Remotely Discerning Radioactive Source Type via Laser-Driven Electron Avalanche in Air
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Zingale, Anthony Vincent↗
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Ion source design for industrial applications
The more frequently used design techniques for the components of broad-beam electron bombardment ion sources are discussed. The approach used emphasizes refractory metal cathodes and permanent-magnet multipole discharge chambers. Design procedures and sample calculations are given for the discharge chamber, ion optics, the cathodes, and the magnetic circuit. Hardware designs are included for the isolator, cathode supports, anode supports, pole-piece assembly, and ion-optics supports. A comparison is made between two-grid and three-grid optics. The designs presented are representative of current technology and are adaptable to a wide range of configurations.
Kaufman, H. R.↗
Development of Ultrafast Capabilities for X-ray Free-Electron Lasers at the Linac Coherent Light source
The ability to produce ultra-short, high-brightness X-ray pulses is revolutionizing the field of ultra-fast X-ray spectroscopy. Free electron laser (FEL) facilities are driving this revolution, but unique aspects of the FEL process make the required characterization and use of the pulses challenging. In this paper, we describe a number of developments in the generation of ultra-short X-ray FEL pulses, and the concomitant progress in the experimental capabilities necessary for their characterization and use at the Linac Coherent Light Source. This includes the development of sub-femtosecond hard and soft X-ray pulses, along with ultra-fast characterization techniques for these pulses. We also describe improved techniques for optical cross-correlation as needed to address the persistent challenge of external optical laser synchronization with these ultra-short X-ray pulses.
Coffee, Ryan N↗
Lactiplantibacillus plantarum uses ecologically relevant, exogenous quinones for extracellular electron transfer
Extracellular electron transfer (EET) is a metabolic process that frequently uses quinones to couple intracellular redox reactions with extracellular electron acceptors. The physiological relevance of this metabolism for microorganisms capable of EET but unable to synthesize their own quinones remains to be determined. To address this question, we investigated quinone utilization by Lactiplantibacillus plantarum, a microorganism required for food fermentations, that performs EET and is also a quinone auxotroph. L. plantarum selectively used 1,4-dihydroxy-2-naphthoic acid (DHNA) and more hydrophilic naphthoquinones for EET reduction of insoluble iron (ferrihydrite). However, quinones used for EET also inhibited L. plantarum growth in non-aerated conditions. Transcriptomic analysis showed that DHNA-induced oxidative stress in L. plantarum, but this was alleviated when the electron acceptor, ferric ammonium citrate (FeAC), was included in the growth medium. Although DHNA and FeAC induced L. plantarum EET, this metabolism was still dependent on direct access to environmental electron shuttles. To determine whether quinone-producing food fermentation bacteria could be sources of those electron shuttles, L. plantarum EET was measured after incubation with Lactococcus lactis and Leuconostoc mesenteroides. Quinone-producing L. lactis, but not a quinone-deficient L. lactis ΔmenC mutant, increased L. plantarum ferrihydrite reduction and medium acidification through an EET-dependent mechanism. L. plantarum EET was also stimulated by L. mesenteroides, resulting in greater environmental acidification and transient increases in L. plantarum cell numbers. Our findings show that L. plantarum overcomes the toxic effects of exogenous quinones to use those compounds for EET-conferred, ecological advantages during the early stages of food fermentations.
59 BASIC BIOLOGICAL SCIENCES↗
TEST RESULTS FOR A CESIUM ELECTRON BOMBARDMENT ION MOTOR
Cesium electron bombardment ion source for electrostatic propulsion
ELECTRON BOMBARDMENT↗
GPAW: An open Python package for electronic structure calculations
We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe–Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn–Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Relativistic electrons near geostationary orbit - Evidence for internal magnetospheric acceleration
The possibility of an internal magnetospheric acceleration mechanism as the source of relativistic electron fluxes in earth's outer magnetosphere is explored. Such a model includes the substorm generation of a spectrally soft electron component, with subsequent inward radial diffusion. At low L values, an outward transport of energetic electrons occurs which leads to a return of the accelerated population to the outer magnetosphere. Data obtained concurrently at geostationary orbit at three widely spaced local times during a relativistic electron event provide support for acceleration by a recirculation process.
Baker, D. N.↗
On the Onset Delays of Solar Energetic Electrons and Protons: Evidence for a Common Accelerator
The processes responsible for the acceleration of solar energetic particles (SEPs) are still not well understood, including whether SEP electrons and protons are accelerated by common or separate processes. Using a numerical particle transport model that includes both pitch-angle and perpendicular spatial diffusion, we simulate, among other quantities, the onset delay for MeV electrons and protons and compare the results to observations of SEPs from widely separated spacecraft. Such observations have previously been interpreted, in a simple scenario assuming no perpendicular diffusion, as evidence for different electron and proton sources. We show that, by assuming a common particle source together with perpendicular diffusion, we are able to simultaneously reproduce the onset delays for both electrons and protons. We argue that this points toward a common accelerator for these particles. Moreover, a relatively broad particle source is required in the model to correctly describe the observations. This is suggestive of diffusive shock acceleration occurring at large shock structures playing a significant role in the acceleration of these SEPs.
R D Strauss↗
Workshop on Biological Science Opportunities Provided by the APS Upgrade
Synchrotron-based macromolecular X-ray crystallography (MX) is the primary method for determining the structure of biological molecules including proteins, nucleic acids, complexes, and large assemblies at high resolution (e.g., 95% of all new Protein Data Bank [PDB] deposits in 2017). The Advanced Photon Source (APS), which is the largest and the most productive light source for MX, will be upgraded to a multi-bend achromat lattice (the Advanced Photon Source Upgrade — APS-U) that will deliver the ultimate diffraction-limited X-ray source with radically transformative MX capabilities. The specific advantages of the APS-U X-ray source include (1) increased brightness allowing delivery of intense beams with dimensions of 1-μm or less, resulting in a 100-fold improvement in signal-to-noise, (2) increased brightness at high energy (especially for energies of 12 keV-40 keV commonly used in MX) that can be exploited to reduce primary radiation damage, and (3) low-convergence X-ray beams that will be essential for the study of large macromolecular complexes at high resolution. A workshop on “Biological Science Opportunities Provided by the APS Upgrade” was organized at Argonne National Laboratory on August 18-20, 2018, to discuss the biological science opportunities offered by the unique new capabilities of the APS-U and how the broad structural biology community could best exploit them. The APS-U takes place in a context where advances in X-ray free-electron laser (XFEL) sources and cryo-electron microscopy (cryo-EM) have added to the constellation of techniques and propelled structural biology in new directions. Synergies exist between these different technologies and it is essential to develop enabling technologies that exploit the unique, world-leading, and synergistic characteristics of each light source to boost capabilities for producing leading-edge biological research for U.S. Department of Energy (DOE) missions in energy and environment, and National Institutes of Health (NIH) missions in human health.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
On the Cause of Two Prompt Shock-Induced Relativistic Electron Depletion Events
Dayside interplanetary (IP) shock-induced injections are known to be a source of highly relativistic electrons in Earth's outer radiation belt, and are possibly the only source of greater than 1 megaelectronvolt electrons in the inner belt. The associated electron energization process is well understood and modeled. Recently, relativistic electron depletion echoes have also been associated with IP shocks, but the processes driving the depletions are less well understood. In this study, we investigate in detail two shock induced greater than 1 megaelectronvolt electron depletion events observed by the Van Allen Probes, March 17, 2015 and May 24, 2013, and draw similarities to night-side substorm related enhancements and depletions. Both events exhibit shock induced enhancements on one of the Van Allen Probes and depletions on the other in greater than 1 megaelectronvolt channels, such observations have not previously been reported. The depletion of greater than 1 megaelectronvolt electrons during the March 17, 2015 event is associated with enhancements in 10 second - 100 second kiloelectronvolt electrons on the same spacecraft. The depletion is consistent with the effects of a lack of seed electrons at larger radial distances combined with inward motion due to asymmetric compression by the shock impact. The immediate enhancements and depletions of 75 kiloelectronvolt - 2.6 greater than 1 megaelectronvolt electrons are explained by the local phase space density radial profile. Observations of electron flux dynamics during the May 24, 2013 event can also be explained by a lack of a seed population at larger radial distances, supported by butterfly distributions observed during the event. The electron's inward radial motion can be attributed to the inward propagating impulse also associated with the greater than 1 megaelectronvolt electron enhancements observed on the complementary probe, rather than global asymmetric compression. This causal mechanism has parallels to substorm related depletions. Alternatively, evidence is provided to attribute the sudden depletions to losses due to a sudden but brief inward motion of the magnetopause.
Schiller, Quintin↗
Energy of microwave-emitting electrons and hard x-ray/microwave source model in solar flares
Based on the rate of increse of the microwave flux relative to the hard X-ray flux at various energies from the onset to the peak of a flare, the mean energy of microwave-emitting electrons is estimated for 22 flares observed simultaneously in hard X-rays and microwaves. The energy of electrons varying in proportion to the 17 GHz emission is found to concentrate below 100 keV, and the mean energy or eletrons emitting 70 keV x-rays is less than about 130 keV for thin-target and less than about 180 keV for thick-target emission models, suggesting that the 17 GHz emission derives from electrons with energy of less than a few hundred keV. The magnetic field strength in the microwave source is found to be 500-1000 G for the thick-target and 1000-2000 G for the thin-target case, and 16 of the 22 events examined can be successfully explained by the thick-target model. Of the six events which cannot be explained by the thick-target model, two events give L of less than about 300 km.
Nitta, N.↗
Design of the Lanthanum hexaboride based plasma source for the large plasma device at UCLA
The Large Plasma Device (LAPD) at UCLA (University of California, Los Angeles) produces an 18 m long, magnetized, quiescent, and uniform plasma at a high repetition rate to enable studies of fundamental plasma physics. Here, we report on a major upgrade to the LAPD plasma source that allows for more robust operation and significant expansion of achievable plasma parameters. The original plasma source made use of a heated barium oxide (BaO) coated nickel sheet as an electron emitter. This source had a number of drawbacks, including a limited range of plasma density (≲4.0 × 10 12 cm −3 ), a limited discharge duration (∼10 ms), and susceptibility to poisoning following oxygen exposure. Further, the new plasma source utilizes a 38 cm diameter lanthanum hexaboride (LaB 6 ) cathode, which has a significantly higher emissivity, allowing for a much larger discharge power density, and is robust to exposure to air. Peak plasma density of up to 3.0 × 10 13 cm −33 in helium gas has been achieved. The typical operating pressure is ∼10 −5 Torr, while dynamic pressure can be achieved through the gas-puffing technique. Discharges as long as 70 ms have been produced, enabling a variety of long-time-scale studies of processes, such as turbulent particle transport. The new source has been in continuous operation for 14 months, having survived air leaks, power outages that led to rapid temperature changes on the cathode and heater, and planned machine openings. We describe the design, construction, and initial operation of this novel new large-area LaB6 plasma source.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Bayesian Optimization of a Free-Electron Laser
The Linac Coherent Light Source X-ray free-electron laser is a complex scienti c apparatus which changes con gurations multiple times per day, necessitating fast tuning strategies to reduce setup time for successive experiments. To this end, we employ a Bayesian approach to maximizing X- ray laser pulse energy by controlling groups of quadrupole magnets. A Gaussian process model provides probabilistic predictions for the machine response with respect to control parameters, enabling a balance of exploration and exploitation in the search for the global optimum. We show that the model parameters can be learned from archived scans, and correlations between devices can be extracted from the beam transport. Furthermore, the result is a sample-e cient optimization routine, combining both historical data and knowledge of accelerator physics to signi cantly outperform existing optimizers.
47 OTHER INSTRUMENTATION↗
Implementation of Design Changes Towards a More Reliable, Hands-off Magnetron Ion Source [Slides]
As the main H- ion source for the accelerator complex, magnetron ion sources have been used at Fermilab since the 1970’s. At the offline test stand, new R&D is carried out to develop and upgrade the present magnetron-type sources of H- ions of up to 80 mA and 35 keV beam energy in the context of the Proton Improvement Plan. The aim of this plan is to provide high-power proton beams for the experiments at FNAL. In order to reduce the amount of tuning and monitoring of these ion sources, a new electronic system consisting of a current-regulated arc discharge modulator allow the ion source to run at a constant arc current for improved beam output and operation. A solenoid-type gas valve feeds H2 gas into the source precisely and independently of ambient temperature. This summary will cover several studies and design changes that have been tested and will eventually be implemented on the operational magnetron sources at Fermilab. Innovative results for this type of io n source include cathode geometries, solenoid gas valves, current controlled arc pulser, cesium boiler redesign, gas mixtures of hydrogen and nitrogen, and duty factor reduction, with the aim to improve source lifetime, stability, and reducing the amount of tuning needed. In this summary, I will highlight the advances made in ion sources at Fermilab and will outline the directions of the continuing R&D effort.
43 PARTICLE ACCELERATORS↗
Implementation of Design Changes Towards a More Reliable, Hands-off Magnetron Ion Source
As the main H- ion source for the accelerator complex, magnetron ion sources have been used at Fermilab since the 1970’s. At the offline test stand, new R&D is carried out to develop and upgrade the present magnetron-type sources of H- ions of up to 80 mA and 35 keV beam energy in the context of the Proton Improvement Plan. The aim of this plan is to provide high-power proton beams for the experiments at FNAL. In order to reduce the amount of tuning and monitoring of these ion sources, a new electronic system consisting of a current-regulated arc discharge modulator allow the ion source to run at a constant arc current for improved beam output and operation. A solenoid-type gas valve feeds H2 gas into the source precisely and independently of ambient temperature. This summary will cover several studies and design changes that have been tested and will eventually be implemented on the operational magnetron sources at Fermilab. Innovative results for this type of ion source include cathode geometries, solenoid gas valves, current controlled arc pulser, cesium boiler redesign, gas mixtures of hydrogen and nitrogen, and duty factor reduction, with the aim to improve source lifetime, stability, and reducing the amount of tuning needed. In this summary, I will highlight the advances made in ion sources at Fermilab and will outline the directions of the continuing R&D effort.
43 PARTICLE ACCELERATORS↗
Reversal ion source - A new source of negative ion beams
A new type of ion source utilizing beams of electrons and target molecules, rather than a diffuse, volume plasma, is described. The source utilizes an electrostatic electron 'mirror' which reverses trajectories in an electron beam, producing electrons at their turning point having a distribution of velocities centered at zero velocity. A gas which attaches zero-velocity electrons is introduced at this turning point. Negative ions are produced by an attachment or dissociative attachment process. For many of the thermal electron-attaching molecules the cross sections can be quite large, varying as the inverse square root of the electron energy or just the s-wave threshold law. The efficiency and current density of the ion source for production of Cl(-) through the large, thermal energy attachment process is estimated. It is argued that the source can be used for the production of negative ions through attachment resonances located at higher energies as well.
Orient, O. J.↗
Electron temperature and concentration in a thermal atomic oxygen source
A thermal atomic oxygen source for materials screening was built for NASA by Boeing Aerospace. The objective here was to use a microwave interferometer and Langmuir probe to characterize the electron concentration in this thermal atomic oxygen source. Typical operating conditions in the thermal atomic oxygen source were found to produce electron concentrations that were well below the detection threshold of the interferometer (10(exp 8) cm (sup -3)). The researchers calibrated (with the interferometer) the Langmuir probe at an artificially high plasma density and then used the circular and the square Langmuir probes to measure the low electron concentrations that exist during materials exposure tests. Electron concentration was measured as a function of power and position. The electrons were lost to the walls through ambipolar diffusion, and their concentration was accurately described by an equation. The electron concentration was proportional to power squared and decayed exponentially with distance.
Pedrow, Patrick Dennis↗
Generation and characterization of magnetized electron beam from a DC high voltage photogun for electron beam cooling application
One of the most challenging requirements for the proposed Electron-Ion Collider is the strong cooling of the proton beam, which is key to achieving the collider’s desired luminosity of order 10 33 -10 34 cm -2 s -1 . Magnetized bunched-beam electron cooling could be a means to achieve the required high luminosity, where strong cooling is accomplished inside a cooling solenoid where the ions co-propagate with an electron beam generated from a source immersed in a magnetic field. To increase the cooling efficiency, a bunched electron beam with high bunch charge and high repetition rate is required. This work describes the production and characterization of magnetized electron beam using a compact 300 kV DC high voltage photogun and bi-alkali antimonide photocathode. Beam magnetization was studied using a diagnostic beamline that includes viewer screens for measuring the shearing angle of the electron beamlet passing through a narrow upstream slit. Simulations and corresponding measurements of beam magnetization are presented as a function of laser spot size and magnetic field strength. Correlated beam emittance with magnetic field (0–0.15 T) at the photocathode was measured for various laser spot sizes. Measurements of photocathode lifetime were carried out at different magnetized electron beam currents up to 28 mA, and bunch charge up to 0.7 nC (not simultaneously).
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗