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J. Giner Navarro - CLIC WS2015 1
Statistical analysis of RF conditioning and breakdowns
Jorge GINER NAVARROCLIC Workshop 2015
26/01/2015
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J. Giner Navarro - CLIC WS2015 2
Overview• Introduction• Conditioning data from test stands• Magnitudes to describe conditioning status• Comparison of different structure conditionings• Conclusions
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J. Giner Navarro - CLIC WS2015 3
Introduction• Performance of CLIC accelerating structures has been tested in
klystron-powered test stands at KEK, SLAC and CERN.• New prototypes of accelerating structures need to be conditioned
to achieve the CLIC main requirements of gradient 100 MV/m at a pulse length of around 200 ns and a low breakdown rate (BDR) of 10-7 bpp/m.
• RF conditioning process needs to be understood in order to minimize time and costs.
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J. Giner Navarro - CLIC WS2015 4
Xbox-1: TD26CC#1 conditioning historyAutomatic operation by a conditioning algorithm
[see J.Tagg presentation]
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11168 BDs
B. Woolley - CLIC WS2014
J. Giner Navarro - CLIC WS2015 5
Rescaled gradient
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BDR= 7e-5 bpp 2e-5 bpp 2e-6 bpp
CONDITIONINGBDR
measure
TD26CC#1 raw data
Equivalent gradient curve with constant pulse length of 250 ns since the beginning
Equivalent gradient curve with constant pulse length of 250 ns since the beginning and constant BDR of 2e-5 bpp
J. Giner Navarro - CLIC WS2015 6
Describing conditioning status
Analogously, we can define a “scaled BDR” :
which gives the BDR evolution at a fixed gradient and pulse width.
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We can define a “scaled Gradient” to describe the conditioning level of the structure:
which gives the Gradient curve keeping constant pulse width and BDR.
Using the scaling law: [ A.Grudiev et al, Phys. Rev. ST AB 12, 102001,1 (2009) ]
These magnitudes gives us the conditioning status of the structure according to our requirements.
J. Giner Navarro - CLIC WS2015 7
NEXTEF (KEK): TD24R05#4 test historyConditioning and fixed-gradient tests carried out in NEXTEF test stand for the TD24R05 structure provides a source of comparison with our data. Data courtesy of T. Higo.
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Normalized gradient here
Scaled gradient
TD24R05_#4
J. Giner Navarro - CLIC WS2015 8
Comparison of conditioning evolution
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Scaled gradient vs cumulative number of PULSES
Scaled gradient vs cumulative number of BREAKDOWNS
Conditioning to high-gradient is given by the pulses not the breakdowns!
𝐸0∗
𝐸0∗
#Pulses
#BDs
J. Giner Navarro - CLIC WS2015 9
Further studiesAccording to these results, pulsing at constant gradient would slowly decrease the breakdown rate in the structure, which means that the surface is well influenced by the RF high-powered pulses.
Study of long term trends, whether there is an asymptotic BDR or not, and the time needed to complete the conditioning is hard to determine.
High-repetition rate systems are more efficient in this study.[see A. Korsback presentation]
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J. Giner Navarro - CLIC WS2015 10
HRR Fixed-Gap system data analysis
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In the Fixed-gap system, at DC Spark lab (CERN), high electric fields are reproduced between two Cu electrodes, pulsing at a repetition rate up to 1 kHz. Analogous studies to RF accelerating structure tests can be driven in less time.Here we compare its conditioning evolution in terms of surface electric field.
Data courtesy of N.Shipman
J. Giner Navarro - CLIC WS2015 11
RF Breakdown statistics
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101
102
103
104
105
106
10-10
10-8
10-6
10-4
10-2
102
104
106
10-8
10-7
10-6
10-5
10-4
10-3
10-2
Pulses
Pro
babi
lity
Den
sity
Fun
ctio
n
101
102
103
104
105
106
10-8
10-7
10-6
10-5
10-4
10-3
10-2
103
104
105
106
107
10-8
10-7
10-6
10-5
10-4
TD26
T24 doglegCrab cavity
8.8e-6 bpp6.5e-6 bpp
2.0e-6 bpp
2.1e-3 bpp 2.3e-3 bpp
3.1e-4 bpp
1.1e-4 bpp
Analysis in Breakdown statistics shows different regimes of the BDR.[See Anders Korsback presentation for full analysis in a DC system]
J. Giner Navarro - CLIC WS2015 12
Conclusions• Working on data analysis from test stands provides a better understanding about
the conditioning process, the goal of which is the feasibility and the proper performance of the accelerating structure in the linear collider.
• Scaling laws are used to compare different structure conditionings and the same trends are found with the number of pulses, but not with the number of breakdowns.
• Different models (dislocations, local tips…) are being studied to describe the effect that the wall’s surface resists more power with increasing pulses.
• High repetition rate systems would provide valuable information in this study. The Fixed-Gap system in the DC Spark lab is running to acquire new fresh data.
• Results from this study lead to more strategies to carry out during the conditioning of the RF structure. Optimization in time (and cost) will be essential when producing new structures.
Thank you for your attention!26/01/2015
Acknowledgement to A. Degiovanni and W. Wuensch for their contribution in this work!
15
Normalized BDR in LOG-LOG scale𝐵𝐷𝑅𝐸𝑎30 𝑡𝑝
5
𝐵𝐷𝑅❑∗ ∝𝑁 𝑝𝑢𝑙𝑠𝑒𝑠
𝑨′
log (𝐵𝐷𝑅¿¿❑∗)∝ 𝑨 ′ log (𝑁¿¿𝑝𝑢𝑙𝑠𝑒𝑠❑)¿ ¿
16
Normalized BDR in LOG-LOG scale – Linear fit
A’ = -8.35 +/- 0.02
A’ = -7.74 +/- 0.02
𝐵𝐷𝑅𝐸𝑎30 𝑡𝑝
5
Pivot model
BDR = 1 (limit for operation by definition)
Emax (assumed limit for gradient)
The exponent X increases with the number of pulses (n)
log(BDR)=X(n)*log(E0)
Emeas = α Emax
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70 80 90 100 110 120 130 140 155
A.D 02.04.2014