13,612 research outputs found
Synchroscan streak camera imaging at a 15-MeV photoinjector with emittance exchange
At the Fermilab A0 photoinjector facility, bunch-length measurements of the
laser micropulse and the e-beam micropulse have been done in the past with a
fast single-sweep module of the Hamamatsu C5680 streak camera with an intrinsic
shot-to-shot trigger jitter of 10-20ps. We have upgraded the camera system with
the synchroscan module tuned to 81.25MHz to provide synchronous summing
capability with less than 1.5ps FWHM trigger jitter and a phase-locked delay
box to provide phase stability of ~1ps over 10s of minutes. These steps allowed
us to measure both the UV laser pulse train at 263nm and the e-beam via optical
transition radiation (OTR). Due to the low electron beam energies and OTR
signals, we typically summed over 50 micropulses with 0.25-1nC per micropulse.
The phase-locked delay box allowed us to assess chromatic temporal effects and
instigated another upgrade to an all-mirror input optics barrel. In addition,
we added a slow sweep horizontal deflection plug-in unit to provide dual-sweep
capability for the streak camera. We report on a series of measurements made
during the commissioning of these upgrades including bunch-length and phase
effects using the emittance exchange beamline and simultaneous imaging of a UV
drive laser component, OTR, and the 800nm diagnostics laser.Comment: 26 p
Identifying network communities with a high resolution
Community structure is an important property of complex networks. An
automatic discovery of such structure is a fundamental task in many
disciplines, including sociology, biology, engineering, and computer science.
Recently, several community discovery algorithms have been proposed based on
the optimization of a quantity called modularity (Q). However, the problem of
modularity optimization is NP-hard, and the existing approaches often suffer
from prohibitively long running time or poor quality. Furthermore, it has been
recently pointed out that algorithms based on optimizing Q will have a
resolution limit, i.e., communities below a certain scale may not be detected.
In this research, we first propose an efficient heuristic algorithm, Qcut,
which combines spectral graph partitioning and local search to optimize Q.
Using both synthetic and real networks, we show that Qcut can find higher
modularities and is more scalable than the existing algorithms. Furthermore,
using Qcut as an essential component, we propose a recursive algorithm, HQcut,
to solve the resolution limit problem. We show that HQcut can successfully
detect communities at a much finer scale and with a higher accuracy than the
existing algorithms. Finally, we apply Qcut and HQcut to study a
protein-protein interaction network, and show that the combination of the two
algorithms can reveal interesting biological results that may be otherwise
undetectable.Comment: 14 pages, 5 figures. 1 supplemental file at
http://cic.cs.wustl.edu/qcut/supplemental.pd
Conversion of a transverse density modulation into a longitudinal phase space modulation using an emittance exchange technique
We report on an experiment to produce a train of sub-picosecond microbunches
using a transverse-to-longitudinal emittance exchange technique. The generation
of a modulation on the longitudinal phase space is done by converting an
initial horizontal modulation produced using a multislits mask. The preliminary
experimental data clearly demonstrate the conversion process. To date only the
final energy modulation has been measured. However numerical simulations, in
qualitative agreement with the measurements, indicate that the conversion
process should also introduce a temporal modulation.Comment: 4 pages, 6 figures. Submitted to the proceedings of the Physics and
Applications of High-Brightness Electron Beams (HBEB09), Nov. 16-19, 2009,
Maui H
Symmetry of `molecular' configurations of interacting electrons in a quantum dot in strong magnetic fields
A molecular description for magic-number configurations of interacting
electrons in a quantum dot in high magnetic fields developed by one of the
authors has been elaborated for four, five and six electron dots. For four
electrons, the magic spin-singlet states are found to alternate between two
different resonating valence bond (RVB)-like states. For the five-electron
spin-polarized case, the molecular description is shown to work for the known
phenomenon of magic-number sequences that correspond to both the N-fold
symmetric ring configuration and a -fold symmetric one with a center
electron. A six-electron dot is shown here to have an additional feature in
which inclusion of quantum mechanical mixing between classical configurations,
which are deformed and degenerate, restores the N-fold symmetry and reproduces
the ground-state energy accurately.Comment: 4 pages, to be published in Physisca
Thermoelectric effect in high mobility single layer epitaxial graphene
The thermoelectric response of high mobility single layer epitaxial graphene
on silicon carbide substrates as a function of temperature and magnetic field
have been investigated. For the temperature dependence of the thermopower, a
strong deviation from the Mott relation has been observed even when the carrier
density is high, which reflects the importance of the screening effect. In the
quantum Hall regime, the amplitude of the thermopower peaks is lower than a
quantum value predicted by theories, despite the high mobility of the sample. A
systematic reduction of the amplitude with decreasing temperature suggests that
the suppression of the thermopower is intrinsic to Dirac electrons in graphene.Comment: 5 pages, 4 figure
The Second-Class Action: How Courts Thwart Wage Rights by Misapplying Class Action Rules
Courts apply to wage rights cases an aggressive scrutiny that not only disadvantages low-wage workers, but is fundamentally incorrect on the law. Rule 23 class actions automatically cover all potential members if the court grants plaintiffsâ class certification motion. But for certain employment rights casesâmainly wage claims but also age discrimination and gender equal pay claimsâ29 U.S.C. § 216(b) allows not class actions but âcollective actionsâ covering just those opting in affirmatively. Yet courts in collective actions assume a gatekeeper role just as they do in Rule 23 class actions, disallowing many actions by requiring a certification motion proving strict commonality among members. This Article argues that conditioning § 216(b) collective actions on certification motions proving commonality is incorrect. Section 216(b) is not an opt-in version of Rule 23; it is a liberalized form of simple Rule 20 joinder, which permits joint suit whenever claims share one common issue and address related events. No text authorizes any § 216(b) certification inquiry, nor is judicial gatekeeping justified by economic logic: Rule 23 classes present principal-agent and asymmetric information problems because lead plaintiffs may inadequately represent unengaged members, but all § 216(b) collective actions members are full plaintiffs with individual claims, obviating the need for judicial scrutiny
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