Speaker
Description
In high power millimetre wave transmission systems, even small unwanted effects can have an impact on the system functionality. Therefore, a precise knowledge of the fields and loss mechanisms for realistic scenarios is required during the design phase. An important issue is the
effect of higher order modes (HOMs) on the performance of the equatorial and upper ECRH launchers for ITER. The exact simulation of mode mixtures in optical systems requires the knowledge of the complex mode amplitudes. These are, however, impossible to predict because HOMs are mostly the result of manufacturing tolerances or assembly errors. Furthermore, time
dependent phenomena, like thermal expansion of waveguides or long-term movement of buildings, play a role. Therefore, a Monte Carlo approach is the only option to assess the robustness of a system with respect to different spectra of HOMs [1].
The PROFUSION code package [2] contains a solver for the reflection of arbitrary fields on almost arbitrarily shaped metallic surfaces. It was already used for the simulation of the in-vessel EC optics for the first ITER plasma of the 2016 baseline [3]. Some new features had to be
implemented for the spurious mode analysis of the ITER launchers as explained below.
Beam truncation is an important issue for ECRH launchers because the microwave has to pass through massive shielding structures, where the openings are kept as small as possible. HOMs are critical here because they have larger beam divergences. A typical way to model truncation
effects is to calculate the field on an imaginary surface and apply the beam truncation using a given boundary curve. This yields both the power loss, by means of integrating the power density outside of the contour, and the resulting deformation of the beam after the truncation. With our original FFT-based propagator, this worked only for planar surfaces perpendicular to the beam axis. Our new propagator is based on the surface equivalence principle and works also for non-planar surfaces.
This allows a whole new class of problems, including the beam truncation by almost arbitrary obstacles, to be simulated with high accuracy.
Another addition was the modelling of the steering mechanism within PROFUSION. Given the mirror parameters for a nominal steering angle of 0° and the direction and position of the rotation axis, we can now generate the complete set of parameters, including the movement of the incident points on the mirror surface, for arbitrary angles. This allows the complete simulation of the launchers without exporting and converting the data for each angle from the CAD model.
The Monte Carlo analysis works by running the solvers only for the pure modes and saving the relevant fields for these fundamental solutions. Due to the linearity of Maxwell’s Equations, we can then obtain the results for a large number of mode mixtures by calculating the linear combinations of these fundamental solutions. This is significantly faster than running the solvers for every single mode mixture. The code was changed to allow access to any intermediate fields at arbitrary locations in the system and standard performance parameters like truncation losses and peak power densities can be calculated for each mode mixture.
This work was partly funded by the ITER organization under the service contract IO/23/CT/4300002949.
References
[1] B. Plaum, "Estimation of the Effects of Spurious Modes in Linear Microwave Systems Using a Monte Carlo Algorithm," in IEEE Journal of Microwaves, vol. 3, no. 3, pp. 1061-1067, July 2023, doi: 10.1109/JMW.2023.3283152
[2] B. Plaum, "Simulation of microwave beams with PROFUSION (2022 Edition)", 2022, [online] Available: http://elib.uni-stuttgart.de/handle/11682/12258.
[3] B. Plaum, M. Preynas and M. Choe, "Calculations for the optical system for the first ITER plasma", EPJ Web Conf., vol. 277, 2023.
