29th FCC-ee Optics Design meeting

Friday 29 Apr 2016, 09:00 → 10:00 Europe/Zurich

6/R-012 - conference room (CERN)

Presents: Bastian Haerer, Bernhard Holzer, Mike Koratzinos, Luis Medina, Mauro Migliorati, Katsunobu Oide, Salim Ogur, Sandra Aumon. 

Trapped electromagnetic modes can be generated by various protrusions in the vacuum chamber (BPMs, flanges, bellows, etc). They are characterized by their frequency f, the quality factor Q, and the shunt impedance R. Typically, f for these modes is in the range of GHz. A candidate for high-Q trapped mode is the beryllium pipe in the interaction region. If the mode with large Q is resonantly excited by the beam with I=1.5 A, the local heating can reach tens of kW. So detuning (making the mode frequency not equal to an integer of the RF frequency) and de-Quing (suppressing the quality factor to low values with absorbers) is important. Unfortunately, the filling pattern of the beam does not effect much the heating, although some research in this direction might be needed. Also, I simulated the effect of 10% (rms) fluctuations of bunch charges on the strength of the beam resonances and hence heating. The effect is very small. 

Bernhard Holzer asked the power loss in SuperB compared to FCC-ee. Gennady replied that the order of magnitude is about 0.1kW whereas FCC-ee is not far from 1MW. However, the computations were done is a worst case scenario. Gennady recommended to not have trapped mode at the IP. 

Mauro asked then about possible issues with excited waveguide modes, that their propagation needs to be studied. Katsunobu replied that SuperKEK uses vacuum pump to absorb them. Katsunobu asked about modes at low frequencies, which should be small and therefore not an issue.

09:00 → 09:30 Local heating due to trapped modes in vacuum chamber

Speaker: Gennady Stupakov (SLAC)

trapped_modes_viewgraphs.pdf