In the pasts decades particle accelerators and especially RF photoinjectors had an impressive development, thanks to which a lot of applications were possible from the industrial one up to the medical use. Historically accelerators were developed for nuclear and particles physics but nowadays only a small part of accelerators are devoted to science, most of them are used for applications. The current request for scientific scope, especially for particle physics, is an higher and higher beam energy. The energy scale of TeV in the center of mass, in the past decades was reached. In the most recent and powerful proton particle accelerator Large Hadron Collider (LHC), was reached the impressive energy value in the center of mass of about 13 TeV and a Luminosity of about 10^34 cm^−2s^−1. The energy request from the scientific community has led to some accelerators project that exceed 50 Km lenght. In order to overcome these accelerator dimensions and especially to reduce costs, a good solution is the plasma acceleration. With this new accelerator technique the accelerating gradients can be a factor 10^3 more intense with respect to the modern RF technology. Nowadays plasma acceleration does not look like a chimera anymore, it was been demonstrated as a proof of principle but a lot of work is still to do in order to reach a good beam quality such that it can be used in accelerator facilities and in applications. Most of the applications in fact demand an high beam quality: ultra low energy spread and ultra high brightness beams. High brightness beams means bunches with an high peak current and a low emittance. These quality parameters are also necessary in order to perform a good matching between beams from accelerator and plasma in the so called external injection scheme for plasma acceleration. For example the energy spread that a bunch acquires during the acceleration is proportional to the length of the bunch that is injected into the plasma. Furthermore with a low transverse emittance the beam can be easily focused in order to reach the transverse matching conditions between beam and plasma. Beam brightness is a fundamental parameter for applications as the Free Electron Laser (FEL) that is able to produce X rays, where the gain length (Lg) is inversely proportional to the electron beam brightness (Lg proportional to B^−1/3) in the Self Amplified Spontaneous Emission (SASE) X-ray regime. These requests in the electron beam quality means that a perfect control of the bunches along the beam line is necessary, starting from the bunch generation in an electron gun up to the accelerator end, especially in the photoinjector region where the beam is not yet relativistic and is in the so called space charge regime. From these requests the beam transport has to be optimized, especially at low energies, performing a fine tuning of the machine parameters and of the positions of the machine elements along the beam line. To do that have to be fixed: a proper position for the first accelerating section, an integrated magnetic field of the gun solenoid, an optimal bunch compression scheme and mostly avoid any misalignments of the accelerating sections and of any magnetic components. In the photoinjector, where the beam is not yet relativistic, any misalignments can generate a transverse kick of the entire beam and a distortion of the beam transverse shape. The bunch can easily degrade its quality parameters in the photoinjector region, which in case of standard applications (e.g. industrial or medical) could be not so detrimental. Differently in case of applications as the ones here discussed (plasma acceleration or FEL) it is a serious matter which compromise the application itself. One of the effects of a low beam quality is a more difficult matching with the linac and subsequently with the plasma channel. In order to meet these stringent beam quality parameter requests, I optimized the beam dynamics of a new ultra high gradient 1.6 cells C-band (5.712 GHz) gun able to reach 240 MV/m as a peak field. By means of the ultra high gradient a better control of the space charge forces inside the bunch is possible. After optimizations on this electron gun a proper emittance compensation scheme was found through simulations with the software General Particle Tracer (GPT). Simulations showed the possibility to have, with a 100 pC beam and an energy of about 150 MeV, an emittance value of about 55 nm and a final beam brightness value of about 5 × 10^16 A/m^2. In order to optimize the present and future SPARC_LAB beam line, I wrote an algorithm able to evaluate transverse misalignments of a gun solenoid with coils powered with opposite currents. This algorithm was checked successfully in a dedicated run at SPARC_LAB. Using this algorithm the estimation of misalignements was about 1 mm and 0.5 mm in the transverse planes. During the SPARC_LAB machine operations we measure a bunch centroid displacement due to the misaligned solenoid. Aligning the solenoid to the found values we will improve the centroid orbit displacement of about 99.4%. Furthermore it will be possible avoid transverse kicks and distortions of the transverse beam shape and emittances i.e. the fundamental parameters to match simultaneously the beam with plasma in both transverse planes. The beam can have different spots and emittances in the x, y transverse planes due to laser on cathode misalignments, or due to some residual misalignements on the gun solenoid or on accelerating sections. I studied the possibility to insert Printed Circuit(PC) skew quadrupoles inside the future SPARC_LAB gun solenoid. By GPT simulations these PC skew quadrupoles will be able to reduce spot differences in the x and y planes, from 14% up to about 1% and a differences in the emittances, in x versus y plane, from 14% up to 5%. A first design of these PC quadrupoles was made, and we are planning to install them in the future SPARC_LAB gun solenoid.
High gradient ultra-high brightness RF photo-injector optimization / Croia, Michele. - (2018 Jan 26).
|Titolo:||High gradient ultra-high brightness RF photo-injector optimization|
|Data di discussione:||26-gen-2018|
|Appartiene alla tipologia:||07a Tesi di Dottorato|