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On transverse mode coupling instability in the VLHC

Description: We present the results of calculation of transverse mode coupling instability (TMCI) thresholds for Very Large Hadron Collider (VLHC). The estimates are done with use of conventional approach and more sophisticated mode dynamics analysis. We found the RF system parameters which allows to get appropriate TMCI threshold number of particles per bunch and attain high luminosity of the collider. Alternative ways to raise the threshold are briefly discussed.
Date: January 1, 1998
Creator: Danilov, V.V. & Shiltsev, V.D.
Partner: UNT Libraries Government Documents Department

What have we learned about the solar interior from solar oscillations?

Description: After a brief review of the discovery and properties of solar oscillations, I summarize the significant advances in our knowledge of the Sun`s interior structure achieved by using solar oscillation frequency data. I discuss the surprising solar interior rotation profile; the precise determination of the convection zone depth; the convection zone helium abundance; evidence for diffusive settling of helium during the Sun`s 4.5 billion year lifetime; and the Sun`s central structure and implications for the solar neutrino problem.
Date: December 31, 1995
Creator: Guzik, J.A.
Partner: UNT Libraries Government Documents Department

Simple Beam-Optic Simulations and Proposed Mechanical Mitigation for the Triplet Oscillation Problem

Description: The purpose of these simulations is to gain a better understanding of the relative contributions to the beam perturbation from the main horizontal oscillation modes (yawing and rolling) of Q1, Q2, and Q3. For this purpose, a simple beam transport program was implemented with an EXCEL spreadsheet to calculate the horizontal beam envelop through the Q1, Q2, Q3 triplet up to the IP, with the possibility of specifying horizontal displacements of the singlets. For now, the weak focusing properties of D0 and DX have been ignored, but could easily be included in the future if necessary. In a first simulation, quadrupole strengths have been adopted that correspond to {beta}* = 2m at the IP. The parameters used listed in Table 1 were obtained from references [1] and [2].
Date: May 1, 2008
Creator: Thieberger,P.; Montag, C.; Snydstrup, L.; Trbojevic, D. & Tuozzolo, J.
Partner: UNT Libraries Government Documents Department

Higher Order Mode Damper Study of the 56 MHz SRF Cavity

Description: This report summarizes the study on the higher order mode (HOM) damper for the 56 MHz SRF cavity. The Q factors and frequencies of the HOMs with the HOM damper are measured and compared to the simulation. The high pass filter prototype for rejecting the fundamental mode is designed and tested. The filter measurement is also compared to the simulation. Based on the measurement, a new location of the HOM damper is chosen.
Date: August 1, 2008
Creator: Choi,E. & Hahn, H.
Partner: UNT Libraries Government Documents Department

Passive modal damping with piezoelectric shunts

Description: The use of piezoelectric materials in conjunction with passive inductance-resistance-capacitance (RLC) circuits to dampen specific vibration modes is explored. The piezoelectric materials convert mechanical energy to electrical energy, which is then dissipated in the RLC circuit through joule heating. An impulse is applied to a simple cantilevered beam and by varying the inductance and resistance values, the natural oscillation frequency fcir the RLC circuit is tuned to dampen the first mode of vibration.
Date: January 1, 2001
Creator: Granier, J. J. (John J.); Haundhausen, R. J. (R. Jason) & Gaytan, G. E. (Gabriel E.)
Partner: UNT Libraries Government Documents Department

Vibration modes of spherical shells and containment vessels.

Description: Spherical pressure vessels are used to fully contain the effects of high explosions. In this paper, the vibrations of a spherical containment vessel undergoing elastic response are investigated. Vibration modes of containment vessels are of particular interest, as it is the superposition and interaction of different modes of response with closely spaced frequencies that has been reported to be the mechanism of 'strain growth'. First, the modal frequencies of a spherical shell for both axisymmetric and nonaxisymmetric response modes are discussed, based on a sequence of papers that have appeared in the open literature. Analytical predictions are then compared with numerical simulations using ABAQUS. It is found that the numerical simulations accurately predict both the axisymmetric and nonaxisymmetric modal frequencies for the complete spherical shell. Next, numerical simulations of modal frequencies for the more complex spherical containment vessel (with nozzles) are compared with the spherical shell results. Numerical simulations for the spherical containment vessel reveal that frequencies are somewhat similar to the complete spherical shell. Limited comparisons with experimentally recorded frequencies for participating modes of vessel dynamic response during high explosive containment testing are presented as well.
Date: January 1, 2001
Creator: Duffey, T. A. (Thomas A.) & Romero, C. D. (Christopher D.)
Partner: UNT Libraries Government Documents Department

Chemical reactions of water molecules on Ru(0001) induced by selective excitation of vibrational modes

Description: Tunneling electrons in a scanning tunneling microscope were used to excite specific vibrational quantum states of adsorbed water and hydroxyl molecules on a Ru(0 0 0 1) surface. The excited molecules relaxed by transfer of energy to lower energy modes, resulting in diffusion, dissociation, desorption, and surface-tip transfer processes. Diffusion of H{sub 2}O molecules could be induced by excitation of the O-H stretch vibration mode at 445 meV. Isolated molecules required excitation of one single quantum while molecules bonded to a C atom required at least two quanta. Dissociation of single H{sub 2}O molecules into H and OH required electron energies of 1 eV or higher while dissociation of OH required at least 2 eV electrons. In contrast, water molecules forming part of a cluster could be dissociated with electron energies of 0.5 eV.
Date: May 7, 2009
Creator: Mugarza, Aitor; Shimizu, Tomoko K.; Ogletree, D. Frank & Salmeron, Miquel
Partner: UNT Libraries Government Documents Department

Tuning the LEDA RFQ 6.7 MeV accelerator

Description: This paper presents the results of tuning the 8 meter long Radio Frequency Quadrupole (RFQ) built for the Low Energy Demonstration Accelerator (LEDA). This 350-MHz RFQ is split into four 2-meter-long-RFQs. Then they are joined with resonant coupling to form an 8-meter-long RFQ. This improves both the longitudinal stability and the transverse stability of this long RFQ. The frequencies of the modes near the RFQ mode are measured. The authors show the effect on the RF fields of an error in the temperature of each one of the 2-meter-long-RFQs. Slug tuners distributed along the outer walls tune the RFQ. The program RFQTUNE is used to determine the length of the tuners. The tuners are machined to length when the final tuning is complete.
Date: December 31, 1998
Creator: Young, L.M. & Rybarcyk, L.
Partner: UNT Libraries Government Documents Department

Observation and calculation of trapped modes near cut-off in the ALS bellow-shield

Description: Observed heating of the RF shields in the bellows of the Advanced Light Source (ALS) storage ring has led to studies of possible causes. One such possibility is resonant impedances near the cut-off frequency of the beam-pipe that arise from small, localized, enlargements of the beampipe cross-section. Calculations of trapped modes in the elliptical-section vacuum chamber, approximated by a rectangular geometry, are described. Measurements of a bellows-shield in the test laboratory are also described, as are temperature measurements of a flexband in the storage ring.
Date: June 1, 1996
Creator: Corlett, J.N.
Partner: UNT Libraries Government Documents Department

A damped detuned structure for the next linear collider

Description: An X-band Damped Detuned Structure (DDS) for NLC has been fabricated as part of a collaboration between KEK and SLAC. The individual cells were diamond point machined and microwave tested at KEK. The cells were diffusion bonded at SLAC. The structure has been cold tested. The time dependence of the beam induced dipole wakefields have been measured with the SLC beam in the test station ASSET. The structure is designed so that the dipole modes have an approximately gaussian density distribution in the frequency domain. This gives an approximately gaussian decrease of the wakefields for short times (about 10 ns), which is produced by the interference among the 206 modes in the lowest dipole mode band of the 206 cell structure. Without damping, however, the wakefields then rise back to a level which is approximately equal to the expected incoherent level from the 206 modes. The damping is accomplished by means of 4 rectangular slots or manifolds (approximately 5 mm by 10 mm) equally spaced in azimuth around the structure and running the full length of the structure. These manifolds act as single mode rectangular waveguides for the lowest band dipole modes, but are cut off for the accelerating mode. The manifolds are coupled to every cell in the structure, except for 3 at each end, by means of radial slots. Each of the four manifolds will have the dipole mode frequencies traveling in both directions and so are terminated on both ends. The structure will be installed in the NLC Test Accelerator this fall.
Date: September 1, 1996
Creator: Miller, R.H.; Adolphsen, C. & Bane, K.L.
Partner: UNT Libraries Government Documents Department

The quasi-coherent oscillations of SS Cygni

Description: Properties of the quasi-coherent oscillations in the extreme ultraviolet flux of the dwarf nova SS Cygni are described. Rapid periodic oscillations are observed in the optical and soft X-ray flux of high accretion rate cataclysmic variables (CVs; nova-like variables and dwarf novae in outburst). These so-called `dwarf nova oscillations` (DNOS) have high coherence (Q {approx} 10{sup 4}-10{sup 6}), periods of {approx} 10-30 s, and amplitudes of @ 10-30% in soft X-rays and @ 0.5% in the optical. DNOs have never been detected in dwarf novae in quiescence, despite extensive searches; they appear on the rising branch of the dwarf nova, outburst, typically persist through maximum, and disappear on the declining branch of the outburst. The period of the oscillation also correlates with outburst state, decreasing on the rising branch and increasing on the declining branch. 2048 The dwarf nova SS Cygni routinely exhibits DNOs during outburst. Optical oscillations have been detected at various times with periods ranging from 7.3 s to 10.9 s. At soft X-ray energies, oscillations have been detected in {ital HEAO 1} LED 1 data at periods of @ 9 s and 11 s and in EXOSAT LE data at periods between 7.4 s and 10.4 s. Here, we describe the properties of the oscillations in the extreme ultraviolet (EUV) flux detected with the {ital Extreme Ultraviolet Explorer} (EUVE) Deep Survey (DS) photometer and Short Wavelength (SW) spectrometer during target-of-opportunity observations of SS Cyg in out- burst in 1993 August and 1994 June/July.
Date: August 1, 1996
Creator: Mauche, C.W.
Partner: UNT Libraries Government Documents Department

Dynamics of density fluctuations in a non-Markovian Boltzmann- Langevin model

Description: In the course of the past few years, the nuclear Boltzmann-Langevin (BL)model has emerged as a promising microscopic model for nuclear dynamics at intermediate energies. The BL model goes beyond the much employed Boltzmann-Uehling-Uhlenbeck (BUU) model, and hence it provides a basis for describing dynamics of density fluctuations and addressing processes exhibiting spontaneous symmetry breaking and catastrophic transformations in nuclear collisions, such as induced fission and multifragmentation. In these standard models, the collision term is treated in a Markovian approximation by assuming that two-body collisions are local in both space and time, in accordance with Boltzmann`s original treatment. This simplification is usually justified by the fact that the duration of a two-body collision is short on the time scale characteristic of the macroscopic evolution of the system. As a result, transport properties of the collective motion has then a classical character. However, when the system possesses fast collective modes with characteristic energies that are not small in comparision with the temperature, then the quantum-statistical effects are important and the standard Markovian treatment is inadequate. In this case, it is necessary to improve the one-body transport model by including the memory effect due to the finite duration of two-body collisions. First we briefly describe the non-Markovian extension of the BL model by including the finite memory time associated with two-body collisions. Then, using this non-Markovian model in a linear response framework, we investigate the effect of the memory time on the agitation of unstable modes in nuclear matter in the spinodal zone, and calculate the collisional relaxation rates of nuclear collective vibrations.
Date: March 1, 1996
Creator: Ayik, S.
Partner: UNT Libraries Government Documents Department

High gradient experiments on NLCTA accelerator structures

Description: This paper presents new results of high-gradient studies performed on a 1.8 m traveling-wave accelerator section with detuned high-order deflecting modes. This structure was designed initially for studies of detuned structures and will be installed in the Next Linear Collider Test Accelerator (NLCTA). The paper describes the test set-up in the Accelerator Structure Test Area (ASTA) including electron gun, prebuncher, pre-accelerator, spectrometer, Faraday cups, 200 MW SLED-II power compression system, Magic-T type phase shifters and attenuators. Rf processing, detailed dark current analysis, radiation problems, and beam acceleration measurements are discussed.
Date: August 1, 1996
Creator: Wang, J.W.; Eichner, J.P. & Fant, K.H.
Partner: UNT Libraries Government Documents Department

Rippled-beam free-electron laser

Description: The authors describe a new microwave generation mechanism involving a scalloping annular electron beam. The beam interacts with the axial electric field of a TM{sub 0n} mode in a smooth circular waveguide through the axial free-electron laser interaction, in which the beam ripple period is synchronous with the phase slippage of the rf mode relative to the electron beam. Due to nonlinearities in the orbit equation, the interaction can be made autoresonant, where the phase and amplitude of the gain is independent of the beam energy.
Date: October 1, 1997
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Seismological modeling of the Delta Scuti star: CD-24 7599

Description: A major goal of asteroseismology is a better understanding of stellar evolution via ``snapshots`` of many stars of different masses in different evolutionary states. For stars of about 2M{sub {circle_dot}} near the sequence, b Scuti stars are the usual suspects. There is an ongoing renaissance in theoretical modeling of 6 Scuti stars brought on by improvements in constitutive physics and by a dramatic increase in the number of modes observed. FG Virginis and CD-24` 7599 are two of the best studied objects, and they have 19 and 13 known frequencies, respectively. . We create models using an updated and modified version of the Iben code described by Guzik & Cox that includes either of the two versions of the OPAL opacities . We use the star`s observed location on the H-R diagram as a starting point for our seismological modeling. Because there is no evidence for observed t = 3 modes, we only consider l = 0, 1, and 2 modes in our analysis. We take into account rotational splitting (about 5 - 10 {mu}Hz) in our frequency matching. Several observed modes must be rotationally split members of a given mode. CD-24` 7599 is less than halfway through core hydrogen burning, and the modes appear to be a set of consecutive 3rd through 5th overtones of {ital l} = 0 through 2 modes. With only 13 modes, we find satisfactory fits with models between 1.9 and 2.0 M{sub {circle_dot}} that fall within the observed luminosity and effective temperature range. By contrast, Guzik & Bradley suggest that FG Virginis is over halfway through core hydrogen burning and the best fitting models lie near 1.80 or 2.00 M{sub {circle_dot}}. We see persistent discrepancies in some low frequency modes, which suggests we may need a small amount of core overshoot or a slight ...
Date: November 1, 1996
Creator: Bradley, P.A. & Guzik, J.A.
Partner: UNT Libraries Government Documents Department

Effective index model predicts modal frequencies of vertical-cavity lasers

Description: Previously, an effective index optical model was introduced for the analysis of lateral waveguiding effects in vertical-cavity surface-emitting lasers. The authors show that the resultant transverse equation is almost identical to the one typically obtained in the analysis of dielectric waveguide problems, such as a step-index optical fiber. The solution to the transverse equation yields the lateral dependence of the optical field and, as is recognized in this paper, the discrete frequencies of the microcavity modes. As an example, they apply this technique to the analysis of vertical-cavity lasers that contain thin-oxide apertures. The model intuitively explains the experimental data and makes quantitative predictions in good agreement with a highly accurate numerical model.
Date: April 18, 2000
Partner: UNT Libraries Government Documents Department

Modal response of interior mass based upon external measurements

Description: Modal response testing has been used to predict the motion of interior masses of a system in which only external instrumentation is allowed. Testing of this form may occasionally be necessary in validation of a computer model, but also has potential as a tool for validating individual assemblies in a QA process. Examination of the external frequency response and mode shapes can offer insight into interior response. The interpretation of these results is improved through parallel analytical solutions. A simple, three-mass model has been examined experimentally and analytically to demonstrate modal theory. These results show the limitations of the external measurement in predicting internal response due to transmissibility. A procedure for utilizing external testing is described. The question posed through this research is whether or not modal correlation analysis can be adapted for use in systems for which instrumentation of critical components is missing.
Date: October 12, 1999
Creator: Chow, C T; Eli, M; Jorgensen, B R & Woehrle, T
Partner: UNT Libraries Government Documents Department

Measurement and identification of HOM`s in RF cavities

Description: One of the major sources of beam impedance in accelerators is the higher-order modes (HOM`s) of the RF cavities. We report here on a number of techniques for the identification of HOM`s and measurement of their properties. Central to these techniques is the application of symmetry principles and the effects of symmetry-breaking perturbations (including mode-mixing) to the {open_quotes}standard{close_quotes} techniques of spectrum measurements and bead pulls.
Date: May 1, 1997
Creator: Goldberg, D.A. & Rimmer, R.A.
Partner: UNT Libraries Government Documents Department

Compensation of longitudinal coupled-bunch instability in the advanced photon source storage ring

Description: A longitudinal couple-bunch (CB) instability was encountered in the 7-GeV storage ring. This instability was found to depend on the bunch fill pattern as well as on the beam intensity. The beam spectrum exhibited a coupled-bunch signature, which could be reproduced by an analytical model. The oscillations were also observed on a horizontal photon monitor. The beam fluctuations exhibited two periodicities, which were found to be correlated with the rf cavity temperatures. This correlation is consistent with the measured temperature dependence of the higher-order mode (HOM) frequencies. The HOM impedance drives the beam when brought into resonance with the CB mode by the temperature variation. Increasing the inlet cavity water temperature suppressed the instability. The experimental results are compared to an analytical model which characterizes the fill-pattern dependence. Studies to identify the offending HOMs are also presented.
Date: August 1, 1997
Creator: Harkay, K.C.; Nassiri, A.; Song, J.J.; Kang, Y.W. & Kustom, R.L.
Partner: UNT Libraries Government Documents Department

Preliminary asteroseismology of G29-38

Description: G 29-38 is a cool pulsating DA white dwarf with a series of modes from 110 s to over 1000 s found by Kleinman et al. Kleinman et al. present suggestive evidence that most of the modes they see are {ell}=l modes, but they could not prove this assertion with their data. the large number of modes (10 to 20) present in G 29-38 gives us hope that we can constrain the mode structure in the manner that Bradley & Winget use for the DBV GD 358. Here, we try to match the observed data with periods predicted from models similar to those of Bradley. We present results of our attempts to find a seismological fit to G 29-38 using the data of Kleinman et al. and look for a model that identifies most of the observed modes as {ell}=l modes. G 29-38 has a discrepancy between the spectroscopically favored mass of about 0.70M{sub {circle_dot}} and the parallactic mass of about 0.80M{sub {circle_dot}}. We attempt to find models that are consistent with either mass determination and see if we can place constraints on the hydrogen layer mass with these assumptions. We also look for models that have hydrogen layer masses that have hydrogen layer masses near 10{sup -4}M{star}, consistent with the value Clemens suggests for the hotter DAV stars. We also look to see if other combinations of stellar mass and hydrogen layer mass are consistent with the observed data and explore their consequences.
Date: August 1, 1996
Creator: Bradley, P.A. & Kleinman, S.J.
Partner: UNT Libraries Government Documents Department

Crystalline beams

Description: A beam of confined charged particles, that are cooled to the extreme of the space-charge dominated regime, where the relative motion of particles within the beam is small compared to their Coulomb potential energies, will crystallize in a unique form of condensed matter. Such a system of particles can be simulated using the method of Molecular Dynamics, which explicitly includes the interaction between all pairs of particles and uses repeating cells to simulate the effects of a long beam. Within the molecular dynamics simulations typically a few thousand particles are followed in time, allowed to equilibrate, and then the velocities are gradually scaled down while still allowing the system to maintain equilibrium. To reach a cold equilibrium value requires 10-100 thousand iterations, corresponding to real times on the order of a few thousand betatron periods.
Date: June 1, 1995
Creator: Schiffer, J.P.
Partner: UNT Libraries Government Documents Department

Global Alfven eigenmodes in WELDELSTEIN 7-AS

Description: In the presence of fast particle populations marginally stable global modes in the shear Alfven branch can be destabilized by wave particle resonances. This is particularly of concern in future large devices, where losses of resonant particles ({alpha}-particles in a reactor) may then limit the available heating power and also may cause damage of the first wall. In tokamaks TAE modes inside toroidicity induced gaps of the shear Alfven continua have been found. In stellarators with very weak shear like W7-AS low-n TAE-gaps do not occur but gaps below the shear Alfven continua with mode numbers m and n, if the resonant values {tau} = n/m do not exist in the plasma volume (k{sub {parallel}} = (m{sm_bullet}{tau} - n )/R {ne} 0 ). Under these conditions GAE modes with frequencies {omega}{sub GAE} < (k{sub {parallel}}{sm_bullet}V{sub A}){sub min} are the favoured modes. The investigation of GAE modes could also be of relevance in the case of advanced tokamak equilibria with flat or inverted q-profiles in the central region.
Date: September 1, 1995
Creator: Weller, A.; Goerner, C. & Jaenicke, R.
Partner: UNT Libraries Government Documents Department