arXiv subjects

M. Kaemingk

Publications and source records attributed to M. Kaemingk.

13 recordsLinked to original sources

arXiv2025

Multi-Objective Optimizations of High Gradient C-band Photoinjector for High Bunch Charge Applications

The high gradients potentially achievable in distributed-coupling C-band photoinjectors make them attractive for many high brightness applications. Here we discuss optimization results for a 1.6 cell C-band photoinjector with a 240 MV/m peak field at the cathode that delivers a 250 pC electron bunch charge. We use a Multi-Objective Genetic Algorithm (MOGA), obtaining a Pareto front of emittance vs. bunch length. We also perform MOGA optimizations including an aperture to retain only a bright beam core. We find this reduces the emittance of the final beam by more than factor of 2 in some cases. For example, we find that at a root mean square bunch length of 1.6 ps, the use of an aperture improves the transverse emittance from 120 nm to 58 nm assuming negligible photocathode intrinsic emittance. The sacrificial charge at the periphery of the electron beam removed by the aperture linearizes the final slice phase space inside of the remaining beam core. The results obtained surpass the experimental state-of-the-art for beamlines with similar bunch charge.

physics.acc-ph↗

arXiv2025

Photoinduced twist and untwist of moir\'e superlattices in TMDC heterobilayers

Two-dimensional moir\'e materials are formed by artificially stacking atomically thin monolayers. A wealth of correlated and topological quantum phases can be engineered via precise choice of stacking geometry. These designer electronic properties depend crucially on interlayer coupling and atomic registry. An important open question is how atomic registry responds on ultrafast timescales to optical excitation and whether the moir\'e geometry can be dynamically reconfigured to tune emergent phenomena in real time. Here we show that femtosecond photoexcitation drives a coherent twist-untwist motion of the moir\'e superlattice in $2^\circ$ and $57^\circ$ twisted WSe$_2$/MoSe$_2$ heterobilayers, resolved directly by ultrafast electron diffraction. Upon above-band-gap photoexcitation, the moir\'e superlattice diffraction features are enhanced within 1 ps and subsequently suppressed several picoseconds after, deviating markedly from typical photoinduced lattice heating. Kinetic diffraction analysis, supported by simulations of the sample dynamics, indicates a peak-to-trough local twist angle modulation of $0.6^\circ$, correlated with a sub-THz frequency moir\'e phonon. This motion is driven by ultrafast charge transfer that transiently increases interlayer attraction. Our results could lead to ultrafast control of moir\'e periodic lattice distortions and, by extension, the local moir\'e potential that shapes excitons, polarons, and correlation-driven behaviors

cond-mat.mtrl-sci↗

arXiv2024

Compensating slice emittance growth in high brightness photoinjectors using sacrificial charge

Achieving maximum electron beam brightness in photoinjectors requires detailed control of the 3D bunch shape and precise tuning of the beam focusing. Even in state-of-the-art designs, slice emittance growth due to nonlinear space charge forces and partial nonlaminarity often remains non-negligible. In this work we introduce a new means to linearize the transverse slice phase space: a sacrificial portion of the bunch's own charge distribution, formed into a wavebroken shock front by highly nonlinear space charge forces within the gun, whose downstream purpose is to dynamically linearize the desired bunch core. We show that linearization of an appropriately prepared bunch can be achieved via strongly nonlaminar focusing of the sacrificial shock front, while the inner core focuses laminarly. This leads to a natural spatial separation of the two distributions: a dense core surrounded by a diffuse halo of sacrificial charge that can be collimated. Multi-objective genetic algorithm optimizations of the ultra-compact x-ray free electron laser (UCXFEL) injector employ this concept, and we interpret it with an analytic model that agrees well with the simulations. In simulation we demonstrate a final bunch charge of 100 pC, peak current $\sim 30$ A, and a sacrificial charge of 150 pC (250 pC total emitted from cathode) with normalized emittance growth of $<20$ nm-rad due to space charge. This implies a maximum achievable brightness approximately an order of magnitude greater than existing FEL injector designs.

physics.acc-ph↗

arXiv2023

Towards construction of a novel nm resolution MeV-STEM for imaging of thick biological samples

Driven by life-science applications, mega-electron-volt Scanning Transmission Electron Microscope (MeV-STEM) has been proposed to image thick biological samples. The high penetration of inelastic scattering signals of MeV electrons could make the MeV-STEM an appropriate microscope for biological samples as thick as 10 {\mu}m or more with a nanoscale resolution, considering the effect of electron energy, beam broadening and low-dose limit on resolution. The best resolution is inversely related to the sample thickness and changes from 6 nm to 24 nm when the sample thickness increases from 1 {\mu}m to 10 {\mu}m. To achieve such a resolution in STEM, the imaging electrons must be focused on the specimen with a nm size and a mrad semi-convergence angle. This requires an electron beam emittance of a few picometer, which is ~1,000 times smaller than the presently achieved nm emittance, in conjunction with less than 10-4 energy spread and 1 nA current. We numerically simulated two different approaches that are potentially applicable to build a compact MeV-STEM instrument: 1) DC gun, aperture, Superconducting radio frequency (SRF) cavities, and STEM column; 2) SRF gun, aperture, SRF cavities, and STEM column. Beam dynamic simulations show promising results, which meet the needs of an MeV-STEM, a few picometer emittance, less than 10-4 energy spread, and 0.1-1 nA current from both options. Also, we designed a compact STEM column based on permanent quadrupole quintuplet not only to demagnify the beam size from 1 {\mu}m at the source point to 2 nm at the specimen, but also to provide the freedom of changing the magnifications at the specimen and a scanning system to raster the electron beam across the sample with a step size of 2 nm and the repetition rate of 1 MHz. This makes it possible to build a compact MeV-STEM and use it to study thick, large-volume samples in cell biology.

physics.ins-det↗

arXiv2022

Four-Dimensional Emittance Measurements of Ultrafast Electron Diffraction Optics Corrected Up to Sextupole Order

Ultrafast electron diffraction (UED) is a technique in which short-pulse electron beams can probe the femtosecond-scale evolution of atomic structure in matter driven far from equilibrium. As an accelerator physics challenge, UED imposes stringent constraints on the brightness of the probe electron beam. The low normalized emittance employed in UED, often at the 10 nm scale and below, is particularly sensitive to both applied field aberrations and space charge effects. The role of aberrations is increasingly important in small probe systems that often feature multiple orders of magnitude variations in beam size during transport. In this work, we report the correction of normal quadrupole, skew quadrupole, and sextupole aberrations via dedicated corrector elements in an ultrafast electron micro-diffraction beamline. To do this, we generate precise 4-dimensional phase space maps of rf-compressed electron beams, and find excellent agreement with aberration-free space charge simulations. Finally, we discuss the role a probe-forming aperture can play in improving the brightness of bunches with appreciable space charge effects.

physics.acc-ph↗

arXiv2022

Multi-scale time-resolved electron diffraction: A case study in moir\'e materials

Ultrafast-optical-pump -- structural-probe measurements, including ultrafast electron and x-ray scattering, provide direct experimental access to the fundamental timescales of atomic motion, and are thus foundational techniques for studying matter out of equilibrium. High-performance detectors are needed in scattering experiments to obtain maximum scientific value from every probe particle. We deploy a hybrid pixel array direct electron detector to perform ultrafast electron diffraction experiments on a WSe$_2$/MoSe$_2$ 2D heterobilayer, resolving the weak features of diffuse scattering and moir\'e superlattice structure without saturating the zero order peak. Enabled by the detector's high frame rate, we show that a chopping technique provides diffraction difference images with signal-to-noise at the shot noise limit. Finally, we demonstrate that a fast detector frame rate coupled with a high repetition rate probe can provide continuous time resolution from femtoseconds to seconds, enabling us to perform a scanning ultrafast electron diffraction experiment that maps thermal transport in WSe$_2$/MoSe$_2$ and resolves distinct diffusion mechanisms in space and time.

physics.ins-det↗

arXiv2021

A kiloelectron-volt ultrafast electron micro-diffraction apparatus using low emittance semiconductor photocathodes

We report the design and performance of a time-resolved electron diffraction apparatus capable of producing intense bunches with simultaneously single digit micron probe size, long coherence length, and $200$ fs rms time resolution. We measure the 5d (peak) beam brightness at the sample location in micro-diffraction mode to be $7 \times 10^{13} \ \mathrm{A}/\text{m}^2\text{-rad}^2$. To generate high brightness electron bunches, the system employs high efficiency, low emittance semiconductor photocathodes driven with a wavelength near the photoemission threshold at a repetition rate up to 250 kHz. We characterize spatial, temporal, and reciprocal space resolution of the apparatus. We perform proof-of-principle measurements of ultrafast heating in single crystal Au samples and compare experimental results with simulations that account for the effects of multiple-scattering.

physics.acc-ph↗

arXiv2020

COHERENT Collaboration data release from the first detection of coherent elastic neutrino-nucleus scattering on argon

Release of COHERENT collaboration data from the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) on argon. This release corresponds with the results of "Analysis A" published in Akimov et al., arXiv:2003.10630 [nucl-ex]. Data is shared in a binned, text-based format representing both "signal" and "backgrounds" along with associated uncertainties such that the included data can be used to perform independent analyses. This document describes the contents of the data release as well as guidance on the use of the data. Included example code in C++ (ROOT) and Python show one possible use of the included data.

nucl-ex↗

arXiv2020

First Measurement of Coherent Elastic Neutrino-Nucleus Scattering on Argon

We report the first measurement of coherent elastic neutrino-nucleus scattering (\cevns) on argon using a liquid argon detector at the Oak Ridge National Laboratory Spallation Neutron Source. Two independent analyses prefer \cevns over the background-only null hypothesis with greater than $3\sigma$ significance. The measured cross section, averaged over the incident neutrino flux, is (2.2 $\pm$ 0.7) $\times$10$^{-39}$ cm$^2$ -- consistent with the standard model prediction. The neutron-number dependence of this result, together with that from our previous measurement on CsI, confirms the existence of the \cevns process and provides improved constraints on non-standard neutrino interactions.

nucl-ex↗

arXiv2019

Sensitivity of the COHERENT Experiment to Accelerator-Produced Dark Matter

The COHERENT experiment is well poised to test sub-GeV dark matter models using low-energy recoil detectors sensitive to coherent elastic neutrino-nucleus scattering (CEvNS) in the $\pi$-DAR neutrino beam produced by the Spallation Neutron Source. We show how a planned 750-kg liquid argon scintillation detector would place leading limits on scalar light dark matter models, over two orders of magnitude of dark matter mass, for dark matter particles produced through vector and leptophobic portals in the absence of other effects beyond the standard model. The characteristic timing structure of a $\pi$-DAR beam allows a unique opportunity for constraining systematic uncertainties on the standard model background in a time window where signal is not expected, enhancing expected sensitivity. Additionally, we discuss future prospects, further increasing the discovery potential of CEvNS detectors. Such methods would test the calculated thermal dark matter abundance for all couplings $\alpha'\leq1$ within the vector portal model over an order of magnitude of dark matter masses.

hep-ex↗

arXiv2019

First Constraint on Coherent Elastic Neutrino-Nucleus Scattering in Argon

Coherent elastic neutrino-nucleus scattering (CEvNS) is the dominant neutrino scattering channel for neutrinos of energy $E_\nu < 100$ MeV. We report a limit for this process using data collected in an engineering run of the 29 kg CENNS-10 liquid argon detector located 27.5 m from the Oak Ridge National Laboratory Spallation Neutron Source (SNS) Hg target with $4.2\times 10^{22}$ protons on target. The dataset yielded $< 7.4$ observed CEvNS events implying a cross section for the process, averaged over the SNS pion decay-at-rest flux, of $<3.4 \times 10^{-39}$ cm$^{2}$, a limit within twice the Standard Model prediction. This is the first limit on CEvNS from an argon nucleus and confirms the earlier CsI non-standard neutrino interaction constraints from the collaboration. This run demonstrated the feasibility of the ongoing experimental effort to detect CEvNS with liquid argon.

hep-ex↗

arXiv2018

COHERENT Collaboration data release from the first observation of coherent elastic neutrino-nucleus scattering

This release includes data and information necessary to perform independent analyses of the COHERENT result presented in Akimov et al., arXiv:1708.01294 [nucl-ex]. Data is shared in a binned, text-based format, including both "signal" and "background" regions, so that counts and associated uncertainties can be quantitatively calculated for the purpose of separate analyses. This document describes the included information and its format, offering some guidance on use of the data. Accompanying code examples show basic interaction with the data using Python.

nucl-ex↗

arXiv2018

COHERENT 2018 at the Spallation Neutron Source

The primary goal of the COHERENT collaboration is to measure and study coherent elastic neutrino-nucleus scattering (CEvNS) using the high-power, few-tens-of-MeV, pulsed source of neutrinos provided by the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). The COHERENT collaboration reported the first detection of CEvNS [Akimov:2017ade] using a CsI[Na] detector. At present the collaboration is deploying four detector technologies: a CsI[Na] scintillating crystal, p-type point-contact germanium detectors, single-phase liquid argon, and NaI[Tl] crystals. All detectors are located in the neutron-quiet basement of the SNS target building at distances 20-30 m from the SNS neutrino source. The simultaneous measurement in all four COHERENT detector subsystems will test the $N^2$ dependence of the cross section and search for new physics. In addition, COHERENT is measuring neutrino-induced neutrons from charged- and neutral-current neutrino interactions on nuclei in shielding materials, which represent a non-negligible background for CEvNS as well as being of intrinsic interest. The Collaboration is planning as well to look for charged-current interactions of relevance to supernova and weak-interaction physics. This document describes concisely the COHERENT physics motivations, sensitivity, and next plans for measurements at the SNS to be accomplished on a few-year timescale.

physics.ins-det↗