Measurement of Electron Clouds in Large Accelerators by Microwave Dispersion

Abstract

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. Low energy background electrons in the beam pipes of high energy accelerators of positively charged beams present a serious challenge to increasing current in these machines. Under the right machine conditions, such as bunch repetition rate, peak current, etc., amplification of the electrons can occur from secondary emission when the electrons strike the beam pipe walls, creating a growth in vacuum pressure along with a number of adverse effects on the circulating beam including severe two-stream instabilities, transverse beam blowup, and heating of cryogenic vacuum chambers. The net result is that the beam intensity is limited and beam quality reduced [1,2]. This effect is important for several future accelerators Electron cloud effects have been primarily observed in a number of high intensity synchrotrons and storage rings [4 -13]. Experimental studies of the electrons have mainly used local detectors (retarding field analyzers) to measure the time dependence, density, and energy spectrum of the electron cloud in a small region near the detector [14 -16]. However, the electron cloud density (ECD) can vary significantly along the beam pipe depending on local beam pipe geometry and surface conditions. Furthermore, the local measurement only detects those electrons that reach the beam pipe walls and can only infer the ECD with the help of computer simulation. Therefore, it is important to develop means of directly measuring the electron clouds over longer sections of the accelerator. Of course, one method of inferring the ring average ECD is from effects on the high energy beam itself, which usually only appear at relatively high beam intensities, however.In this Letter, we present a novel idea [17] and its successful demonstration for measuring the ECD over a much longer section of a storage ring. This idea is based on measuring the time delay (i.e., phase shift) of a microwave signal propagating in the beam pipe due to the change of the index of refraction caused by the electron cloud. In practice, it would be challenging to measure the absolute phase shift of the signal, which is expected to be at most only a few degrees over a hundred meters, in an accelerator environment. Our idea relies instead on measuring the modulation of the phase shift of the microwave signal through the electron plasma by taking advantage of the modulation of the density of the electron cloud from gaps in the fill pattern of the circulating posi...

Characterization of electron clouds in the Cornell Electron Storage Ring Test Accelerator usingTE-wave transmission

“…To obtain an estimate of the phase shift approximations can be introduced, especially when one is only interested in its maximum value and the time scale of its variations, which is directly linked to the ECD rise and fall times. Previous works [12,17] have assumed a simple sinusoidal phase modulation, which would only generate an upper and a lower sideband for small modulation indexes. In that case the estimate of the maximum ECD only depends on the sidebands amplitude relative to the carrier SB c and the relative phase shift is simply…”

“…In this paper we present results from CesrTA obtained using the microwave transmission technique [12]. This technique is based on the transmission of a microwave signal in the lowest TE waveguide mode in a section of a synchrotron beam pipe and measuring the phase modulation caused by the periodic variation of the density of the electron plasma.…”

Application of new simulation algorithms for modeling rf diagnostics of electron clouds

“…We simulated 460,000 rf cycles (28.67 million simulation time steps), which translates to about 650 modulation periods. In this case, the side band peaks are not well resolved, as can be seen in figure (2). Note that in this figure the spectra are only evaluated for 2 24 data points in the time series, not the full number of simulation time steps.…”

“…Recently a technique to measure this modulation, e.g. [2], referred to as traveling wave rf diagnostics has been developed. This non-destructive technique measures average electron cloud density by injecting rf and measuring changes to the phase of the signal due to the presence of the electron plasma.…”

Experimental observations of in situ secondary electron yield reduction in the PEP-II particle accelerator beam line

“…Finally, we would like to mention that microwave dispersion measurements [28] estimated an average electron cloud density of 6.6E+11 e/m 3 in a 50m PEP-II straight section located just downstream of our experimental chamber. This estimation of the cloud density should be considered as a lowest limit value for the cloud density in our experimental chamber upstream, due to its proximity to the bend and thus subjected to a higher rate of photoelectrons.…”