Electron cloud density measurements in accelerator beam-pipe using resonant microwave excitation

Abstract

An accelerator beam can generate low energy electrons in the beam-pipe, generally called electron cloud, that can produce instabilities in a positively charged beam. One method of measuring the electron cloud density is by coupling microwaves into and out of the beam-pipe and observing the response of the microwaves to the presence of the electron cloud. In the original technique, microwaves are transmitted through a section of beam-pipe and a change in EC density produces a change in the phase of the transmitted signal. This paper describes a variation on this technique in which the beam-pipe is resonantly excited with microwaves and the electron cloud density calculated from the change that it produces in the resonant frequency of the beam-pipe. The resonant technique has the advantage that measurements can be localized to sections of beam-pipe that are a meter or less in length with a greatly improved signal to noise ratio.

The conversion of CESR to operate as the Test Accelerator, CesrTA. Part 3: electron cloud diagnostics

“…Drive (5 Watts) to Spectrum Analyzer E Figure 22. BPM buttons can be connected in vertical pairs to drive the TE 10 mode by driving top bottom buttons out of phase [19]. Figure 23.…”

“…Figure 23. Schematic diagram illustrating a typical measurement setup, where bandpass filters are used to limit the voltage of the direct beam signal [19].…”

“…With a fixed drive frequency at or near resonance, the phase of the resonant response will be shifted by an amount that is also proportional to the EC density as given by ∆φ ≈ 2Q∆ω/ω . The details of this analysis are presented elsewhere [19].…”

“…With a fixed drive frequency, a change in resonant frequency produces a change in the phase of the response. The phase of the response includes the convolution of the changing EC density with the response time of the resonance[19].…”

Fast Transverse Beam Instability Caused by Electron Cloud Trapped in Combined Function Magnets

“…The method was initially proposed for measuring the cumulative average density of the electron cloud, which can vary significantly along the beampipe depending on local geometry and surface conditions, over a large section of an accelerator[39]. It proved to be valuable for studying the clouds at many large scale high intensity accelerators: MI at Fermilab[40], CESR at Cornell[41], PS and SPS at CERN[42], etc. The technique is also useful for measuring the cloud in the regions where an installation of a pick-up detector is technically challenging, for example inside the magnets.The first attempt to perform a microwave measurement in Recycler was made Eldred et al in 2014 [29].…”