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EuCARD >> News >> Newsletters >> Issue 2 >> Article 4

Breaking news for Proton "Surfatrons"

It has been known for some time that plasmas (gases of free ions and electrons) can support very large electric fields. This property enables plasmas to accelerate particles to relativistic energies over shorter distances than current technologies, paving the way for future high performance, smaller accelerators.

The driving seat

Research into plasma acceleration centres on creating waves within the plasma by shooting a "driver" laser or bunches of negatively or positively charged particles into the plasma itself (see table).

In the same way that a surfer picks up speed in the wake of a wave, so particles subsequently injected into the plasma are rapidly accelerated by travelling across the driver’s "wakefield". This surfing analogy has led to the colloquial name of "surfatrons" for accelerators using plasma wakefield acceleration.

Initially, laser driven plasma wakefield acceleration was proposed and experimental verification of the ideas followed. In recent experiments, electric field gradients of 100 Gigavolts per metre (GV/m) have been achieved. These have so far been limited to distances of a few cm, but the progress has been very impressive, for example, a recent milestone report "Requirements for electron beam diagnostics" from EuCARD WP11 shows the work the project is doing within this field. In order to accelerate an electron bunch to 1 TeV, these gradients would have to be maintained over distances of tens of metres, or many acceleration stages would have to be combined.

It was later recognized that the plasma could also be excited by an electron bunch. Given an intense enough bunch of electrons, the plasma is both created and excited by the passage of the bunch. In the case of electron driven plasma wakefield acceleration, a gradient of 50 GV/m was achieved and sustained for almost one metre in experiments at SLAC. However, the energy given to the accelerated bunch is limited to a maximum of twice the energy of the driver bunch.

Positive new findings

In contrast to plasmas driven by electrons, only limited investigations of the plasma wave excitation by a positively charged beam (mainly positrons) have been performed.

Importantly, there have not yet been any beam tests with proton-driven plasmas. The electric field distribution should be similar to that created by electrons.

Physically, the negatively charged electron driver "blows out" the background plasma electrons creating a low density region behind the driver. For proton drivers, instead of "blowing out" plasma electrons, they "pull them in" to the centre of the bunch.

However, given that protons can be accelerated to TeV energies in conventional accelerators, it is conceivable to accelerate electron bunches in the wake of the proton driving bunch (e.g. in the wake of an LHC proton beam) to up to several TeV in one pass through the plasma.

The electric field strength (left) and electron density (right) in the plasma from simulation studies of proton driven field acceleration. The accelerating field ranged from +2 GV/m (red) to -2 GV/m (blue). The accelerated bunch is seen as a black spot in the right-hand image. Image courtesy: A. Caldwell et al. Thumbnail image main page courtesy of Timo Balk, stock.xchng

The plasma wake produced by a 1 TeV proton bunch has been investigated and reported in a recent publication (A. Caldwell, K. Lotov, A. Pukhov, F. Simon, Nat. Phys. 5 (2009) 363). The electric fields are a factor of 100 higher than those considered for the International Linear Collider (ILC), and could lead to the acceleration of a bunch of electrons to several hundred GeV within a few hundred metres (starting with a 1 TeV proton bunch).

These exciting results have spurred discussions of the need for a demonstration experiment. A Letter of Intent for experimentation at CERN centred on studies of proton bunch interactions with plasmas and subsequent electron acceleration is currently being planned. Possible beams include the PS and SPS beams at CERN. In a first round of measurements, modulations of a long (20 cm root mean squared (rms)) proton bunch would be searched for. This effect is predicted in particle-in-cell simulations and their observation would provide an excellent test of the simulations. The goals for subsequent rounds of experimentation would include generating stronger electric fields in the plasmas by first longitudinally compressing the proton bunch, and eventually demonstrating acceleration of an electron bunch in the wake of the proton bunch. A collaboration aiming at carrying out these experiments will be formed in the near future. Anyone interested in participating should contact Allen Caldwell for more information. A future topical workshop might be organized with the support of EuCARD WP4 AccNet.

- Ralph Aßmann, CERN & EuCARD WP8; Allen Caldwell and Guoxing Xia, Max-Planck-Institut für Physik München; Konstantin Lotov, Budker Institute of Nuclear Physics and Novosibirsk State University; Alexander Pukhov, Institut für Theoretische Physik I, Heinrich-Heine-Universität Düsseldorf; Frank Zimmermann, CERN & EuCARD WP4; Kate Kahle, CERN & EuCARD WP2