5,940 research outputs found
Pulsed Beam Tests at the SANAEM RFQ Beamline
A proton beamline consisting of an inductively coupled plasma (ICP) source,
two solenoid magnets, two steerer magnets and a radio frequency quadrupole
(RFQ) is developed at the Turkish Atomic Energy Authority's (TAEA) Saraykoy
Nuclear Research and Training Center (SNRTC-SANAEM) in Ankara. In Q4 of 2016,
the RFQ was installed in the beamline. The high power tests of the RF power
supply and the RF transmission line were done successfully. The high power RF
conditioning of the RFQ was performed recently. The 13.56 MHz ICP source was
tested in two different conditions, CW and pulsed. The characterization of the
proton beam was done with ACCTs, Faraday cups and a pepper-pot emittance meter.
Beam transverse emittance was measured in between the two solenoids of the
LEBT. The measured beam is then reconstructed at the entrance of the RFQ by
using computer simulations to determine the optimum solenoid currents for
acceptance matching of the beam. This paper will introduce the pulsed beam test
results at the SANAEM RFQ beamline. In addition, the high power RF conditioning
of the RFQ will be discussed.Comment: 6 pages, 6 figures. Proceedings of the International Particle
Accelerator Conference 2017 (IPAC'17), May 14-19, 2017, TUPAB015, p. 134
Optical characteristics of nanocrystalline AlxGa1-xN thin films deposited by hollow cathode plasma-assisted atomic layer deposition
Cataloged from PDF version of article.Gallium nitride (GaN), aluminum nitride (AlN), and AlxGa(1-x)N films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 degrees C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and AlxGa1-xN films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 degrees C). For all films, the average optical transmission was similar to 85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and AlxGa1-xN were lower compared to GaN thin films. The refractive index of as-deposited films decreased from 2.33 to 2.02 (lambda = 550 nm) with the increased Al content x (0 400 nm). Postdeposition annealing at 900 degrees C for 2 h considerably lowered the refractive index value of GaN films (2.33-1.92), indicating a significant phase change. The optical bandgap of as-deposited GaN film was found to be 3.95 eV, and it decreased to 3.90 eV for films annealed at 800 degrees C for 30 min and 2 h. On the other hand, this value increased to 4.1 eV for GaN films annealed at 900 degrees C for 2 h. This might be caused by Ga2O3 formation and following phase change. The optical bandgap value of as-deposited AlxGa1-xN films decreased from 5.75 to 5.25 eV when the x values decreased from 1 to 0.68. Furthermore, postdeposition annealing did not affect the bandgap of Al-rich films. (C) 2014 American Vacuum Society
Hollow cathode plasma-assisted atomic layer deposition of crystalline AIN, GaN and AI Ga1- N thin films at low temperatures
Cataloged from PDF version of article.The authors report on the use of hollow cathode plasma for low-temperature plasma-assisted atomic layer
deposition (PA-ALD) of crystalline AlN, GaN and AlxGa1 xN thin films with low impurity concentrations.
Depositions were carried out at 200 C using trimethylmetal precursors and NH3 or N2/H2 plasma. X-ray
photoelectron spectroscopy showed the presence of 2.5–3 at.% O in AlN and 1.5–1.7 at.% O in GaN
films deposited using NH3 and N2/H2 plasma, respectively. No C impurities were detected within the
films. Secondary ion mass spectroscopy analyses performed on the films deposited using NH3 plasma
revealed the presence of O, C (both <1 at.%), and H impurities. GIXRD patterns indicated polycrystalline
thin films with wurtzite crystal structure. Hollow cathode PA-ALD parameters were optimized for AlN
and GaN thin films using N2/H2 plasma. Trimethylmetal and N2/H2 saturation curves evidenced the selflimiting
growth of AlN and GaN at 200 C. AlN exhibited linear growth with a growth per cycle (GPC) of
1.0 A. For GaN, the GPC decreased with the increasing number of deposition cycles, indicating Ëš
substrate-enhanced growth. The GPC calculated from a 900-cycle GaN deposition was 0.22 A. Ëš
Ellipsometric spectra of the samples were modeled using the Cauchy dispersion function, from which
the refractive indices of 59.2 nm thick AlN and 20.1 nm thick GaN thin films were determined to be 1.94
and 2.17 at 632 nm, respectively. Spectral transmission measurements of AlN, GaN and AlxGa1 xN thin
films grown on double side polished sapphire substrates revealed near-ideal visible transparency with
minimal absorption. Optical band edge values of the AlxGa1 xN films shifted to lower wavelengths with
the increasing Al content, indicating the tunability of band edge values with the alloy composition
Low-Temperature Deposition of Hexagonal Boron Nitride via Sequential Injection of Triethylboron and N2/H2 Plasma
Cataloged from PDF version of article.Hexagonal boron nitride (hBN) thin films were deposited on
silicon and quartz substrates using sequential exposures of
triethylboron and N
2
/H
2
plasma in a hollow-cathode plasma-
assisted atomic layer deposition reactor at low temperatures
(
≤
450
°
C). A non-saturating film deposition rate was observed
for substrate temperatures above 250
°
C. BN films were charac-
terized for their chemical composition, crystallinity, surface
morphology, and optical properties. X-ray photoelectron spec-
troscopy (XPS) depicted the peaks of boron, nitrogen, carbon,
and oxygen at the film surface. B 1s and N 1s high-resolution
XPS spectra confirmed the presence of BN with peaks located
at 190.8 and 398.3 eV, respectively. As deposited films were
polycrystalline, single-phase hBN irrespective of the deposition
temperature. Absorption spectra exhibited an optical band edge
at
~
5.25 eV and an optical transmittance greater than 90% in
the visible region of the spectrum. Refractive index of the hBN
film deposited at 450
°
C was 1.60 at 550 nm, which increased
to 1.64 after postdeposition annealing at 800
°
C for 30 min.
These results represent the first demonstration of hBN deposi-
tion using low-temperature hollow-cathode plasma-assisted
sequential deposition technique. © 2014 The American Ceramic Society
Project PROMETHEUS: Design and Construction of a Radio Frequency Quadrupole at TAEK
The PROMETHEUS Project is ongoing for the design and development of a 4-vane
radio frequency quadrupole (RFQ) together with its H+ ion source, a low energy
beam transport (LEBT) line and diagnostics section. The main goal of the
project is to achieve the acceleration of the low energy ions up to 1.5 MeV by
an RFQ (352 MHz) shorter than 2 meter. A plasma ion source is being developed
to produce a 20 keV, 1 mA H+ beam. Simulation results for ion source,
transmission and beam dynamics are presented together with analytical studies
performed with newly developed RFQ design code DEMIRCI. Simulation results
shows that a beam transmission 99% could be achieved at 1.7 m downstream
reaching an energy of 1.5 MeV. As the first phase an Aluminum RFQ prototype,
the so-called cold model, will be built for low power RF characterization. In
this contribution the status of the project, design considerations, simulation
results, the various diagnostics techniques and RFQ manufacturing issues are
discussed.Comment: 4 pages, 8 figures, Proceedings of the 2nd International Beam
Instrumentation Conference 2013 (IBIC'13), 16-19 Sep 2013, WEPC02, p. 65
Quartz Cherenkov Counters for Fast Timing: QUARTIC
We have developed particle detectors based on fused silica (quartz) Cherenkov
radiators read out with micro-channel plate photomultipliers (MCP-PMTs) or
silicon photomultipliers (SiPMs) for high precision timing (Sigma(t) about
10-15 ps). One application is to measure the times of small angle protons from
exclusive reactions, e.g. p + p - p + H + p, at the Large Hadron Collider, LHC.
They may also be used to measure directional particle fluxes close to external
or stored beams. The detectors have small areas (square cm), but need to be
active very close (a few mm) to the intense LHC beam, and so must be radiation
hard and nearly edgeless. We present results of tests of detectors with quartz
bars inclined at the Cherenkov angle, and with bars in the form of an "L" (with
a 90 degree corner). We also describe a possible design for a fast timing
hodoscope with elements of a few square mm.Comment: 24 pages, 14 figure
Tkachenko modes as sources of quasiperiodic pulsar spin variations
We study the long wavelength shear modes (Tkachenko waves) of triangular
lattices of singly quantized vortices in neutron star interiors taking into
account the mutual friction between the superfluid and the normal fluid and the
shear viscosity of the normal fluid. The set of Tkachenko modes that propagate
in the plane orthogonal to the spin vector are weakly damped if the coupling
between the superfluid and normal fluid is small. In strong coupling, their
oscillation frequencies are lower and are undamped for small and moderate shear
viscosities. The periods of these modes are consistent with the observed
~100-1000 day variations in spin of PSR 1828-11.Comment: 7 pages, 3 figures, uses RevTex, v2: added discussion/references,
matches published versio
Fabrication of flexible polymer-GaN core-shell nanofibers by the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition
Here we demonstrate the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) processes by fabricating flexible polymer-GaN organic-inorganic core-shell nanofibers at a processing temperature much lower than that needed for the preparation of conventional GaN ceramic nanofibers. Polymer-GaN organic-inorganic core-shell nanofibers fabricated by the HCPA-ALD of GaN on electrospun polymeric (nylon 6,6) nanofibers at 200 °C were characterized in detail using electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence measurements, and dynamic mechanical analysis. Although transmission electron microscopy studies indicated that the process parameters should be further optimized for obtaining ultimate uniformity and conformality on these high surface area 3D substrates, the HCPA-ALD process resulted in a ∼28 nm thick polycrystalline wurtzite GaN layer on polymeric nanofibers of an average fiber diameter of ∼70 nm. Having a flexible polymeric core and low processing temperature, these core-shell semiconducting nanofibers might have the potential to substitute brittle ceramic GaN nanofibers, which have already been shown to be high performance materials for various electronic and optoelectronic applications. This journal is © The Royal Society of Chemistry
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This paper presents physics-based surrogate modeling algorithms for systems governed by parameterized partial differential equations (PDEs) commonly encountered in design optimization and uncertainty analysis. We first outline unsupervised learning approaches that leverage advances in the machine learning literature for a meshfree solution of PDEs. Subsequently, we propose continuum and discrete formulations for systems governed by parameterized steady-state PDEs. We consider the case of both deterministically and randomly parameterized systems. The basic idea is to embody the design variables or uncertain parameters in additional dimensions of the governing PDEs along with the spatial coordinates. We show that the undetermined parameters of the surrogate model can be estimated by minimizing a physics-based objective function derived using a multidimensional least-squares collocation or the Bubnov-Galerkin scheme. This potentially allows us to construct surrogate models without using data from computer experiments on a deterministic analysis code. Finally, we also outline an extension of the present approach to directly approximate the density function of random algebraic equations
Forward Neutron Production at the Fermilab Main Injector
We have measured cross sections for forward neutron production from a variety
of targets using proton beams from the Fermilab Main Injector. Measurements
were performed for proton beam momenta of 58 GeV/c, 84 GeV/c, and 120 GeV/c.
The cross section dependence on the atomic weight (A) of the targets was found
to vary as where is for a beam momentum of
58 GeV/c and 0.540.05 for 120 GeV/c. The cross sections show reasonable
agreement with FLUKA and DPMJET Monte Carlos. Comparisons have also been made
with the LAQGSM Monte Carlo.Comment: Accepted for publication in Physical Review D. This version
incorporates small changes suggested by referee and small corrections in the
neutron production cross sections predicted by FLUK
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