Using tewave resonances for the measurement of electron cloud density

In the past few years, electron cloud (EC) density has been measured by means of its effect on TE waves propagated through the accelerator vacuum chamber. This technique has been used in several laboratories around the world (CERN, SLAC, FNAL, Cornell, INFN-LNF). Recent measurements at CESRTA and DAFNE show that the simple theory relating the phase shift of a propagating TE wave to the electron cloud density provides an inadequate description of the data. Standing waves set up between discontinuities in the vacuum chamber make the propagation length ill-defined and the standing wave pattern is not confined to the portion of beampipe between the input and output couplers. In this paper we present evidence that the response function near cutoff is the result of coupling to standing waves trapped in the vacuum chamber and that the analysis should be based on the response of a resonant cavity rather than waveguide transmission. This evidence includes measurements at DAFNE, CESRTA, a test waveguide, computer EM simulations, and analytical calculations.