arXiv subjects

David Meier

Publications and source records attributed to David Meier.

At least 19 recordsLinked to original sources

arXiv2025

Offset Finding of Beamline Parameters on the METRIXS Beamline at BESSY II Using Machine Learning

Beamline alignment is challenging as the beamline components must be set up ideally so that the rays follow the desired optical path. Automated methods using a digital twin allow for faster diagnostics and improved beam properties compared to manual tuning. We introduce an automated method of finding the offsets to improve this digital twin model. These offsets represent the unknown but constant differences between the beamline parameter positions as set up at the physical beamline and the corresponding parameter positions of its digital twin. Our method assumes the capability to execute precise relative movements with a known step size for these parameters, although the absolute position information is unknown. By combining the surrogate model with a global optimizer, we successfully determine offsets for 34 beamline parameters on a simulated METRIXS beamline at the BESSY II synchrotron radiation source in Berlin.

physics.acc-ph↗

arXiv2025

Reconstructing Time-of-Flight Detector Values of Angular Streaking Using Machine Learning

Angular streaking experiments enable for experimentation in the attosecond regions. However, the deployed Time-of-flight detectors are susceptible to noise and failure. These shortcomings make the outputs of the Time-of-flight detectors hard to understand for humans and further processing, such as for example the extraction of beam properties. In this article, we present an approach to remove high noise levels and reconstruct up to three failed Time-of-flight detectors from an arrangement of 16 Time-of-flight detectors. Due to its fast evaluation time, the presented method is applicable online during a running experiment. It is trained with simulation data, and we show the results of denoising and reconstruction of our method on real-world experiment data.

physics.data-an↗

arXiv2024

Attosecond soft X-ray pulses generated by chirp-dispersed manipulation in an XFEL reveal nonlinear core-electron dynamics in neon

Free-electron lasers have demonstrated their capability of generating intense attosecond X-ray pulses, which are the key to studying electron dynamics at their natural time scale and in specifically targeted electronic states, but come at the expanse of complicated generation schemes and stochastic pulse shapes. Here, we demonstrate a novel and simple operation concept based on the manipulation of the electron-bunch chirp-dispersion and working with the full 4.5 MHz repetition rate at the European XFEL in Germany. With a high-fidelity single-shot temporal characterisation, we detect X-ray pulses with durations of down to 200 attoseconds and peak powers reaching into the terawatt regime at ~1 keV photon energy. As a direct application, we present simultaneous measurements of nonlinear X-ray-matter interaction via time-resolved electron spectroscopy. Using the derived temporal pulse information and restricting the durations to a regime where individual X-ray pulses are shorter than the single-core-hole life time in neon atoms, we reveal an otherwise hidden peak-intensity dependence in the nonlinear dynamics of double-core-hole formation. Our results open the field of attosecond science to the investigation of electronic processes not only in the ground state but also in systems driven far off their equilibrium. They shed light on highly transient intermediate steps in complex electronic dynamics and thus promise to help build the conceptual bridge between fundamental physical processes and chemical photo-reactions.

physics.optics↗

arXiv2022

Investigation of process history and phenomenology of plutonium oxides using vector quantizing variational autoencoder

Accurate, high throughput, and unbiased analysis of plutonium oxide particles is needed for analysis of the phenomenology associated with process parameters in their synthesis. Compared to qualitative and taxonomic descriptors, quantitative descriptors of particle morphology through scanning electron microscopy (SEM) have shown success in analyzing process parameters of uranium oxides. We utilize a VQ-VAE to quantitatively describe plutonium dioxide (PuO2) particles created in a designed experiment and investigate their phenomenology and prediction of their process parameters. PuO2 was calcined from Pu(III) oxalates that were precipitated under varying synthetic conditions that related to concentrations, temperature, addition and digestion times, precipitant feed, and strike order; the surface morphology of the resulting PuO2 powders were analyzed by SEM. A pipeline was developed to extract and quantify useful image representations for individual particles with the VQ-VAE to perform multiple classification tasks simultaneously. The reduced feature space could predict process parameters with greater than 80% accuracies for some parameters with a single particle. They also showed utility for grouping particles with similar surface morphology characteristics together. Both the clustering and classification results reveal valuable information regarding which chemical process parameters chiefly influence the PuO2 particle morphologies: strike order and oxalic acid feedstock. Doing the same analysis with multiple particles was shown to improve the classification accuracy on each process parameter over the use of a single particle, with statistically significant results generally seen with as few as four particles in a sample.

cond-mat.mtrl-sci↗

arXiv2022

ALCHEMI finds a "shocking" carbon footprint in the starburst galaxy NGC~253

Centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in a peculiar observable composition. We analyze abundances of $CO_2$, a reliable tracer of ice chemistry, from data collected as part of the ALMA large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC~253 with an angular resolution of 1.6$''$. We constrain the $CO_2$ abundances in the gas phase using its protonated form $HOCO^+$. The distribution of $HOCO^+$ is similar to that of methanol, which suggests that $HOCO^+$ is indeed produced from the protonation of $CO_2$ sublimated from ice. The $HOCO^+$ fractional abundances are found to be $(1-2)\times10^{-9}$ at the outer part of the central molecular zone (CMZ), while they are lower ($\sim10^{-10}$) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk star-forming regions. From the range of $HOCO^+/CO_2$ ratios suggested from chemical models, the gas-phase $CO_2$ fractional abundance is estimated to be $(1-20)\times10^{-7}$ at the outer CMZ, and orders of magnitude lower near the center. We estimate the $CO_2$ ice fractional abundances at the outer CMZ to be $(2-5)\times10^{-6}$ from the literature. A comparison between the ice and gas $CO_2$ abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.

astro-ph.GA↗

arXiv2022

Unsupervised real-world knowledge extraction via disentangled variational autoencoders for photon diagnostics

We present real-world data processing on measured electron time-of-flight data via neural networks. Specifically, the use of disentangled variational autoencoders on data from a diagnostic instrument for online wavelength monitoring at the free electron laser FLASH in Hamburg. Without a-priori knowledge the network is able to find representations of single-shot FEL spectra, which have a low signal-to-noise ratio. This reveals, in a directly human-interpretable way, crucial information about the photon properties. The central photon energy and the intensity as well as very detector-specific features are identified. The network is also capable of data cleaning, i.e. denoising, as well as the removal of artefacts. In the reconstruction, this allows for identification of signatures with very low intensity which are hardly recognisable in the raw data. In this particular case, the network enhances the quality of the diagnostic analysis at FLASH. However, this unsupervised method also has the potential to improve the analysis of other similar types of spectroscopy data.

physics.data-an↗

arXiv2022

Optimizing a Superconducting Radiofrequency Gun Using Deep Reinforcement Learning

Superconducting photoelectron injectors are a promising technique for generating high brilliant pulsed electron beams with high repetition rates and low emittances. Experiments such as ultra-fast electron diffraction, experiments at the Terahertz scale, and energy recovery linac applications require such properties. However, optimization of the beam properties is challenging due to the high amount of possible machine parameter combinations. In this article, we show the successful automated optimization of beam properties utilizing an already existing simulation model. To reduce the amount of required computation time, we replace the costly simulation by a faster approximation with a neural network. For optimization, we propose a reinforcement learning approach leveraging the simple computation of the derivative of the approximation. We prove that our approach outperforms common optimization methods for the required function evaluations given a defined minimum accuracy.

physics.acc-ph↗

arXiv2021

Artificial intelligence for online characterization of ultrashort X-ray free-electron laser pulses

X-ray free-electron lasers (XFELs) as the world's brightest light sources provide ultrashort X-ray pulses with a duration typically in the order of femtoseconds. Recently, they have approached and entered the attosecond regime, which holds new promises for single-molecule imaging and studying nonlinear and ultrafast phenomena such as localized electron dynamics. The technological evolution of XFELs toward well-controllable light sources for precise metrology of ultrafast processes has been, however, hampered by the diagnostic capabilities for characterizing X-ray pulses at the attosecond frontier. In this regard, the spectroscopic technique of photoelectron angular streaking has successfully proven how to non-destructively retrieve the exact time-energy structure of XFEL pulses on a single-shot basis. By using artificial intelligence techniques, in particular convolutional neural networks, we here show how this technique can be leveraged from its proof-of-principle stage toward routine diagnostics even at high-repetition-rate XFELs, thus enhancing and refining their scientific accessibility in all related disciplines.

physics.data-an↗

arXiv2019

Astro2020 Science White Paper: Cold Gas Outflows, Feedback, and the Shaping of Galaxies

There is wide consensus that galaxy outflows are one of the most important processes determining the evolution of galaxies through cosmic time, for example playing a key role in shaping the galaxy mass function. Our understanding of outflows and their drivers, however, is in its infancy --- this is particularly true for the cold (neutral atomic and molecular) phases of outflows, which present observational and modeling challenges. Here we outline several key open questions, briefly discussing the requirements of the observations necessary to make progress, and the relevance of several existing and planned facilities. It is clear that galaxy outflows, and particularly cold outflows, will remain a topic of active research for the next decade and beyond.

astro-ph.GA↗

arXiv2019

Astro2020 Science White Paper: Physical Conditions in the Cold Gas of Local Galaxies

We describe a next major frontier in observational studies of galaxy evolution and star formation: linking the physical conditions in the cold, star-forming interstellar medium to host galaxy and local environment. The integrated gas content of galaxies has been surveyed extensively over the last decades. The link between environment and cold gas density, turbulence, excitation, dynamical state, and chemical makeup remain far less well understood. We know that these properties do vary dramatically and theoretical work posits a strong connection between the state of the gas, its ability to form stars, and the impact of stellar feedback. A next major step in the field will be to use sensitive cm-, mm-, and submm-wave spectroscopy and high resolution spectroscopic imaging to survey the state of cold gas across the whole local galaxy population. Such observations have pushed the capabilities of the current generation of telescopes. We highlight three critical elements for progress in the next decade: (1) robust support and aggressive development of ALMA, (2) the deployment of very large heterodyne receiver arrays on single dish telescopes, and (3) development of a new interferometric array that dramatically improves on current capabilities at cm- and mm-wavelengths (~ 1-115 GHz).

astro-ph.GA↗

arXiv2019

Relativistic Jets in the Accretion & Collimation Zone: New Challenges Enabled by New Instruments

Jets are a ubiquitous part of the accretion process, seen in a wide variety of objects ranging from active galaxies (AGN) to X-ray binary stars and even newly formed stars. AGN jets are accelerated by the supermassive black hole of their host galaxy by a coupling between the magnetic field and inflowing material. They are the source for many exciting phenomena and can profoundly influence the larger galaxy and surrounding cluster. This White Paper points out what advances can be achieved in the field by new technologies, concentrating on the zone where jets are accelerated to relativistic speeds and collimated. The ngVLA and new space VLBI missions will give higher angular resolution, sensitivity and fidelity in the radio, penetrating this zone for additional objects and allowing us to resolve fundamental questions over the physics of jet acceleration and collimation. Interferometry in other bands would allow us to probe directly flaring components. We also emphasize the need for polarimetry, which is essential to revealing the role and configuration of magnetic fields.

astro-ph.GA↗

arXiv2018

Relativistic Jets in Active Galactic Nuclei

The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These Active Galactic Nuclei, AGN, can form jets which are observed on scales from AU to Mpc and from meter wavelengths to TeV gamma ray energies. High resolution radio imaging and multi-wavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: AGN jets are formed when the black hole spins and the accretion disk is strongly magnetized, perhaps on account of gas accretingat high latitude beyond the black hole sphere of influence; AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind; higher power jets can emerge from their galactic nuclei in a relativistic, supersonic and proton-dominated state and they terminate in strong, hot spot shocks; lower power jets are degraded to buoyant plumes and bubbles; jets may accelerate protons to EeV energies which contribute to the cosmic ray spectrum and which may initiate pair cascades that can efficiently radiate synchrotron gamma rays; jets were far more common when the universe was a few billion years old and black holes and massive galaxies were growing rapidly; jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright.

astro-ph.HE↗

arXiv2018

Science with an ngVLA: Gas Density Across the Universe

Gas density is widely believed to play a governing role in star formation. However, the exact role of density in setting the star formation rate remains debated. We also lack a general theory that explains how the gas density distribution in galaxies is set. The primary factor preventing the resolution of these issues is the limited number of observations of the gas density distribution across diverse environments. Centimeter- and millimeter-wave spectroscopy offer the most promising way forward in this field, but the key density-sensitive transitions are faint compared to the capabilities of current telescopes. In this chapter, we describe how a next-generation Very Large Array (ngVLA) represents the natural next step forward in this sensitivity-limited field. Such a facility would provide a crucial link between the `Milky Way' and `Extragalactic' views of star formation and dramatically advance our understanding of the drive and role of gas density in galaxies, building on current results from ALMA, NOEMA, the Green Bank Telescope, and other current facilities working in this area.

astro-ph.GA↗

arXiv2018

Survey of Water and Ammonia in Nearby galaxies (SWAN): Resolved Ammonia Thermometry, and Water and Methanol Masers in IC 342, NGC 6946 and NGC 2146

The Survey of Water and Ammonia in Nearby galaxies (SWAN) studies atomic and molecular species across the nuclei of four star forming galaxies: NGC\,253, IC\,342, NGC\,6946, and NGC\,2146. As part of this survey, we present Karl G. Jansky Very Large Array (VLA) molecular line observations of three galaxies: IC\,342, NGC\,6946 and NGC\,2146. NGC\,253 is covered in a previous paper. These galaxies were chosen to span an order of magnitude in star formation rates and to select a variety of galaxy types. We target the metastable transitions of ammonia NH$_{3}$(1,1) to (5,5), the 22\,GHz water (H$_2$O) ($6_{16}-5_{23}$) transition, and the 36.1\,GHz methanol (CH$_3$OH) ($4_{-1}-3_{0}$) transition. {We use the NH$_{3}$\ metastable lines to perform thermometry of the dense molecular gas.} We show evidence for uniform heating across the central kpc of IC\,342 with two temperature components for the molecular gas, similar to NGC 253,} of 27\,K and 308\,K, and that the dense molecular gas in NGC\,2146 has a temperature $<$86 K. We identify two new water masers in IC\,342, and one new water maser in each of NGC\,6946 and NGC\,2146. The two galaxies NGC\,253 and NGC\,2146, with the most vigorous star formation, host H$_2$O kilomasers. Lastly, we detect the first 36\,GHz CH$_3$OH\ masers in IC\,342 and NGC\,6946. For the four external galaxies the total CH$_3$OH\ luminosity in each galaxy suggests a correlation with galactic star formation rate, whereas the morphology of the emission is similar to that of HNCO, a weak shock tracer.

astro-ph.GA↗

arXiv2014

Solution to the quantum Zermelo navigation problem

The solution to the problem of finding a time-optimal control Hamiltonian to generate a given unitary gate, in an environment in which there exists an uncontrollable ambient Hamiltonian (e.g., a background field), is obtained. In the classical context, finding the time-optimal way to steer a ship in the presence of a background wind or current is known as the Zermelo navigation problem, whose solution can be obtained by working out geodesic curves on a space equipped with a Randers metric. The solution to the quantum Zermelo problem, which is shown here to take a remarkably simple form, is likewise obtained by finding explicit solutions to the geodesic equations of motion associated with a Randers metric on the space of unitary operators. The result reveals that the optimal control in a sense `goes along with the wind'.

quant-ph↗

arXiv2010

The Nuclear Spectroscopic Telescope Array (NuSTAR)

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (5 -- 80 keV) telescope to orbit. NuSTAR will offer a factor 50 -- 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12-arcminute field of view. After launch, NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. A number of additional observations will be included in the primary mission, and a guest observer program will be proposed for an extended mission to expand the range of scientific targets. The payload consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus rocket into a low-inclination Earth orbit. Data will be publicly available at GSFC's High Energy Astrophysics Science Archive Research Center (HEASARC) following validation at the science operations center located at Caltech.

astro-ph.IM↗

arXiv2009

A Vigorous Explorer Program

Explorers have made breakthroughs in many fields of astrophysics. The science from both these missions contributed to three Nobel Prizes - Giacconi (2002), Mather, and Smoot (2006). Explorers have: marked the definitive beginning of precision cosmology, discovered that short gamma-ray bursts are caused by compact star mergers and have measured metalicity to redshifts z>6. NASA Explorers do cutting-edge science that cannot be done by facility-class instruments. The Explorer program provides a rapid response to changing science and technology, to enable cutting-edge science at moderate cost. Explorers also enable innovation, and engage & train scientists, managers and engineers, adding human capital to NASA and the nation. The astrophysics Explorer launch rate now being achieved is 1 per 3 years, and budget projections are in the $150M/year range for the next five years. A newly Vigorous Explorer Program should be created to: 1. Reach the long-stated goal of annual astrophysics launches; 2. Find additional launch options for Explorers and actively encourage cost savings in launchers and spacecraft, such as new commercial vehicles and innovative partnerships. 3. Mitigate risk via stronger technical development and sub-orbital programs, and through longer, more thorough, Phase A programs, potentially reducing the need for a 30% contingency; 4. Strive to protect the funding for missions that have reached Phase B, to prevent significant launch slips and cancellations, with a goal of 4 to 5 years from Phase B to launch; 5. Review the project management procedures and requirements to seek cost reductions, including the risk management strategy and the review and reporting process; 6. Review and possibly modify the cost caps for all Explorer classes to optimize scientific returns per dollar. [ABRIDGED]

astro-ph.IM↗

arXiv2007

Spitzer Observations of Powerful Radio Sources

We have measured the mid-infrared radiation from an orientation-unbiased sample of powerful 3C RR galaxies and quasars using the IRS and MIPS instruments aboard the Spitzer Space Telescope. We fit the Spitzer data as well as other measurements from the literature with synchrotron and dust components. At 15 microns, quasars are typically four times brighter than radio galaxies with the same isotropic radio power. Based on our fits, half of this difference can be attributed to the presence of non-thermal emission in the quasars but not the radio galaxies. The other half is consistent with dust absorption in the radio galaxies but not the quasars.

astro-ph↗