and guide their propagation over long distances. Here we demonstrate the stable generation of laser filament-guided electrical discharge columns in air initiated by high energy (up to 250 mJ) 1030 nm wavelength laser pulses of 7 ps duration at repetition rates up to 1 kHz and we discuss the processes leading to breakdown. A current proportional to the laser pulse energy is observed to arise as soon as the laser pulse arrives, initiating a high impedance phase of the discharge. Full breakdown, characterized by impedance collapse, occurs 100 ns to several µs later. A record 4.7-fold reduction in breakdown voltage for dc-biased discharges, which remains practically independent of the repetition rate up to 1 kHz, is observed to be primarily caused by a single laser pulse that produces a large (∼80%) density depression. The radial gaps between the filamentary plasma channel and the hollowed electrodes employed are shown to play a significant role in the breakdown dynamics. A rapid increase of 3-4 orders of magnitude in current is observed to follow the formation of localized radial current channels linking the filament to the electrodes. The increased understanding and control of kHz repetition rate filament-guided discharges can aid their use in applications.

Dynamics of Nanosecond Laser Pulse Propagation and of Associated Instabilities in a Magnetized Underdense Plasma.

The propagation and energy coupling of intense laser beams in plasmas are critical issues in inertial confinement fusion. Applying magnetic fields to such a setup has been shown to enhance fuel confinement and heating. Here we report on experimental measurements demonstrating improved transmission and increased smoothing of a high-power laser beam propagating in a magnetized underdense plasma. We also measure enhanced backscattering, which our kinetic simulations show is due to magnetic confinement of hot electrons, thus leading to reduced target preheating.

Transport of an intense proton beam from a cone-structured target through plastic foam with unique proton source modeling.

Laser-accelerated proton beams are applicable to several research areas within high-energy density science, including warm dense matter generation, proton radiography, and inertial confinement fusion, which all involve transport of the beam through matter. We report on experimental measurements of intense proton beam transport through plastic foam blocks. The intense proton beam was accelerated by the 10ps, 700J OMEGA EP laser irradiating a curved foil target, and focused by an attached hollow cone. The protons then entered the foam block of density 0.38g/cm^{3} and thickness 0.55 or 1.00mm. At the rear of the foam block, a Cu layer revealed the cross section of the intense beam via proton- and hot electron-induced Cu-K_{α} emission. Images of x-ray emission show a bright spot on the rear Cu film indicative of a forward-directed beam without major breakup. 2D fluid-PIC simulations of the transport were conducted using a unique multi-injection source model incorporating energy-dependent beam divergence. Along with postprocessed calculations of the Cu-K_{α} emission profile, simulations showed that protons retain their ballistic transport through the foam and are able to heat the foam up to several keV in temperature. The total experimental emission profile for the 1.0mm foam agrees qualitatively with the simulated profile, suggesting that the protons indeed retain their beamlike qualities.

Influence of spatial-intensity contrast in ultraintense laser-plasma interactions.

Increasing the intensity to which high power laser pulses are focused has opened up new research possibilities, including promising new approaches to particle acceleration and phenomena such as high field quantum electrodynamics. Whilst the intensity achievable with a laser pulse of a given power can be increased via tighter focusing, the focal spot profile also plays an important role in the interaction physics. Here we show that the spatial-intensity distribution, and specifically the ratio of the intensity in the peak of the laser focal spot to the halo surrounding it, is important in the interaction of ultraintense laser pulses with solid targets. By comparing proton acceleration measurements from foil targets irradiated with by a near-diffraction-limited wavelength scale focal spot and larger F-number focusing, we find that this spatial-intensity contrast parameter strongly influences laser energy coupling to fast electrons. We find that for multi-petawatt pulses, spatial-intensity contrast is potentially as important as temporal-intensity contrast.

Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate.

We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 µm-wavelength Yb:YAG laser at repetition rates up to f=1kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f=0.1kHz encounter a nearly unperturbed environment, at f=1kHz, a channel with an axial air density hole of ∼20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f=1kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

Electron acceleration at oblique angles via stimulated Raman scattering at laser irradiance >10

The generation of hot, directional electrons via laser-driven stimulated Raman scattering (SRS) is a topic of great importance in inertial confinement fusion (ICF) schemes. Little recent research has been dedicated to this process at high laser intensity, in which back, side, and forward scatter simultaneously occur in high energy density plasmas, of relevance to, for example, shock ignition ICF. We present an experimental and particle-in-cell (PIC) investigation of hot electron production from SRS in the forward and near-forward directions from a single speckle laser of wavelength λ_{0}=1.053µm, peak laser intensities in the range I_{0}=0.2-1.0×10^{17}Wcm^{-2} and target electron densities between n_{e}=0.3-1.6%n_{c}, where n_{c} is the plasma critical density. As the intensity and density are increased, the hot electron spectrum changes from a sharp cutoff to an extended spectrum with a slope temperature T=34±1keV and maximum measured energy of 350 keV experimentally. Multidimensional PIC simulations indicate that the high energy electrons are primarily generated from SRS-driven electron plasma wave phase fronts with k vectors angled ∼50^{∘} with respect to the laser axis. These results are consistent with analytical arguments that the spatial gain is maximized at an angle which balances the tendency for the growth rate to be larger for larger scattered light wave angles until the kinetic damping of the plasma wave becomes important. The efficiency of generated high energy electrons drops significantly with a reduction in either laser intensity or target electron density, which is a result of the rapid drop in growth rate of Raman scattering at angles in the forward direction.

Diminishing returns drive altruists to help extended family.

Altruism between close relatives can be easily explained. However, paradoxes arise when organisms divert altruism towards more distantly related recipients. In some social insects, workers drift extensively between colonies and help raise less related foreign brood, seemingly reducing inclusive fitness. Since being highlighted by W. D. Hamilton, three hypotheses (bet hedging, indirect reciprocity and diminishing returns to cooperation) have been proposed for this surprising behaviour. Here, using inclusive fitness theory, we show that bet hedging and indirect reciprocity could only drive cooperative drifting under improbable conditions. However, diminishing returns to cooperation create a simple context in which sharing workers is adaptive. Using a longitudinal dataset comprising over a quarter of a million nest cell observations, we quantify cooperative payoffs in the Neotropical wasp Polistes canadensis, for which drifting occurs at high levels. As the worker-to-brood ratio rises in a worker's home colony, the predicted marginal benefit of a worker for expected colony productivity diminishes. Helping related colonies can allow effort to be focused on related brood that are more in need of care. Finally, we use simulations to show that cooperative drifting evolves under diminishing returns when dispersal is local, allowing altruists to focus their efforts on related recipients. Our results indicate the power of nonlinear fitness effects to shape social organization, and suggest that models of eusocial evolution should be extended to include neglected social interactions within colony networks.

Preoperative assessment of skeletal muscle mass during magnetic resonance enterography in patients with Crohn's disease.

Measurement of the psoas muscle area has been applied to estimate lean muscle mass as a surrogate marker of sarcopenia, but there is a paucity of evidence regarding the influence of sarcopenia on clinical outcomes following inflammatory bowel disease surgery. The aim of this study was to evaluate the association between MRI enterography defined sarcopenia and postoperative complications in patients undergoing elective ileocaecal resection for Crohn's disease. To obtain cross sectional area measurement of the psoas muscle, the freehand area tool was used to trace the margin of each psoas muscle at the level of L4, with the sum recorded as Total Psoas Area (TPA). The total cross sectional muscle area of the abdominal wall was recorded as Skeletal Muscle Area (SMA), while myosteatosis was measured by normalising the psoas muscle intensity with the mean intensity of the cerebrospinal fluid. The primary outcome was the incidence of 30-day postoperative complications in patients in the lowest quartile of TPA and SMA. 31 patients were included and ten patients (32.25%) developed postoperative complications within 30 days of surgery. The cut-off values for the lowest quartile for TPA were 11.93 cm2 in men and 9.77 cm2 in women, including a total of 8 patients (25.8%) with 5 patients in this group (62.5%) developing postoperative complications and 3 patients (37.5%) Clavien-Dindo class ≥ 3 complications. The cut-off values for the lowest quartile for SMA were 73.49 cm2 in men and 65.85 cm2 in women, with 4 patients out of 8 (50%) developing postoperative complications. Psoas muscle cross sectional area and skeletal mass area can be estimated on Magnetic Resonance Enterography as surrogate markers of sarcopenia with high inter-observer agreement.

Feasibility of intraoperative ultrasound of the small bowel during Crohn's disease surgery.

BACKGROUND: Intraoperative assessment of the extent and location of Crohn's disease is not standardised and relies on a mixture of surgeons' experience, tactile feedback and macroscopic appearance. To overcome this variability, we developed a protocol for full intraoperative ultrasound scan of the small bowel and we here report the results of "Assessing the Feasibility and Safety of Using Intraoperative Ultrasound in Ileocolic Crohn's Disease-The IUSS CROHN Study". METHODS: This is a prospective single centre observational study with enrolment of all patients undergoing elective surgery for terminal ileal Crohn's disease from January 2019 to March 2020. Patients underwent laparoscopic ileocolic resection, according to a standardised technique. Ultrasound intraoperative quantitative assessment was performed according to the METRIC (MREnterography or ulTRasound in Crohn's disease) scoring guide. RESULTS: Intraoperative ultrasound was successfully performed in 6 patients from the ileocaecal valve to the proximal jejunum. The median time required was 23.5 min (range 17-37 min) as compared to 6.5 min (5-12 min) required for the macroscopic evaluation performed by the surgeon. In 3 patients, intraoperative ultrasound identified more disease than surgical evaluation. CONCLUSIONS: This feasibility study demonstrated the safety of intraoperative ultrasound and allowed the development of a standardised protocol for intraoperative ultrasound and the data collection required to inform a randomised multicentre study.

High order mode structure of intense light fields generated via a laser-driven relativistic plasma aperture.

The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of extreme light-matter interactions, including ultrafast laser-driven particle acceleration, attosecond pulse generation, plasma photonics, high-field physics and laboratory astrophysics. Here, we experimentally demonstrate modifications to the polarisation and temporal properties of intense light measured at the rear of an ultrathin target foil irradiated by a relativistically intense laser pulse. The changes are shown to result from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities.

The Portsmouth protocol for intra-operative ultrasound of the small bowel in Crohn's disease.

AIM: Bowel preservation is paramount in Crohn's disease surgery as affected patients are typically young adults at risk of having several abdominal surgical procedures during their lifetime. Intra-operative assessment of the extent and location of Crohn's disease is not standardized and is left to a mixture of the surgeon's experience, tactile feedback, macroscopic appearance and preoperative imaging. The aim of this study was to describe the technical steps of a standardized protocol for intra-operative ultrasound assessment of the small bowel in patients undergoing surgery for ileocolic Crohn's disease. METHOD: After laparoscopic mobilization of the bowel, a periumbilical incision is performed for extracorporeal division of the mesentery and the resection and anastomosis. A gastrointestinal consultant radiologist, with expertise in Crohn's disease imaging and abdominal ultrasound, performs full intra-operative assessment of the small bowel by applying a sterile ultrasound probe directly to the bowel, prior to resection being performed by the surgeon. The bowel is assessed through the wound protector with a sterile technique and the length, location and number of segments is documented together with further quantitative assessment using the METRIC (MR enterography or ultrasound in Crohn's disease) scoring guide. RESULTS: A step-by-step protocol for intra-operative ultrasound evaluation of the entire small bowel is described. CONCLUSIONS: A standardized approach to intra-operative evaluation of the extent and location of Crohn's disease is desirable. Intra-operative ultrasound may provide added value for assessment of proximal and multifocal Crohn's disease.

Near-Sun observations of an F-corona decrease and K-corona fine structure.

Remote observations of the solar photospheric light scattered by electrons (the K-corona) and dust (the F-corona or zodiacal light) have been made from the ground during eclipses1 and from space at distances as small as 0.3 astronomical units2-5 to the Sun. Previous observations6-8 of dust scattering have not confirmed the existence of the theoretically predicted dust-free zone near the Sun9-11. The transient nature of the corona has been well characterized for large events, but questions still remain (for example, about the initiation of the corona12 and the production of solar energetic particles13) and for small events even its structure is uncertain14. Here we report imaging of the solar corona15 during the first two perihelion passes (0.16-0.25 astronomical units) of the Parker Solar Probe spacecraft13, each lasting ten days. The view from these distances is qualitatively similar to the historical views from ground and space, but there are some notable differences. At short elongations, we observe a decrease in the intensity of the F-coronal intensity, which is suggestive of the long-sought dust free zone9-11. We also resolve the fine-scale plasma structure of very small eruptions, which are frequently ejected from the Sun. These take two forms: the frequently observed magnetic flux ropes12,16 and the predicted, but not yet observed, magnetic islands17,18 arising from the tearing-mode instability in the current sheet. Our observations of the coronal streamer evolution confirm the large-scale topology of the solar corona, but also reveal that, as recently predicted19, streamers are composed of yet smaller substreamers channelling continual density fluctuations at all visible scales.

An optically multiplexed single-shot time-resolved probe of laser-plasma dynamics.

We introduce a new approach to temporally resolve ultrafast micron-scale processes via the use of a multi-channel optical probe. We demonstrate that this technique enables highly precise time-resolved, two-dimensional spatial imaging of intense laser pulse propagation dynamics, plasma formation and laser beam filamentation within a single pulse over four distinct time frames. The design, development and optimization of the optical probe system is presented, as are representative experimental results from the first implementation of the multi-channel probe with a high-power laser pulse interaction with a helium gas jet target.

Dual Ion Species Plasma Expansion from Isotopically Layered Cryogenic Targets.

A dual ion species plasma expansion scheme from a novel target structure is introduced, in which a nanometer-thick layer of pure deuterium exists as a buffer species at the target-vacuum interface of a hydrogen plasma. Modeling shows that by controlling the deuterium layer thickness, a composite H^{+}/D^{+} ion beam can be produced by target normal sheath acceleration (TNSA), with an adjustable ratio of ion densities, as high energy proton acceleration is suppressed by the acceleration of a spectrally peaked deuteron beam. Particle in cell modeling shows that a (4.3±0.7) MeV per nucleon deuteron beam is accelerated, in a directional cone of half angle 9°. Experimentally, this was investigated using state of the art cryogenic targetry and a spectrally peaked deuteron beam of (3.4±0.7) MeV per nucleon was measured in a cone of half angle 7°-9°, while maintaining a significant TNSA proton component.

Near-100 MeV protons via a laser-driven transparency-enhanced hybrid acceleration scheme.

The range of potential applications of compact laser-plasma ion sources motivates the development of new acceleration schemes to increase achievable ion energies and conversion efficiencies. Whilst the evolving nature of laser-plasma interactions can limit the effectiveness of individual acceleration mechanisms, it can also enable the development of hybrid schemes, allowing additional degrees of control on the properties of the resulting ion beam. Here we report on an experimental demonstration of efficient proton acceleration to energies exceeding 94 MeV via a hybrid scheme of radiation pressure-sheath acceleration in an ultrathin foil irradiated by a linearly polarised laser pulse. This occurs via a double-peaked electrostatic field structure, which, at an optimum foil thickness, is significantly enhanced by relativistic transparency and an associated jet of super-thermal electrons. The range of parameters over which this hybrid scenario occurs is discussed and implications for ion acceleration driven by next-generation, multi-petawatt laser facilities are explored.

A regional EUS service using a collaborative network.

Endoscopic ultrasound (EUS) is increasingly used in the management of hepatobiliary lesions, allowing staging and tissue acquisition. It is operator-dependent, and fine needle aspiration (FNA) of solid lesions provides an auditable standard; high-volume centres have shown excellent results for solid pancreatic lesion FNA with sensitivities of 92%-97%. The British Society of Gastroenterology guidelines stress that clinical quality should determine service provision, with geographical accessibility a secondary consideration. We set up the Wessex EUS network, working from a single hepatobiliary (HPB) pancreatic multidisciplinary team, with EUS provided in four local centres providing agreed standards and audit. Pancreatic solid lesion FNA results showed a pooled sensitivity of 94%, comparable with high-volume single centres. This demonstrates a network with good clinical governance is a plausible solution to providing a specialist service such as EUS and may be a roadmap that other specialist services under pressure could follow.