Discharge plasmas as a medium a collider based on wakefield accelerators

Wakefield accelerators use the immense fields produced in plasmas to accelerate particles to high energy. Energy gains > GeV are now routinely produced at laser facilities around the world over only centimetre distances. However, the relatively low energy available in even the most powerful laser pulse is a limitation on the particle's energy gain.

By contrast the ion beams available at CERN have energy that exceeds 10's of kJ (if not more). This means that instead of accelerating particles over centimetres, there is the potential to accelerate over tens if not hundreds of metres. This would produce an accelerator relevant to high-energy physics experiments.

The AWAKE experiment at CERN has already produced exciting results using the SPS beam demonstrating 2 GeV energy gain of electrons in a 10m cell. To overcome the fact that the SPS beam is too long to ideally drive a wakefield, a process called self-modulation has been used to produce beamlets within the SPS beam that are resonant with a wakefield. This is initiated in a Rb cell by a laser that ionises the plasma as the proton beam is passing through it. However, the use of the ionising laser puts a limit on the acceleration length.

We have proposed (along with IST Lisbon) using a discharge plasma that would enable acceleration lengths well in excess of the 10m available now. But for controlled acceleration the plasma density must be controlled to better than 1%, and it may be further necessary to have ramps in plasma density to optimise the acceleration process. Hence, in this project, we will look to diagnose and model the generation of large scale (metre long) plasmas, using the prototype discharge we have here in the Blackett Lab.

We will also model the interaction of the SPS proton beam with our accelerator stage. We will then look further to optimise the plasma profile to improve the acceleration. These investigations will be performed with a view to implementing both the discharge and the diagnostics on future runs of AWAKE at CERN.