multipacting in pip-ii 650 mhz.pptx...
Post on 25-Aug-2018
215 Views
Preview:
TRANSCRIPT
Multipacting with space charge effect in PIP-II 650 MHz cavities and their power couplers
Gennady Romanov
January 26, 2016
Content
• On parameters of MP simulations without space charge effects.• PIP-II 650 MHz cavities models and workflow.• Simulations with space charge effects ON, parameters.• MP in the cell of low β cavity and comparison with simulations
without space charge effects.• MP in the cell of high β cavity.• MP in power couplers.• Suppressing multipacting in the cells.• Conclusion
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities2
Exponential growth rate
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities3
Three reasons to be very careful about exponential growth rate:1. MP is space charge limited and self-destructive process, and in fact there is no exponential growth of number of
particles.2. Speculations on MP intensity based on α value are misleading, because there is no correlation between α and total
MP current.3. In some cases growth of number of particles is not exponential even without space charge effect.
From CST webinar on MP
CST automatic detection of MP
te
CST criterion
Space charge effect ON
Effective SEY
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities4
In some cases particle number growth is not exponential even without space charge effect.
From CST webinar on MP
This s a real device designed, built and tested in aerospace industry.
; SEYcollision
emission
II
Effective <SEY> :• Detects multipacting regardless character of particle growth.• Being compared to SEY of material it indicates how “good” dynamic conditions in a cavity are for MP
. (Paolo) )( if , SEY
0t
T
eNtNe
<SEY> >1 - MP<1 – no MP
Models and workflow
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities5
00.5
11.5
22.5
3
0 200 400 600 800 1000
SEY
Incident energy, eV
True SEY vs incident energy
Nb wet treatedNb bakedNb discharge cleaned
PIC simulations with space charge ON requires more iterations per time step. But due to the limited number of particles they are surprisingly faster than the ones without space charge effect.
For each model the simulations are performed with every SEY. The energy of collisions Wcoll, power of collisions Pcoll, power of secondary emission Pemitand secondary emission current Iemit averaged over last RF period and total number of particles are calculated as the functions of effective voltage of a cavity (energy gain).
From A.Lunin&I.Gonin
β=0.61
β=0.92
EM solver
PIC solver
Furman model
Space charge is ONNew criterion:<I_emission> = const ≠ 0
Min size 0.2 mm
Min size 0.35 mm
Select to start movie
MP in low β cell, nominal energy gain 11.9 MeV
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities6
0
100
200
300
400
500
600
0 5 10 15 20
W, e
V
Energy gain, MeV
Low β, W_collision, wet treatment
Charge OFFCharge ON
0
0.5
1
1.5
2
2.5
0
2
4
6
8
10
0 5 10 15 20
<SE
Y>
I, A
Energy gain, MeV
Low β, I_emission and <SEY>, wet treatment
I emission<SEY>
0
2
4
6
8
10
0 5 10 15 20
I, A
Energy gain, MeV
Low β, I_emission
Wet treatmentBakedDischarge cleaned
0100200300400500600
0 5 10 15 20
W, e
V
Energy gain, MeV
Low β, W_collision
Wet treatmentBakedDischarge cleaned
Experimental (SRF2011, MOPO070) MP barriers in single cells: Eacc = 7-8 MV/m (4.9-5.6 MeV, the barriers are weak)=>
MP band
I_emission is notproportional to SEY (!)
MP in high β cell, nominal energy gain 19.9 MeV
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities7
00.511.522.53
02468
1012
0 5 10 15 20 25
<SE
Y>
I, A
Energy gain, MeV
I_emission and <SEY>, wet treatment
I emission<SEY>
0
100
200
300
400
500
0 5 10 15 20 25
W, e
V
Energy gain, MeV
W_collision, wet treatment
Charge ONCharge OFF
02468
1012
0 5 10 15 20 25
I, A
Energy gain, MeV
I_emission
Wet treatmentBakedDischarge cleaned
0
100
200
300
400
500
0 5 10 15 20 25
W, e
V
Energy gain, MeV
W_collision
Wet treatmentBakedDischarge cleaned
Non-resonant MP
MP in the power couplers
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities8
β = 0.92
β = 0.61
The low beta power coupler is practically free of MP – it’s very week and observed in the coaxial part . In the high beta power coupler more powerful MP develops also exclusively in the coaxial line. In both cases a potential on the central electrode is supposed to be applied to suppress this multipacting.
0.001
0.01
0.1
1
10
100
0 5 10 15 20 25
I_em
issi
on, A
Energy gain, MeV
I_emission, log scale
Low betaHigh beta
0
0.5
1
1.5
2
3 5 7 9 11 13 15
I, A
Energy gain, MeV
Impact of convexity
Normal
Convexity
Suppressing MP with convexity
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities9
S. Kazakov
Hφ
≈10 times
≈3 times
E
W_coll = 65 eV W_coll = 103 eV
Conclusion
• PIC simulations of MP with space charge ON require equal or even less CPU time compare to ones with space charge OFF.
• Beam dynamic of MP with space charge ON is very different.• From practical point of view it is important that low boundary
of MP is the same with and without space charge, while total MP band is more narrow with space charge ON.
• With proper surface preparation MP should not be a problem for both high beta and low beta cavities.
• MP found in the high beta power coupler, but planned electric potential on the coupler central electrode should suppress it.
• A convexity works with space charge ON, though not as effectively as predicted by simulation without charge effects.
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities10
Backup 1
26/I-2016G.Romanov | Multipacting in PIP-II 650 MHz cavities11
SEY_max=2.8 (wet treatment)
SEY_max=1.25 (discharge cleaned)
For higher SEY multipactor occupies bigger area, also it is more distributed over phases ( in time). That’s why more particles (and higher I_emis) at the same current density.
top related