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W. Wuensch, rf development meeting 25-3-2015
Considerations on running normal conducting cavities cold
2
Recent results of the cryo-cooled normal conducting structure
Structure is running now at an accelerating gradient of 300 MV/m with a breakdown rate ~ 10-6 /pulse/meter.
S. Tantawi, CLIC workshop 2015
W. Wuensch, rf development meeting 25-3-2015
• Power• Gradient
W. Wuensch, rf development meeting 25-3-2015
Our standard conditions:
Average 12 GHz power per meter
Of which 1.6kW/m is dissipated in the copper, the rest goes to the beam and loads.
Mains to main linac 12 GHz efficiency η=33.7%=1/3
So the mains power going into copper dissipation is 4.8kW/m
W. Wuensch, rf development meeting 25-3-2015
Resistive losses in copper decrease as you cool it. Interesting number for superconducting magnet beam screens.
• Factor 4 in surface resistance. Roll-off due to anomalous skin effect and lattice defects.
• 50 K is near optimum point for operation – maximum gain in Q before it flattens out.
Ongoing test: Cryogenic Testing of normal conducting accelerating structures
•To design the structure we
used our detailed
measurements for copper
conductivity at 11.424 GHz
using specialized cavities
•Conductivity increases (by a
factor of 17.6 at 25K), enough
to reduce cyclic stresses.
•The yield strength of copper
also increases.
5 104
1 105
1.5 105
2 105
2.5 105
0 50 100 150 200 250 300
7N_LG_S2Before and after ~10m etch
Q0(7NLGS2_Mar232010)Q0(7NLGS2_Etch_Apr302010)Q
0
T
RF in
(Slide S. Tantawi, my arrow)
Same surface resistance change!
W. Wuensch, rf development meeting 25-3-2015
We need to pump the dissipated heat. Carnot efficiency at 50 to 300 K:
But real cryogenic systems don’t achieve full Carnot efficiency. The overall power factor for the FCC 50 K cryogenic system is estimated as factor 20, which corresponds to 25% of Carnot.
Power to heat pump dissipated rf:
Reduced need to produce rf power:
Total mains power to cover dissipation 9.2kW/m cooled, compared to 4.8kW/m not cooled.
n.b. – this is at the same gradient! Power goes up as gradient2
HEP Budget Briefing, Mar 6 2015 8
Understanding the Physics of High Gradients has Established the Limits of Normal Conducting Copper Structures
50 100 150 200 250 300 35010 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
10 0
Gradient [M V /m ]
Bre
akdo
wn
Pro
babi
lity
[1/p
ulse
/met
er]
a0.105 , t2.0 mm , Ag 1150 ns1st slctd1ma0.105 , t2.0 mm , Clamped Ag 3150 ns slctda0.105 , t2.0 mm , 1150 nsa0.105 , t2.0 mm , Cryo 2 45 K150ns slctdm
Cu@45KHard CuAg#1
Soft Cu
Hard CuAg#2
100 200 300 400 500 600 70010 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
10 0
P eak E lec tric F ield [M V /m ]
Bre
akdo
wn
Pro
babi
lity
[1/p
ulse
/met
er]
a0.105 , t2.0 mm , Ag1150 ns1st slctd1ma0.105 , t2.0 mm , Clamped Ag3150 ns slctda0.105 , t2.0 mm , 1150 nsa0.105 , t2.0 mm , Cryo2 45 K150ns slctdm
Cu@45KHard CuAg#1
Soft Cu
Hard CuAg#2Hard Cu
Narrow error bar will be obtained after recalibration; work in progress
• Basic physics experiments move to testing normal & SC engineered materials (2016-2017).
• Cost effective implementation of accelerator structures capable of operating efficiently at these gradients (basic development 2016-2017, and then growing effort to 2020)
• Build RF sources that can power these structures to high gradients (basic development 2016-2017, and then growing effort to 2020)
• New architectures for future facilities (colliders, light sources, etc.) will emerge when efficient RF systems to power linacs operating at these gradients become available
• Immediately, this technology will lead to RF guns with unprecedented brightness.
+60%
W. Wuensch, rf development meeting 25-3-2015
We didn’t measure breakdown rate and quote “maximum.” From memory was probably around 10-2
W. Wuensch, rf development meeting 25-3-2015
250 300 350 400 450
10
8
6
4
2
50 K
300 KEsurface [MV/m]
Log(BDR)
𝐵𝐷𝑅∝𝑒−𝐸 𝑓 +𝜀 0𝐸❑
2 ∆𝑉𝑘𝑏𝑇
𝐸 𝑓=0.8𝑒𝑉
∆𝑉=0.8×10−24𝑚3
E30
+20%
+70%
(being near BDR=1 field worries me)
W. Wuensch, rf development meeting 25-3-2015
1kHz dc pulserT controlled sample holder
Running all the time, Marx generator coming in addition for fast rise and fall.
At CERN since 2011 but not exploited due to lack of manpower.
W. Wuensch, rf development meeting 25-3-2015
Current conclusions:• Gradient gain now quoted in SLAC experiment seems reasonable
compared to theory and is not inconsistent with CERN experiment.
• But limited gain in Q means pumping heat costs a lot of power. We have many knobs to gain gradient at the expense of efficiency - less beam loading, shorter structures, shorter pulses, etc. Determining which path is least inefficient would require further study.
• Then - micron tolerances, installation cost?
• But cooled systems are extremely useful for understanding breakdown physics. SLAC experiment seems to be consistent with Helsinki theory.
• The variable temperature head in the dc spark system combined with the high repetition rate pulser would be an excellent, low-cost way of investigating this effect quickly – only need manpower.
