656 research outputs found
Laser-induced electron emission from a tungsten nanotip: identifying above threshold photoemission using energy-resolved laser power dependencies
We present an experiment studying the interaction of a strongly focused 25 fs
laser pulse with a tungsten nanotip, investigating the different regimes of
laser-induced electron emission. We study the dependence of the electron yield
with respect to the static electric field applied to the tip. Photoelectron
spectra are recorded using a retarding field spectrometer and peaks separated
by the photon energy are observed with a 45 % contrast. They are a clear
signature of above threshold photoemission (ATP), and are confirmed by
extensive spectrally resolved studies of the laser power dependence.
Understanding these mechanisms opens the route to control experiment in the
strong-field regime on nanoscale objects.Comment: 9 pages, 6 figure
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A Transmissive Optics Approach for Time-Slicing the LCLS X-Ray Pulse
This paper investigates the use of off-axis zone plate optical systems to deliver time-sliced LCLS FEL pulses to users under the 3 energy chirp scenarios elucidated by P. Emma. We present formulas for designing off-axis zone plate optical systems that achieve a given time-slice duration and intensity. The results show that it is feasible to fabricate zone-plate systems capable of providing intense spots of time-sliced 8.275 KeV photons under the scenario of a 2.0% chirp, but that it is beyond current and envisioned fabrication capabilities to create zone-plate systems of similar performance under the scenarios offering energy chirps of < 0.25%. Finally we present results of numerical calculations of the electric fields delivered to the user by an off-axis zone plate optical system producing time-slices of {le} 50 {center_dot} fs with photon densities of 200 photons/{angstrom}{sup 2} under the 2% energy chirp scenario
A study of atmospheric neutrinos with the IMB detector
A sample of 401 contained neutrino interactions collected in the 3300 metric ton fiducial mass IMB detector was used to study neutrino oscillations, geomagnetic modulation of the flux and to search for point sources. The majority of these events are attributed to neutrino interactions. For the most part, these neutrinos are believed to originate as tertiary products of cosmic ray interactions in the atmosphere. The neutrinos are a mixture of v sub e and v sub micron
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Transmission Grating Measurements of Undulator K
This study was undertaken to understand the practicalities of determine K differences in the undulator modules by measuring single-shot x-ray spectra of the spontaneous radiation with a transmissive grating spectrometer under development to measure FEL spectra. Since the quality of the FEL is dependent on a uniform K value in all the undulator modules, being able to measure the relative undulator K values is important. Preliminary results were presented in a presentation, ''Use of FEL Off-Axis Zone Plate Spectrometer to Measure Relative K by the Pinhole/Centroid Method'', at the ''LCLS Beam-Based Undulator K Measurements Workshop'' on November 14, 2005 (UCRL-PRES-217281). This study applies equally well to reflective gratings of the appropriate period and inclinations
Bounds on Dark Matter from the ``Atmospheric Neutrino Anomaly''
Bounds are derived on the cross section, flux and energy density of new
particles that may be responsible for the atmospheric neutrino anomaly. Decay of primordial
homogeneous dark matter can be excluded.Comment: 10 pages, TeX (revtex
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Interaction of VUV-FEL radiation with B4C and SiC at 32nm wavelength
The output fluence and pulse duration of XFELs such as LCLS and TESLA will pose significant challenges to the optical components which may be damaged by the XFEL beam [1]. It is expected that low-atomic-number materials such as SiC, B{sub 4}C, and diamond exhibit weak absorption and therefore are damaged least. It has been suggested that the fundamental damage mechanism that determines the fluence damage threshold for single-shot exposures is thermal melting of the materials [2]. For multiple-shot exposures, the damage threshold is potentially lower than the melt threshold due to fatigue effects associated with mechanical stresses during to thermal cycling [3]
Measuring the to Ratio in a High Statistics Atmospheric Neutrino Experiment
By exploiting differences in muon lifetimes it is possible to distinguish
from charged current interactions in underground
neutrino detectors. Such observations would be a useful tool in understanding
the source of the atmospheric neutrino anomaly.Comment: 6 pages no figure
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Space-Charge Effects in the Gas Detector
Discussion of space-charge effects in a photoluminescence cell that will be used as a non-disruptive total energy monitor at the LCLS facility is presented. Regimes where primary photoelectrons will be confined within the X-ray beam aperture are identified. Effects of the space-charge on the further evolution of the electron and ion populations are discussed. Parameters of the afterglow plasma are evaluated. Conditions under which the detector output will be proportional to the pulse energy are defined
LOTIS Upper Limits and the Prompt OT from GRB 990123
GRB 990123 established the existence of prompt optical emission from
gamma-ray bursts (GRBs). The Livermore Optical Transient Imaging System (LOTIS)
has been conducting a fully automated search for this kind of simultaneous low
energy emission from GRBs since October 1996. Although LOTIS has obtained
simultaneous, or near simultaneous, coverage of the error boxes obtained with
BATSE, IPN, XTE, and BeppoSAX for several GRBs, image analysis resulted in only
upper limits. The unique gamma-ray properties of GRB 990123, such as very large
fluence (top 0.4%) and hard spectrum, complicate comparisons with more typical
bursts. We scale and compare gamma-ray properties, and in some cases afterglow
properties, from the best LOTIS events to those of GRB 990123 in an attempt to
determine whether the prompt optical emission of this event is representative
of all GRBs. Furthermore, using LOTIS upper limits in conjunction with the
relativistic blast wave model, we weakly constrain the GRB and afterglow
parameters such as density of the circumburster medium and bulk Lorentz factor
of the ejecta.Comment: 5 pages, 2 figures, To appear in Proceedings of the 5th Huntsville
Gamma-Ray Burst Symposiu
Uncovering Extreme Nonlinear Dynamics in Solids Through Time-Domain Field Analysis
Time-domain analysis of harmonic fields with sub-cycle resolution is now
experimentally viable due to the emergence of sensitive, on-chip techniques for
petahertz-scale optical-field sampling. We demonstrate how such a time-domain,
field-resolved analysis uncovers the extreme nonlinear electron dynamics
responsible for high-harmonic generation within solids. Time-dependent density
functional theory was used to simulate harmonic generation from a solid-state
band-gap system driven by near- to mid-infrared waveforms. Particular attention
was paid to regimes where both intraband and interband emission mechanisms play
a critical role in shaping the nonlinear response. We show that a time-domain
analysis of the harmonic radiation fields identifies the interplay between
intra- and interband dynamical processes underlying the nonlinear light
generation. With further analysis, we show that changes to the dominant
emission regime can occur after only slight changes to the peak driving
intensity and central driving wavelength. Time-domain analysis of harmonic
fields also reveals, for the first time, the possibility of rapid changes in
the dominant emission mechanism within the temporal window of the driving pulse
envelope. Finally, we examine the experimental viability of performing
time-domain analysis of harmonic fields with sub-cycle resolution using
realistic parameters
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