Measurements of the propagation of EM waves through the vacuum chamber of the PEP-II Low Energy Ring for beam diagnostics

Abstract

We present the results of our measurements of the electron cloud density in the PEP-II low energy ring (LER) by propagating a TE wave into the beam pipe. By connecting a signal generator to a beam position monitor button we can excite a signal above the vacuum chamber cut-off frequency and measure its propagation through the beam pipe with a spectrum analyzer connected to another button about 50 meters away. The measurement can be performed with different beam conditions and also at different settings of the solenoids used to reduce the build up of electrons. The presence of a modulation in the TE wave transmission, synchronous with the beam revolution frequency, which appear to increase in depth when the solenoids are switched off, seem to be directly correlated to the electron cloud density in the region between the two BPM's. In this paper we present and discuss the measurements taken in the Interaction Region 12 straight of the LER during 2006 and the first part of 2007.

Final Report for "Modeling Electron Cloud Diagnostics for High-Intensity Proton Accelerators"

“…Similarly the TM01 mode has a cutoff frequency of 2.578 GHz, and we drive the current source at 2.836 GHz in the x-direction (propagation direction) to excite only that mode. Samples of the wave amplitudes for these modes are shown in figures (2) and (3). Figure (2) shows the components of electric and magnetic fields for TE11 RF signals.…”

“…Electron cloud effects are considered to be one of the most important factors that will limit machine performance for high-intensity accelerators, and electron cloud mitigation methods can have a large influence on the cost and design of such accelerators. Recently, researchers have developed a new diagnostic technique that uses microwaves to measure electron cloud densities [1,2,3], and there are a number of experimental programs both in the United States (FermiLab, SLAC, LBL, Cornell) and abroad (CERN) that are using injected microwaves to measure both electron cloud densities and the effects of cloud mitigation techniques, such as coatings to reduce production of secondary electrons.…”