Electron-Cloud R&D

Diagnostics

In several efforts our electron cloud modeling and experimental strengths can be brought to bear to both simulate and measure aspects of electron cloud physics and diagnostics. One example concerns a new microwave diagnostic for the electron cloud density. The technique consists of injecting a microwave signal into the vacuum chamber and detecting it at another location, typically several tens of meters away. One can then infer properties of the electron cloud by comparing the input with the output signal. A microwave frequency just above cut-off provides optimal conditions for this technique. Previous efforts by Caspers and Kroyer (the inventors of this technique) to measure the electron cloud density with this technique in the CERN SPS achieved some success, but the interpretation of the results was unclear. This technique offers the promise of being able to directly measure the average electron density in a storage ring, rather than indirectly by detection of the electrons striking the vacuum wall. We have analytically computed the microwave phase shift under simplifying assumptions, simulated the transmission of the microwave signal with the electromagnetic code VORPAL (developed by Tech X Corp.) for more realistic situations, and validated the predictions at the SLAC PEP-II positron ring. This technique provides a real-time, non-invasive measurement of the electron-cloud density in a sector of the ring, and complements other techniques based on localized electron detectors at the surface of the chamber. A publication in Phys. Rev. Letters has been accepted for publication. More detailed simulations, now in progress by our group, aim to clarify the possible advantages of using various microwave modes in order to extract more detailed information about the electron-cloud distribution.