Old Dominion University, Norfolk, Virginia
LBNL, Berkeley, California
ANL, Argonne
Muons, Inc, Batavia
Fermilab, Batavia
Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and FRA DOE contract number DE-AC02-07CH11359
Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual vacuum and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of magnetic field, frequency, and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) and pressure barrier capable of operating both at high pressure and in vacuum been designed and built, and preliminary testing has been completed. A series of detailed experiments is planned at the Argonne Wakefield Accelerator.
LBNL, Berkeley, California
CLASSE, Ithaca, New York
Funding: Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
We report on the optimization of TE Wave measurements at the Cesr-TA ring at Cornell University. The CESR storage ring is currently used as a testbed for technologies to be used in the damping rings of the International Linear Collider. The TE Wave measurement method utilizes capacitive buttons (BPMs) in the ring to excite and detect a propagating electromagnetic wave corresponding to the beampipe's fundamental TE mode. The presence of low-energy electrons along the wave path changes its propagation characteristics, which can be detected by analyzing the received signal. By choosing the machine fill pattern (gaps and bunch trains length) it is possible to modulate the density of the electron cloud and derive information on its rise and fall times by observing the detected signal spectrum. The possibility of circulating both electron and positron beams in the ring enabled us to separate the contribution of primary photoelectrons, which are independent on the circulating particle nature, from the transverse resonant mechanism, which can increase the primary electron density many times over and which only takes place with a circulating positron beam.
LBNL, Berkeley, California
Y.Y. Labs, Inc., Fremont, California
Funding: Work supported by the U.S. Department of Energy under Contract No.
By coupling the emitted synchrotron light into an optical fiber, it is possible to transmit the signal at substantial distances from the light port, without the need to use expensive beamlines. This would be especially beneficial in all those cases when the synchrotron is situated in areas not easily access because of their location, or due to high radiation levels. Furthermore, the fiber output can be easily switched, or even shared, between different diagnostic instruments. We present the latest results on the coupling and dispersion measurements performed at the Advanced Light Source in Berkeley.
LBNL, Berkeley, California
CERN, Geneva
BNL, Upton, Long Island, New York
The Luminosity Monitor for the LHC has been built at LBL and is going to be installed in the LHC in early 2009. The device designed for the high luminosity regions (ATLAS and CMS) is a gas ionization chamber, that is designed with the ability to resolve bunch by bunch luminosity as well as survive extreme levels of radiation. During the experimental R&D phase of its design, the prototype of this detector has been tested extensively in RHIC as well as in the SPS. Result of these experiments are shown here, with comments on the implications for early operations of the LHC.
Dimtel, San Jose
LBNL, Berkeley, California
IHEP Beijing, Beijing
BEPC-II is a two ring electron-positron collider designed to operate at 1 A beam currents. Longitudinal and transverse coupled-bunch instabilities have been observed in both electron and positron rings. In this paper we present measurements of both transverse and longitudinal instabilities with the identification of active eigenmodes, measurements of growth and damping rates, as well as of the residual beam motion levels. The measurements will then be used to estimate the growth rates at the design beam currents (yet to be achieved). We will also demonstrate how such data is used for specifying power amplifier and kicker parameters.
SLAC, Menlo Park, California
LBNL, Berkeley, California
BNL, Upton, Long Island, New York
CERN, Geneva
Funding: Work supported by Department of Energy contract DE–AC03–76SF00515 and the US LARP program.
The SPS at high intensities exhibits transverse single-bunch instabilities with signatures consistent with an Ecloud driven instability. While the SPS has a coupled-bunch transverse feedback system, control of Ecloud-driven motion requires a much wider control bandwidth capable of sensing and controlling motion within each bunched beam. This paper draws beam dynamics data from the measurements and simulations of this SPS instability, and develops initial performance requirements for a feedback system with 2-4 GS/sec sampling rates to damp Ecloud-driven transverse motion in the SPS at intensities desired for high-current LHC operation. Requirements for pickups, kickers and signal processing architectures are presented. Initial lab measurements of proof-of-principle lab model prototypes are presented for the wideband kicker driver signal functions.
CLASSE, Ithaca, New York
LBNL, Berkeley, California
KEK, Ibaraki
ANL, Argonne
Fermilab, Batavia
CalPoly, San Luis Obispo, CA
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
SLAC, Menlo Park, California
Cornell University, Ithaca, New York
Cockcroft Institute, Warrington, Cheshire
Funding: Support provided by the US National Science Foundation, the US Department of Energy, and the Japan/US Cooperation Program.
In March of 2008, the Cornell Electron Storage Ring (CESR) concluded twenty eight years of colliding beam operations for the CLEO high energy physics experiment. We have reconfigured CESR as an ultra low emittance damping ring for use as a test accelerator (CesrTA) for International Linear Collider (ILC) damping ring R&D. The primary goals of the CesrTA program are to achieve a beam emittance approaching that of the ILC Damping Rings with a positron beam, to investigate the interaction of the electron cloud with both low emittance positron and electron beams, to explore methods to suppress the electron cloud, and to develop suitable advanced instrumentation required for these experimental studies (in particular a fast x-ray beam size monitor capable of single pass measurements of individual bunches). We report on progress with the CESR conversion activities, the status and schedule for the experimental program, and the first experimental results that have been obtained.
Tech-X, Boulder, Colorado
LBNL, Berkeley, California
FZK, Karlsruhe
Funding: This work funded by the Department of Energy under Small Business Innovation Research Contract No. DE-FG02-08ER85042.
Microwave transmission in accelerator beam pipes is providing a unique method for determining electron cloud characteristics, such as density, plasma temperature, and potentially the efficacy of electron cloud mitigation techniques. Physically-based numerical modeling is currently providing a way to interpret the experimental data, and understand the plasma-induced effects on rf signals. We report here recent applications of numerical simulation of microwave transmission in the presence of electron clouds. We examine the differences in phase shift induced by TE11 and TM01 modes in circular cross section beam pipes for uniform density electron clouds. We also detail numerical simulation of the cyclotron resonance and examine how the width of the resonance changes with applied dipole magnetic fields strength and cloud temperature.
LBNL, Berkeley, California
Funding: Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Uneven beam fill patterns in storage rings, such as gaps in the fill patterns, leads to periodic, or transient loading of the modes of the RF cavities. We show that an analogous effect can occur in the loading of a dipole cavity mode when the beam passes off the electrical center of the cavity mode. Although this effect is small, it results in a variation of the transverse offset of the beam along the bunch train. For ultralow emittance beams, such as optimized third generation light sources and damping rings, this effect results in a larger projected emittance of the beam compared with the single bunch emittance. The effect is particularly strong for the case when a strong dipole mode has been purposely added to the ring, such as a deflecting, or ‘‘crab'' cavity. We derive an approximate analytic solution for the variation of the beam-induced deflecting voltage along the bunch train. We also show via a tracking simulation the combined effect of the periodic loading of the fundamental and dipole modes.
LBNL, Berkeley, California
CERN, Geneva
Funding: Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Single and multi-bunch instabilities on bunch trains driven by electron clouds have been observed in the CERN SPS for some years. In this paper, we present initial results to implement a damping system in a computer simulation of a single bunch vertical instability using the HEADTAIL code. The code simulates the interaction between a proton bunch and a uniform electron cloud that has built up inside of the beam pipe. In all simulations we use typical SPS parameter sets for three different values of the beam momentum : 26 GeV/c, 55 GeV/c and 120 GeV/c. The feedback is implemented as a corrective kick calculated from the vertical centroid of each slice of the electron bunch with a one turn delay. The bandwidth of the feedback is varied by filtering the slice information along the bunch. Initial results indicate that the instability can be damped with a minimum bandwidth of 300 MHz with a relatively high gain.