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Hot forging wire and arc additive manufacturing (HF-WAAM)
No full textDuring WAAM, the material is locally forged immediately after deposition, and in-situ viscoplastic deformationoccurs at high temperatures. In the subsequent layer deposition, recrystallization of the previous solidificationstructure occurs that refines the microstructure. Because of its similarity with hot forging, this variant wasnamed hot forging wire and arc additive manufacturing (HF-WAAM). A customized WAAM torch was developed,manufactured, and tested in the production of samples of AISI316L stainless steel. Forging forces of 17 N and 55N were applied to plastically deform the material. The results showed that this new variant refines the solidificationmicrostructure and reduce texture effects, as determined via high energy synchrotron X-ray diffractionexperiments, without interrupting the additive manufacturing process. Mechanical characterization was performedand improvements on both yield strength and ultimate tensile strength were achieved. Furthermore, itwas observed that HF-WAAM significantly affects porosity; pores formed during the process were closed by thehot forging process. Because deformation occurs at high temperatures, the forces involved are small, and theWAAM equipment does not have specific requirements with respect to stiffness, thereby allowing the incorporationof this new variant into conventional moving equipment such as multi-axis robots or 3-axis tableused in WAAM
Microscopic force for aerosol transport
No full textA key ingredient for single particle diffractive imaging experiments is the successful and efficient delivery of sample. Current sample-delivery methods are based on aerosol injectors in which the samples are driven by fluid-dynamic forces. These are typically simulated using Stokes' drag forces and for micrometer-size or smaller particles, the Cunningham correction factor is applied. This is not only unsatisfactory, but even using a temperature dependent formulation it fails at cryogenic temperatures. Here we propose the use of a direct computation of the force, based on Epstein's formulation, that allows for high relative velocities of the particles to the gas and also for internal particle temperatures that differ from the gas temperature. The new force reproduces Stokes' drag force for conditions known to be well described by Stokes' drag. Furthermore, it shows excellent agreement to experiments at 4 K, confirming the improved descriptive power of simulations over a wide temperature range
Novel ultrafast fiber laser sources utilizing fiber nonlinearities
No full textDue to the lack of proper active gain media, mid-IR frequency combs in the molecular “fingerprint” region are normally implemented by difference frequency generation (DFG) process inside nonlinear crystals. In addition to granting an access to the long-wave (>10 μm) side of the mid-IR spectrum, DFG also offers a wide wavelength tunability, a high-repetition-rate (HRR) operation (>10 MHz), and a passive cancelation of the carrier-envelope phase offset, all of which enable a simplified experimental configuration for constructing mid-IR frequency combs.Currently, DFG-based mid-IR frequency combs especially at wavelengths beyond 10 μm suffer from insufficient available power, mainly caused by un-optimized wavelength-shifting processes and an inefficient parametric interaction of the DFG process. Moreover, during conventional nonlinear fiber-optic wavelength-shifting methods, an excessive amount of relative timing jitter (RTJ) noise is introduced to the signal pulses, which later translates to the noise of the generated mid-IR idler pulses during the DFG process, and may significantly deteriorate the comb performance of the mid-IR frequency comb.In this thesis, we numerically investigate several important issues that are related with constructing high-power, low-noise DFG-based mid-IR frequency combs. We first discuss two nonlinear fiber-optic methods, namely, soliton self-frequency shift (SSFS) and self-phase-modulation enabled spectral selection (SESS), by comparing their capability of generating wavelength-shifted signal pulses with a high pulse energy and a low RTJ noise. To elucidate an efficient approach for generating high-power mid-IR sources, we study the power scalability of DFG by investigating DFG processes under different launching conditions. Two sets of DFG with different pump wavelengths, 1.03 μm and 2 μm, are compared. Besides the power scalability, we also compare the noise transfer property of these two sets of DFG. We identify how the timing jitter noise of an offset-free frequency comb affects its comb characteristics through Monte-Carlo simulations. The comb line-width can be well estimated by using a geometrical _-separation line method. Finally, we demonstrate a 1-GHz passively offset-free laser source which is built based upon a Yb:fiber laser system and an alternative DFG schematic.Enabled by novel nonlinear fiber-optic wavelength-shifting methods, the pulse energy of the wavelength-shifted signal pulses can be increased from sub-nJ level to tens-of-nJ or even hundreds-of-nJ level. Using such signal pulses to seed an optimized DFG system, a 50-MHz HRR mid-IR frequency comb with an average output power of >2.2-W is viable. We believe that such a source will open new avenues for novel scientific applications
In Situ X-ray Diffraction and X-ray Absorption Spectroscopic Studies of a Lithium-Rich Layered Positive Electrode Material: Comparison of Composite and Core–Shell Structures
No full textLithium- and manganese-rich transition-metal oxide (LMR-NMC) electrodes have been designed either as heterostructures of the primary components (“composite”) or as core–shell structures with improved electrochemistry reported for both configurations when compared with their primary components. A detailed electrochemical and structural investigation of the 0.5Li2MnO3–0.5LiNi0.5Mn0.3Co0.2O2 composite and core–shell structured positive electrode materials is reported. The core–shell material shows better overall electrochemical performance compared to its corresponding composite material. While both configurations gave the same initial charge capacity of ∼300 mAh/g when cycled at a rate of 10 mA/g at 25 °C, the core–shell sample gives a discharge capacity of 232 mAh/g compared to 208 mAh/g delivered by the composite sample. Also, the core–shell sample gave better rate capability and a smaller first-cycle irreversible capacity loss than the composite sample. The improved performance of the core–shell material is attributed to its lower surface reactivity and limited structural change since the more stable Li2MnO3 shell screens the more reactive Ni-rich core material from interacting with either air or electrolyte at high potentials, thereby preventing electrode surface modification. In situ X-ray diffraction correlated with electrochemical data revealed that the composite sample shows stronger volumetric changes in the lattice parameters during charging to 4.8 V. In addition, X-ray absorption spectroscopy showed an incomplete Ni reduction process after the first discharge for the composite sample. From these results, it was shown that this leads to a more severe degradation in the composite material that affects Li+ intercalation in the subsequent discharge, thereby resulting in its poorer performance. Furthermore, to confirm these results, another LMR-NMC material with a different composition (having a Ni-poor core)—0.5Li2MnO3-0.5LiNi0.33Mn0.33Co0.33O2—was investigated. The core–shell structured positive electrode material also gave an improved electrochemical performance compared to the corresponding composite positive electrode material. These results show that the core–shell configuration could effectively be used to improve the performance of the LMR-NMC materials to enable future high-energy applications
THz streak camera performance for single-shot characterization of XUV pulses with complex temporal structures
No full textThe THz-field-driven streak camera has proven to be a powerful diagnostic-technique that enables the shot-to-shot characterization of the duration and the arrival time jitter of free electron laser (FEL) pulses. Here we investigate the performance of three computational approaches capable to determine the duration of FEL pulses with complex temporal structures from single-shot measurements of up to three simultaneously recorded spectra. We use numerically simulated FEL pulses in order to validate the accuracy of the pulse length retrieval in average as well as in a single-shot mode. We discuss requirements for the THz field strength in order to achieve reliable results and compare our numerical study with the analysis of experimental data that were obtained at the FEL in Hamburg - FLASH
ILC Study Questions for Snowmass 2021
No full textTo aid contributions to the Snowmass 2021 US Community Study on physics at the International Linear Collider and other proposed colliders, we present a list of study questions that could be the basis of useful Snowmass projects. We accompany this with links to references and resources on physics, and a description of a new software framework that we are preparing for studies at Snowmass
Microstructural Investigations of Novel High Temperature Alloys Based on NiAl-(Cr,Mo)
No full textApart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae byadding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo) reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trendpredicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientationrelationship between NiAl matrix and (Cr,Mo) reinforcing phase was (100)||(100) and ||. An orientation relationship described by a rotation of almost 60 about was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observed
Status and prospects of the Belle II experiment. Dark sectors and rare penguins
No full textThe Belle II experiment at the SuperKEKB accelerator in Tsukuba, Japan, is a second-generation B factory experiment. Belle II will benefit from an approximate factor of 40 increase in the instantaneous luminosity with respect to the previous Belle / KEKB experiment, due to significantly reduced beam interaction profile, and higher beam current. In this talk, I will present some results and prospects for dark sector physics measurements on early data, as well as some select important physics measurements over the lifetime of the Belle II experiment. I will focus on areas where there is complementarity with LHCb. In particular the rare B decay modes that can help to shed light on recent anomalies observed by LHCb
Field-enabled quantum interference in atomic Auger decay
No full textWe demonstrate that an external terahertz (THz) field enables the formation of interference between two distinct Auger pathways leading to the same final ionic state. The kinetic energy of Auger electrons ejected from either of two spin-orbit split one-hole states of magnesium cations is recorded. In the presence of the THz field, a clear oscillatory structure in the Auger spectrum emerges, which we find to be in very good agreement with an analytical model based on perturbation theory. For this interference to occur, the THz field has to chirp the energy of both Auger electrons and photoelectrons simultaneously, in order to create states with indistinguishable quantum properties