Electron-cloud Effect - Measurement Techniques

Measurement Techniques

There are many different ways of measuring the electron cloud in a vacuum chamber. Each one gives insight into a different aspect of the electron cloud.

Retarding field analyzers are local grids in the chamber wall that allow some of the cloud to escape. These electrons can be filtered by an electric field and the resultant energy spectrum can be measured. Retarding field analyzers can be installed in drift regions, dipoles, quadrupoles, and wiggler magnets. A limitation is that retarding field analyzers measure only local cloud, and because they measure current, there is inherently some time averaging involved. The RFA can also interact with the measurement it is taking through secondary electrons from the retarding grid being expelled from the RA and being kicked back into the device by the beam.

Witness bunch studies measure the tune shift along successive bunches in a train and in a witness bunch that is places at varying locations behind the train. Since tune shift is related to the ring-averaged central cloud density if the tune shift is known the central cloud density can be calculated. An advantage of witness bunch studies is the tune shifts can be measured bunch by bunch and so the time evolution of the cloud can be measured.

The vacuum chamber in an accelerator can be used as a waveguide for radio-frequency transmission. Transverse-electric waves can be propagated in the chamber. The electron cloud acts as a plasma and causes a density dependent phase shift in the RF. The phase shift can be measured as frequency sidebands which can then be converted back into a plasma density.