Plasma-based accelerator structures can sustain electron plasma waves with electric fields several orders of magnitude higher than those achievable with present day RF technologies. 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. The current goal of the world-wide R&D programs is to demonstrate the stable and repeatable production of 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 scheme proposed by SPARC_LAB is based on the external injection of the electrons inside the plasma in order to achieve low beam emittance and low energy spread. The acceleration process is divided into two stages. First, the electron beam is generated and accelerated to an energy of the order of about 100 MeV, using a linear accelerator that employs state-of-the-art photocathode extraction and RF acceleration/compression. The second stage consists in injecting the electron beam into the plasma. Plasma-based accelerators are usually grouped according to the excitation mechanism of the electron plasma wave: Laser Wakefield Accelerators (LWFA) if driven by laser pulses, or Particle-driven 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. The same plasma module can also be used, in a different configuration, as a very compact lens for the electrons.
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 resonant PWFA a train of high-brightness fs-long electron bunches with stable length, charge and spacing will be generated by means of the Comb Beam technique directly at the photoinjector; the overall length and spacing will be controlled and optimized under RF compression, i.e. velocity bunching, to achieve the required bunch train quality needed to behave in the plasma as both driving and witness beam.
In LWFA, the ponderomotive force of an high intensity 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. LWFA in external injection uses a single short electron bunch, generated by means of the velocity bunching technique, in the plasma-acceleration capillary, where the plasma wakefield is excited by the high power laser pulse.
The SPARC_LAB group is also developing a plasma-lens device that can be used in an accelerator facility to strongly focus an electron beam, squeezing its size down to few microns. Such a tool, known as active-plasma lens, consists of a neutral gas confined in a narrow structure like a capillary. The gas is then ionized into a plasma by a current-discharge flowing through the capillary itself. The discharge induces an azimuthal magnetic field whose strength is directly proportional to the flowing current. Such a magnetic field can then be used to focus particle beams and replace conventional devices (like solenoids and quadrupoles) with much more compact structures.
Contact person: R. Pompili, riccardo.pompili@lnf.infn.it, Tel. (+39) 069403-2465
Last update: 09/2019