RF modes for plasma processing P. Berrutti

Monopole 2nd passband 9th mode HOMs Coupler Qext 2nd monopole passband Outline Monopole 1st pass band Coupler Qext range Monopole 2nd passband 9th mode HOMs Coupler Qext 2nd monopole passband End-group single cell considerations Conclusions P. Berrutti 11/27/2018

Monopole 1st pass band The first monopole passband for 9-cell LCLS-II cavities, received from the vendor, is included in the frequency range going from 1270 to 1302 MHz The plot on the right shows S21 measurement taken on CAV008, having the 1/9pi mode at 1274.356 MHz and pi mode at 1298.263 MHz. In general one could assume that a bandwidth of 32 MHz is sufficient to cover the first monopole pass band. P. Berrutti 11/27/2018

Monopole 1st pass band electric field on axis Going from 1/9pi to pi mode the electric field distribution changes, allowing plasma localization, preferably, in different cells. P. Berrutti 11/27/2018

Coupler Qext range FPC Qext is adjustable, the full range will probably allow reaching min Qext of approx. 3-4E6 for accelerating mode 1.3 GHz. HOMs couplers do not provide good coupling for the first pass band. QHOM1 1.1e11 3.4e10 2.2e10 2.1e10 3.0e10 6.8e10 3.1e11 2.0e13 1.4e13 QHOM2 3.1e10 7.1e10 3.3e11 Freq., [GHz] 1.2763 1.2784 1.2816 1.2856 1.2898 1.2938 1.2971 1.2992 1.3000 P. Berrutti 11/27/2018

2nd monopole pass band 9th mode In principle a good candidate to confine the plasma around the iris area. Field asymmetry due to the different shape of end-cells. Power couples through one of the two HOMs couplers because of field asymmetry. 3D field distribution of 2nd monopole pass band pi mode. Field distribution on axis (blue) and at 30 mm offset (green), beam pipe radius 35 mm. P. Berrutti 11/27/2018

HOM couplers Qext 2nd monopole pass band HOM couplers have good coupling for this pass band, around 2E5, it would help igniting the plasma in the cavity, rather than in the coupler: less reflected power. Plasma ignition Curve from SNS coupler Qext=7E5 HOM1 and HOM2 have different Qext because of field asymmetry Freq. [GHz] 2.382 2.387 2.395 2.405 2.418 2.431 2.443 2.453 2.458 QHOM1 8.2e5 2.6e5 1.2e5 1.7e6 3.8e5 4.2e5 6.6e5 1.5e6 7.6e6 QHOM2 6.8e8 3.0e8 5.0e6 3.2e7 2.9e4 8.7e4 1.4e5 1.7e5 3.0e5 P. Berrutti 11/27/2018

End-group single cell considerations One single cell has been built using end-groups: first and last half-cells of a 9-cell cavity have been welded together. HOM couplers, Power coupler and pick up ports are available. Possibility of coupling HOMs through dedicated couplers. Installation of FPC on a single cell. 2nd monopole passband pi mode shows almost even field distribution, despite the asymmetry. P. Berrutti 11/27/2018

Conclusions 1st monopole passband can be excited: several fields patterns allowing plasma confinement in different cells. Qext of LCLS-II coupler can reach approx. 4E6. HOM couplers allow for better coupling (approx. 2E5), need to use HOMs. The pi mode of the 2nd monopole pass band has been considered, more modes can be examined to see if any is suitable, preferably no dipoles and quadrupoles. The end group single cell can provide a simple proof of principle experiment. P. Berrutti 11/27/2018