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Flavor mixing and charm decay

Description: The results of mixing matrix determination and their implications on heavy quark decays are given. The decays of charm mesons D/sup 0/, D/sup +/, F/sup +/ into two pseudoscalar mesons are discussed in the framework of SU(3) symmetry. The charm decays are also discussed in terms of quark diagrams. It is demonstrated that the differences observed in the lifetimes of D/sup 0/ and D/sup +/, and in the branching ratios B(D/sup 0/ ..-->.. K/sup -/K/sup +/) and B(D/sup 0/ ..-->.. ..pi../sup -/..pi../sup +/) can be easily incorporated. 3 figures.
Date: January 1, 1980
Creator: Chau Wang, L.C.
Partner: UNT Libraries Government Documents Department

Baeklund transformations, conservation laws and linearization of the self-dual Yang-Mills and chiral fields

Description: Baecklund Transformations (BT) and the derivation of local conservation laws are first reviewed in the classic case of the Sine-Gordon equation. The BT, conservation laws (local and nonlocal), and the inverse-scattering formulation are discussed for the chiral and the self-dual Yang-Mills fields. Their possible applications to the loop formulation for the Yang-Mills fields are mentioned. 55 references, 1 figure.
Date: January 1, 1980
Creator: Wang, L.C.
Partner: UNT Libraries Government Documents Department

Geometric Effects on Electron Cloud

Description: The development of an electron cloud in the vacuum chambers of high intensity positron and proton storage rings may limit the machine performances by inducing beam instabilities, beam emittance increase, beam loss, vacuum pressure increases and increased heat load on the vacuum chamber wall. The electron multipacting is a kind of geometric resonance phenomenon and thus is sensitive to the geometric parameters such as the aperture of the beam pipe, beam shape and beam bunch fill pattern, etc. This paper discusses the geometric effects on the electron cloud build-up in a beam chamber and examples are given for different beams and accelerators.
Date: July 6, 2007
Creator: Wang, L
Partner: UNT Libraries Government Documents Department

Effects of Beam Filling Pattern on Beam Ion Instability and Beam Loading In PEP-X

Description: A proposed high-brightness synchrotron light source (PEP-X) is under design at SLAC. The 4.5-GeV PEP-X storage ring has four theoretical minimum emittance (TME) cells to achieve the very low emittance and two double-bend achromat (DBA) cells to provide spaces for IDs. Damping wigglers will be installed in zero-dispersion straights to reduce the emittance below 0.1 nm. Ion induced beam instability is one critical issue due to its ultra small emittance. Third harmonic cavity can be used to lengthen the bunch in order to improve the beam's life time. Bunch-train filling pattern is proposed to mitigate both the fast ion instability and beam loading effect. This paper investigates the fast ion instability and beam loading for different beam filling patterns.
Date: June 2, 2009
Creator: Wang, L.
Partner: UNT Libraries Government Documents Department

Electron Cloud in the Wigglers of the Positron Damping Ring of the International Linear Collider

Description: The ILC positron damping ring comprises hundreds of meters of wiggler sections, where many more photons than in the arcs are emitted, and with the smallest beampipe aperture of the ring. A significant electron-cloud density can therefore be accumulated via photo-emission and via beam-induced multipacting. In field-free regions the electron-cloud build up may be suppressed by adding weak solenoid fields, but the electron cloud remaining in the wigglers as well as in the arc dipole magnets can still drive single-bunch and multi-bunch beam instabilities. This paper studies the electron-cloud formation in an ILC wiggler section for various scenarios, as well as its character, and possible mitigation schemes.
Date: July 6, 2007
Creator: Wang, L.
Partner: UNT Libraries Government Documents Department

Adaptive Impedance Analysis of Grooved Surface using the Finite Element Method

Description: Grooved surface is proposed to reduce the secondary emission yield in a dipole and wiggler magnet of International Linear Collider. An analysis of the impedance of the grooved surface based on adaptive finite element is presented in this paper. The performance of the adaptive algorithms, based on an element-element h refinement technique, is assessed. The features of the refinement indicators, adaptation criteria and error estimation parameters are discussed.
Date: July 6, 2007
Creator: Wang, L.
Partner: UNT Libraries Government Documents Department

Distortion of Crabbed Bunch Due to the Electron Cloud

Description: In order to improve the luminosity, two crab cavities have been installed in KEKB HER and LER [1]. Since there is only one crab cavity in each ring, the crab cavity generates a horizontally titled bunch along the whole ring. The achieved specific luminosity with crabbed bunch is higher, but it is not as high as that from beam-beam simulation [2]. One of the suspicions is the electron cloud. The electron cloud in LER (positron beam) may distort the crabbed bunch and cause the luminosity drop. This note briefly estimates the bunch shape distortion due to the electron cloud in KEKB LER.
Date: May 28, 2008
Creator: Wang, L & Raubenheimer, T.
Partner: UNT Libraries Government Documents Department

Transverse Single Bunch Instability in PEP-X

Description: A proposed high-brightness synchrotron light source (PEP-X) is under design at SLAC. The 4.5- GeV PEP-X storage ring has four theoretical minimum emittance (TME) cells to achieve the very low emittance and two double-bend achromat (DBA) cells to provide spaces for IDs. Damping wigglers will be installed in zero-dispersion straights to reduce the emittance below 0.1nm. In this paper, we present a preliminary estimation of the threshold of the transverse mode coupling instability (TMCI). Three approaches have been used in the estimation and they agree well with each other.
Date: June 2, 2009
Creator: Wang, L. & Stupakov, G.
Partner: UNT Libraries Government Documents Department

Trapping of Electron Cloud LLC/Cesrta Quadrupole and Sextupole Magnets

Description: The Cornell Electron Storage Ring (CESR) has been reconfigured as an ultra low emittance damping ring for use as a test accelerator (CesrTA) for International Linear Collider (ILC) damping ring R&D [1]. One of the primary goals of the CesrTA program is to investigate the interaction of the electron cloud with low emittance positron beam to explore methods to suppress the electron cloud, develop suitable advanced instrumentation required for these experimental studies and benchmark predictions by simulation codes. This paper reports the simulation of the electron-cloud formation in CESRTA and ILC quadrupole and sextupole magnets using the 3D code CLOUDLAND. We found that electrons can be trapped with a long lifetime in a quadrupole and sextupole magnet due to the mirror field trapping mechanism. We study the effects of magnet strength, bunch current, ante-chamber effect, bunch spacing effect and secondary emission yield (SEY) in great detail. The development of an electron cloud in magnets is the main concern where a weak solenoid field is not effective. Quadrupole and sextupole magnets have mirror field configurations which may trap electrons by the mirror field trapping mechanism [2]. Fig.1 shows the orbit of a trapped electron in a quadrupole magnet. The electron makes gyration motion (called transverse motion) and also moves along the field line (called longitudinal motion). At the mirror point (middle of the field line), there is a maximum longitudinal energy and minimum transverse energy. When the electron moves away from the mirror point, its longitudinal energy reduces and the transverse energy increases as the magnetic field increases. If the magnetic field is strong enough, the longitudinal energy becomes zero at one point and then the electron is turned back by the strong field. Note that the electrons are trapped in the region near the middle of the field lines. Although ...
Date: August 18, 2011
Creator: Wang, L. & Pivi, M.
Partner: UNT Libraries Government Documents Department

STUDY ON THE VOLUMETRIC RATIO OF THE HYBRID GM REGENERATOR OVER THE COLD CHAMBER WORKING AT 4K.

Description: This paper presents a computational analysis on the hybrid GM regenerator at liquid helium region. The effects of the ratio of the regenerator volume over the cold chamber volume on cooling performance were simulated numerically. The results show that there exits a minimum ratio of the regenerator volume over the cold chamber volume for a G-M cryocooler at 4K. The cooling capacity of the refrigerator drops sharply with smaller regenerator, and rises slowly with larger regenerator. The effects of the material combinations of the hybrid regenerator on the volumetric ratio were also studied. The design principle of the geometry size of the regenerator working at 4K was discussed.
Date: July 12, 1999
Creator: WANG,L.
Partner: UNT Libraries Government Documents Department

THE EXPERIMENTAL STUDY OF A SINGLE STAGE G-M REFRIGERATOR WITH THE REGENERATOR SET OUTSIDE THE CYLINDER.

Description: In this paper, a single-stage G-M refrigerator with the regenerator set outside the cylinder is presented. The experimental system for testing the performance of the cryocooler was constructed. The lowest temperature was 14K when the operating frequency was 0.6 Hz. The cooling capacity of 4.4W has been obtained at 20K. The effects of operating parameters of the refrigerator on cooling performance were also experimentally studied.
Date: July 12, 1999
Creator: WANG,L.
Partner: UNT Libraries Government Documents Department

Stability of uranium silicides during high energy ion irradiation

Description: Changes induced by 1.5 MeV Kr ion irradiation of both U{sub 3}Si and U{sub 3}Si{sub 2} have been followed by in situ transmission electron microscopy. When irradiated at sufficiently low temperatures, both alloys transform from the crystalline to the amorphous state. When irradiated at temperatures above the temperature limit for ion beam amorphization, both compounds disorder with the Martensite twin structure in U{sub 3}Si disappearing from view in TEM. Prolonged irradiation of the disordered crystalline phases results in nucleation of small crystallites within the initially large crystal grains. The new crystallites increase in number during continued irradiation until a fine grain structure is formed. Electron diffraction yields a powder-like diffraction pattern that indicates a random alignment of the small crystallites. During a second irradiation at lower temperatures, the small crystallizes retard amorphization. After 2 dpa at high temperatures, the amorphization dose is increased by over twenty times compared to that of initially unirradiated material.
Date: November 1, 1991
Creator: Birtcher, R.C & Wang, L.M.
Partner: UNT Libraries Government Documents Department

Quench Protection for the MICE Cooling Channel CouplingMagnet

Description: The MICE coupling coil is fabricated from Nb-Ti, which hashigh quench propagation velocities within the coil in all directionscompared to coils fabricated with other superconductors such as niobiumtin. The time for the MICE coupling coil to become fully normal throughnormal region propagation in the coil is shorter than the time needed fora safe quench (as defined by a hot-spot temperature that is less than 300K). A MICE coupling coil quench was simulated using a code written at theInstitute of Cryogenics and Superconductive Technology (ICST) at theHarbin Institute of Technology (HIT). This code simulates quench backfrom the mandrel as well as normal region propagation within the coil.The simulations included sub-division of the coil. Each sub-division hasa back to back diodes and resistor across the coil. Current flows in theresistor when there is enough voltage across the coil to cause current toflow through the diodes in the forward direction. The effects of thenumber of coil sub-divisions and the value of the resistor across thesub-division on the quench were calculated with and without quench back.Sub-division of the coupling coil reduces the peak voltage to ground, thelayer-to-layer voltage and the magnet hot-spot temperature. Quench backreduces the magnet hot-spot temperature, but the peak voltage to groundand layer-to-layer voltage are increased, because the magnet quenchesfaster. The resistance across the coil sub-division affects both thehot-spot temperature and the peak voltage to ground.
Date: November 20, 2007
Creator: Green, M.A.; Wang, L. & Guo, X.L.
Partner: UNT Libraries Government Documents Department

2D Optical Streaking for Ultra-Short Electron Beam Diagnostics

Description: We propose a novel approach to measure short electron bunch profiles at micrometer level. Low energy electrons generated during beam-gas ionization are simultaneously modulated by the transverse electric field of a circularly-polarized laser, and then they are collected at a downstream screen where the angular modulation is converted to a circular shape. The longitudinal bunch profile is simply represented by the angular distribution of the electrons on the screen. We only need to know the laser wavelength for calibration and there is no phase synchronization problem. Meanwhile the required laser power is also relatively low in this setup. Some simulations examples and experimental consideration of this method are discussed. At Linac Coherent Light Source (LCLS), an S-band RF transverse deflector (TCAV) is used to measure the bunch length with a resolution 10 femtosecond (fs) rms. An X-band deflector (wavelength 2.6cm) is proposed recently to improve the resolution. However, at the low charge operation mode (20pC), the pulse length can be as short as fs. It is very challenging to measure femtosecond and sub-femtosecond level bunch length. One of the methods is switching from RF to {mu}m level wavelength laser to deflect the bunch. A powerful laser ({approx}10s GW) is required to deflect such a high energy beam (GeV) in a wiggler. Synchronization is another difficulty: the jitter between the bunch and the laser can be larger than the laser wavelength, which makes single-shot measurement impossible. To reduce the laser power, we propose to use ionized electrons from high energy electron beam and gas interaction for high energy electron bunch diagnostics. Similarly, the femtosecond X-ray streak camera uses X-ray ionization electrons to measure the X-ray pulse. The electrons generated by beam-gas ionization have low energy (eVs). Therefore, a lower laser power is possible to deflect such low energy electrons. Note that ...
Date: December 14, 2011
Creator: Ding, Y.T.; Huang, Z.; Wang, L. & /SLAC
Partner: UNT Libraries Government Documents Department

Optimization for Single-Spike X-Ray FELs at LCLS with a Low Charge Beam

Description: The Linac Coherent Light Source is an x-ray free-electron laser at the SLAC National Accelerator Laboratory, which is operating at x-ray wavelengths of 20-1.2 Angstrom with peak brightness nearly ten orders of magnitude beyond conventional synchrotron radiation sources. At the low charge operation mode (20 pC), the x-ray pulse length can be <10 fs. In this paper we report our numerical optimization and simulations to produce even shorter x-ray pulses by optimizing the machine and undulator setup at 20 pC charge. In the soft x-ray regime, with combination of slotted-foil or undulator taper, a single spike x-ray pulse is achievable with peak FEL power of a few 10s GW. Linac Coherent Light Source (LCLS), the world's first hard x-ray Free electron laser (FEL), has started operation since 2009. With nominal operation charge of 250 pC, the generated x-ray pulse length is from 70 fs to a few hundred fs. This marks the beginning of a new era of ultrashort x-ray sciences. In addition, a low charge (20pC) operation mode has also been established. Since the collective effects are reduced at the low charge mode, we can increase the compression factor and still achieve a few kA peak current. The expected electron beam and x-ray pulses are less than 10 fs. There are growing interests in even shorter x-ray pulses, such as fs to sub-fs regime. One of the simple solutions is going to even lower charge. As discussed, single-spike x-ray pulses can be generated using 1 pC charge. However, this charge level is out of the present LCLS diagnostic range. 20 pC is a reasonable operation charge at LCLS, based on the present diagnostic system. At 20 pC in the soft x-ray wavelength regime, we have experimentally demonstrated that FEL can work at undercompression or over-compression mode, such as 1 ...
Date: December 14, 2011
Creator: Wang, L.; Ding, Y.; Huang, Z. & /SLAC
Partner: UNT Libraries Government Documents Department

Distortion of Crabbed Bunch Due to Electron Cloud and Global Crabbing

Description: Crab cavities may be used improve the luminosity in colliding beam colliders with crab crossing. In a global crab crossing correction, only one crab cavity is installed in each ring and the crab cavities generate a horizontally titled bunch oscillating around the ring. The electron cloud in positively charged rings may distort the crabbed bunch and cause the luminosity drop. This paper briefly estimates the distortion of positron bunch due to the electron cloud with global crab and estimates the effect in the KEKB and possible LHC upgrades.
Date: August 1, 2008
Creator: Wang, L.; Raubenheimer, T.O.; Cai, Y. & /SLAC
Partner: UNT Libraries Government Documents Department

Precise Calculation of Traveling-Wave Periodic Structure

Description: The effects of the round edge beam hole on the frequency and wake field are studied using variational method, which allows for rounded iris disk hole without any approximation in shape treatment. The frequency and wake field of accelerating mode and dipole mode are studied for different edge radius cases, including the flat edge shape that is often used to approximately represent the actual structure geometry. The edge hole shape has weak effect on the frequency, but much effect on the wake field. Our study shows that the amounts of wake fields are not precise enough with the assumption of the flat edge beam hole instead of round edge.
Date: July 6, 2007
Creator: Wang, L.; Li, Z.; Seryi, A. & /SLAC
Partner: UNT Libraries Government Documents Department