Crab cavities are a radiofrequency equipment 
that will “crab” the particle bunch
This is really a pioneering technology
 It has been proposed for the LHC by Rama Calaga
As the bunches fly by each other, you would like that basically
 every particle in this bunch is able to go through the other bunch
This maximises the probability it will collide with a proton from the other beam
If you have a crossing angle, obviously one bunch will see only a part of the other bunch, while with the crab cavities
by tilting them into the collision point, you make sure that they really traverse through each other
This maximises the probability they will collide and enhances the luminosity
The crab cavities are an equipment that has never been done before. This shape has never been done before
They are a fully new development
The crab cavities that we are using today are made of bulk niobium. At 2 kelvin, this material exhibits superconducting properties
This is extremely favourable to sustain the type of very high fields that we want to generate inside the cavity
The crab cavities create a transverse field that is oscillating at 400 megahertz
That means that every 2.5 nanoseconds they change the direction of the field which goes up and down
The bunches in the LHC pass by every 25 nanoseconds
The beam goes very fast. It goes with the speed of light
A beam that is typically ten centimetres long, or a bit longer – 15 centimetres – will pass in front of me in one billionth of a second
No time to look at it!
And the crab cavity must grab this beam and it must rotate it and give it the right angle
And 400 metres away another crab cavity must give it a counter shot, exactly equal and opposite, exactly!
Otherwise the beam won’t be stable
