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Studies of emittance growth in the ATF

Description: Several different mechanisms of emittance growth in the Accelerator Test Facility (ATF) at KEK are investigated: the author calculates rise times of the fast beam-ion instability for the damping ring (DR), and discusses the emittance growth caused by coherent synchrotron radiation in the beam-transport line (BT), the effect of quadrupole wake fields in the injector linac, and, finally, a single-bunch head-tail ion effect that can occur in both the DR and the BT. A first attempt to measure the quadrupole wake on the real machine is also reported.
Date: March 1, 1997
Creator: Zimmermann, F.
Partner: UNT Libraries Government Documents Department

Coherent instabilities at the FNAL Booster

Description: This paper presents results of experimental and theoretical investigations of transverse beam stability at injection to Fermilab Booster and discusses a novel scheme for transition crossing allowing to avoid the longitudinal emittance growth related to the transition. At reduced chromaticity a multibunch high order head-tail mode develops with growth time of 12 turns at fractional part of tune close to zero. An estimate of the growth rate based on known sources of impedance results in significantly smaller value and cannot explain observed instability growth rate.
Date: June 1, 2006
Creator: Lebedev, V.; Burov, A.; Pellico, W.; Yang, X. & /Fermilab
Partner: UNT Libraries Government Documents Department

Formation of Plasmoid Chains in Magnetic Reconnection

Description: A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers (104 ≤ S ≤ 108) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfvenically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as S3/8 and the instability growth rate in the linear stage as S1/4, in agreement with the theory by Loureiro et al. [Phys. Plasmas 14, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasistationary picture for S>104.
Date: September 9, 2009
Creator: R. Samtaney, N.F. Loureiro, D. A. Uzdensky, A.A. Schekochihin, and S.C. Cowley
Partner: UNT Libraries Government Documents Department

Strong stabilization of the Rayleigh-Taylor instability by material strength at Mbar pressures

Description: Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) instability growth rate due to high pressure effective lattice viscosity are presented. Using a laser created ramped drive, vanadium samples are compressed and accelerated quasi-isentropically at {approx}1 Mbar pressures, while maintaining the sample in the solid-state. Comparisons with simulations and theory indicate that the high pressure, high strain rate conditions trigger a phonon drag mechanism, resulting in the observed high effective lattice viscosity and strong stabilization of the RT instability.
Date: November 19, 2009
Creator: Park, H S; Lorenz, K T; Cavallo, R M; Pollaine, S M; Prisbrey, S T; Rudd, R E et al.
Partner: UNT Libraries Government Documents Department

EXPERIMENTAL TESTS OF VANADIUM STRENGTH MODELS AT HIGH PRESSURES AND STRAIN RATES

Description: Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) instability growth rate due to high pressure material strength or effective lattice viscosity in metal foils are presented. On the Omega Laser in the Laboratory for Laser Energetics, University of Rochester, target samples of polycrystalline vanadium are compressed and accelerated quasi-isentropically at {approx}1 Mbar pressures, while maintaining the samples in the solid-state. Comparison of the results with constitutive models for solid state strength under these conditions show that the measured RT growth is substantially lower than predictions using existing models that work well at low pressures and long time scales. High pressure, high strain rate data can be explained by the enhanced strength due to a phonon drag mechanism, creating a high effective lattice viscosity.
Date: March 2, 2010
Creator: Park, H; Barton, N R; Becker, R C; Bernier, J V; Cavallo, R M; Lorenz, K T et al.
Partner: UNT Libraries Government Documents Department

Relationship of self-focusing to spatial instability modes

Description: The spatial distribution of focal spots formed when laser beams self- focus in materials is shown to originate from the growth of certain instability modes. These modes are determined by a simple mathematical relationship derived from instability theory, which has been verified experimentally. Because of these instabilities, the threshold power for self-focusing is inversely proportional to the self-focusing length for high-power laser beams. (auth)
Date: November 1, 1973
Creator: Campillo, A.J.; Shapiro, S.L. & Suydam, B.R.
Partner: UNT Libraries Government Documents Department

Transverse modes and instabilities of a bunched beam with space charge and resistive wall impedance

Description: Transverse instability of a bunch in a ring accelerator is considered with space charge and wakefield taken into account. It is assumed that space charge tune shift significantly exceeds the synchrotron tune. Bunch spectrum, instability growth rate, and effects of chromaticity are studied with different bunch and wake forms. Fast instability caused by coupling of transverse modes is studied in detail. It is shown that, for monotonic wakes, the transverse mode coupling instability is possible only with a certain sign of the wake. Its threshold and growth rate are calculated precisely over a wide range of parameters.
Date: November 1, 2011
Creator: Balbekov, V.
Partner: UNT Libraries Government Documents Department

Studies of fluctuation processes in nuclear collisions. Progress report, March 1, 1995--March 31, 1996

Description: This progress report studies of fluctuation processes in nuclear collisions discusses the following topics: quantal effects on growth of instabilities in nuclear matter; collisional damping of giant resonances in a non-Markovian approach; and medium-modified interaction induced by fluctuations.
Date: March 1, 1996
Creator: Ayik, S.
Partner: UNT Libraries Government Documents Department

Transverse multibunch instabilities for non-rigid bunches

Description: In this paper, the authors present a method for computing growth rates and frequency shifts of a beam containing multiple non-rigid bunches. With this approach, they calculate non-rigid multibunch effects which can impact phenomena which are traditionally treated as single-bunch effects, such as the transverse mode-coupling instability. This approach is important for high current storage rings such as PEP-II at SLAC (the B-Factory) which have very strong interbunch forces. Typical calculations treat multibunch and single bunch effects separately, and thus eliminate important interactions between the two. To illustrate the technique, the authors calculate growth rates and frequency shifts using PEP-II as an example.
Date: May 1, 1995
Creator: Berg, J.S. & Ruth, R.D.
Partner: UNT Libraries Government Documents Department

Nonlinear theory of kinetic instabilities near threshold

Description: A new nonlinear equation has been derived and solved for the evolution of an unstable collective mode in a kinetic system close to the threshold of linear instability. The resonant particle response produces the dominant nonlinearity, which can be calculated iteratively in the near-threshold regime as long as the mode doe snot trap resonant particles. With sources and classical relaxation processes included, the theory describes both soft nonlinear regimes, where the mode saturation level is proportional to an increment above threshold, and explosive nonlinear regimes, where the mode grows to a level that is independent of the closeness to threshold. The explosive solutions exhibit mode frequency shifting. For modes that exist in the absence of energetic particles, the frequency shift is both upward and downward. For modes that require energetic particles for their existence, there is a preferred direction of the frequency shift. The frequency shift continues even after the mode traps resonant particles.
Date: May 1, 1997
Creator: Berk, H.L.; Pekker, M.S. & Breizman, B.N.
Partner: UNT Libraries Government Documents Department

Plasma Vertical Control with Internal and External Coils in Nest Step Tokamaks

Description: Vertical stability and control are examined for a tokamak configuration intended to be a generic representation of next step devices. Vertical stability calculations show that a critical resistive wall location can be determined for realistic structures, and that the introduction of small amounts of low resistivity material to an all steel structure can significantly reduce the vertical instability growth rate. Vertical control simulations show that internal control coils require significantly less feedback power than external coils, and that low resistivity materials can allow very low feedback powers or coils to be located externally with reasonable feedback powers.
Date: November 3, 2000
Creator: Kessel, C.E.; Heitzenroeder, P. & Jun, C.
Partner: UNT Libraries Government Documents Department

Fast transverse instability in the NSNS Accumulator Ring

Description: This paper reports on the results of investigation of possible fast transverse instabilities in the NSNS Accumulator Ring. The instability may be caused by the presence of stripline devices like kicker magnets, the active damper system, and by the RF cavities, and the sharp steps of the vacuum pipe. The instability can be overcome by adopting aluminum as the material of the vacuum pipe.Still the growth time of the instability remains short especially for the mode in proximity of the betatron tune.
Date: August 1997
Creator: Ruggiero, A. G. & Blaskiewicz, M.
Partner: UNT Libraries Government Documents Department

Negative energy modes and gravitational instability of interpenetrating fluids

Description: The authors study the longitudinal instabilities of two interpenetrating fluids interacting only through gravity. When one of the constituents is of relatively low density, it is possible to have a band of unstable wave numbers well separated from those involved in the usual Jeans instability. If the initial streaming is large enough, and there is no linear instability, the indefinite sign of the free energy has the possible consequence of explosive interactions between positive and negative energy modes in the nonlinear regime. The effect of dissipation on the negative energy modes is also examined.
Date: September 1, 1998
Creator: Casti, A.R.R.; Spiegel, E.A. & Morrison, P.J.
Partner: UNT Libraries Government Documents Department

Particle motion inside and near a linear half-integer stopband

Description: This paper studies the motion of a particle whose tune is inside and near a linear half-integer stopband. Results are found for the tune and beta functions in the stable region close to an edge of the stopband. Results are found for the growth rates and for the exponentially growing particle motion inside the stopband. It is shown that the eigenvalues and the eigenfunctions of the transfer matrix are real inside the stopband. All the results found are also valid for small accelerators where the large accelerator approximation is not used.
Date: July 1, 1995
Creator: Parzen, G.
Partner: UNT Libraries Government Documents Department

Active Damping of the E-P Instability at the Los Alamos Proton Storage Ring

Description: A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). The feedback system configuration, setup procedures, and optimization of performance are described. Results of several experimental tests of system performance are presented including observations of instability threshold improvement and grow-damp experiments, which yield estimates of instability growth and damping rates. A major effort was undertaken to identify and study several factors limiting system performance. Evidence obtained from these tests suggests that performance of the prototype was limited by higher instability growth rates arising from beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizontal plane, which had no feedback.
Date: March 17, 2008
Creator: Macek, R.J.; Alamos, /Los; Assadi, S.; Ridge, /Oak; Byrd, J.M.; /LBL, Berkeley et al.
Partner: UNT Libraries Government Documents Department

Incoherent and coherent tune shifts

Description: The longitudinal and transverse microwave instabilities are the two important limiting factors for the performance of an accelerator. Comparing with the fairly unified approach for the longitudinal microwave instabilities, different approaches have been used to define the transverse microwave instabilities. One reason of this is related to the role played by the space charge incoherent and coherent tune shifts. In this article, the transverse microwave instabilities will be discussed by defining separately the roles of the space charge incoherent and coherent tunes, which are represented by the space charge transverse impedances. Preliminary results for the AGS as proton driver are presented by using this approach.
Date: December 31, 1995
Creator: Zhang, S.Y.
Partner: UNT Libraries Government Documents Department

Magnetohydrodynamic calculations with a nonmonotonic q profile and equilibrium, sheared toroidal flow

Description: The linear and nonlinear stability of a nonmonotonic q profile is examined using a reduced set of magnetohydrodynamic (MHD) equations with an equilibrium, sheared toroidal flow. The reversed shear profile is shown to be unstable to a rich variety of resistive MHD modes including pressure-driven instabilities and tearing instabilities possessing a tearing/interchange character at low Lundquist number, S, and taking on a double/triple tearing structure at high S. Linear calculations show that the destabilizing effect of toroidal velocity shear on tearing modes is enhanced at finite pressure seen previously for tearing modes at high S. Nonlinear calculations show the generation of a large, m = 1, n = 0, Reynolds-stress-driven poloidal flow in the absence of significant flow damping. Calculations in which the poloidal flow was heavily damped show that sub-Alfvenic, sheared toroidal flows have a minimal effect on weakly-coupled, localized instabilities.
Date: July 1, 1998
Creator: Held, E.D.; Leboeuf, J.N. & Carreras, B.A.
Partner: UNT Libraries Government Documents Department

Observation and control of resistive wall modes

Description: Two approaches to achieving long-time scale stabilization of the ideal kink mode with a real, finite conductivity wall are considered: plasma rotation and active feedback control, DIII-D experiments have demonstrated stabilization of the resistive wall mode (RWM) by sustaining beta greater than the no-wall limit for up to 200 ms, much longer than the wall penetration time of a few ms. These plasmas are typically terminated by an m = 3, n = 1 mode as the plasma rotation slows below a few kHz. Recent temperature profile data shows an ideal MHD mode structure, as expected for the resistive wall mode at beta above the no-wall limit. The critical rotation rate for stabilization is in qualitative agreement with recent theories for dissipative stabilization in the absence of magnetic islands. However, drag by small-amplitude RWMs or damping of stable RWMs may contribute to an observed slowing of rotation at high beta, rendering rotational stabilization more difficult. An initial open-loop active control experiment, using non-axisymmetric external coils and a new array of saddle loop detectors, has yielded encouraging results, delaying the onset of the RWM.
Date: December 1998
Creator: Strait, E. J.; Garofalo, A. M. & Austin, M. E.
Partner: UNT Libraries Government Documents Department

Investigation of the effect of resistive MHD modes on spherical torus performance in CDX-U

Description: Resistive MHD modes and associated effects on spherical torus performance are investigated in the CDX-U device for Ip {le} 100 kA. Presently, the growth of resistive MHD modes (n=1/m=3 or n=1/m=2) as the edge q[q(a)] is lowered toward 3.5 appears to limit the maximum current achievable in CDX-U. For low q(a) discharges, a prominent rotating hot spot can be seen with the soft x-ray array, indicative of a magnetic island associated with a n=1/m=1 mode. The edge mode, which is n=1/m=3 or n=1/m=2, can be seen by the soft x-ray and edge magnetic pick up coil array. The growth of those modes in space and amplitude eventually leads to an Internal Reconnection Event (IRE). Prior to the IRE, strong mode-mixing takes place suggesting magnetic island overlap. The IRE causes a rapid heat loss from the core causing a strong plasma elongation and current spike due to the plasma inductance drop. With an appropriate discharge control, a MHD quiescent high confinement regime with over twice the central electron temperature relative to the MHD active regime has been found. To assess the halo-induced effects during the MHD events, a pair of segmented Rogowski coils were installed on the center stack. The observed halo-induced current fraction is generally small (less than 5% of the total plasma current) even for the case of forced disruption.
Date: January 1, 1997
Creator: Ono, M.; Stutman, D. & Hwang, Y.S.
Partner: UNT Libraries Government Documents Department

Nonlinear m = 1 mode and fast reconnection in collisional plasmas

Description: Time evolution of the m = 1 resistive kink mode is shown to be compromised of two exponential growth phases separated by a transition period during which the growth becomes temporarily algebraic. A modified Sweet-Parker model that takes into account some of the changes in the geometry of the core plasma and the growing island is offered to explain the departure from the algebraic growth of the early nonlinear phase.
Date: February 1, 1997
Creator: Aydemir, A.Y.
Partner: UNT Libraries Government Documents Department

Stabilization of external kink modes in magnetic fusion experiments using a thin conducting shell

Description: In nearly all magnetic fusion devices the plasma is surrounded by a conducting shell of some description. In most cases this is the vacuum vessel. What effect does a conducting shell have on the stability of external kink modes? Is there any major difference between the effect of a perfectly conducting shell and a shell of finite conductivity? What happens if the shell is incomplete? These, and other, questions are explored in detail in this lecture using simple resistive magnetohydrodynamic (resistive MHD) arguments. Although the lecture concentrates on one particular type of magnetic fusion device, namely, the tokamak, the analysis is fairly general and could also be used to examine the effect of conducting shells on other types of device (e.g. Reversed Field Pinches, Stellerators, etc.).
Date: November 1, 1995
Creator: Fitzpatrick, R.
Partner: UNT Libraries Government Documents Department