No 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...
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