acceleration gradient limitations in room temperature and superconducting acceleration structures...
TRANSCRIPT
Acceleration gradient limitations in room temperature
and superconducting accelerationstructures
Nikolay SolyakFermilab
RT Breakdown studies in recent years
• NLC/GLC and CLIC type TW and SW accelerating structures @ 11 GHz (SLAC) and 30 GHz (CERN)• Waveguide, Single cell cavities (SW, TW), • Special cavities (choke-cavity, TM01, PBG)
– Frequency dependence – Stored energy, Power and Group velocity– Pulse length dependence– Geometry (low/high impedance structure, slots, choke, phase advance, …)– Materials (Cu, CuZn, Au, Mo, Ti, AL, W, SS, …), coating– Surface preparation, Oxidation– HP processing
• Surface and data analysis, Simulations and Theoretical models (field emission models, plasma formation, pulse heating, etc…)
Aggressive R&D program to understand gradient limitations and physics of breakdown (collaboration of many institutions: SLAC,CERN,KEK,...)
Still we have no theory which can explain all data for all set of parameters and make a prediction (example: T53vg3MC (good) T26vg3 (not good))
(J.P.Delahaye)
Test Results at 11 GHz
power fit = E30
T53VG3MC-11GHzHDS60S
Summary on gradient scaling (CLIC)
consttE pa 6/130~ aEBDR
For a fixed pulse length For a fixed BDR
constBDR
tE pa 530
New constrain - modified Poynting vector which is geometry independent and can be scaled among all (TW and SW) Cu structures.
SS ImRe0000 cSW
TW
SWTW
c gHEC
CHES
• A model of the breakdown trigger based on the pulsed heating of the potential breakdown site by the field emission currents
• describes both TW and SW accelerating structure experimental results• Good agreement experiments with analytical estimate
A.Grudiev, W.Wuensch
TD18vg2.4 T18vg2.4 T28vg3 TD28vg3 CLIC_G
Sc=6.2MW/mm2 @ tpP=100ns
Average unloaded gradient [MV/m]
109 106 105 103 120
Prediction of average unloaded gradient at rect. pulse length of 100ns and BDR=1e-6 based on the results achieved in T53vg3MC: 102.3MV/m at 100ns and BDR=1e-6: Sc=6.2MW/mm2@100ns.
Predictions for test structures
Best tested CLIC type structure TD18vg2.4: Eacc ~ 105 MV/m at 100 ns
From Alexei Grudiev
“Practical” Gradient limitations in RT accelerating structure
• Stored energy (SW), RF power and group velocity (TW)– fraction of energy dissipated in tip vs. geometry?– < 1J energy dissipated in breakdown site– In TWS power absorbed in spot, before plasma built
• Breakdown triggered by field emission - Es
– Fatigue, modification of boundaries and grains
• Magnetic pulse heating – T~ (Hs2)()1/4 (plastic
deformation limit: T~50ºC for annealed Cu and ~130ºC)
• Possible other limitations (dark current, Multipactoring, couplers, windows, etc…)
• Growing tips
Theory of Breakdown
• A lot of experimental data is available, more will come soon– Need more statistics for each tested geometry
• Few models are presented, but non of them is able to explain all experiments and predict gradient limits – Do we need one theory to explain everything?– Few breakdown mechanisms few theories– Clear criteria to define which mechanism is dominated for
chosen parameters space
• Facts, which should be explained by theory– Breakdown in TM01 cavity (no surface electric field)– Factor of ~2 difference in electric field in low/ high impedance
structures– Gradient limitation in cavity with small gaps (choke cavity)– ….
• Let’s try to scale gradient limit results obtained for room temperature structures to SC cavity:
RT: Eacc~100 MV/m (Epk~200 MV/m) at 100ns.
SC: Eacc~ 20 MV/m (Epk~40 MV/m) @ 1ms or Eacc~ 7 MV/m (Epk~14 MV/m) @ 1sec
• BDR – usually not considered for SC cavity• Frequency of SC cavity is lower stored energy higher
• Conclusion: Physics of breakdown in SRF is different
Scaling results from NC to SC structures
consttE pa 6/1
Achieved Limit of SRF electric field•No known theoretical limit•1990: Peak surface field ~130 MV/m in CW and 210 MV/m in 1ms
pulse. J.Delaen, K.Shepard,”Test a SC rf quadrupole device”, Appl.Phys.Lett,57 (1990)
•2007: Re-entrant cavity: Eacc= 59 MV/m (Epk=125 MV/m,Hs=206.5mT). (R.L. Geng et. al., PAC07_WEPMS006) – World record in accelerating gradient
CW 4.2 K
EP
1.3 GHz cavities – 4 production batch
“Practical” gradient limitations for SC cavities
• Surface magnetic field – 200 mT (absolute limit?)
• Field emission, X-ray, starts at ~ 20 MV/m
• Thermal breakdown (strong limits for F>2GHz)
• Multipactoring (in cavity or couplers)
• Medium and high field Q-slopes (cryogenic losses)
• Lorentz detuning and microphonics (frequency change)
• Quality of surface treatment and Assembly
From Alex Gurevich
Energy gap is a function: = (T,Es)
Magnito-optical Imaging – A.Polyanskii & P.Lee
Limit in gradient and Magnetic field
Re-entrant single-cell
K. Saito
Pushing up SRF gradient limit
New Geometry
J.Sekutowicz, V.Shemelin, K.Saito
Traveling Wave Accelerating Structure
~ 25% (max 42%) higher accelerating gradient (vs. TESLA cavity)
• Shorter cells (105 deg phase advance) to improve transit-time factor
• No limitation (up to 10m) in cavity length
• Need tuning to cancel reflected wave
A.Kanareykin, N.Solyak, V.Yakovlev, P.Avrakhov
Field Emission
•Caused by macro-particles at the surface
•Starts typically >20 MV/m
•Exponential Q-drop
•X-ray
•Dark current
•Disappear after HPR
•Clean assembly
Thermal breakdown
Particle and hole on thesurface
Sub-mm size defects lead to quench Temperature Mapping
Hot spots ( large grain cavity )
Reduced after baking
Q-slope restored by 40 V anodization
1E+09
1E+10
1E+11
0 5 10 15 20 25 30Eacc (MV/m)
Q0
Baseline 40 V anodization1st 120C 12h bake 2nd 120C 12h bake
T = 1.7 K
1
4
7
10
13
16
19
22
25
28
31
34
S1 S
3 S5 S
7 S9 S11 S
13 S15
0
0.02
0.04
0.06
0.08
0.1
0.12
T (K)
AzimuthBottom Iris
Top Iris
Equator
Q0 = 6.1 109
Eacc = 30.3 MV/m
Large-grain single-cell
1
4
7
10
13
16
19
22
25
28
31
34
S1 S
3 S5 S
7 S9 S1
1 S1
3 S1
5
0
0.03
0.06
0.09
0.12
0.15
0.18
T (K)
AzimuthBottom Iris
Top Iris
Equator
Q0 = 2.9 109
Eacc = 24 MV/m
Large Grain : Hot SpotsLarge Grain : Hot Spots
G. Ciovati - LINAC (2006)
Conclusion
0,..,
xHTPx
TT
x cmdiss
2,2
1RFRFmsdiss HHTRP
bsbsKRFcms TTTThHHTR ,,..,2
1 2
hk
b
Thickness
Thermal Breakdown in pure Niobium
Solution for k=const (thermal conductivity)
Test result of 3.9 GHz accelerating cavity
1.E+09
1.E+10
1.E+11
0 2 4 6 8 10 12 14 16 18 20
Eacc, MV/m
Q
1.45 K
1.8 K
2.0 K
1
10
100
1000
10000
2.0 3.0 4.0 5.0 6.0 7.0
Tc/T
nOhm
s
RS
BCS
3-cell cavity built at FNAL
Final cavity preparation done at FNAL (BCP,HPWR)
Residual resistance R_res ~ 6 n Achieved: H_peak = 103 mT, E_acc = 19 MV/m
(Goal: Hpeak= 68 mT, Eacc=14 MV/m)
Magnetic field is likely limited by thermal breakdown
No Field Emission Q ~ 8*10 9 at E_acc =15 MV/m
Maximum accelerating field not depend on Temp
9-cell/3.9GHz: Eacc= 21 MV/m
TESLA: Eacc= 24 MV/mH = 103 mT
No Temperature dependence – why?
resc
RFlinBCSs
nlins R
H
HTRBTR
2
0,
2
,1
481,
resRF
cH
HT
linBCSs
nlins R
H
H
T
eTRBTR
c
RF
2/7
0,
27,2
2)(
4,
0,
0
232 ,/
,c
RF
b H
Hh
TkT
For 2 cases: β·h<<1 and β·h>>1 it gives:
1. Non-linear Surface Resistance
res
linbcss
nlins Rdt
thT
thTtTRBTR
0
2, cos
cossinhsin
2,
The non-linear BCS resistance (RF pair breaking) in the clean limit is given by
Where:
*P.Bauer et.all, “Discussion of possible evidence for non-linear BSC resistance in SRF cavity”, SRF 2005 workshop
TklinBCSs
beT
fATR
2
,
Δ ~1.5 meV is the superconducting energy gap
Surface Resistance used in models
Non-linear case - blue
- Linear BCS:
Measured values of Kapitza conductance and values used in model
2. Kapitza Resistance of Niobium surface
*A. Aizaz, “Improved Heat Transfer in SRF Cavities, NSCL, MSU, 2006** Bousson et. All, “Kapitza Conductance and Thermal Conductivity of Materials Used For SRF Cavities Fabrication”, SRF workshop, 1999
Summarized parameters a, n obtained experimentally**.
Kapitza Conductance
0,01
0,1
1
1,4 1,6 1,8 2 2,2Tb (K)
hk (W
/cm
^2/K
)Used in Model 3cellsStrained*As recieved RRR232*Bousson data**after Ti*after HT*Used in Model 9cells
Measured values of thermal conductivity* (left) and values used in models for 3 cells and 9 cells case(right)
* A. Aizaz, “Improved Heat Transfer in SRF Cavities, NSCL, MSU, 2006
3. Thermal Conductivity of the bulk Niobium
Thermal Conductivity Model
10
100
1000
1 10T(K)
W/mK
3 cells Cavity9 cells Cavity
Cavity #4 QvsE July 10 2008
1.E+08
1.E+09
1.E+10
0 2 4 6 8 10 12 14 16 18 20 22 24
Eacc, MV/m
Q
1.8K
2K
1,E+09
1,E+10
1,E+11
0 2 4 6 8 10 12 14 16 18 20Eacc, MV/m
Q
1.45 K
1.8 K
2.0 K
Example 1: 3-cell 3.9 GHz cavity
1,E+09
1,E+10
0 20 40 60 80 100 120
H surface (mT)
Q
T=1.44 KT=1.8 KT=2 K
Example 2: 9-cell 3.9 GHz cavity
1,E+09
1,E+10
0 20 40 60 80 100 120
H surface (mT)
Q
T=1.8 KT=2 K
Results for 3.9 GHz cavity
3-cell cavity
9-cell cavity
I. Gonin - FNAL
Multipacting
MP-exponential increase of electrons under certain resonance conditions
M.Liepe
Multipacting in ICHIRO cavity
Example: MP in HOM coupler
Gradient Limitation due to MP in HOM coupler.
Result: Coupler was destroyed
Reproducibility problem – Scattering of gradient
0
5
10
15
20
25
30
35
40
45
Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06
Eac
c[M
V/m
]
BCP
EP
10 per. Mov. Avg. (BCP)
10 per. Mov. Avg. (EP)
Gradients achieved over time in DESY cavities
ILC Goal
K.Saito
ILC Word-wide S0 / S1 R&D program
• Aim to develop procedure which will provide 85 % acceptance yield in cavity performance– S0 – single cavity– S1 – 9-cell cavity
– Fabricate 4x4 cavities and demonstrate yield at 4x4 test each (statistics)
S0 - Single-cell R&D program (KEK)
ILC BCD receipt
• final EP (~20μm) after a heavy material removal ( > 100μm).
• HPR (70kg/cm2,1hr)• Bake (120OC,48hr).
Eacc = 46.5± 8.0 MV/m (17%)
EP FLASH receipt
• EP(20μm)+EP(3μm, fresh acid)
• HPR (70kg/cm2,1hr)• Bake (120OC,48hr)
Eacc = 46.7±1.9 MV/m (4%)
20 25 30 35 40 45 50 55 600.98
1
1.02
1.04
1.06
1.08
1.1
1.12
1.14
1.16
1.18
1.2
Project Cost -vs- Linac Gradient
Gradient ( MV/m)
Rela
tive C
ost
ILC Main Linac
~ Eacc2 - Cryogenic losses
~ Eacc Linac/Tunnel length
Summary
• Gradient limitation in RT structure is not fully understood yet, but…– mechanisms are identified– multi-parametric studies underway – better understanding of physics– Modeling and prediction of gradient limits in real
structure
• Gradient limitation in SC cavities– Few mechanisms are studied
• Critical magnetic field• FE, thermal breakdown, MP, Q-slope, …
– New ideas to push gradient limits• Geometry optimization (LL, Re-entrant, TW structure)• multilayer
– Surface treatment and Cleaning are the key problems in SC cavity to reach stable performance.