An electron cloud is an unwanted accumulation of free electrons around an accelerator beam of positively charged particles. It forms when the beam ionises residual molecules inside the vacuum of the beampipe and the freed electrons are accelerated in the electromagnetic field of the beam particles. If some of the accelerated electrons forcefully hit the inside of the beampipe, they free further electrons and molecules, amplifying the electron cloud effect.
If the electron cloud build-up around the beam is considerable, it can destabilise the beam.
A way to reduce the electron cloud is to perform scrubbing of the accelerator, which reduces the amount of residual molecules in the vacuum.