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Stabilizing effect of a double-harmonic RF system in the CERN PS

Description: Motivated by the discussions on scenarios for LHC upgrades, beam studies on the stability of flat bunches in a double-harmonic RF system have been conducted in the CERN Proton Synchrotron (PS). Injecting nearly nominal LHC beam intensity per cycle, 18 bunches are accelerated on harmonic h = 21 to 26GeV with the 10MHz RF system. On the flat-top, all bunches are then transformed to flat bunches by adiabatically adding RF voltage at h = 42 from a 20 MHz cavity in anti-phase to the h = 21 system. The voltage ratio V (h42)/V (h21) of about 0.5 was set according to simulations. For the next 140 ms, longitudinal profiles show stable bunches in the double-harmonic RF bucket until extraction. Without the second harmonic component, coupled-bunch oscillations are observed. The flatness of the bunches along the batch is analyzed as a measure of the relative phase error between the RF systems due to beam loading. The results of beam dynamics simulations and their comparison with the measured data are presented.
Date: April 1, 2009
Creator: Bhat, C.; /Fermilab; Caspers, F.; Damerau, H.; Hancock, S.; Mahner, E. et al.
Partner: UNT Libraries Government Documents Department

Ringing in the pulse response of long and wideband coaxial transmission lines due to group delay dispersion

Description: In particle accelerators coaxial cables are commonly used to transmit wideband beam signals covering many decades of frequencies over long distances. Those transmission lines often have a corrugated outer and/or inner conductor. This particular construction exhibits a significant amount of frequency dependent group delay variation. A comparison of simulations based on theoretical models, numerical simulations and S{sub 21} network analyzer measurements up to 2.5 GHz is presented. It is shown how the non-linear phase response and varying group delay leads to ringing in the pulse response and subsequent distortion of signal s transmitted through such coaxial transmission lines.
Date: May 4, 2009
Creator: Kotzian,G.; de Maria,R.; Caspers, F.; Federmann, S. & Hofle, W.
Partner: UNT Libraries Government Documents Department

Measurements of the Electron Cloud Density in the PEP-II Low Energy Ring

Description: Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation of these machines at high currents. Because of the size of these accelerators, it is difficult to probe the low energy electron clouds over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave that is independently excited and transmitted over a section of the accelerator. We infer the absolute phase shift with relatively high accuracy from the phase modulation of the transmission due to the modulation of the electron cloud density from a gap in the positively charged beam. We have used this technique for the first time to measure the average electron cloud density over a 50 m straight section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center. We have also measured the variation of the density by using low field solenoid magnets to control the electrons.
Date: April 10, 2012
Creator: Byrd, J.; De Santis, S.; Sonnad, K.; /LBL, Berkeley; Caspers, F.; Kroyer, T. et al.
Partner: UNT Libraries Government Documents Department

Microwave Transmission Measurements of the Electron Cloud Density In The Positron Ring of PEP-II

Description: Clouds of electrons in the vacuum chambers of accelerators of positively charged particle beams present a serious limitation for operation of these machines at high currents. Because of the size of these accelerators, it is difficult to probe the low energy electron clouds over substantial lengths of the beam pipe. We applied a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave which is independently excited and transmitted over a straight section of the accelerator. The modulation in the wave transmission which appear to increase in depth when the clearing solenoids are switched off, seem to be directly correlated to the electron cloud density in the section. Furthermore, we expect a larger phase shift of a wave transmitted through magnetic dipole field regions if the transmitted wave couples with the gyration motion of the electrons. We have used this technique to measure the average electron cloud density (ECD) specifically for the first time in magnetic field regions of a new 4-dipole chicane in the positron ring of the PEP-II collider at SLAC. In this paper we present and discuss the measurements taken in the Low Energy Ring (LER) between 2006 and 2008.
Date: July 3, 2008
Creator: Pivi, M.T.F.; Krasnykh, A.K; /SLAC; Byrd, J.; Santis, S.De; Sonnad, K.G. et al.
Partner: UNT Libraries Government Documents Department

Measurement of Electron Clouds in Large Accelerators by Microwave Dispersion

Description: Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation at high currents. Furthermore, it is difficult to probe their density over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave transmitted over a section of the accelerator and used it to measure the average electron cloud density over a 50 m section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center.
Date: March 19, 2008
Creator: De Santis, S.; Byrd, J.M.; /LBL, Berkeley; Caspers, F.; /CERN; Krasnykh, A. et al.
Partner: UNT Libraries Government Documents Department

Status of LHC crab activity simulations and beam studies

Description: The LHC crab cavity program is advancing rapidly towards a first prototype which is anticipated to be tested during the early stages of the LHC phase I upgrade and commissioning. The general project status and some aspects related to crab optics, collimation, aperture constraints, impedances, noise effects. beam transparency and machine protection critical for a safe and robust operation of LHC beams with crab cavities are addressed here.
Date: May 4, 2009
Creator: Calaga,R.; Assman, R.; Barranco, J.; Barranco, J.; Calaga, R.; Caspers, F. et al.
Partner: UNT Libraries Government Documents Department