SPS Enamelled flanges Simulations & Measurements. Fritz Caspers and Jose E. Varela

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Cody Long
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1 SPS Enamelled flanges Simulations & Measurements Fritz Caspers and Jose E. Varela

2 Outline Introduction Simulations Measurements Conclusions

3 Outline Introduction Simulations Measurements Conclusions

4 Introduction We are tracking down a, possibly, microwave instability at 1.4GHz in the SPS. We are working now to asses the possibility that the enamelled flanges are the ones responsible of this instability. Flanges are located all around the ring. Approximately there are 1500 enamelled flanges in the SPS.

5 Introduction There are eight different beam pipes in the SPS. In addition, in some cases there are bellows attached to the flanges. There are two types of bellows. The impedance issues of the bellows were already studied in 1974 by G. Dome. Resistive cylinders were added to some bellows. However, whether or not these dampers are still present in the machine is unknown. Many different cases have to be assessed

6 Introduction Enamelled Flange + Long Bellow + No Resistor Enamelled Flange + Short Bellow + No Resistor Enamelled Flange + Short Bellow + Short Fixed Resistor Enamelled Flange + Long Bellow + Long Fixed Resistor

7 Introduction Simulations Outline Model Flange + Straight section beam pipe Flange + QD beam pipe Flange + QD beam pipe + Short bellow Measurements Conclusions

8 Simulations - Model The flanges are electromagnetically complex. The flanges have a 156mm diameter. There is a 2.5mm separation between each side. The enamel layer is typically mm thick.

9 Simulations - Model

10 Simulations - Model Accurate modelling of the flange implies 30Mcell simulations. From the EM point of view we have an open structure. Thus, enough space must be left outside.

11 Outline Introduction Simulations Model Flange + Straight section beam pipe Flange + QD beam pipe Flange + QD beam pipe + Short bellow Measurements Conclusions

12 Simulations In the first place we consider a flange between two straight section beam pipes, i.e. φ=156mm circular waveguides. The cut-off frequency of the beam pipes is GHz. First higher order mode at 2.34 GHz. Both marked with green circles in the figure.

13 Simulations Assuming that the simulation using PMLs is correct. The longitudinal electric field is along a longitudinal plane: Ez Enamel layer The enamel layer + the diamondlike middle ring make sort of a laterally-open ridge waveguide. The field maximum is marked by the red circle. The beam sees the fringing fields of the waveguide. Fringing fields

Ridge Waveguides Well known waveguides in communication

14 Ridge Waveguides Well known waveguides in communication systems. When compared to rectangular waveguides, we have: Broader mono-mode bandwidth -> More info can be transmitted. Higher conductor losses -> Due to the higher field density in the central region. Ff = 1.03 GHz Fh = 2.05 GHz Ff = 0.76 GHz Fh = 2.06 GHz Ff = 0.54 GHz Fh = 2.11 GHz

15 Outline Introduction Simulations Model Flange + Straight section beam pipe Flange + QD beam pipe Measurements Conclusions

16 Simulations Let us now consider a flange near a QD, where there are circular beam pipes with a φ=83mm diameter. In this case the first beam pipe mode with the appropriate symmetry has a cut-off frequency of 2.765GHz. The difference between the size of the beam pipe and the radius of the flange creates a cylindrical pillbox cavity. The resonance frequency of the ideal TM010 mode is GHz.

17 Simulations Several simulations have been carried out. Significant differences have been obtained in the longitudinal impedance for different beam sizes. Wake potentials did not converge in any case. 10% relative error between peaks

Simulations Plot of Ez over a longitudinal plane for a certain time for the σ = 20mm simulation. Flange Resonance With some vorstellungskraft we can see two resonances.

18 Simulations Plot of Ez over a longitudinal plane for a certain time for the σ = 20mm simulation. Flange Resonance With some vorstellungskraft we can see two resonances. Cavity Resonance The one that we saw before in the flange, and the one produced by new cavity. Both fields are out of phase in the time domain. Currently, a simulation is running with frequency field monitors to check.

19 Simulations Additional eigen-mode simulations of the structure have been carried out. Since the structure must be closed to run this type of simulation additional resonances are created. Significant influence of the outer walls has been found. If the boundaries are too far away -> Low frequency resonances are created. If the boundaries are too close -> The resonances of interest are perturbed. Two resonances have been found to be independent of the boundary conditions at 1.34 and 1.67 GHz.

20 Simulations The 1.34GHz resonance:

21 Simulations The 1.67GHz resonance:

22 Outline Introduction Simulations Model Flange + Straight section beam pipe Flange + QD beam pipe Measurements Conclusions

23 Measurements Wire measurement on a complete BPH.

24 Measurements Transmission and reflection coefficients of the complete BPH. The 1.4GHz peak is the only one that moves when a perturbation is introduced near the enamelled flange.

25 Outline Introduction Simulations Model Flange + Straight section beam pipe Flange + QD beam pipe Measurements Conclusions

26 Conclusions The problem under consideration is very complex. Simulations have not provided consistent results. New measurements will be carried out soon. Work in progress any ideas are welcomed.

Waveguides. Metal Waveguides. Dielectric Waveguides

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