Modeling microwave/electron-cloud interaction

Starting from the separate codes BI-RME and ECLOUD or PyECLOUD, we are developing a novel joint simulation tool, which models the combined effect of a charged particle beam and of microwaves on an electron cloud. Possible applications include the degradation of microwave transmission in tele-communication satellites by electron clouds; the microwave-transmission tecchniques being used in particle accelerators for the purpose of electroncloud diagnostics; the microwave emission by the electron cloud itself in the presence of a magnetic field; and the possible suppression of electron-cloud formation in an accelerator by injecting microwaves of suitable amplitude and frequency. A few early simulation results are presented.

Contribution to modeling multipactor and corona discharges in high power electromagnetic fields

Francisco Edén Sorolla Rosario

Multipactor and microwave corona breakdown are two high frequency discharges occurring within microwave components for space applications as well as within particle accelerators. The appearance of these phenomena results in the exponential growth of the electron population triggered by the interaction of the environmental charged particles with the microwave component. The consequence of these discharges is an electron cloud build-up that, for space applications, loads the Radio Frequency (RF) cavities. This entails the increase of the system noise level and the return loss, generating undesirable harmonics and creating a local augmentation of the temperature that may destroy internal components, with the subsequent failure of the mission. Concerning the particle accelerators, the electron cloud induces the deflection and the deformation of the particle beam, reducing its beam-to beam collision efficiency and causing the malfunctioning of the accelerator. The lack of a software or a method to predict the multipactor power breakdown threshold of RF components leads the scientific community to deal with high-cost test campaigns. Although the detection of the radiated power produced by the multipactor current was proposed as a breakdown detection method, no absolute levels of radiation were available to compare with measurements and validate this technique to assess the breakdown onset. In this thesis, an analytical expression to quantify the radiated power spectrum produced by a multipactor discharge within a parallel-plate waveguide has been developed. As there must be a mechanism to limit the growth of the electron population and the radiated power spec- trum depends on the number of electrons, the multipactor steady-state is also analyzed in this thesis. As multi- pactor breakdown is assumed to be detected when the saturation in the amount of electrons has been reached, an analytical expression of the electron population in the steady-state has been proposed in this thesis. This could help to predict the levels of radiation that must be detected when the breakdown occurs, and could be compared with the measurements to establish a rigorous multipactor detection criterion. Another part of the thesis deals with the development of a globally convergent algorithm to calculate very efficiently a large amount of roots of the cross-product of Bessel functions. This algorithm can be employed in the calculation of the radiated power spectrum produced by a multipactor discharge within a coaxial waveguide using the appropriated Green function, which needs the evaluation of these roots. Regarding the corona breakdown, this thesis intends to show the critical dependency of the breakdown power threshold on the ionization rate model. The ionization rate is a gas parameter that provides the amount of electrons released per unit time due to the ionization produced in the gas molecules as function of the elec- tric field amplitude. Three models of the ionization rate found in the literature are considered in this thesis and the corona breakdown power thresholds predicted are compared. Noticeable differences between the results obtained with each model have been found in some cases, which are analyzed in this work. Finally, in line with this thesis, a new high power SMA-like coaxial connector (in mass and size) has been developed. This thesis shows the design of the “Power Sub-miniature” (PSM) connector, which is capable to withstand high input powers without the risk of corona or multipactor breakdown to occur. The PSM has been manufactured and tested showing excellent multipactor withstanding capabilities. This connector is, in fact, able to withstand, at least, up to 1500 W input power for a pulsed signal of 2% duty-cycle in P-band (438 MHz) as well as in L-band (1124 MHz), without showing any trace of multipactor discharge. This implies an improvement of 50% compared to other powerful connectors such as the TNC. Concerning corona breakdown, the PSM is able to withstand more than 80 W CW without showing any sign of glowing or noticeable discharge, what is a challenge for coaxial connectors of this size at these input powers.

EPFL2012

Beam-cavity interactions in high power cyclotrons

Lukas Stingelin

The ring cyclotron of the Paul Scherrer Institute (PSI) accelerates an intense proton beam from 72MeV up to 590MeV. This happens in four cavities of very high quality factor, oscillating in the fundamental mode. The beam can excite parasitic oscillation modes (HOMs), because of its time structure. Measurements showed that their field can leak out into the vacuum chamber. Until now, there is no tool available to predict the potentially harmful effect of these HOMs onto the beam operation of the cyclotron. It is foreseeable that these effects might play a role if even higher beam currents have to be accelerated. This dissertation therefore deals with the numerical analysis and measurement of beam-cavity interactions. First calculations for a single cavity, interacting with a proton bunch were performed with MAFIA's eigenmode- (E3), time domain- (T3) and particle-in-cell (TS3) solvers. However, the structured grid and the limited computing performance of MAFIA make realistic simulations impossible. A simplified computation method is developed in this dissertation since a self-consistent simulation is impossible on today's computers: The parallel eigensolver Omega3P of the Stanford Linear Accelerator Center (SLAC) allowed us to calculate eigenmodes of the entire ring cyclotron for the first time ever. The rf fields are expanded onto a superposition of these modes and the excitation is calculated in frequency domain. Trajectories of the particles in the static magnetic field, superposed with the space charge fields and the beam excited HOMs, are then simulated. However, the quantitative accuracy of this model is still limited. On the one hand, because of the simplification in the geometry of the simulated rf structure, which otherwise would lead to a problem size going beyond the available computing resources. On the other hand, because it is not yet possible to simulate strongly absorbing boundaries more accurately. The simulation results confirm that up to proton beam currents of 2mA, corresponding to the routinely accelerated beam intensities, only a small deformation of the charge distribution appears. This thesis leads to a new simulation tool for further studies of intensity increases in high power cyclotrons.

EPFL2004

Analysis of a Novel Toroidal Configuration for Hadron Therapy Gantries

Enrico Felcini

Hadron therapy refers to a medical treatment that uses hadron beams (i.e. protons and ions) to deliver localized energy that suppresses cancerous cells, sparing the neighbouring healthy tissues from unwanted radiation. The major technical components of a hadron therapy centre are the particle accelerator (cyclotron, synchrotron, or linac) and the beam delivery system that controls, shapes and orients the particles towards the area to be treated. The beam delivery can consist of fixed transfer lines, or it can include a gantry, a transfer line that rotates around the patient and allows radiation from multiple directions. The present work investigates a new toroidal gantry for hadron therapy, named GaToroid. This novel gantry configuration allows the dose delivery from a discrete number of angles avoiding magnets as well as patient rotation. Compared to traditional gantries that require rotating magnets, this improvement is made possible by a toroidal magnet operating in steady-state. This design constitutes the ideal conditions for the use of superconductors to generate a significantly higher magnetic field compared to normal-conducting solutions, as well as to reduce the weight and footprint of the magnets. The study of a GaToroid system requires the integration of several aspects of physics and engineering. In this framework, the focus of this research is on the design of the superconducting coils integrated with beam optics and particle tracking analyses. The first part of the thesis illustrates the optimization of the toroidal magnet. Coupling two-dimensional particle tracking and magnetic field calculations, an algorithm was developed to identify optimal gantry configurations that maximize the energy acceptance of the system. Two solutions composed of 16 coils, differing in high and low values of engineering current density, were investigated. In line with current clinical requirements, the beams converged at the isocenter within 1 mm over the whole treatment energy spectrum for both configurations. The second part of the thesis describes the algorithm implemented for the two- and three-dimensional particle tracking. Building upon the results of the magnetic optimization, a linear beam optics formalism was developed to determine the focusing properties of GaToroid. The third part of the thesis focuses on the engineering design of the low current density solution. Using two thermo-electric models, lumped and one-dimensional, the Nb-Ti and ReBCO cable geometries were validated, together with the quench protection system. Furthermore, analytical and numerical studies on mechanics made it possible to estimate the overall footprint and weight of the system. Results show that, compared with the state-of-the-art gantries, the proposed GaToroid solution has the potential to be more compact and lighter by at least a factor two. Finally, the last part of the thesis describes the design of a scaled-down demonstrator wound with ReBCO tapes. Studies on quench protection, mechanics and experimental implementation aimed at testing the use of ReBCO technology for GaToroid coils are discussed. In conclusion, this work presents the first overall description of a GaToroid system, ranging from the analytical definition, magnetic optimization, particle tracking and magnet engineering. The investigation of this new toroidal paradigm for gantries represents a quantum step toward more compact and less expensive solutions for hadron therapy centres.

EPFL2021