Prediction of multipactor in the iris region of rf deflecting mode cavities

Multipactor is a major cause of field limitation in many superconducting rf cavities. Multipacting is a particular issue for deflecting mode cavities as the typical behavior is not well studied, understood, or parametrized. In this paper an approximate analytical model for the prediction of multipactor in the iris region of deflecting mode cavities is developed. This new but simple model yields a clear explanation on the broad range of rf field levels over which the multipactor can occur. The principle multipactors under investigation here are two-point multipactors associated with cyclotron motion in the cavity’s rf magnetic field. The predictions from the model are compared to numerical simulations and good agreement is obtained. The results are also compared to experimental results previously reported by KEK and are also found in good agreement.

Figure 1

${\mathrm{HM}}_{11}$ mode transverse electric field determined from (7, 8) with iris walls shown in red and example electron trajectory in blue.

Figure 2

A typical trajectory showing the orbit center $C$ and ${\varphi }_{\mathrm{neg}}$. The iris curvature has been exaggerated for illustrative purposes.

Figure 3

An example of a resonant trajectory in the iris of a dipole cavity, calculated using the tracking code, for a 70 mm radius iris, rf frequency of 800 MHz, and a magnetic field of 54 mT.

Figure 4

Acceleration terms for a launch phase of $-45$ degrees where the radius is calculated from the center of the electron orbit.

Figure 5

Launch phase of resonant electron trajectories as a function of peak magnetic field for a 70 mm radius iris and rf frequency of 800 MHz.

Figure 6

Phase focusing of the electrons towards the resonant phase for electrons starting at the incorrect phase.

Figure 7

Phase focusing of the electrons towards the resonant orbit.

Figure 8

Electron trajectories for various initial electron energies.

Figure 9

Orbit radii as a function of initial electron velocity.

Figure 10

Orbit radius against peak surface magnetic field calculated using tracking code and with analytical calculations.

Figure 11

Impact energy as a function of electron starting energy.

Figure 12

Comparison between the numerically calculated impact energy and the analytically predicted, for multipactor in a rf deflecting cavity with an iris of 70 mm radius.

Figure 13

Second order multipactor trajectory.

Figure 14

Electron trajectories from CST Particle Studio. The left inset shows the position of the multipactor on the iris.

Figure 15

Maximum $⟨\mathrm{SEY}⟩$; as a function of transverse voltage from CST Particle Studio for various 800 MHz cavity geometries. The iris radius, $R$, and the iris rounding, $A$, are given for each geometry.

Figure 16

Maximum $⟨\mathrm{SEY}⟩$ as a function of peak surface magnetic field from CST Particle Studio for various 800 MHz cavity geometries. The iris radius, $R$, and the iris rounding, $A$, are given for each geometry. The red line shows the predicted onset of multipactor.