683 research outputs found
Impedance of a Rectangular Beam Tube with Small Corrugations
We consider the impedance of a structure with rectangular, periodic
corrugations on two opposing sides of a rectangular beam tube. Using the method
of field matching, we find the modes in such a structure. We then limit
ourselves to the the case of small corrugations, but where the depth of
corrugation is not small compared to the period. For such a structure we
generate analytical approximate solutions for the wave number , group
velocity , and loss factor for the lowest (the dominant) mode
which, when compared with the results of the complete numerical solution,
agreed well. We find: if , where is the beam pipe width and is
the beam pipe half-height, then one mode dominates the impedance, with
( is the depth of corrugation),
, and , which (when replacing by
) is the same scaling as was found for small corrugations in a {\it round}
beam pipe. Our results disagree in an important way with a recent paper of
Mostacci {\it et al.} [A. Mostacci {\it et al.}, Phys. Rev. ST-AB, {\bf 5},
044401 (2002)], where, for the rectangular structure, the authors obtained a
synchronous mode with the same frequency , but with .
Finally, we find that if is large compared to then many nearby modes
contribute to the impedance, resulting in a wakefield that Landau damps.Comment: 18 pages, 6 figures, 1 bibliography fil
Calculation of wakefields in 2D rectangular structures
We consider the calculation of electromagnetic fields generated by an
electron bunch passing through a vacuum chamber structure that, in general,
consists of an entry pipe, followed by some kind of transition or cavity, and
ending in an exit pipe. We limit our study to structures having rectangular
cross-section, where the height can vary as function of longitudinal coordinate
but the width and side walls remain fixed. For such structures, we derive a
Fourier representation of the wake potentials through one-dimensional
functions. A new numerical approach for calculating the wakes in such
structures is proposed and implemented in the computer code ECHO(2D). The
computation resource requirements for this approach are moderate and comparable
to those for finding the wakes in 2D rotationally symmetric structures.
Numerical examples obtained with the new numerical code are presented.Comment: 31 pages, 10 figure
Corrugated Pipe as a Beam Dechirper
We have studied the use of a metallic pipe with small corrugations for the purpose of passively dechirping, through its wakefield, a short, intense electron bunch. The corrugated pipe is attractive for this purpose because its wake: (i) has near maximal possible amplitude for a given aperture and (ii) has a relatively large oscillation wave length, even when the aperture is small. We showed how the corrugated structure can satisfy dechirping requirements encountered in the NGLS project at LBNL. We found that a linear chirp of -40 MeV/mm can be induced by an NGLS-like beam, by having it pass through a corrugated, metallic pipe of radius 3 mm, length 8.2 m, and corrugation parameters full depth 450 {mu}m and period 1000 {mu}m. This structure is about 15 times as effective in the role of dechirper as an S-band accelerator structure used passively
Instanton-based Techniques for Analysis and Reduction of Error Floors of LDPC Codes
We describe a family of instanton-based optimization methods developed
recently for the analysis of the error floors of low-density parity-check
(LDPC) codes. Instantons are the most probable configurations of the channel
noise which result in decoding failures. We show that the general idea and the
respective optimization technique are applicable broadly to a variety of
channels, discrete or continuous, and variety of sub-optimal decoders.
Specifically, we consider: iterative belief propagation (BP) decoders, Gallager
type decoders, and linear programming (LP) decoders performing over the
additive white Gaussian noise channel (AWGNC) and the binary symmetric channel
(BSC).
The instanton analysis suggests that the underlying topological structures of
the most probable instanton of the same code but different channels and
decoders are related to each other. Armed with this understanding of the
graphical structure of the instanton and its relation to the decoding failures,
we suggest a method to construct codes whose Tanner graphs are free of these
structures, and thus have less significant error floors.Comment: To appear in IEEE JSAC On Capacity Approaching Codes. 11 Pages and 6
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Coherent Instabilities of ILC Damping Ring
The paper presents the first attempt to estimates the ILC damping ring impedance and compare thresholds of the classical instabilities for several designs initially proposed for the DR. The work was carried out in the spring of 2006. Since then the choice of the DR is narrowed. Nevertheless, the analysis described may be useful for the next iterations of the beam stability. Overall, the conventional instabilities will have little impact on the ring performance provided the careful design of the ring minimizes the impedance below acceptable level indicated above. The only exception is the transverse CB instability. The longitudinal CB is less demanding. However, even the transverse CB instability would have threshold current above nominal provided the aperture in the wigglers is increased from 8 mm to 16 mm. The microwave instability needs more studies. Nevertheless, we should remember that the ILC DR is different from existing high-current machines at least in two respects: absence of the beam-beam tune spread stabilizing beams in colliders, and unusual strict requirements for low emittance. That may cause new problems such as bunch emittance dilution due to high-frequency wakes (BPMs, grooves), etc. Even if such a possibility exists, it probably universal for all machines and ought be addressed in the design of vacuum components rather than have effect on the choice of the machine design
Simulation studies for dielectric wakefield programme at CLARA facility
Short, high charge electron bunches can drive high magnitude electric fields
in dielectric lined structures. The interaction of the electron bunch with this
field has several applications including high gradient dielectric wakefield
acceleration (DWA) and passive beam manipulation. The simulations presented
provide a prelude to the commencement of an experimental DWA programme at the
CLARA accelerator at Daresbury Laboratory. The key goals of this program are:
tunable generation of THz radiation, understanding of the impact of transverse
wakes, and design of a dechirper for the CLARA FEL. Computations of
longitudinal and transverse phase space evolution were made with Impact-T and
VSim to support both of these goals.Comment: 10 Pages, 4 Figures, Proceedings of EAAC2017 Conferenc
Experimental Observation of Energy Modulation in Electron Beams Passing Through Terahertz Dielectric Wakefield Structures
We report observation of a strong wakefield induced energy modulation in an
energy-chirped electron bunch passing through a dielectric-lined waveguide.
This modulation can be effectively converted into a spatial modulation forming
micro-bunches with a periodicity of 0.5 - 1 picosecond, hence capable of
driving coherent THz radiation. The experimental results agree well with
theoretical predictions.Comment: v3. Reviewers' suggestions incorporated. Accepted by PR
Assessment of HCC response to Yttrium-90 radioembolization with gadoxetate disodium MRI: correlation with histopathology.
Transarterial <sup>90</sup> Y radioembolization (TARE) is increasingly being used for hepatocellular carcinoma (HCC) treatment. However, tumor response assessment after TARE may be challenging. We aimed to assess the diagnostic performance of gadoxetate disodium MRI for predicting complete pathologic necrosis (CPN) of HCC treated with TARE, using histopathology as the reference standard.
This retrospective study included 48 patients (M/F: 36/12, mean age: 62 years) with HCC treated by TARE followed by surgery with gadoxetate disodium MRI within 90 days of surgery. Two radiologists evaluated tumor response using RECIST1.1, mRECIST, EASL, and LI-RADS-TR criteria and evaluated the percentage of necrosis on subtraction during late arterial, portal venous, and hepatobiliary phases (AP/PVP/HBP). Statistical analysis included inter-reader agreement, correlation between radiologic and pathologic percentage of necrosis, and prediction of CPN using logistic regression and ROC analyses.
Histopathology demonstrated 71 HCCs (2.8 ± 1.7 cm, range: 0.5-7.5 cm) including 42 with CPN, 22 with partial necrosis, and 7 without necrosis. EASL and percentage of tumor necrosis on subtraction at the AP/PVP were independent predictors of CPN (p = 0.02-0.03). Percentage of necrosis, mRECIST, EASL, and LI-RADS-TR had fair to good performance for diagnosing CPN (AUCs: 0.78 - 0.83), with a significant difference between subtraction and LI-RADS-TR for reader 2, and in specificity between subtraction and other criteria for both readers (p-range: 0.01-0.04). Radiologic percentage of necrosis was significantly correlated to histopathologic degree of tumor necrosis (r = 0.66 - 0.8, p < 0.001).
Percentage of tumor necrosis on subtraction and EASL criteria were significant independent predictors of CPN in HCC treated with TARE. Image subtraction should be considered for assessing HCC response to TARE when using MRI.
• Percentage of tumor necrosis on image subtraction and EASL criteria are significant independent predictors of complete pathologic necrosis in hepatocellular carcinoma treated with <sup>90</sup> Y radioembolization. • Subtraction, mRECIST, EASL, and LI-RADS-TR have fair to good performance for diagnosing complete pathologic necrosis in hepatocellular carcinoma treated with <sup>90</sup> Y radioembolization
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