3D multiphysics modeling of superconducting cavities with a massively parallel simulation suite

Radiofrequency cavities based on superconducting technology are widely used in particle accelerators for various applications. The cavities usually have high quality factors and hence narrow bandwidths, so the field stability is sensitive to detuning from the Lorentz force and external loads, including vibrations and helium pressure variations. If not properly controlled, the detuning can result in a serious performance degradation of a superconducting accelerator, so an understanding of the underlying detuning mechanisms can be very helpful. Recent advances in the simulation suite ace3p have enabled realistic multiphysics characterization of such complex accelerator systems on supercomputers. In this paper, we present the new capabilities in ace3p for large-scale 3D multiphysics modeling of superconducting cavities, in particular, a parallel eigensolver for determining mechanical resonances, a parallel harmonic response solver to calculate the response of a cavity to external vibrations, and a numerical procedure to decompose mechanical loads, such as from the Lorentz force or piezoactuators, into the corresponding mechanical modes. These capabilities have been used to do an extensive rf-mechanical analysis of dressed TESLA-type superconducting cavities. The simulation results and their implications for the operational stability of the Linac Coherent Light Source-II are discussed.

Figure 1

Illustration of the LCLS-II dressed cavity showing the outer helium vessel wrapped with magnetic shielding, the fundamental power coupler (on the right) and the tuner (on the left) that has both coarse (motor) and fine (piezoactuator) axial positioning control.

Figure 2

The vertical cross section (top) and full (bottom) simulation model of the LCLS-II cavity including the electromechanical interface, boundary conditions, materials and the simplified piezotuner model.

Figure 3

Spring model of the cavity in a helium tank equipped with bellows and piezotuner.

Figure 4

Vacuum model of the TESLA cavity meshed with curved tetrahedral elements. Also shown are the boundary conditions and the electromechanical interface.

Figure 5

Complex magnitudes of electric (top) and magnetic (bottom) fields (a.u.) for the 1.3 GHz acceleration mode color coded on a linear scale.

Figure 6

Cavity detuning normalized to the maximum structural displacement as a function of the mechanical eigenmode frequency. Longitudinal cavity modes are highlighted in red.

Figure 7

Illustrations of the structural displacements for three selected modes. The magnitudes of the displacements (a.u.) are color coded on a linear scale.

Figure 8

Direction and magnitude of the electromagnetic pressure (a.u.) color coded on a linear scale for the fundamental 1.3 GHz accelerator mode.

Figure 9

Direction and magnitude of the structural displacements due to a static Lorentz force.

Figure 10

Direction and magnitude of the structural displacements due to a static piezoload.

Figure 11

Maximum displacement magnitude (blue) and the corresponding phase of the load (red) as a function of the piezoactuator frequency.

Figure 12

Detuning magnitude as a function of the piezoactuator frequency ${\mathrm{\Phi }}_{PZ}$ calculated directly from the structural deformations at each frequency (red) and by summing the modal response functions (blue).

Figure 13

Measurements and simulations of cavity detuning as a function of the piezoactuator frequency—the curves are described in the text.