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Hi Chris,
It is not true that it was totally expected. We thought it would become an issue eventually in the LHC and this was studied during the design phase. What was a little surprising was the strength of the cloud with the 50ns spacing. However, we have now realized that we pushed the bunch intensities very high, very early in the LHC and this may play a part.
Now what is electron cloud .... Mmm , ok here goes.
Low energy electrons can be liberated in the vacuum chamber by a variety of mechanisms – ionization of the residual gas is one such mechanism. Photo-electrons from the synchrotron light is another ... Now imagine these electrons floating about in the vacuum when a proton bunch passes. This has an enormous electrical field from all those positive charges squeezed together. This field is attractive to the electrons and will accelerate them as the bunch passes. These electrons can then go on to hit the walls of the chamber. When they hit the chamber they can liberate more electrons. Some of these can, in turn, be accelerated by succeeding proton bunches in the same way. Hence, if the conditions are right there can be an avalanche build-up of electrons in the vacuum chamber. This is the e-cloud. There are many parameters which play a part in this build up the bunch intensity and spacing are important as is the physical size of the vacuum chamber and the work function (or secondary electron yield) of the surface of the chamber. This last parameter basically says how many electrons will be given off for 1 impacting electron. The SEY we see in the machine seems to be around 2.5 – which is very high. The SEY is also the parameter we work on when scrubbing. Effectively we can alter the SEY of the surface simply by provoking the effect ... We slowly condition the surface and lower the SEY.
I attach a sketch picture which might help ...
Why do we care? Well all these electrons banging around in warm regions of the machine lead to a pressure rise – liberating condensed gas from the vacuum chamber surface. In cold regions of the machine they act to transfer energy to the cold surface and hence a heat load to the cryo system. In addition the density of the cloud can affect the beam itself leading to instabilities and emittance growth.
Hope this helps
Cheers
