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.
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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 λ
~ ...1 μm. Longer-λ
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 CO
laser (λ
≈ 10 μm). Through optical scattering experiments, we observed wakes that 4-ps CO
pulses with < 1/2 terawatt (TW) peak power drove in hydrogen plasma of electron density down to 4 × 10
cm
(1/100 atmospheric density) via a self-modulation (SM) instability. Shorter, more powerful CO
pulses drove wakes in plasma down to 3 × 10
cm
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.
Summary
We introduce noncontact optical microscopy and optical scattering to characterize asphalt binder microstructure at temperatures ranging from 15°C to 85°C for two compositionally different ...asphalt binders. We benchmark optical measurements against rheometric measurements of the magnitude of the temperature‐dependent bulk complex shear modulus |G*(T)|. The main findings are: (1) Elongated (∼5 × 1 μm), striped microstructures (known from AFM studies as ‘bees’ because they resemble bumble‐bees) are resolved optically, found to reside primarily at the surface and do not reappear immediately after a single heating–cooling cycle. (2) Smaller (∼1 μm2) microstructures with no observable internal structure (hereafter dubbed ‘ants’), are found to reside primarily in the bulk, to persist after multiple thermal cycles and to scatter light strongly. Optical scattering from ‘ants’ decreases to zero with heating from 15°C to 65°C, but recovers completely upon cooling back to 15°C, albeit with distinct hysteresis. (3) Rheometric measurements of |G*(T)| reveal hysteresis that closely resembles that observed by optical scatter, suggesting that thermally driven changes in microstructure volume fraction cause corresponding changes in |G*(T)|.
Lay description
Most paved roads are built using asphalt mixtures that consist of mineral aggregates held together by a binder, or ‘glue’, that governs a road's mechanical properties. This ‘glue’, called bitumen or asphalt binder, is a viscous hydrocarbon emulsion distilled from petroleum. Pavement grade bitumen must be fluid enough to be pumped, worked and mixed during paving, yet stiff enough after paving to resist rutting in hot weather, and soft enough to resist cracking in cold weather. These demanding, and partly conflicting, requirements have engendered extensive research to understand and optimize the chemical makeup and microscopic characteristics of bitumen that control its macroscopic mechanical properties.
Under an optical microscope, bitumen that appears uniform in texture to the unaided eye exhibits a rich array of bacteria‐size inclusions. Here, we find that some of these inclusions reside on the surface, are best observed with green light, and can be prevented from forming by preparing samples under a cover slip. Other smaller inclusions pervade the bulk, are best observed with near infrared light, and survive indefinite thermal cycling. We employ near infrared optical microscopy to measure the volume fraction of two chemically distinct pavement‐grade bitumen varieties that these bulk inclusions occupy as we vary sample temperature between 15°C and 65°C. Upon slow heating, this volume fraction at first grows, then shrinks to zero. Upon slow cooling, the volume fraction is as much as 78–92% smaller (depending on the bitumen variety) at a given temperature than during heating, although it returns to its original value after cooling back to 15°C. In parallel with these microscopic measurements, we employ a rheometer to measure variations in stiffness of these two bitumen varieties as we vary their temperature over the same range. We observe that the stiffness of both varieties reduces upon heating, and recovers upon cooling. But most significantly, we observe a 15–23% discrepancy in stiffness at any given temperature between the heating and cooling cycles that mirrors the behavior of the microscopic inclusions. This behavior reproduces during subsequent thermal cycles. The significance of these results is that they establish a direct, quantitative correlation, and suggest a causal connection, between the volume fraction occupied by the inclusions and the stiffness of bitumen. They also demonstrate that optical microscopy can play a key role in fast, noncontact screening and testing of asphalt binders.
Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multi-gigaelectronvolt ...energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.
Summary
We combined optical and atomic force microscopy to observe morphology and kinetics of microstructures (typically referred to as bees) that formed at free surfaces of unmodified Performance ...Graded (PG) 64‐22 asphalt binders upon cooling from 150°C to room temperature (RT) at 5°C min–1, and changes in these microstructures when the surface was terminated with a transparent solid (glass) or liquid (glycerol) overlayer. The main findings are: (1) at free binder surfaces, wrinkled microstructures started to form near the crystallization temperature (∼45°C) of saturates such as wax observed by differential scanning calorimetry, then grew to ∼5 µm diameter, ∼25 nm wrinkle amplitude and 10–30% surface area coverage upon cooling to RT, where they persisted indefinitely without observable change in shape or density. (2) Glycerol coverage of the binder surface during cooling reduced wrinkled area and wrinkle amplitude three‐fold compared to free binder surfaces upon initial cooling to RT; continued glycerol coverage at RT eliminated most surface microstructures within ∼4 h. (3) No surface microstructures were observed to form at binder surfaces covered with glass. (4) Submicron bulk microstructures were observed by near‐infrared microscopy beneath the surfaces of all binder samples, with size, shape and density independent of surface coverage. No tendency of such structures to float to the top or sink to the bottom of mm‐thick samples was observed. (5) We attribute the dependence of surface wrinkling on surface coverage to variation in interface tension, based on a thin‐film continuum mechanics model.
Lay Description
Asphalt binder, or bitumen, is the glue that holds aggregate particles together to form a road surface. It is derived from the heavy residue that remains after distilling gasoline, diesel and other lighter products out of crude oil. Nevertheless, bitumen varies widely in composition and mechanical properties. To avoid expensive road failures, bitumen must be processed after distillation so that its mechanical properties satisfy diverse climate and load requirements. International standards now guide these mechanical properties, but yield varying long‐term performance as local source composition and preparation methods vary. In situ diagnostic methods that can predict bitumen performance independently of processing history are therefore needed. The present work focuses on one promising diagnostic candidate: microscopic observation of internal bitumen structure. Past bitumen microscopy has revealed microstructures of widely varying composition, size, shape and density. A challenge is distinguishing bulk microstructures, which directly influence a binder's mechanical properties, from surface microstructures, which often dominate optical microscopy because of bitumen's opacity and scanning‐probe microscopy because of its inherent surface specificity. In previously published work, we used infrared microscopy to enhance visibility of bulk microstructure. Here, as a foil to this work, we use visible‐wavelength microscopy together with atomic‐force microscopy (AFM) specifically to isolate surface microstructure, to understand its distinct origin and morphology, and to demonstrate its unique sensitivity to surface alterations. To this end, optical microscopy complements AFM by enabling us to observe surface microstructures form at temperatures (50°C–70°C) at which bitumen's fluidity prevents AFM, and to observe surface microstructure beneath transparent, but chemically inert, liquid (glycerol) and solid (glass) overlayers, which alter surface tension compared to free surfaces. From this study, we learned, first, that, as bitumen cools, distinctly wrinkled surface microstructures form at the same temperature at which independent calorimetric studies showed crystallization in bitumen, causing it to release latent heat of crystallization. This shows that surface microstructures are likely precipitates of the crystallizable component(s). Second, a glycerol overlayer on the cooling bitumen results in smaller, less wrinkled, sparser microstructures, whereas a glass overlayer suppresses them altogether. In contrast, underlying smaller bulk microstructures are unaffected. This shows that surface tension is the driving force behind formation and wrinkling of surface precipitates. Taken together, the work advances our ability to diagnose bitumen samples noninvasively by clearly distinguishing surface from bulk microstructure.
Plasma waves generated in the wake of intense, relativistic laser1,2 or particle beams3,4 can accelerate electron bunches to gigaelectronvolt energies in centimetre-scale distances. This allows the ...realization of compact accelerators with emerging applications ranging from modern light sources such as the free-electron laser to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre wakefields can accelerate witness electron bunches that are either externally injected5,6 or captured from the background plasma7,8. Here we demonstrate optically triggered injection9–11 and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ‘plasma photocathode’ decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical11 density down-ramp injection12–16 and is an important step towards the generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness17. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh-brightness beams.
We report observations of coherent optical transition radiation interferometry (COTRI) patterns generated by microbunched ∼ 200 − MeV electrons as they emerge from a laser-driven plasma accelerator. ...The divergence of the microbunched portion of electrons, deduced by comparison to a COTRI model, is ∼ 9 × smaller than the ∼ 3 mrad ensemble beam divergence, while the radius of the microbunched beam, obtained from COTR images on the same shot, is < 3 μ m . The combined results show that the microbunched distribution has estimated transverse normalized emittance ∼ 0.4 mm mrad .
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