294 research outputs found
A Highly Flexible Bunch Compressor for the APS LEUTL FEL
The LEUTL FEL at the APS has achieved gain at 530 nm with peak beam current
\~100 A. In an effort to push to 120 nm and beyond, we have designed and are
commissioning a bunch compressor to increase the peak current to 600 A or more.
The bunch compressor uses a four-dipole chicane at an energy of 100 to 210 MeV.
To provide options for control of emittance growth due to coherent synchrotron
radiation (CSR), the chicane has variable R56. The central pair of dipoles is
movable, accommodating variable R56 with less concern about emittance dilution
from nonuniformity of the dipole field. The symmetry of the chicane is also
variable via longitudinal motion of the final dipole, which is predicted to
have an effect on emittance growth. Following the chicane, a three-screen
emittance measurement system should permit resolution of the difference in
emittance growth between various chicane configurations. A vertical bending
magnet analysis line is present to permit imaging of correlations between
transverse and energy coordinates. This paper reviews the features and expected
performance of the chicane. A companion paper discusses the physics design in
detail.
Work supported by the U.S. Department of Energy, Office of Basic Energy
Sciences, under Contract No. W-31-109-ENG-38.Comment: LINAC2000 THC05 3 pages 1 figur
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Summary of beam quality diagnostics and control working group
The working group on beam quality, diagnostics, and control at the 12th Advanced Accelerator Concepts Workshop held a series of meetings during the Workshop. The generation of bright charged-particle beams (in particular electron and positron beams), along with state-of-the-art beam diagnostics and synchronization were discussed
Realizing Higher-Level Gauge Symmetries in String Theory: New Embeddings for String GUTs
We consider the methods by which higher-level and non-simply laced gauge
symmetries can be realized in free-field heterotic string theory. We show that
all such realizations have a common underlying feature, namely a dimensional
truncation of the charge lattice, and we identify such dimensional truncations
with certain irregular embeddings of higher-level and non-simply laced gauge
groups within level-one simply-laced gauge groups. This identification allows
us to formulate a direct mapping between a given subgroup embedding, and the
sorts of GSO constraints that are necessary in order to realize the embedding
in string theory. This also allows us to determine a number of useful
constraints that generally affect string GUT model-building. For example, most
string GUT realizations of higher-level gauge symmetries G_k employ the
so-called diagonal embeddings G_k\subset G\times G \times...\times G. We find
that there exist interesting alternative embeddings by which such groups can be
realized at higher levels, and we derive a complete list of all possibilities
for the GUT groups SU(5), SU(6), SO(10), and E_6 at levels k=2,3,4 (and in some
cases up to k=7). We find that these new embeddings are always more efficient
and require less central charge than the diagonal embeddings which have
traditionally been employed. As a byproduct, we also prove that it is
impossible to realize SO(10) at levels k>4. This implies, in particular, that
free-field heterotic string models can never give a massless 126 representation
of SO(10).Comment: 69 pages, LaTeX, 5 figures (Encapsulated PostScript). Revised to
match published versio
String Unification, Higher-Level Gauge Symmetries, and Exotic Hypercharge Normalizations
We explore the extent to which string theories with higher-level gauge
symmetries and non-standard hypercharge normalizations can reconcile the
discrepancy between the string unification scale and the GUT scale extrapolated
from the Minimal Supersymmetric Standard Model (MSSM). We determine the
phenomenologically allowed regions of (k_Y,k_2,k_3) parameter space, and
investigate the proposal that there might exist string models with exotic
hypercharge normalizations k_Y which are less than their usual value k_Y=5/3.
For a broad class of heterotic string models (encompassing most realistic
string models which have been constructed), we prove that k_Y >= 5/3. Beyond
this class, however, we show that there exist consistent MSSM embeddings which
lead to k_Y < 5/3. We also consider the constraints imposed on k_Y by demanding
charge integrality of all unconfined string states, and show that only a
limited set of hypercolor confining groups and corresponding values of k_Y are
possible.Comment: 59 pages, standard LaTeX, 4 figures (Encapsulated PostScript).
Expanded reference
NASA Goddard Space Flight Center's Compendium of Recent Single Event Effects Results
We present the results of single event effects (SEE) testing and analysis investigating the effects of radiation on electronics. This paper is a summary of test results
Space-Borne Electron Accelerator Design
Renewed interest in active experiments with relativistic particle beams in space has led to the development of solid-state radio-frequency (RF) linear accelerators (linac) that can deliver MeV electron beams but operate with low-voltage DC power supplies. The solid-state RF amplifiers used to drive the accelerator are known as high-electron mobility transistors (HEMTs), and at C-band (5–6 GHz) are capable of generating up to 500 watts of RF power at 10% duty factor in a small package, i.e., the size of a postage stamp. In operation, the HEMTs are powered with 50 V DC as their bias voltage; they thus can tap into the spacecraft batteries or electrical bus as the primary power source. In this paper we describe the initial testing of a compact space-borne RF accelerator consisting of individual C-band cavities, each independently powered by a gallium nitride (GaN) HEMT. We show preliminary test results that demonstrate the beam acceleration in a single C-band cavity powered by a single HEMT operating at 10% duty factor. An example of active beam experiments in space that could benefit from the HEMT-powered accelerators is the proposed Magnetosphere-Ionosphere Connection (CONNEX) experiment (Dors et al., 2017)
Intersecting Branes Flip SU(5)
Within a toroidal orbifold framework, we exhibit intersecting brane-world
constructions of flipped SU(5) \times U(1) GUT models with various numbers of
generations, other chiral matter representations and Higgs representations. We
exhibit orientifold constructions with integer winding numbers that yield 8 or
more conventional SU(5) generations, and orbifold constructions with fractional
winding numbers that yield flipped SU(5) \times U(1) models with just 3
conventional generations. Some of these models have candidates for the 5 and
{\bar 5} Higgs representations needed for electroweak symmetry breaking, but
not for the 10 and {\bar 10} representations needed for GUT symmetry breaking.
We have also derived models with complete GUT and electroweak Higgs sectors,
but these have undesirable extra chiral matter.Comment: 18 pages, Latex file, some clarifying comments added, version to
appear in Nuclear Physics
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