Generation of high brightness electron beams from plasma-based accelerators

Research project


Plasma-based accelerators represent the new frontier for the acceleration of high quality, i.e. high brightness, electron beams because of their capability to sustain extremely large accelerating gradients. In conventional Radio-Frequency (RF) linear accelerators, accelerating gradients are currently limited to ~100 MV/m, mainly due to breakdown occurring on the walls of the structure. Ionized plasmas, however, can sustain electron plasma waves with electric fields three orders of magnitude higher than those achievable with actual technologies. Moreover, the accelerating field strength is tunable by adjusting the plasma density.

In the last decade, a great progress has been done in several international laboratories to demonstrate the acceleration of electron beams with accelerating gradients of the order of several tens GV/m. These fields are generated within plasma structures (such as Langmuir waves or electron cavitation) propagating with velocity close to the light speed behind laser or charged-particle driving pulses.

Plasma-based accelerators are usually grouped according to the excitation mechanism of the electron plasma wave is excited: Laser Wakefield Accelerators (LWFA) if driven by laser pulses, or Plasma Wakefield Accelerators (PWFA) if driven by particle bunches. A high power driving pulse can excite a plasma wave in which electrons are trapped and gain energy as long as they are in phase with the accelerating and focusing field. In LWFA, the ponderomotive force of a laser pulse, traveling through an under-dense plasma, can excite a plasma wave with longitudinal electric fields larger than 10 GV/m. Electrons can then be accelerated up to ultra-relativistic energies on a centimeter scale. Terawatt-class lasers are needed in order to provide the required electric field. In PWFA the plasma wave is excited by the space charge forces of the driving electron bunch that displace the plasma electrons. In that way the driving electron pulse can transfer a large fraction of its kinetic energy to a subsequent bunch (witness bunch) placed at a proper distance.

In just fifteen years, laser-driven plasma accelerators have advanced from making 10 MeV beams with ~100% energy spread to GeV bunches with a few percent energy spread. The steady increase in maximum energy was enabled by the availability of a number of multi-hundred terawatt laser facilities around the world.

The progress for beam-driven experiments has been even more remarkable, thanks to the development of high quality ultra-short electron bunches, with the maximum energy gained in the plasma increasing from a couple hundred MeV to over 40 GeV in just two years.

Even though the principle of plasma-based acceleration has been proven by several groups, the so accelerated beams still suffer from large angular divergence, large energy spread, poor reproducibility, which prevent their use as an alternative to conventional RF accelerators which typically provide stable and high quality electron beams.

The current goal of the world wide R&D programs is to demonstrate the stable and repeatable production of high quality electron beams, i.e. high brightness beams, as those required for example by Free Electron Lasers which is one of the most demanding application of electron particle accelerators.

The INFN laboratory at Frascati (LNF) has the unique possibility to achieve plasma-based acceleration of high brightness beams. The facility SPARC_LAB is currently being commissioned at LNF; it consists in a conventional high brightness photo-injector, SPARC, and a multi-hundred terawatt laser, FLAME. The combined availability of these primary beam sources and related instrumentation, together with advanced expertise in the accelerator/laser/plasma physics and technology, will lead to unprecedented potentials of research and discoveries.

At the moment LNF is the only laboratory in the world able to combine high brightness, hundreds of MeV SPARC electron beam with a TW laser FLAME. SPARC itself has been conceived as a test bench for advanced beam manipulation and it can profit from extremely versatile layout and beam diagnostics. Moreover, many technical elements, e.g. beam line installation, timing and synchronization among photons and electrons are already funded and being commissioned by many of the people involved in this research.

The proposed plasma acceleration experiment will rearrange mostly existing hardware to perform proof-of-principle of novel acceleration schemes. The aim of this proposal is to have funds to get high talented, young researchers and assure the necessary manpower to perform such ground breaking experiments in an extremely short time scale.

In this vision, the presence of three research units from universities is of fundamental importance to attract talented PhDs and young scientists. Our proposal can profit of the support of national (e.g. ENEA, CNR) and international (e.g. UCLA, CNRS, IST) institutions. Some of them have long standing collaborations in the framework of LNF activities and some others are willing to join this particularly challenging experiment.

Our proposal has the ambition to produce high brightness electron beams by means of two techniques. A first scheme consists in injecting a witness electron bunch in a plasma where the plasma wave is excited by a high power laser pulse, i.e. external injection in a LWFA. The second scheme relies on the induction of coherent plasma oscillations with multiple electron bunches, that is a resonant PWFA. This very promising method consists in using a comb beam, i.e. a train of equidistant bunches, to increase the accelerating gradient. An active scheme to produce such a comb beam was conceived at LNF and first successfully tested at SPARC, The additional benefit of resonant PWFA relies on the use of lower charge bunches in the train with respect to traditional PWFA, with the advantage of a better control of acceleration and transport.

The generation, acceleration and full characterization of electron beams by the two above schemes is the aim of this proof-of-principle experiment. Thus, our research will focus on different topics ranging from advanced beam manipulation and low emittance photo-cathodes, to high accuracy diagnostics for electrons inside and outside the plasma. A relevant numerical simulation effort is needed to define the experimental parameters, to design diagnostics and to support and guide the beam operation. Performing the experiment itself is challenging as well; nevertheless LNF is the unique Italian laboratory having all the expertise to guide the solution of timing and synchronisation, beam handling, data taking and all the other issues arising in a modern high brightness electron accelerator.

This proposal collects many researchers already working together in the previous experiments hosted in the recent years at LNF in the framework of SPARC activity; therefore the activity coordination should be greatly simplified. Moreover, the unexpected interest that our experiment is receiving from international institutions strengthens the possibility of making an important breakthrough with respect to the state of the art in plasma-based acceleration techniques.

Accelerators using plasmas, and lasers, hold great promise to accelerate electrons, positrons, (and even protons and ions) to high energies over short distances and with high quality. This innovative technique for beam acceleration represents one of the possible technologies that could revolutionize directly or indirectly many fields of science. These include, in particular, Free Electron Lasers and Linear Colliders, short X-ray production for biology, material science, plasma studies, and industry. All such areas can profit from such plasma accelerator features as compactness and short pulse duration of both electron bunches and associated radiation sources.
Effective start/end date1/1/12 → …




plasma accelerators
electron beams
plasma waves
electron plasma
electric fields
free electron lasers
radio frequencies
time measurement
light speed