622 research outputs found
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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
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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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
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
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
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
Astrophysical constraints on superlight gravitinos
I review the constraints on the mass of gravitinos that follow from
considerations on energy loss in stars and from Big Bang Nucleosynthesis
arguments.Comment: Invited talk at the 5th Workshop on High Energy Physics
Phenomenology(WHEPP-5), Pune, India, 12-26 January 199
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
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Soft X-ray Mirrors for the Linac Coherent Light Source
The Linac Coherent Light Source (LCLS) is a 0.15-1.5 nm wavelength free-electron laser (FEL) being constructed at the Stanford Linear Accelerator Center (SLAC) by a multi-institution consortium, including Lawrence Livermore National Laboratory (LLNL). One of LLNL's responsibilities involves the design and construction of two grazing-incidence mirror systems whose primary intent is to reduce radiation levels in the experimental halls by separating the FEL beam from unwanted high-energy photons. This paper discusses one of these systems, the Soft X-ray Offset Mirror System (SOMS) that will operate in the wavelength range 0.62-1.5 nm (0.827-2.00 keV). The unusual properties of the FEL beam translate to stringent specifications in terms of stability, material choice and mirror properties. It also precludes using approaches previously developed for synchrotron light sources. This situation has led us to a unique mirror design, consisting of a reflective boron carbide layer deposited on a silicon substrate. In the first part of this paper, we discuss the basic system requirements for the SOMS and motivate the need for these novel reflective elements. In the second part of this paper, we discuss the development work we have performed, including simulation and experimental verification of the boron carbide coating properties, and the expected performance of the final system
LOTIS Search for Early Time Optical Afterglows: GRB 971227
We report on the very early time search for an optical afterglow from GRB
971227 with the Livermore Optical Transient Imaging System (LOTIS). LOTIS began
imaging the `Original' BATSE error box of GRB 971227 approximately 14 s after
the onset of gamma-ray emission. Continuous monitoring of the position
throughout the evening yielded a total of 499 images (10 s integration).
Analysis of these images revealed no steady optical afterglow brighter than
R=12.3 +- 0.2 in any single image. Coaddition of different combinations of the
LOTIS images also failed to uncover transient optical emission. In particular,
assuming a constant early time flux, no optical afterglow brighter than R=14.2
+- 0.2 was present within the first 1200 s and no optical afterglow brighter
than R=15.0 +- 0.2 was present in the first 6.0 h.
Follow up observations by other groups revealed a likely X-ray afterglow and
a possible optical afterglow. Although subsequent deeper observations could not
confirm a fading source, we show that these transients are not inconsistent
with our present knowledge of the characteristics of GRB afterglows. We also
demonstrate that with the upgraded thermoelectrically cooled CCDs, LOTIS is
capable of either detecting very early time optical afterglow or placing
stringent constraints on the relationship between the gamma-ray emission and
the longer wavelength afterglow in relativistic blast wave models.Comment: 17 pages, 3 eps figures, revisions based on reviewers comment
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