In the field of the particle accelerators is growing the interest in the development of compact accelerator machines that have to also be able to exceed the current limits of the conventional accelerator structures. In this context, the research activity is leading towards plasma-based devices able to produce accelerating gradients in the GV/m scale, concerning the MV/m scale of the RF-based accelerators. Such plasma-based structures can be also used to focus charged particle beams with large energies. For these reasons, these new compact devices are currently replacing conventional accelerators and standard focusing devices. In this regard, recently, several proofs of principle experiments have been performed in focusing electron beams through active plasma lenses, consisting of gas-filled capillaries in which the plasma is produced by an electrical discharge.
The state of the art concerning the plasma sources for plasma-based accelerators refers to prototypes of the plasma devices that show several problems related to its restrained dimensions, to the stability shot-to-shot of the plasma, to the reproducibility of the plasma properties, as the electron density and uniformity of the longitudinal and transverse profiles inside the plasma source.
This research activity is devoted to the development of a new generation of plasma sources able to overcome the current limits:
1. The maximum length of the current gas-filled capillaries is few centimeters that allow us to obtain beam energies around hundreds of MeV, but if we want to reach very high energies of beams, in the GeV-scale, it is necessary to extend the longitudinal dimension of the plasma sources, up to meters scale. For these so long capillaries, a new technique to produce the high voltage discharge has to be designed. As long as the plasma source is few centimeters long, the high voltage pulse can be directly applied at the extremities of the capillary, but for higher lengths, the capillary has to be segmented and the voltage will be applied to each part to ionizing the gas inside the entire capillary. The development of such a technique will be studied for this research.
2. The plasma properties, as the plasma stability shot-to-shot and the uniformity of the density along the longitudinal dimension of the capillary, strongly affect the electron beam passing through the plasma source. In this research activity, new techniques to improve these plasma parameters will be developed. Indeed, the stability shot-to-shot is produced by the changes of the discharge ignition, which can be reduced by using a laser technique. In terms of the uniformity of the plasma profile, new shapes of the plasma sources will be studied to adapt the density behavior to the beam properties.
3. The capillary characterization is performed by measuring the plasma density through interferometric and spectroscopic techniques, like the Stark broadening method. This technique gives good results when the plasma electron density is in the range 1e+14~ 1e+19 cm^-3, therefore, in this activity, such measurement method will be developed for lower values of plasma densities.