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DOE OSTI2022

Molecular beam homoepitaxy of N-polar AlN: Enabling role of aluminum-assisted surface cleaning

N-polar aluminum nitride (AlN) is an important building block for next-generation high-power radio frequency electronics. We report successful homoepitaxial growth of N-polar AlN by molecular beam epitaxy (MBE) on large-area, cost-effective N-polar AlN templates. Direct growth without any in situ surface cleaning leads to films with inverted Al polarity. It is found that Al-assisted cleaning before growth enables the epitaxial film to maintain N-polarity. The grown N-polar AlN epilayer with its smooth, pit-free surface duplicates the structural quality of the substrate, as evidenced by a clean and smooth growth interface with no noticeable extended defects generation. Near band-edge photoluminescence peaks are observed at room temperature on samples with MBE-grown layers but not on the bare AlN templates, implying the suppression of nonradiative recombination centers in the epitaxial N-polar AlN.

42 ENGINEERING↗

DOE OSTI2023

Extreme high vacuum for polarized electron sources

Nuclear physics experiments often require highly polarized electron beams to do precise measurements of the structure and size of nucleons and the nucleus, as well as for searches for physics beyond the standard model. Jefferson Lab?s electron source, with polarization near 90%, has been providing polarized electron beams for CEBAF for over two decades. Development is underway for polarized electron sources at MESA at Mainz and the Electron Ion Collider at Brookhaven National Lab, and there is potential for polarized electron beam in the future at facilities including the International Linear Collider, an electron upgrade at CERN, and the SuperKEKB collider in Japan. At Jefferson Lab there are even plans to make use of polarized electrons to make a polarized positron source for experimental nuclear physics. High polarization electron beams are generated using photoemission from strained superlattice GaAs based photocathodes, and photocathode lifetime is limited by the ionization of residual gas in the system, which is then accelerated into the photocathode. Extreme high vacuum (near 1x10-10 Pa) is required to operate the Jefferson Lab polarized electron source with an acceptable lifetime, and the upcoming projects will need various combinations of higher current, higher bunch charge and longer photocathode lifetimes. To meet the vacuum requirements for polarized electron sources, every component for a polarized electron source must be optimized, including chamber materials, pumps, bakeout procedure and the high voltage electrode geometry and processing. Each change in these components must be evaluated offline before being used in the accelerator, and effects on pressure are difficult to evaluate even using XHV-optimized hot filament ionization gauges. In fact, we do not get a final evaluation of system modifications until an electron source is built, installed and lifetime measurements are made over the course of months or years of operation. I will be discussing the evolution of vacuum in the Jefferson Lab polarized source system toward XHV pressures and discuss the characterization and limitations measured for commercially available XHV vacuum gauges. Finally, I?ll present the effect of system pressure on photocathode lifetime and highlight how XHV pressure standards can benefit the ongoing efforts to improve vacuum for the next generation of polarized electron sources.

Stutzman, Marcy↗

DOE OSTI2021

IMPROVED LIFETIME OF A HIGH SPIN POLARIZATION SUPERLATTICE PHOTOCATHODE

Highly spin polarized electron beams are required for the operation of a wide range of accelerators and instruments. The production of such electrons requires the use of Negative Electron Affinity (NEA) activated GaAs-based cathodes operated in photoelectron guns. Because of their extreme sensitivity to poor vacuum conditions the degradation of the photoemission process is so strong that NEA activated GaAs-based photocathodes can only survive in the extreme vacuums typical of DC gun. State-of-the-art on photocathode technology for spin polarized beam productions are summarized. Recent results on the use of robust NEA coating based on the Cs-Te and Cs-Sb leading to improved operational lifetime of a high spin polarization photocathode are reviewed.

47 OTHER INSTRUMENTATION↗

NASA NTRS1965

Polarization of the incident beam in non-normal reflectance measurements

Incident light ray polarization in nonnormal reflection coefficient measurements

POLARIZATION↗

DOE OSTI2026

Measurement of the Neutron Elastic Electromagnetic Form Factor Ratio at Large Momentum Transfer

Exploring nucleon structure is vital both for understanding its origin and existence as well as for the advancement of the sciences. It helps us answer key questions such as how quark and gluon dynamics create 99% of the nucleon mass. Electron- nucleon scattering has been widely used for precision studies of the nucleon and nuclear structure since the Nobel Prize winning investigations by Robert Hofstadter and collaborators in the 1950s. These studies provide information about the spatial charge and current densities of the nucleon in terms of the electromagnetic form factors. The form factors are functions of four momentum transfer squared (Q2). Extending the electromagnetic form factor measurements to higher Q2 plays a critical role in furthering the understanding of nucleon structure. This motivated the Super BigBite Spectrometer (SBS) program at Jefferson Lab. The open nature of the spectrometers and the direct line of sight from the target to the tracking detector locations in experimental setups such as SBS creates high levels of background at the detectors. This necessitates the use of tracking detectors with high rate capability and good position resolution. Gas Electron Multiplier (GEM) detectors are an excellent choice for tracking detectors in such experiments. Understanding the performance of the GEM detectors is important not just for SBS experiments but also for future high-luminosity experiments. This thesis reports the exploratory results from the measurement of the neutron elastic electromagnetic form factor ratio (Gn E/Gn M) at high momentum transfer. A longitudinally polarized electron beam was scattered off a polarized 3He target, used as an effective polarized neutron target. In this experiment, the polarized 3He target achieved a world record polarization weighted luminosity at a beam current of 45 µA. Double spin asymmetry of the scattered neutron events is used to extract the neutron form factor ratio. Measurements were taken at Q2 = 3.0, 6.8, 9.8 (GeV/c)2. The lowest Q2 measurement is in good agreement with the existing world data, and the higher-Q2 measurements extend the Q2 reach well beyond the existing world data and are expected to remain unmatched for a long time.

Gamage, Vimukthi Haththotuwa [Univ. of Virginia, C↗

DOE OSTI2025

Beam Commissioning Result of Polarized Target at SpinQuest

SpinQuest is a fixed-target experiment at Fermilab to measure the Drell-Yan process using transversely polarized NH$_3$ and ND$_3$ targets and unpolarized 120-GeV proton beam. In the Drell-Yan process, a quark in one scattering hadron and an anti-quark in the other hadron annihilate into a virtual photon and then decay into a muon (lepton) pair. The angular distribution of final-state muon pairs with respect to the target polarization is sensitive to the Sivers function of light antiquarks in the nucleon, which is one of the eight Transverse Momentum Dependent (TMD) parton distribution functions. The intensity of the proton beam is as high as $10^{12}$ protons/second, in order to accumulate the required statistics of Drell-Yan events. The polarized target system at SpinQuest has been carefully designed to accommodate such a high beam intensity. It is equipped with an evaporation refrigerator with a cooling power of around 5 W at 1 K. The target temperature is maintained at 1 K even under heat load caused by the proton beam and the DNP microwave. The target system and the spectrometer were commissioned with the proton beam from May through July 2024. The experimental setup of SpinQuest and the performance of the target system during the beam commissioning is reported.

Nakano, Kenichi [Virginia U.]↗

DOE OSTI2021

Measurements of the Beam Normal Asymmetry using Polarized Elastic Electron Scattering off Various Spin-0 Nuclei at 1 GeV

In this first part of this thesis, I report on the 1 GeV beam normal asymmetry measurements (AT) on various spin-0 nuclei during PREX-II. In a QED framework, this P-even, T-odd observable is a direct probe of the imaginary part of the two-photon exchange amplitude, since time reversal symmetry dictates AT must vanish in the one-photon approximation. At ultrarelativistic energies, the beam normal asymmetry is sensitive to the hadronic structure of the target nucleus. Calculations of the two-photon exchange amplitude are challenging since one has to consider all intermediate hadronic states in the calculation. The treatment of the states are both model and kinematics dependent. Therefore direct measurements of the beam normal asymmetry can test theory models. For experiments which measure the parity-violating asymmetry APV,the beam normal asymmetry can enter as a false asymmetry for APV when the beam is not 100% longitudinally polarized.

Richards, Ryan↗

DOE OSTI2024

Polarized internal target experiments based on EIC beams

The Electron-Ion Collider is under construction at BNL. It will have high-energy high-intensity polarized beams of electrons and hadrons. These beams will allow a high accuracy investigation of nucleon structure in the low- to very-low-x DIS regime. At the same time, similar to the realization at HERA, these beams could be used with an internal target for a very productive investigation of medium- to high-x nucleon structure. Due to a novel regime of electron beam operation and its high polarization and intensity, the Figure-of-Merit of an internal target experiment at EIC will be 500+ times higher than was obtained by HERMES.

Wojtsekhowski, Bogdan↗

DOE OSTI2020

Systematic error analysis in the absolute hydrogen gas jet polarimeter at RHIC

The Polarized Atomic Hydrogen Gas Jet Target polarimeter (HJET) is operated at the Relativistic Heavy Ion Collider (RHIC) since 2004 to measure the absolute polarization of each colliding proton beam. Polarimeter detectors and data acquisition were upgraded in 2015 to increase the solid angle, energy range, and to improve the energy and time resolution. These upgrades along with an improved beam intensity and polarization allowed us to greatly reduce the statistical and systematic errors for the proton polarization measurements in RHIC Runs 15 (E beam = 100 GeV) and 17 (255 GeV). For a typical 8 hour RHIC store, the measured proton beam average polarization was about P beam ~ 55 ± 2.0 stat ± 0.3 syst %. The elastic pp analyzing power, AN(t)~0.04, was determined with a precision of about |δA N (t)|~0.0002 in the momentum transfer squared range 0.001<-t<0.020 GeV 2 . In this paper we present a detailed systematic error analysis of the polarization measurements at HJET. Perspectives of using the HJET based absolute polarimeters in the future Electron Ion Collider (EIC) will be also discussed.

43 PARTICLE ACCELERATORS↗

NASA NTRS1989

Optical multiple object tracking techniques

Two multichannel multiple-object tracking techniques are reviewed. In the diffraction grating technique, the input scene is picked up by a TV camera and imaged onto a liquid-crystal light valve (LCLV), and the output side of the light valve is illuminated with a suitably polarized and collimated coherent laser beam to yield a reflected beam with polarization modulated according to the intensity of the incoherent input. This reflected beam passes through a beam splitter cube and an analyzer, resulting in an intensity modulated coherent image. An array of spectrum islands containing the information of the input appears after crossing a contact screen/lens combination. In the multiple-focus hololens technique, the scene of moving objects is sent into the LCTVSLM through a camera; a collimated laser beam is incident upon the LCTV screen; a low-pass filter is inserted between the LCTVSLM and the hololens for the removal of the high order diffractions due to the grid structure of the LCTV. The feasibility of the LCTVSLM and multiple-focus hololens technique is demonstrated.

Liu, Hua-Kuang↗

NASA NTRS1990

Heating of the polar wind due to ion beam instabilities

Recent DE 1 observations (Chen et al., 1990) in the polar cap region have detected upflowing field-aligned O(+) and H(+) beams of about 10 eV energy; it was suggested that these beams may interact with the polar wind and provide free energy to the polar wind which will alter its 'classical' description. This paper investigates both the linear and the nonlinear effects of upflowing O(+) and H(+) ion beams on an O(+) and H(+) polar wind by applying isotropic Maxwellian distributions for modeling the O(+) and H(+) polar-wind ions and using drifting Maxwellian distributions for the electrons, O(+), and H(+) beam ions. It is shown that, in the presence of O(+) and H(+) beams, the slow ion acoustic and slow ion cyclotron modes can couple to the normal modes of the plasma and be driven unstable. The O(+) ions are preferentially heated over the H(+) ions; this preferential heating couses enhanced outflow of O(+) ions along polar cap field lines by effectively increasing the O(+) scale height.

Chen, Margaret W.↗

DOE OSTI2023

Evaluation of a High-Power Target Design for Positron Production at CEBAF

A source for polarized positron beams at the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab is being designed. The Polarized Electrons for Polarized Positrons (PEPPo) concept is used to produce polarized e$^+$e$^-$-pairs from the bremsstrahlung radiation of a longitudinally polarized electron beam interacting within a high-Z conversion target. The scheme under consideration includes a 4 mm thick tungsten target that absorbs 17 kW deposited by a 1 mA continuous-wave electron beam with an energy of 120 MeV. The concept of a rotating tungsten rim mounted on a water-cooled copper disk was explored. The results of ANSYS thermal and mechanical analyses are discussed together with FLUKA evaluations of the radiation damages.

Ushakov, A.↗

DOE OSTI2023

Evaluation of a High-Power Target Design for Positron Production at CEBAF

Ushakov, A.↗

NASA NTRS1987

Enhanced beam coupling modulation using the polarization properties of photorefractive GaAs

Observation is reported of a rotation in the polarization of the two photorefractive recording beams in GaAs for a configuration with the internally generated space-charge field along the line 110 crystallographic orientation. This rotation is a result of simultaneous constructive and destructive beam coupling in each beam for the optical electric field components along the two electrooptically induced principal dielectric axes of the crystal. By turning one of the beams on and off, the intensity of the other beam after the crystal and a polarization analyzer can be modulated by as much as 500 percent. This result is of particular importance for optical information processing applications.

Partovi, Afshin↗

DOE OSTI2024

Charge lifetime improvement of the Continuous Electron Beam Accelerator Facility photogun with a biased anode

GaAs photocathodes in dc high-voltage photoguns are highly susceptible to ion back-bombardment, which reduces the photocathode quantum efficiency and limits the useful operating lifetime for producing polarized electron beams. This paper demonstrates that applying a small positive bias to the photogun anode can significantly suppress ion back-bombardment and increase charge lifetime. This technique was studied extensively using the Continuous Electron Beam Accelerator Facility photogun, where highly polarized electron beams created using a strained-superlattice GaAs / GaAsP photocathode were used and charge lifetimes improved by almost a factor of 2. A new simulation code was developed to model ion production and tracking in order to better understand and explain the factors that led to the performance improvement. Results of the experiments and simulations are discussed in detail. Published by the American Physical Society 2024

43 PARTICLE ACCELERATORS↗

DOE OSTI2025

Characterization of electron spin polarization from positive electron affinity GaAs photocathodes

Negative Electron Affinity (NEA) GaAs photocathodes are widely used to generate spin-polarized electron beams, typically achieving Electron Spin Polarizations (ESPs) in the range of 35%–40%. However, when operated in a Positive Electron Affinity (PEA) state, where a potential barrier inhibits low-energy electrons from escaping into vacuum, measured ESPs can exceed 50%. This effect can occur naturally during photocathode operation, as the NEA surface activation layers can degrade easily over time, increasing the electron affinity. In this work, we investigate and characterize the behavior of enhanced ESPs under PEA conditions. We present experimental measurements of ESP using a retarding-field Mott polarimeter on GaAs photocathodes with controlled and varying electron affinities. These results are complemented by theoretical explanations considering the material band structure, the light excitation profile, and spin depolarization mechanisms.

Electronic band structure↗

DOE OSTI2023

Robustness of cosmic birefringence measurement against Galactic foreground emission and instrumental systematics

The polarization of the cosmic microwave background (CMB) can be used to search for parity-violating processes like that predicted by a Chern-Simons coupling to a light pseudoscalar field. Such an interaction rotates E modes into E modes in the observed CMB signal through an effect known as cosmic birefringence. Even though isotropic birefringence can be confused with the rotation produced by a miscalibration of the detectors' polarization angles, the degeneracy between both effects is broken when Galactic foreground emission is used as a calibrator. Here, in this work, we use realistic simulations of the High-Frequency Instrument of the Planck mission to test the impact that Galactic foreground emission and instrumental systematics have on the recent birefringence measurements obtained through this technique. Our results demonstrate the robustness of the methodology against the miscalibration of polarization angles and other systematic effects, like intensity-to-polarization leakage, beam leakage, or cross-polarization effects. However, our estimator is sensitive to the EB correlation of polarized foreground emission. Here we propose to correct the bias induced by dust EB by modeling the foreground signal with templates produced in Bayesian component-separation analyses that fit parametric models to CMB data. Acknowledging the limitations of currently available dust templates like that of the Commander sky model, high-precision CMB data and a characterization of dust beyond the modified blackbody paradigm are needed to obtain a definitive measurement of cosmic birefringence in the future.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Phase control of spin waves in the van der Waals antiferromagnet NiPS 3

Here we demonstrate phase control of magnons in the van der Waals antiferromagnet NiPS3 using optical excitation by polarized light. The sign of the coherent precession of spin amplitude changes upon (i) reversing the helicity of a circularly polarized pump beam, or (ii) rotating the polarization of a linearly polarized pump by π/2. Because these two excitation pathways have comparable generation efficiency, the phase of spin precession can be continuously tuned from 0 to 2π by controlling the polarization state of the pump pulse. The ability to excite magnons with a desired phase has potential applications in the design of a spin-wave phased array and ultrafast spin information processing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗