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On Sustaining Short, Intense Bunches in Linear and Circular Accelerators

Description: The ability of existing analytical and numerical tools to predict beam performance at the short bunch lengths and high peak currents characteristic of contemporary accelerator designs is discussed. Recent advances in calculating the high frequency behavior of impedance and in describing bunched-beam collective dynamics are highlighted. A critical review is presented of outstanding problems that must be addressed before a thorough description of short, intense bunches is obtained.
Date: May 1, 1991
Creator: Bisognano, Joseph
Partner: UNT Libraries Government Documents Department

D to A Line Tuning

Description: Although in the past the D to A line was well tuned, through the course of years of operations it has not kept up with changes in the Debuncher and Accumulator. The beam is now steered well off center in SEM's 806 and 807, and is possibly scraping in TQ5. In addition, there is some evidence that the beta functions are not matched, causing emmitance blowup of the beam. As the cycle time of the main ring has decreased these limitations have become a significant limitation to the stacking rate.
Date: May 7, 1991
Creator: Halling, Mike
Partner: UNT Libraries Government Documents Department

Modeling large heterogeneous RF structures

Description: Large heterogeneous structures are difficult to model on a numerical grid because of the limitations on computing resources, so that alternate approaches such as equivalent circuits and mode-matching have been developed to treat this problem. This paper will describe the three methods and will analyze a structure representative of the SLAC and JLC detuned structures to compare the efficacy of each approach.
Date: November 1, 1996
Creator: Li, Zenghai; Ko, Kwok; Srinivas, V. & Higo, Toshiyasu
Partner: UNT Libraries Government Documents Department

``Electron Lens`` to Compensate Bunch-to-Bunch Tune Spread in TEV33

Description: In this article we discuss an electron beam lens for compensation of bunch-to-bunch tune spread in the Tevatron antiproton beam. Time-modulated current of an electron beam can produce defocusing forces necessary to compensate effects caused by parasitic beam-beam interactions with proton beam. We estimate maim parameters of the electron beam and consider resulting beam footprint. Emittance growth rate due to the electron current fluctuations is discussed.
Date: October 1, 1997
Creator: Shiltsev, V.
Partner: UNT Libraries Government Documents Department

Simulations of the longitudinal instability in the new SLC damping rings

Description: In 1992 a longitudinal, single bunch instability was observed in the SLC damping rings. Beyond a threshold current of 3 x 10{sup 10} a {open_quotes}saw-tooth{close_quotes} variation in bunch length and energy spread was observed, a phenomenon that made it practically impossible to operate the SLC collider above threshold. For the 1994 run a new, low-impedance vacuum chamber was installed in both damping rings both to alleviate this problem and to shorten the bunch length. According to recent measurements the bunch length has indeed become shorter, but the {open_quotes}saw-tooth{close_quotes} instability is still seen, now beginning at currents of 1.5-2.0 x 10{sup 10}. Fortunately, it appears to be benign and does not seem to limit SLC performance. In an earlier paper the authors investigated the single bunch behavior of the SLC damping rings with the old vacuum chamber using time domain tracking and a Vlasov equation approach. When compared to measurements they found: good agreement in the average values of bunch length, energy spread, and synchronous phase shift as functions of current; a 30% discrepancy in threshold current; in agreement, a mode with frequency near 2.5 times the synchrotron frequency (the so-called {open_quotes}sextupole{close_quotes} mode) as signature of the instability and the slope of the mode frequency as function of current. In the present paper they repeat the exercise of the earlier paper but with a new wakefield. The impedance which used to be inductive has become resistive, leading to different phenomena. In a recent paper the instability in a purely resistive ring is analyzed using a Vlasov equation approach. It is demonstrated that such an instability is a weak instability, with a growth rate proportional to intensity squared, and one that can be described as the coupling of two quadrupole modes with different radial mode numbers. The authors compare their results with ...
Date: June 1, 1995
Creator: Bane, K.L.F. & Oide, K.
Partner: UNT Libraries Government Documents Department

Recent transition crossing results from the Fermilab Main Ring

Description: This paper outlines the known mechanisms for emittance dilution and beam loss during transition crossing. These include scraping, Johnsen`s nonlinear effect, bunch length mismatch, microwave and negative-mass instabilities, and beam loading. A recent experiment on transition crossing performed with 13 proton bunches injected into the Main Ring is discussed.
Date: March 1, 1997
Creator: Bhat, C.M. & Ng, K.Y.
Partner: UNT Libraries Government Documents Department

Subpicosecond compression experiments at Los Alamos National Laboratory

Description: The authors report on recent experiments using a magnetic chicane compressor at 8 MeV. Electron bunches at both low (0.1 nC) and high (1 nC) charges were compressed from 20 ps to less than 1 ps (FWHM). A transverse deflecting rf cavity was used to measure the bunch length at low charge; the bunch length at high charge was inferred from an induced energy spread of the beam. The longitudinal centrifugal-space charge force is calculated using a point-to-point numerical simulation and is shown not to influence the energy-spread measurement.
Date: September 1, 1995
Creator: Carlsten, B.E.; Russell, S.J. & Kinross-Wright, J.M.
Partner: UNT Libraries Government Documents Department

Observations of bunch lengthening effects in the APS 7-GeV storage ring

Description: Measurements of the bunch length and horizontal beam size at a dispersive point in the lattice versus single-bunch current have been done on the Advanced Photo Source (APS) 7-GeV storage ring. These data are relevant to issues (limits) of obtaining high peak currents for storage-ring-based FELs. Bunch lengths from {sigma}{sub {tau}} {approx} 25 to 70 ps were measured using a Hamamatsu C5680 dual-sweep streak camera. Additional complementary data on energy spread deduced from horizontal beam size at a dispersive point in the lattice were also tracked versus single-bunch current. Both optical synchrotron radiation (OSR) and x-ray synchrotron radiation (XSR) techniques were used. For the set of data taken at a synchrotron frequency of 1.2 kHz (indicating rf gap voltage {<=} 6 MV), the significant bunch lengthening without a comparable horizontal size change ({Delta}E growth) is consistent with the potential well distortion model rather than the predictions of a microwave instability calculation. With higher rf gap voltage, peak currents up to 400 A were observed. A bunch-lengthening effect in multibunch mode was also observed as the stored-beam current approached the available rf power limit.
Date: December 31, 1996
Creator: Lumpkin, A.H.; Yang, B.X. & Chae, Y.C.
Partner: UNT Libraries Government Documents Department

Intrabeam Scattering in the NLC Main Damping Rings

Description: We use Bane's approximation to the Bjorken-Mtingwa theory of intrabeam scattering to calculate the emittance growth as a function of bunch charge in the KEK ATF. We find that our results are consistent with the experimental data. We then calculate the emittance growth in the NLC Main Damping Rings using the same formulae; we allow for some uncertainty in the ATF data by using two different values for the Coulomb log factor in the formulae for the emittance growth rates. We find that despite the IBS emittance growth, it should still be possible to achieve the specified transverse and longitudinal emittances in the NLC Main Damping Rings at the specified bunch charge.
Date: June 8, 2006
Creator: Wolski, Andrzej
Partner: UNT Libraries Government Documents Department

Experiment and simulations of sub-ps electron bunch train generation at Fermilab photoinjectors

Description: Recently the generation of electron bunch trains with sub-picosecond time structure has been experimentally demonstrated at the A0 photoinjector of Fermilab using a transverse-longitudinal phase-space exchange beamline. The temporal profile of the bunch train can be easily tuned to meet the requirements of the applications of modern accelerator beams. In this paper we report the A0 bunch-train experiment and explore numerically the possible extension of this technique to shorter time scales at the Fermilab SRF Accelerator Test Facility, a superconducting linear electron accelerator currently under construction in the NML building.
Date: October 1, 2011
Creator: Sun, Y.-E; Church, M.; /Fermilab; Piot, P.; Prokop, C.R. & U., /Fermilab /Northern Illinois
Partner: UNT Libraries Government Documents Department

LHC Abort Gap Cleaning Studies During Luminosity Operation

Description: The presence of significant intensities of un-bunched beam is a potentially serious issue in the LHC. Procedures using damper kickers for cleaning both the Abort Gap (AG) and the buckets targeted for injection, are currently in operation at flat bottom. Recent observations of relatively high population of the AG during physics runs brought up the need for AG cleaning during luminosity operation. In this paper the results of experimental studies performed in October 2011 are presented.
Date: May 11, 2012
Creator: Gianfelice-Wendt, E.; /Fermilab; Bartmann, W.; Boccardi, A.; Bracco, C.; Bravin, E. et al.
Partner: UNT Libraries Government Documents Department

Multipacting Simulation Study for 56 MHz Quarter Wave Resonator using 2D Code

Description: A beam excited 56 MHz Radio Frequency (RF) Niobium Quarter Wave Resonator (QWR) has been proposed to enhance RHIC beam luminosity and bunching. Being a RF cavity, multipacting is expected; therefore an extensive study was carried out with the Multipac 2.1 2D simulation code. The study revealed that multipacting occurs in various bands up to peak surface electric field 50 kV/m and is concentrated mostly above the beam gap and on the outer conductor. To suppress multipacting, a ripple structure was introduced to the outer conductor and the phenomenon was successfully eliminated from the cavity.
Date: January 2, 2009
Creator: Naik,D. & Ben-Zvi, I.
Partner: UNT Libraries Government Documents Department


Description: With the experimental success of longitudinal, bunched beam stochastic cooling in RHIC it is natural to ask whether the system works as well as it might and whether upgrades or new systems are warranted. A computer code, very similar to those used for multi-particle coherent instability simulations, has been written and is being used to address these questions.
Date: September 10, 2007
Partner: UNT Libraries Government Documents Department

Measurement of Transverse Beam Size in Accumulator as a Function of Momenta During Stacking

Description: The horizontal and vertical emittance as a function of momentum was measured in the accumulator while stacking with a small stack. The data suggests that the transverse emittance of the beam is blown up between the injection orbit and the stacking orbit. The technique shows promise, and should be repeated with a large stack to measure the emittance in the accumulator at a time when we have large losses.
Date: January 22, 1992
Creator: Halling, Mike
Partner: UNT Libraries Government Documents Department


Description: The SPS at high intensities exhibits transverse single-bunch instabilities with signatures consistent with an Ecloud driven instability. They present recent MD data from the SPS, details of the instrument technique and spectral analysis methods which help reveal complex vertical motion that develops within a subset of the injected bunch trains. The beam motion is detected via wideband exponential taper striplines and delta-sigma hybrids. The raw sum and difference data is sampled at 50 GHz with 1.8 GHz bandwidth. sliding window FFT techniques and RMS motion techniques show the development of large vertical tune shifts on portions of the bunch of nearly 0.025 from the base tune of 0.185. Results are presented via spectrograms and bunch slice trajectories to illustrate development of the unstable beam and time scale of development along the injected bunch train. The study shows that the growing unstable motion occupies a very broad frequency band of 1.2 GHz. These measurements are compared to numerical simulation results, and the system parameter implications for an Ecloud feedback system are outlined.
Date: May 3, 2010
Creator: Fox, J.D.; Bullitt, A.; Mastorides, T.; Ndabashimiye, G.; Rivetta, C.; Turgut, O. et al.
Partner: UNT Libraries Government Documents Department

Bunching for shorter damping rings for the ILC

Description: A variant rearrangement of the bunch trains for the ILC that enables much shorter damping rings is presented. In a particular example the {approx}2820 bunches are regrouped into {approx}550 subtrains of five adjacent bunches. These subtrains are extracted from the damping rings at {approx}1.8 {micro}s intervals, obtaining the 1ms macrobunch length of the baseline TESLA collider scenario. If the baseline damping rf frequency is 325 MHz and the kicker rise and fall times are {approx}20 ns, a ring circumference of {approx}5.8km is required. Variations of the scheme could easily reduce the circumference to {approx}4km, and faster kickers could reduce it even further.
Date: May 1, 2005
Creator: Neuffer, D.
Partner: UNT Libraries Government Documents Department