Davide Passi

Multiphysics modeling based design of a key-holes magnetron

This paper proposes a particular design technique of an 8 slots resonant cavities X-Band Magnetron. Such study is based on a Multiphysics (MP) simulation and consider thermal-structural effects due to the cathode heating, taking into account that electromagnetic behavior and thus device efficiency depend critically to the operating temperature and to the related thermal induced displacements of the materials. The proposed study involves Thermal Stress (TS), Eigen-frequency (EF) and Particle Tracing (PT) analysis performed with the Finite Element Method (FEM). These computations have been performed on COMSOL. Electric field related to the main resonant modes and particle trajectories have been computed in thermo mechanical operative conditions. Magnetron working points have been estimated.

Multiphysics design of a spatial combiner predisposed for thermo-mechanically affected operation

This paper describes the design based on a Multiphysics simulation for a waveguide X-Band Fin Taper (FT) Spatial Power Combiner. The proposed device uses Fin Lines to microstrip transition (FLuS) to convert the energy from a rectangular waveguide (WG) TE10 fundamental mode to a microstrip (μS) transmission line (TL) quasi-TEM (q-TEM) mode, in order to be amplified by Solid-State Power Amplifiers. An alteration of the electromagnetic behavior can be produced by the temperature increase and the geometrical displacement induced by the thermal expansion of the structure due to the power dissipation of the MMIC amplifiers. A proper multiphysics model is proposed to select materials and shapes of the probes and their support (carrier), considering the thermo-mechanical operative condition. A virtual prototyping technique is proposed: electric field and S-parameters’ computation in such critical conditions are shown. The proposed study has allowed for the proper thermo-mechanical design for such amplifiers; hence, best materials and technologies have been chosen.

The Squarax spatial power combiner

A broad-band transmission line spatial power combiner (SPC) is proposed in this paper, which uses a square coaxial (Squarax) transmission line (TL). This structure has some advantages over the traditional circular coaxial spatial power combiner, which have been described in this paper. Fin-Line to microstrip transitions are inserted into the Squarax TL, in order to allow an easy integration of Monolithic Microwave Integrated Circuit (MMIC) Solid State Power Amplifler (SSPA). The Squarax SPC geometry allows the feeding of a higher number of MMIC than in a Waveguide SPC, so that this structure ensures high power outputs and small sizes, together with theoretical DC frequency cut-ofi. In this work, the design and simulation of a passive 4{18GHz Squarax SPC are reported.

Advanced design of a low energy electron source

A multiphysics-based modeling design of a low energy electron source using a thermionic cathode is described in this paper. The proposed device produces a narrow beam employable in delicate applications where dimensions are critical. The effects of multiple physics influencing factors due to the cathode heating over the beam dynamics have been predicted through a multiphysics design approach. This paper would provide the needed knowledge for virtual prototyping of such devices. For this aim, several strategies have been adopted to obtain a simple model, which shows clearly the investigated mechanisms. According to this study, the appropriate materials and shapes can be chosen.

Multiphysics Design of a Magnetron High Power Transfer System

This paper proposes a particular design technique of an X-Band Magnetron to Linear Accelerator (LINAC) High Power Transfer System consisting in an 8 resonant cavities Magnetron connected to a dedicated vacuum Dielectric Window. Such study is based on a Multiphysics modeling and considers thermal-structural effects due to the cathode heating for the Magnetron and the Joule effect for the Dielectric Window. In this paper, is shown how to compensate the thermal induced degradation of the device performances, by exploiting the consequent Thermo - mechanical deformation of the opportunely designed device shape, which modifies constructively the electromagnetic fields to re-increase performances. The proposed study provided the evaluation of the Magnetron Working Points and Dielectric Window Scattering parameters in Thermo - mechanical operative conditions.

A true-time-delay networks design technique

This paper proposes a technique to design wide band switched-line (SL) true-time-delay (TTD) networks, commonly used for phased array antenna (PAA) applications. This study investigates the constant-delay behavior of switched-line phase shifters based on single-pole double-throw (SPDT) switches. Circuit sizing starts by considering the effective S-parameters of the switches, to use their non-idealities as an integral part of the phase shift linearly dependent to the frequency and by considering, from the beginning, the possible spatial positioning of elements that allows the circuit feasibility as a design target. The aim of this study is to provide a technique suitable for the design of well-matched TTD networks with a flat delay in wide bandwidth. In this paper, we propose new design formulas for which we show a single-frequency implementation. A computational strategy is used to obtain numerical solutions of the derived equations with this study. Finally, a monolithic X-band TTD circuit example is shown.

Real-Time Beam Monitor for Charged Particle Medical Accelerators

A novel real-time charged particle detector for medical accelerators is proposed in this paper. The system, by means of a passive resonant cavity, performs a non destructive measurement of the beam current and provides a real time monitoring of the absorbed dose delivered to the patient. The system complies with requirements of the relevant International Standards for being used as a Radiation Detector for Charged Particles. The formulated theory allows the system design versus the accelerator features by providing analytical formulas and a development strategy. A prototype has been realized and tested in order to check the compliance with the expected behavior in the operating conditions of the Medical Accelerator. The system has been proved to be equivalent to the standard ionization based monitor chambers, furthermore offering several advantages: the absence of bias high voltage, more compactness, small size and the fact that this system measures the physical observable quantity directly related with the dose, the beam current.

Computational Model of a Buncher Cavity for Millimetric Klystron

A computational model of a Buncher cavity for millimetric klystron following a Multiphysics approach is proposed in this paper. At these narrow dimensions, the device is critically exposed to multiple physics effects, due to the power dissipations and external environment, influencing the electromagnetic performances. The cavity is integrated with a carbon nanotube cold cathode in order to reduce thermal expansion and cooled by an opportune airflow that regulates the temperature distribution to compensate the resonant frequency shift. Electromagnetic fields and scattering parameters have been tested in operative conditions by concurrent computation of coupled thermodynamic, fluid dynamic, structural mechanics and electromagnetic simulations. This approach has suggested the appropriate materials and geometrical shapes.

Injection/bunching section design of a Sub-millimetric klystron

The design of the injection and bunching section for a Sub-millimetric klystron is described in this paper. A Carbon nanotube cold cathode is employed to produce the required beam current. An opportune shape of the device has been chosen to allow for the micro-fabrication, while ensuring the correct Buncher excitation. In this range of critical dimensions, multiple physics influencing factors, due to the heating effects and power dissipations over the beam dynamics, may alter the desired behavior of the device. A multiphysics design approach has been employed to ensure the future correct operation selecting appropriate materials and shapes. Several strategies have been adopted to obtain a simple but reliable model.