1. RF modes for plasma processing
P. Berrutti

2. 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

3. 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 MHz
The plot on the right shows S21 measurement taken on CAV008, having the 1/9pi mode at MHz and pi mode at 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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