Dottorato in FISICA DEGLI ACCELERATORI

Titolo della tesi: Development and Characterization of Plasma Sources for Plasma-based Accelerators

In the field of 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 con ventional accelerator structures. The Ph.D. research activity has been developed in the background of plasma-based accelerators (PBAs). In this context, the research activity is leading toward plasma-based devices able to produce accelerating gradients in the GV/m scale, with respect to the MV/m scale of the RF-based accelerators. Such plasma-based structures also can be used to focus charged particle beams with high energies. For these reasons, new compact devices promise to become competitive in the near future concerning the conventional accelerators and standard focusing devices. In particular, concerning the focusing techniques, several proofs of principle experiments have been recently performed in focusing electron beams by means of active plasma lenses, consisting of gas-filled capillaries in which the plasma is produced by an electrical discharge. For this innovative linear electron accelerator based on plasma acceleration technology, the high accelerating gradients required to accelerate charged particle beams are produced by electron density modulations (electron plasma waves) inside plasmas, which in turn can be created by an intense laser pulse (laser wakefield acceleration, LWFA) or by energetic particle beams (plasma wakefield acceleration, PWFA). This work concerns both the theoretical studies and the experimental developments of an overall plasma module to implement the particle acceleration based on plasma technology. In particular, these structures for the confinement and the characterization of plasmas will have to be developed to adapt the plasma properties, such as the electron density profiles, temperature, pressure, etc., to the properties of the particle bunch that has to be accelerated. In Chapter 1, a description of the plasma wakefield acceleration basic concepts is presented. Plasma-based accelerators can be developed by using two main techniques, depending on the strategy used to dynamically modulate the stationary plasma. In the case of this research interest (plasma wakefield acceleration, PWFA), the electron bunch to be accelerated, the witness beam, is preceded by a driver one able to perturb the initial plasma density, creating a ‘wake’ where a strong electric field is produced. In chapter 2, the overall experimental apparatus to produce, con fine and characterize a plasma for particle accelerators has been described. The gas-filled capillary plasma source represents the main component of the plasma module, where the plasma channel will be formed. In order to create the plasma inside the capillary, one or more gas inlets feed the channel with a neutral gas (Hydrogen, Argon, Nitrogen, etc.), which is ionized by a high-voltage discharge. In this regard, the design of the high-voltage discharge circuit plays a crucial role in controlling the plasma properties, that in turn contributes to determining the bunch quality during the acceleration process since is related to the discharge current pulse profile. Therefore, the Ph.D. activity has been addressed in the optimization of the geometric modeling of the gas injection system and the design of the high-voltage circuit to reach higher values of the discharge current inside the capillary 14 during the gas ionization. The measurements of the plasma density in the range of 10-19 -3 10 cm have been performed by the spectroscopic methods. The plasma density distribution (in the case of Hydrogen gas) has been retrieved by the Stark broadening techniques, based on the analysis of the Hydrogen Balmer alpha and Balmer beta lines (chapter 3). in chapter 4, the results concerning gas-filled plasma-discharge capillaries characterization are presented. The longitudinal electron density measurements have been performed to investigate the effects of geometric properties of the plasma source. In particular, the dimensions, the total number, and the positioning of the gas inlets play a crucial role to optimize the shot-to-shot stability and the longitudinal uniformity of the plasma along the capillary. Another essential aspect that has been investigated concerns the plasma ramps (steep plasma gradients) produced at the ends of the capillary during the gas ionization. In fact, this phenomenon represents an essential point for bunch quality preservation because the lack of stability and the uniformity of the electron density in these areas produces significant degradation of the electron bunch emittance. Therefore, the study on the plasma ramps (plumes) formation and their effects on the electron bunch propagating within it will be considered in this research activity to propose a possible solution based on the capillary shape modifications that can optimize the density profile at its extremities.