Illustration of the Laser Wakefield Acceleration.

Conventional accelerators employ oscillating radio radiofrequency (RF) fields to accelerate charged particles. The accelerating rate in these devices is restricted by electrical breakdown in the accelerating tube. This limits the amount of acceleration over any given space, requiring very long accelerators to reach high energies.

A new concept for particle accelerator was conceived in 1979 by Toshiki Tajima and John M. Dawson [1]. The idea was to use an ionized gas, or plasma, to maintain the high electric fields required to accelerate particles. The advantage of plasma accelerators is that their acceleration fields can be much stronger than those of conventional RF accelerators. The electric fields are created by driving a laser pulse or a particle beam through a gas or a pre-ionized plasma. The driving beam creates ripples in the plasma density, displacing the negatively charged electrons from the positively charged ions. The local imbalance between positive and negative charges in the wake of the driving beam creates huge electric fields, of the order of 100 gigavolts per meter. Any electrons trapped in between the middle and the back of the wake will be accelerated forward like a surfer riding a wave, hence the name “wakefield” acceleration (*). In external injection schemes, electrons are strategically injected after the driver beam to arrive at the wake at the time of maximum expulsion of the plasma electrons.

(*) When the plasma wave is formed by an electron or proton bunch the technique is called plasma wakefield acceleration (PWFA); if a laser pulse is used instead it is called laser wakefield acceleration (LWFA).

[1] T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979).

Plasma Accelerator  –

                         the 100 billion volt machine

Advantages of plasma accelerators
  • Acceleration rates 2 – 3 orders of magnitude higher than conventional accelerators, reducing the required acceleration length by 100 to 1000 times.
  • Plasma accelerators overcome the breakdown limit that restricts the accelerating gradients in metallic RF structures.
  • Ultrashort electron bunches, opening up exciting new opportunities for research, i.e. the observation of ultrafast processes in biomolecules.
  • The lasers required for driving plasma accelerators have become available from European  companies, offering a supply chain that is not only comparable to RF industry but also developing in a more dynamic and innovative way.

The first experimental demonstration of wakefield acceleration (PWFA) was reported by a group from Argonne National Laboratory (USA) in 1988 [2]. In 2007, a 42 GeV electron beam was obtained at SLAC using PWFA in just 85 cm [3] whereas a conventional accelerator would have required 2.6 km to reach the same energy.  Scientists at Lawrence Berkeley National Laboratory (USA) used LWFA to accelerate electrons to 1 GeV in about 3.3 cm[4]. In 2014, the BELLA Laser Center at the Lawrence Berkeley National Laboratory, produced electron beams up to 4.25 GeV [5]. 

[2]    J. B. Rosenzweig et al., Phys. Rev. Lett. 61, 98 (1988).
[3]    I. Blumenfeld et al., Nature 445, 741 (2007).
[4]    W. P. Leemans et al., Nature Physics 418, 696 (2006).
[5]    W. P. Leemans et al., Phys. Rev. Lett. 113, 245002 (2014). 

A plasma is an ionized gas, that is, a state of matter in which electrons are detached from their atoms, which hence become ions. Familiar forms of plasma are lightning and neon lights. Although not so common on Earth, plasma is the most abundant form of ordinary matter in the Universe. 

Plasma accelerators presently offer lower beam energy and lower beam quality than conventional accelerators. Shot-to-shot stability and optimization has only recently become a priority. The operation of plasma accelerators is so far limited to working hours and days, and the switching-on and off generates numerous stability problems.

EuPRAXIA addresses specifically these limitations by an extensive program of research covered in the different work packages.

Current Limitations

Illustration of the Laser Wakefield Accelerator.

Plasma cell. With a few millimeters in length, it can sustain accelerating rates of ~ 100 GeV/m, several orders of magnitude larger than conventional accelerators.

© DESY, Heiner Müller-Elsner