New technique for particle acceleration

Cluster members Allen Caldwell and Frank Simon of the Max-Planck-Institut für Physik together with a team of German and Russian physicists have developed a new technique for particle acceleration, called proton-driven-plasma-wakefield acceleration (PWFA). This new development may lead to much more compact – and therefore much cheaper – electron accelerators. Whereas the Large Hadron Collider (LHC) will cause protons to collide, the idea behind the PWFA is to accelerate electrons. In case this technique succeeds it may one day allow machines a fraction of the size of today’s accelerators to create the highest-energy particles ever.

In a conventional accelerator, particles are accelerated by the electric field in radio frequency resonators. The maximum field in these resonators is limited by breakdowns on the cavity walls due to the high fields. The radically new kind of acceleration skirts the electric field issue by using plasma – gas in which electrons have been ripped from their nuclei. This soup of ionized gas can handle electric fields about a thousand times stronger than can conventional accelerators, meaning the accelerators can potentially be a thousand times shorter. “The length of an accelerator is closely related to the overall cost of such a machine”, explains Frank Simon, Junior Research Group Leader at the Excellence Cluster Universe. “So achieving higher acceleration gradients is a way of getting more bang for the buck” says Simon.

Figure 1: Plasma bubble created by the highly relativistic proton bunch (red), used to accelerate electrons (yellow).  

Electron acceleration by proton-driven PWFA is in its earliest theoretical stages and far from experimental verification. Using protons to drive the acceleration is an attractive new idea because extremely high energy proton beams already exist today at the Tevatron and the LHC. Using such beams could allow to accelerate electrons to energies in the TeV regime in one single acceleration stage, something not possible with other techniques of plasma acceleration currently under study. Perhaps the biggest issue is the proton bunch length, which must be very small to allow the electrons to overshoot and create the wakefield. Hadron colliders have bunches that are centimeters in length. But bunches are needed that are hundred micrometers in length.

Recently, spectacular success has been achieved in the study of plasma-based accelerators. Using electrons from the Stanford Linear Accelerator, a doubling of the energy of some of the electrons was achieved over a distance of less than one meter in 2007, an energy increase for which the conventional SLAC machine needs a length of 2 miles. The accelerating gradient was about 50 GV/m, more than a factor of 1000 more than the state-of-the art ILC acceleration structures achieve today.

The results of the simulation studies were published in Nature Physics (Nature Physics, Mai 2009; DOI: 10.1038/NPHYS1248). For more information also see:

http://blog.wired.com/wiredscience/2009/04/tinyaccelerator.html and
http://www.quantumdiaries.org/2009/04/16/future-accelerators-surfin-the-plasma-wave/