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Maraging steels are known for their exceptional strength but suffer from limited work hardening and ductility. Here, we report an intermittent printing approach to tailor the microstructure and mechanical properties of maraging 250 steel via engineering of the thermal history during plasma arc additive manufacturing (PAAM). Through introducing a dwell time between adjacent layers, the maraging 250 steel is cooled below the martensite start temperature, triggering a thermally driven, in-situ martensitic transformation during the printing process. Re-heating or thermal cycling during subsequent layer deposition impedes complete martensitic transformation, enabling coexistence of martensite and retained austenite phases with elemental segregation. The enrichment of Ni in the austenite phase promotes stabilization of the retained austenite upon cooling down to room temperature. The retained austenite is yet metastable during deformation, leading to stress-induced martensitic transformation under loading. Specifically, a 3 min interlayer dwell time produces a maraging 250 steel with approximately 8% retained austenite, resulting in improved work hardening via martensitic transformation induced plasticity (TRIP) during deformation. Meanwhile, the higher cooling rate induced by the dwell time results in substantially refined grain structures with an increased dislocation density, leading to a simultaneously improved yield strength. Notably, the yield strength increases from 836 MPa (0 min dwell) to 990 MPa (3 min dwell), and the uniform elongation increases from 3.2% (0 min dwell) to 6.5% (3 min dwell). This intermittent deposition strategy demonstrates the potential to tune the microstructure and mechanical properties of maraging steels through engineering the thermal history during additive manufacturing. 

Abstract Slow earthquakes, including low-frequency earthquakes, tremor, and geodetically detected slow-slip events, have been widely detected, most commonly at depths of 40–60 km in active subduction zones around the Pacific Ocean Basin. Rocks exhumed from these depths allow us to search for structures that may initiate slow earthquakes. The evidence for high pore-fluid pressures in subduction zones suggests that they may be associated with hydraulic fractures (e.g., veins) and with metamorphic reactions that release or consume water. Loss of continuity and resulting slip at rates exceeding 10−4 m s–1 are required to produce the quasi-seismic signature of low-frequency earthquakes, but the subseismic displacement rates require that the slip rate is slowed by a viscous process, such as low permeability, limiting the rate at which fluid can access a propagating fracture. Displacements during individual low-frequency earthquakes are unlikely to exceed 1 mm, but they need to be more than 0.1 mm and act over an area of ~105 m2 to produce a detectable effective seismic moment. This limits candidate structures to those that have lateral dimensions of ~300 m and move in increments of <1 mm. Possible candidates include arrays of sheeted shear veins showing crack-seal structures; dilational arcs in microfold hinges that form crenulation cleavages; brittle-ductile shear zones in which the viscous component of deformation can limit the displacement rate during slow-slip events; slip surfaces coated with materials, such as chlorite or serpentine, that exhibit a transition from velocity-weakening to velocity-strengthening behavior with increasing slip velocity; and block-in-matrix mélanges. 

Abstract The application high intensity ultrafast lasers to compact plasma-based electron accelerators has recently been an extremely active area of research. Here, for the first time, we show experimentally and theoretically that carefully sculpting an intense ultrafast pulse in the spatio-temporal domain allows ponderomotive pressure to be used for direct acceleration of electron bunches from rest to relativistic energies. With subluminal group velocity and above-threshold intensity, a laser pulse can capture and accelerate electrons, pushing on them like a snowplow. Acceleration of electrons from rest requires a substantial reduction of group velocity. In this demonstration experiment, we achieve a group velocity of ∼0.6c in a tilted pulse by focusing the output of a novel asymmetric pulse compressor we developed for the petawatt-class ALEPH system at Colorado State University. This direct laser-electron approach opens a route towards exploiting optical spatio-temporal control techniques to sculpt electron beams with desired properties such as narrow energy and angular distributions. The tilted-pulse snowplow technique can be scaled from small-scale to facility-scale amplifiers to produce short electron bunches in the 10 keV−10 MeV range for applications including ultrafast electron diffraction and efficient injection into laser wakefield accelerators for acceleration beyond the GeV level. 

Ultrafast laser pulse beams are four-dimensional, space–time phenomena that can exhibit complicated, coupled spatial and temporal profiles. Tailoring the spatiotemporal profile of an ultrafast pulse beam is necessary to optimize the focused intensity and to engineer exotic spatiotemporally shaped pulse beams. Here we demonstrate a single-pulse, reference-free spatiotemporal characterization technique based on two colocated synchronized measurements: (1) broadband single-shot ptychography and (2) single-shot frequency resolved optical gating. We apply the technique to measure the nonlinear propagation of an ultrafast pulse beam through a fused silica window. Our spatiotemporal characterization method represents a major contribution to the growing field of spatiotemporally engineered ultrafast laser pulse beams. 

We generalize our method for propagating spatially chirped Gaussian beams to properly calculate the evolution of geometric spectral phase through a lens. By expanding the spectral phase around the local central frequency, we analytically calculate the spatio-temporal field. Applications to intentionally detuned pulse compressors are discussed. 

High-intensity pulse-beams are ubiquitous in scientific investigations and industrial applications ranging from the generation of secondary radiation sources (e.g., high harmonic generation, electrons) to material processing (e.g., micromachining, laser-eye surgery). Crucially, pulse-beams can only be controlled to the degree to which they are characterized, necessitating sophisticated measurement techniques. We present a reference-free, full-field, single-shot spatiospectral measurement technique called broadband single-shot ptychography (BBSSP). BBSSP provides the complex wavefront for each spectral and polarization component in an ultrafast pulse-beam and should be applicable across the electromagnetic spectrum. BBSSP will dramatically improve the application and mitigation of spatiospectral pulse-beam structure. 

Ultrafast pulse-beam characterization is critical for diverse scientific and industrial applications from micromachining to generating the highest intensity laser pulses. The four-dimensional structure of a pulse-beam, $\stackrel{~<\#comment/>}{E}(x,y,z,\mathrm{\omega <\#comment/>})$, can be fully characterized by coupling spatiospectral metrology with spectral phase measurement. When temporal pulse dynamics are not of primary interest, spatiospectral characterization of a pulse-beam provides crucial information even without spectral phase. Here we demonstrate spatiospectral characterization of pulse-beams via multiplexed broadband ptychography. The complex spatial profiles of multiple spectral components, $\stackrel{~<\#comment/>}{E}(x,y,\mathrm{\omega <\#comment/>})$, from modelocked Ti:sapphire and from extreme ultra-violet pulse-beams are reconstructed with minimum intervening optics and no refocusing. Critically, our technique does not require spectral filters, interferometers, or reference pulses. 

The Pierre Auger Observatory (Auger) and the Telescope Array Project (TA) are the two largest ultra-high-energy cosmic ray observatories in the world. They operate in the Southern and Northern hemispheres, respectively, at similar latitudes but with different surface detector (SD) designs. This difference in detector design changes their sensitivity to the various components of extensive air showers. The over-arching goal of the Auger@TA working group is to cross-calibrate the SD arrays of the two observatories in order to identify or rule out systematic causes for the apparent differences in the flux measured at Auger and TA. The project itself is divided into two phases. Phase-I finished in 2020 and consisted of a station-level comparison facilitated by the deployment of two Auger stations, one prototype station with a single central PMT and a standard Auger station, in the middle of the TA SD near the Central Laser Facility, along with a modified TA station to provide external triggers from the TA SD. This provided the opportunity to observe the same extensive air showers with both Auger and TA detectors to directly compare their measurements. Phase-II of Auger@TA is currently underway and aims at building a self-triggering micro-Auger-array inside the TA array. This micro-array consists of eight Auger stations, seven of which use a 1-PMT prototype configuration and form a single hexagon with a traditional 1.5 km Auger spacing. The 8th station is of the standard Auger 3-PMT configuration and is placed at the center of the hexagon, along with a TA station to form a triplet. Each Auger station will also be outfitted with an AugerPrime Surface Scintillator Detector. A custom communication system using readily available components will be used to provide communication between the stations and remote access to each station via a central communications station. The deployment of the micro-array took place at the end of September 2022. A simulation study was carried out to gauge the expected performance of the Auger@TA micro-array and to derive trigger effi ciencies and event rates. 

We present a ptychographic phase retrieval algorithm which solves the square root problem in second order pulse measurement techniques and reconstructs the fields of multiple incoherent pulses simultaneously from a single dispersion scan trace.