1. Possible Monopole Modes for Plasma Processing in TESLA Cavities
Zenghai Li, Liling Xiao, Cho-Kuen Ng
SLAC National Accelerator Laboratory
Dec.05, 2016

2. Modeling Issues for Plasma Processing
RF Modeling using ACE3P
Calculate passband modes in cavity and determine the required power for plasma processing.
Evaluate multipacting barriers as a function of SEY, which is reduced after surface processing.
Plasma simulation using WARP
Using RF field from ACE3P, determine onset of plasma ignition of neon gas.
Introduce oxygen plasma into the background neon plasma, study the dynamics of oxygen atoms and determine their impact density on cavity walls.

3. TESLA Cavity Monopole Passband Modes
R_beampipe=39mm
Fc(dipole)=2.2GHz
Fc(mono)=2.9GHz
Evaluate the first three passband monopole modes (<Fc) to find suitable modes for plasma cleaning Fc

4. TESLA Cavity Model Upstream HOM1 FPC Downstream HOM2
Up stream beampipe L=141.6mm, down stream L=105.6mm
Two beampipe ends shorted for plasma cleaning
Ignition in the two HOM couplers is preferred

5. Monopole Modes Damping Through HOMs
1st passband modes have poor coupling to HOMs.
2nd and 3rd passband modes have better coupling to HOMs.
There are modes coupling to dipole and quadruple modes in 2nd and 3rd monopole passband.

6. 2nd & 3rd Passband Monopole Modes Damping Through HOMs
F=2741MHz
F=2765MHz
F=2401MHz
Couple to Quad.
F=2387MHz
Couple to dipole

7. 1st Passband Monopole Modes (Same Passband Modes)

8. Plasma cleaning is done at room temperature
Take Nb 300k.

9. max.Es=10kV/m to initiate a glow discharge using π-mode
F (GHz)
Qin_FPC
300k
Pc_Nb(W)
Pin_FPC (kW)
1.3
3e6
1e3 23 19
F (GHz)
Qin_FPC
Q0_Cu
Pc_Cu(W)
Pin_FPC (kW)
1.3
3e6
3e4
0.74
0.019
β = Q0/Qin << 1, Pin = Pc/[4β/(1+β)^2] ~ Pc/(4β)
Copper: β~1e-2, Pc~0.7W, Pin~19W
β~3e-4, Pc~23W, Pin~19kW
Paolo’s estimation is 1.5kW. Higher Q0?
In the same passband modes, only pi-mode can be used for initiation in FPC coupler. Required power is about 18kW.

10. 2nd Passband Monopole Modes

11. max.Es=10kV/m to initiate a glow discharge using 2nd pass band modes
Not include the two mixed modes
F (Hz)
Qext_1
Qext_2
Q0_Nb
@ 300k
Pc_Nb (W)
@ 300k, Es=10kV/m
Pin_1 (kW)
Pin_2 (kW)
5.97e5
1.18e6
963
34.0
5.11
1.01e5
4.11e5
993
29.7
1.01
3.78e4
6.54e5
979
29.9
0.33
2.71e4
1.70e5
850
30.1
0.24
2.74e4
7.61e4
985
30.3
0.21
3.39e4
5.76e4
981
30.6
0.23
5.25e4
4.67e4
975
31.0
0.29
1.22e5
4.06e4
969
31.3
8.34e5
4.45e4
964
31.5
0.25
In the 2nd monopole passband, required power in HOM coupler for plasma ignition is about 300W.

12. 3rd Passband Monopole Modes

13. max.Es=10kV/m to initiate a glow discharge using 3rd pass band modes
Not include the two mixed modes
F (Hz)
Qext_1
Qext_2
300k
Pc_Nb 300k, Es=10kV/m
Pin_1 (kW)
Pin_2 (kW)
2.70e5
2.86e5
1393
83.4
4.04
8.86e4
9.91e4
1389
83.9
1.34
5.87e4
7.47e4
1384
54.1
0.57
5.38e4
8.82e4
37.8
0.37
6.06e4
1.85e5
1390
32.6
0.36
8.08e6
1.84e5
1398
21.0
0.68
2.38e5
1.63e5
1259
32.2
1.41
2.68e5
2.03e5
815
14.5
0.50
1.01e6
2.50e5
1457
12.9
0.54
In the 3rd monopole passband, required power in HOM coupler for plasma ignition is about 600W.

14. Candidate Monopole Passband Modes
Monopole Band
in FPC for Es=10kV/m 300k
in HOMs for Es=10kV/m
300k
1st 1.30GHz
18kW (Qin=3e6)
2nd 2.39~2.46GHz
200~330W
3rd 2.69~2.77GHz
360~700W
1st
2nd
3rd
The required power for ignition is estimated based on 6e4s/m of Nb electrical conductivity at room temperature.