Beam dynamics studies for advanced - compact RF photoinjector for PWFA

Advanced high-brightness RF photoinjectors play a crucial role in generating high peak current and low transverse emittance electron beams, which are essential for driving Plasma Wakefield Ac- celeration (PWFA) stages. The thesis work focused on the study of electron beam dynamics for advanced injectors in PWFA-driven applications. The main activity involved optimizing the in- jector of EuPRAXIA@SPARC_LAB and conducting a feasibility study for upgrading the injector using C-band technology. Starting with a two-bunches beam, named as comb beam, comprising a 200 pC driver and a 30 pC witness, directly generated at the cathode, the dynamics was optimized to meet the output parameters required for the subsequent plasma stage. This optimization was performed using simulation codes like ASTRA and a genetic algorithm called GIOTTO. The study addressed not only the optimization of longitudinal and transverse parameters to match the plasma requirements but also the optimization of the cathode distribution for the comb-like working point. Once the comb dynamics was optimized and the injector parameter table was finalized, my thesis work focused on studying the jitter, both RF and cathode distribution, of the machine to assess its stability and reproducibility. Due to a double velocity bunching compression scheme, this type of injector is highly sensitive to radiofrequency (RF) jitter. To stabilize the injector and improve the driver and witness bunches separation, a high-harmonic cavity (X-band) was inserted between the gun and the first accelerating section. This cavity proved to be essential for improving the working point, and its use has been finalized and included in the EuPRAXIA@SPARC_LAB Technical De- sign Report (TDR) that is in preparation. Looking ahead to a potential upgrade of the machine to achieve higher repetition rates (400 Hz), a feasibility study on the dynamics of a full C-band injec- tor has been conducted. In addition to the advantage of high repetition rates, C-band technology allows for higher accelerating gradients in both the gun and accelerating structures while reducing the injector’s overall length. Achieving these enhancements requires the optimization of both active and passive machine elements, including laser systems, the gun solenoid, and RF components. This study focuses on the possibility of using C-band technology for such advanced applications com- paring the C-band injector’s performance with the state-of-the-art S-band technology. Start-to-end beam dynamics simulations have been conducted to identify the optimal configuration for a C-band photoinjector. For this configuration, the nominal working point of 200+30 pC was studied, achiev- ing the same output parameters as the S-band injector. Considering the more modest compression phase expected for the C-band injector, this solution provides improved stability. As in the previous case, a high-harmonic cavity was included to enhance the working point stability. In this scenario, the cavity’s effects on dynamics were evaluated, including improved beam separation, emittance, and the peak current of the witness beam. Regarding my thesis, I also conducted beam dynamics studies for the final focusing section before the plasma stage in the SPARC_LAB linac. The final focusing study was carried out using two different configurations: a triplet of permanent magnet quadrupoles and a triplet of electromagnetic quadrupoles. The first system was tested with a 50 pC beam (witness-like beam for the SPARC_LAB comb), while the second was tested with a comb-like working point comprising a 200 pC driver and a 50 pC witness. During a six-month visiting period at the University of California, Los Angeles, I focused on the characterization of photocathodes at the PEGASUS laboratory, with particular emphasis on yttrium which offers advantages in visible laser operation, increasing per- pulse energy and avoiding high-harmonic conversions, useful for high-repetition-rate applications. My research also involved developing a machine learning-based analysis method for the PEGASUS photogun to reconstruct the emittance and, consequently, de- termine the average transverse energy of a photocathode using solenoid scan data. This technique could be implemented in future applications for non-intercepting virtual diagnostic in the low energy region for PWFA working point.