1,063 research outputs found
Assessment of Axial Wave Number and Mean Flow Uncertainty on Acoustic Liner Impedance Education
A key parameter in designing and assessing advanced broadband acoustic liners to achieve the current and future noise reduction goals is the acoustic impedance presented by the liner. This parameter, intrinsic to a specific liner configuration, is dependent on sound pressure level and grazing flow velocity. Current impedance eduction approaches have, in general, provided excellent results and continue to be employed throughout the acoustic liner community. However, some recent applications have indicated a possible dependence of the educed impedance on the direction of incident waves relative to the mean flow. The purpose of the current study is to investigate this unexpected behavior for various impedance eduction methods based on the Pridmore-Brown and convected Helmholtz equations. Specifically, the effects of flow profile and axial wavenumber uncertainties on educed impedances for upstream and downstream sources are investigated. The uniform flow results demonstrate the importance of setting a correct Mach number value in obtaining consistent educed impedances for upstream and downstream sources. In fact, the consistency of results over the two source locations was greatly improved by a slight modification of the uniform flow Mach number. In addition, uncertainty in educed axial wavenumber was also illustrated to correlate well with differences in the educed impedances, even with modified uniform flow Mach number. Finally, while less straightforward than in the uniform flow case, it appears that modification of the mean flow profile may also improve consistency of results for upstream and downstream results when shear flow is included
Spatial Control of Photoemitted Electron Beams using a Micro-Lens-Array Transverse-Shaping Technique
A common issue encountered in photoemission electron sources used in electron
accelerators is the transverse inhomogeneity of the laser distribution
resulting from the laser-amplification process and often use of frequency up
conversion in nonlinear crystals. A inhomogeneous laser distribution on the
photocathode produces charged beams with lower beam quality. In this paper, we
explore the possible use of microlens arrays (fly-eye light condensers) to
dramatically improve the transverse uniformity of the drive laser pulse on UV
photocathodes. We also demonstrate the use of such microlens arrays to generate
transversely-modulated electron beams and present a possible application to
diagnose the properties of a magnetized beam.Comment: arXiv admin note: text overlap with arXiv:1609.0166
Longitudinal phase space manipulation in energy recovering linac-driven free-electron lasers
Energy recovering an electron beam after it has participated in a
free-electron laser (FEL) interaction can be quite challenging because of the
substantial FEL-induced energy spread and the energy anti-damping that occurs
during deceleration. In the Jefferson Lab infrared FEL driver-accelerator, such
an energy recovery scheme was implemented by properly matching the longitudinal
phase space throughout the recirculation transport by employing the so-called
energy compression scheme. In the present paper,after presenting a
single-particle dynamics approach of the method used to energy-recover the
electron beam, we report on experimental validation of the method obtained by
measurements of the so-called "compression efficiency" and "momentum
compaction" lattice transfer maps at different locations in the recirculation
transport line. We also compare these measurements with numerical tracking
simulations.Comment: 31 pages, 13 figures, submitted to Phys. Rev. Special Topics A&
Current-induced nuclear-spin activation in a two-dimensional electron gas
Electrically detected nuclear magnetic resonance was studied in detail in a
two-dimensional electron gas as a function of current bias and temperature. We
show that applying a relatively modest dc-current bias, I_dc ~ 0.5 microAmps,
can induce a re-entrant and even enhanced nuclear spin signal compared with the
signal obtained under similar thermal equilibrium conditions at zero current
bias. Our observations suggest that dynamic nuclear spin polarization by small
current flow is possible in a two-dimensional electron gas, allowing for easy
manipulation of the nuclear spin by simple switching of a dc current.Comment: 5 pages, 3 fig
Influence of Source Propagation Direction and Shear Flow Profile in Impedance Eduction of Acoustic Liners
The acoustic impedance of liners is a key parameter for their design, and depends on the flow conditions, i.e., the sound pressure level and the presence of a grazing flow. The surface impedance of a locally reacting liner is defined as a local intrinsic property relating the acoustic pressure to the normal acoustic particle velocity at the liner surface. Impedance eduction techniques are now widely used to retrieve the impedance of liners in aeroacoustic facilities in the presence of a shear grazing flow. While surface impedance is intrinsic by definition, the educed impedance has recently been shown to depend on the direction of the incident waves relative to the mean flow. Different studies have investigated this issue by considering different acoustic propagation models used in the education process in the hope of matching the educed values. The purpose of the present work is to continue the previous investigations by evaluating the influence of the shear flow profile on the educed impedance, while considering a Bayesian inference process in order to evaluate the uncertainty on the educed values. The identified uncertainties were not able to totally account for the observed discrepancies between educed impedances
The hole Fermi surface in BiSe probed by quantum oscillations
Transport and torque magnetometry measurements are performed at high magnetic
fields and low temperatures in a series of p-type (Ca-doped) BiSe
crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van
Alphen quantum oscillations enables us to determine the Fermi surface of the
bulk valence band states as a function of the carrier density. At low density,
the angular dependence exhibits a downturn in the oscillations frequency
between and , reflecting a bag-shaped hole Fermi surface.
The detection of a single frequency for all tilt angles rules out the existence
of a Fermi surface with different extremal cross-sections down to ~meV.
There is therefore no signature of a camel-back in the valence band of our bulk
samples, in accordance with the direct band gap predicted by calculations.Comment: A supplemental material file giving a more detailed description of
our work is available upon reques
Intrinsic Gap of the nu=5/2 Fractional Quantum Hall State
The fractional quantum Hall effect is observed at low field, in a regime
where the cyclotron energy is smaller than the Coulomb interaction. The nu=5/2
excitation gap is measured to be 262+/-15 mK at ~2.6 T, in good agreement with
previous measurements performed on samples with similar mobility, but with
electronic density larger by a factor of two. The role of disorder on the
nu=5/2 gap is examined. Comparison between experiment and theory indicates that
a large discrepancy remains for the intrinsic gap extrapolated from the
infinite mobility (zero disorder) limit. In contrast, no such large discrepancy
is found for the nu=1/3 Laughlin state. The observation of the nu=5/2 state in
the low-field regime implies that inclusion of non-perturbative Landau level
mixing may be necessary to better understand the energetics of half-filled
fractional quantum hall liquids.Comment: 5 pages, 4 figures; typo corrected, comment expande
Contrasting Behavior of the 5/2 and 7/3 Fractional Quantum Hall Effect in a Tilted Field
Using a tilted field geometry, the effect of an in-plane magnetic field on
the even denominator nu = 5/2 fractional quantum Hall state is studied. The
energy gap of the nu = 5/2 state is found to collapse linearly with the
in-plane magnetic field above ~0.5 T. In contrast, a strong enhancement of the
gap is observed for the nu = 7/3 state. The radically distinct tilted-field
behaviour between the two states is discussed in terms of Zeeman and
magneto-orbital coupling within the context of the proposed Moore-Read pfaffian
wavefunction for the 5/2 fractional quantum Hall effect
Fractional quantum Hall effect in CdTe
The fractional quantum Hall (FQH) effect is reported in a high mobility CdTe
quantum well at mK temperatures. Fully-developed FQH states are observed at
filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state
for which the lowest energy excitation is not a spin-flip. This can be
accounted for by the relatively high intrinsic Zeeman energy in this single
valley 2D electron gas. FQH minima are also observed in the first excited (N=1)
Landau level at filling factor 7/3 and 8/3 for intermediate temperatures.Comment: Submitte
Colossal magnetoresistance in an ultra-clean weakly interacting 2D Fermi liquid
We report the observation of a new phenomenon of colossal magnetoresistance
in a 40 nm wide GaAs quantum well in the presence of an external magnetic field
applied parallel to the high-mobility 2D electron layer. In a strong magnetic
field, the magnetoresistance is observed to increase by a factor of ~300 from 0
to 45T without the system undergoing any metal-insulator transition. We discuss
how this colossal magnetoresistance effect cannot be attributed to the spin
degree-of-freedom or localization physics, but most likely emanates from strong
magneto-orbital coupling between the two-dimensional electron gas and the
magnetic field. Our observation is consistent with a field-induced 2D-to-3D
transition in the confined electronic system
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