The main concept behind electron acceleration is based on the fact that a powerful laser pulse can drive an “electron wake” into a plasma medium, similar to a “water wake” created by a boat surfing in the sea (see movie below). This laser-generated wake produces a huge electric field, the so-called “laser wake field,” which in turn accelerates electrons very efficiently within a short distance (typically less than 1 cm). A laser “wake-field” can accelerate electrons with an energy gain of about 100 MeV in 1 mm, which means this acceleration technique is extremely compact compared to conventional accelerators (a standard RF Linac accelerates electrons with a gain of about 35 MeV in 1 m).
The result of the rapid evolution in laser-driven electron acceleration is evident and demonstrated by the fact that electron bunches with energies of about 1 GeV are currently available in small and medium scale laboratories. Furthermore, they are being used for a generation of secondary sources that were previously available only in large scale infrastructures. This research field is very dynamic and promising; in fact, GeV-class electron beams were experimentally demonstrated around 10 years ago using a laser with peak power of about 40 TW, and recently more than 4 GeV electrons were accelerated using 300 TW on target . Thus, the laser systems that will be available at ELI-beamlines, with a peak power of 1 PW (1000 TW) at a high repetition rate (10 Hz) and 10 PW at a low repetition rate (1 shot per min), will certainly offer completely new possibilities to the research community.
Laser-accelerated electron beams can potentially be used for various societal applications (e.g. through the generation of x-ray and gamma ray secondary sources) and, at the same time, enable and validate innovative physical mechanisms proposed by the scientific community, as extensively described in the ELI-White Book  . The electron acceleration program being implemented at ELI-Beamlines is conceived to accommodate, in a long term perspective, experiments covering both aspects. Thus, the HELL (High-energy ELectron-acceleration by Laser) (add a hyperlink to the page on the “HELL platform”) platform being developed at ELI-Beamlines will facilitate the performance of experiments oriented to the use of secondary sources (thanks to the user-friendly “beamline” features that will be focused on user needs), as well as advanced experiment that will require the use of the main electron source (thanks to the flexible “platform” features) to develop and test innovative schemes, in order to improve acceleration techniques and to verify new models.