A large number of electrons called an electron cloud are accumulated in beam
chambers in positron storage rings. These electrons interact with the beam and can
make it unstable. The density information of the electron cloud near the beam is
therefore fundamental for studying the beam instabilities and mitigation techniques
related to the electron cloud. Recently Kanazawa et al. proposed a method to measure
the density of the electron cloud near the beam. The method measures high-energy
electrons selectively by a retarding field analyzer located on a chamber wall, noting
that the electrons near the beam get a strong kick by the beam. They calculated the
density of the electron cloud simply assuming that the electrons that receive a kick are
stationary.
We examined the measurement technique by a new computer code developed by us.
The analysis showed that the volume near the beam occupied by the detected
electrons, i.e. the observed volume, was strongly deformed due to the horizontal
velocity of the electrons and nevertheless the observed volume calculated assuming
the stationary electrons can still be used for calculating the density of the electron
cloud in their measurement conditions. The modeling of electron cloud generation in
a field-free region of KEK B-Factory (KEKB) LER (Low Energy Ring), analysis of
the measurement, comparison of the measured density with simulations and a
possible improvement in the design of electron monitor are presented in details.
NUMERICAL ANALYSIS OF THE MEASUREMENT OF NEAR BEAM ELECTRON CLOUD DENSITY AT KEKB LOW ENERGY RING
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