110 research outputs found
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Design of 118 MHz twelfth harmonic cavity of APS PAR
Two radio frequency (RF) cavities are needed in the Positron Accumulator Ring (PAR) of the Advanced Photon Source. One is for the first harmonic frequency at 9.8 MHz, and the other is for the twelfth harmonic frequency at 118 MHz. This note reports on the design of the 118 MHz RF cavity. Computer models are used to find the mode frequencies, impedances, Q-factors, and field distributions in the cavity. The computer codes MAFIA, URMEL, and URMEL-T are useful tools which model and simulate the resonance characteristics of a cavity. These codes employ the finite difference method to solve Maxwell`s equations. MAFIA is a three-dimensional problem solver and uses square patches to approximate the inner surface of a cavity. URMEL and URMEL-T are two-dimensional problem solvers and use rectangular and triangular meshes, respectively. URMEL-T and MAFIA can handle problems with arbitrary dielectric materials located inside the boundary. The cavity employs a circularly cylindrical ceramic window to limit the vacuum to the beam pipe. The ceramic window used in the modeling will have a wall thickness of 0.9 cm. This wall thickness is not negligible in determining the resonant frequencies of the cavity. In the following, results of two- and three-dimensional modeling of the cavities using the URMEL-T and MAFIA codes are reported
Spectral stability of noncharacteristic isentropic Navier-Stokes boundary layers
Building on work of Barker, Humpherys, Lafitte, Rudd, and Zumbrun in the
shock wave case, we study stability of compressive, or "shock-like", boundary
layers of the isentropic compressible Navier-Stokes equations with gamma-law
pressure by a combination of asymptotic ODE estimates and numerical Evans
function computations. Our results indicate stability for gamma in the interval
[1, 3] for all compressive boundary-layers, independent of amplitude, save for
inflow layers in the characteristic limit (not treated). Expansive inflow
boundary-layers have been shown to be stable for all amplitudes by Matsumura
and Nishihara using energy estimates. Besides the parameter of amplitude
appearing in the shock case, the boundary-layer case features an additional
parameter measuring displacement of the background profile, which greatly
complicates the resulting case structure. Moreover, inflow boundary layers turn
out to have quite delicate stability in both large-displacement and
large-amplitude limits, necessitating the additional use of a mod-two stability
index studied earlier by Serre and Zumbrun in order to decide stability
Existence and stability of viscoelastic shock profiles
We investigate existence and stability of viscoelastic shock profiles for a
class of planar models including the incompressible shear case studied by
Antman and Malek-Madani. We establish that the resulting equations fall into
the class of symmetrizable hyperbolic--parabolic systems, hence spectral
stability implies linearized and nonlinear stability with sharp rates of decay.
The new contributions are treatment of the compressible case, formulation of a
rigorous nonlinear stability theory, including verification of stability of
small-amplitude Lax shocks, and the systematic incorporation in our
investigations of numerical Evans function computations determining stability
of large-amplitude and or nonclassical type shock profiles.Comment: 43 pages, 12 figure
Recommended adult immunization schedule, United States, 2020
In October 2019, the Advisory Committee on Immunization
Practices (ACIP) voted to approve the Recommended
Adult Immunization Schedule for Ages 19
Years or Older, United States, 2020. The 2020 adult immunization
schedule, available at www.cdc.gov/vaccines
/schedules/hcp/imz/adult.html, summarizes ACIP recommendations
in 2 tables and accompanying notes (Figure).
The full ACIP recommendations for each vaccine are available
at www.cdc.gov/vaccines/hcp/acip-recs/index.html.
The 2020 schedule has also been approved by the director
of the Centers for Disease Control and Prevention
(CDC) and by the American College of Physicians (www
.acponline.org), American Academy of Family Physicians
(www.aafp.org), American College of Obstetricians and
Gynecologists (www.acog.org), and American College of
Nurse-Midwives (www.midwife.org)
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Rf impedance measurements of the various vacuum chambers for the Advanced Photon Source (APS)
Coupling impedances (Z) for the 7-GeV APS storage ring have been numerically estimated. In order to confirm these calculations, measurements of the coupling impedance of various vacuum components around the main storage ring was done with a synthetic pulse technique. A section of the beam + ante-chamber, a vacuum valve and without the internal RF screen, and a photon absorber were used as a device under test (DUT) to obtain the preliminary results. The result in the frequency domain (FD) measurement was compared with previous time domain (TD) measurement and with the computer simulation of the APS impedance budget. The Z of k dependence on the pulse length and the thickness of the center wire was examined. 5 refs., 4 figs., 2 tabs
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