Plasma electron acceleration driven by a long-wave-infrared laser.

Laser-driven plasma accelerators provide tabletop sources of relativistic electron bunches and femtosecond x-ray pulses, but usually require petawatt-class solid-state-laser pulses of wavelength λL ~ 1 µm. Longer-λL lasers can potentially accelerate higher-quality bunches, since they require less power to drive larger wakes in less dense plasma. Here, we report on a self-injecting plasma accelerator driven by a long-wave-infrared laser: a chirped-pulse-amplified CO2 laser (λL ≈ 10 µm). Through optical scattering experiments, we observed wakes that 4-ps CO2 pulses with < 1/2 terawatt (TW) peak power drove in hydrogen plasma of electron density down to 4 × 1017 cm-3 (1/100 atmospheric density) via a self-modulation (SM) instability. Shorter, more powerful CO2 pulses drove wakes in plasma down to 3 × 1016 cm-3 that captured and accelerated plasma electrons to relativistic energy. Collimated quasi-monoenergetic features in the electron output marked the onset of a transition from SM to bubble-regime acceleration, portending future higher-quality accelerators driven by yet shorter, more powerful pulses.

Stable Positron Acceleration in Thin, Warm, Hollow Plasma Channels.

Hollow plasma channels are attractive for lepton acceleration because they provide intrinsic emittance preservation regimes. However, beam breakup instabilities dominate the dynamics. Here, we show that thin, warm hollow channels can sustain large-amplitude plasma waves ready for high-quality positron acceleration. We verify that the combination of warm electrons and thin hollow channels enables positron focusing structures. Such focusing wakefields unlock beam breakup damping mechanisms. We demonstrate that such channels emerge self-consistently during the long-term plasma dynamics in the blowout's regime aftermath, allowing for experimental demonstration.

Producing multi-coloured bunches through beam-induced ionization injection in plasma wakefield accelerator.

This paper discusses the properties of electron beams formed in plasma wakefield accelerators through ionization injection. In particular, the potential for generating a beam composed of co-located multi-colour beamlets is demonstrated in the case where the ionization is initiated by the evolving charge field of the drive beam itself. The physics of the processes of ionization and injection are explored through OSIRIS simulations. Experimental evidence showing similar features are presented from the data obtained in the E217 experiment at the FACET facility of the SLAC National Laboratory. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Betatron radiation and emittance growth in plasma wakefield accelerators.

Beam-driven plasma wakefield acceleration (PWFA) has demonstrated significant progress during the past two decades of research. The new Facility for Advanced Accelerator Experimental Tests (FACET) II, currently under construction, will provide 10 GeV electron beams with unprecedented parameters for the next generation of PWFA experiments. In the context of the FACET II facility, we present simulation results on expected betatron radiation and its potential application to diagnose emittance preservation and hosing instability in the upcoming PWFA experiments. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Observation of Betatron X-Ray Radiation in a Self-Modulated Laser Wakefield Accelerator Driven with Picosecond Laser Pulses.

We investigate a new regime for betatron x-ray emission that utilizes kilojoule-class picosecond lasers to drive wakes in plasmas. When such laser pulses with intensities of ∼5×10^{18} W/cm^{2} are focused into plasmas with electron densities of ∼1×10^{19} cm^{-3}, they undergo self-modulation and channeling, which accelerates electrons up to 200 MeV energies and causes those electrons to emit x rays. The measured x-ray spectra are fit with a synchrotron spectrum with a critical energy of 10-20 keV, and 2D particle-in-cell simulations were used to model the acceleration and radiation of the electrons in our experimental conditions.

Role of Direct Laser Acceleration of Electrons in a Laser Wakefield Accelerator with Ionization Injection.

We show the first experimental demonstration that electrons being accelerated in a laser wakefield accelerator operating in the forced or blowout regimes gain significant energy from both the direct laser acceleration (DLA) and the laser wakefield acceleration mechanisms. Supporting full-scale 3D particle-in-cell simulations elucidate the role of the DLA of electrons in a laser wakefield accelerator when ionization injection of electrons is employed. An explanation is given for how electrons can maintain the DLA resonance condition in a laser wakefield accelerator despite the evolving properties of both the drive laser and the electrons. The produced electron beams exhibit characteristic features that are indicative of DLA as an additional acceleration mechanism.

Effects of poor hygiene on cytokine phenotypes in children in the tropics.

We describe immune phenotypes (innate and adaptive cytokines) according to environmental exposure using latent class analysis. A total of 310 schoolchildren living in Ecuador were assayed for spontaneous cytokine production as well as mitogen (SEB)-stimulated cytokines in whole blood cultures. We collected data on environmental exposures by questionnaire and on intestinal parasites by examination of stool samples. Latent class analysis (LCA) was used to group children according to their innate (IL-6, IL-8, IL-10 and TNF-α) and adaptive (IL-5, IL-13, IL-17, IFN-γ and IL-10) cytokine profile. We also conducted multiple-group LCA and LCA with covariates to evaluate the effect of predictors on profile membership. We identified both hyporesponsive and Th2-modified immune phenotypes produced by peripheral blood leukocytes (PBLs) that were associated with intestinal worms and birth order, providing insights into how poor hygiene mediates immunologic effects on immune-mediated diseases.

Individual and contextual determinants of the duration of diarrhoeal episodes in preschool children: a longitudinal study in an urban setting.

This study investigated individual and contextual factors associated with the duration of diarrhoeal episodes in 693 young children living in a large Brazilian city who were followed-up for at least 3 months. The outcome is analysed as a continuous variable, by means of a hierarchical conceptual model organizing the factors in meaningful blocks. A total of 2397 episodes were recorded (median duration 2 days, interquartile range 1-3 days). Low percentage of households connected to the sewerage system in the neighbourhood, low family purchasing power, high agglomeration, mother aged <19 years, low zinc content in child's diet, and episode severity were significantly associated with longer duration (0·26-0·69 days more). Purchasing power effect was largely mediated by environmental conditions, characteristics of the child, and hygienic behaviour. Environmental conditions acted as a possible effect modifier, enhancing the effect on duration of diarrhoea of the child not having being vaccinated against measles or breastfed for >6 months.