LS1 Report: the clouds are lifting

To combat the problem of electron clouds, which perturbate the environment of the particle beams in our accelerators, the Vacuum team have turned to amorphous carbon. This material is being applied to the interior of 16 magnets in the SPS during LS1 and will help prevent the formation of the secondary particles which are responsible for these clouds.

This photo shows the familiar coils of an SPS dipole magnet in brown. The vacuum chamber is the metallic rectangular part in the centre. The small wheeled device you can see in the vacuum chamber carries the hollow cathodes  along the length of the chamber.

When a particle beam circulates at high energy in a vacuum chamber, it unavoidably generates secondary particles. These include electrons produced by the ionisation of residual molecules in the vacuum or indirectly generated by synchrotron radiation. When these electrons hit the surface of the vacuum chamber, they produce other electrons which, through an avalanche-like process, result in a cloud of particles.

And electron clouds are the source of numerous inconveniences:

1. They release gas. And when gas is released, pressure increases, as does radiation, and this has an impact on infrastructures, resulting in background noise.
    
2. They are a source of energy, or to put it another way, of heat, which results in excess consumption of liquid helium for cooling.
    
3. They have a negative charge, so they interact with the beam, which has a positive charge. This translates into oscillation and expansion of the particle bunches, increasing the probability of quenches in the superconducting magnets, as well as reducing luminosity.
    

So how can this problem be resolved? In the LHC, the method currently used is the conditioning or “scrubbing” of the beam tube. “When the vacuum chamber is bombarded by electrons, this produces the avalanche phenomenon," says José Miguel Jiménez, new Head of the TE Department. “However, if the bombardment is sufficiently intense, it gradually induces the opposite effect, i.e. it inhibits the avalanche. But this might not be very efficient at very high energies.”

The violet light is produced by the argon plasma used when spraying the amorphous carbon. This plasma is created in the hollow cathodes, which are negatively polarised in relation to the vacuum chamber.

So in order to adapt to the energies anticipated in the accelerator complex from 2015 onwards, the Vacuum, Surfaces and Coatings (TE-VSC) Group have been working on another solution: amorphous carbon. “The avalanche phenomenon is possible because the surface of the vacuum chambers allows it to happen,” explains José Miguel Jiménez. “Metallic materials have a maximum secondary emission coefficient of between 1.9 and 2.1, which means that every particle hitting these materials creates up to 1.9 to 2.1 new ones. Amorphous carbon, however, has a coefficient of less than 1, which makes the avalanche phenomenon impossible.”

The problem of electron clouds could be resolved for good if the inner walls of the vacuum chambers are coated with a fine layer of amorphous carbon. In 2013, 16 magnets from the SPS were therefore “repainted” on the inside. “The work was not without its difficulties,” admits José Miguel Jiménez. “Although this technique is widely used in industry, it has never been used in long vacuum chambers like those in the SPS.”

The VSC Group therefore had to develop a special tool, called a hollow cathode, which is inserted into the vacuum chamber and then moves along the 7-metre length of each module, coating it with carbon. “During the operation,” José Miguel Jiménez adds, “gases which are harmful for the coating are absorbed by filaments on each side of the vacuum chamber, which “pump” them out as the operation progresses.”

The 16 treated magnets are now ready for installation in the SPS, where time will tell how resistant this coating is. If it gives satisfactory results, the technique could then be adapted for certain magnets in the LHC.
 

by Anaïs Schaeffer