1. W. Wuensch, rf development meeting 25-3-2015 Considerations on running normal conducting cavities cold

2. 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

3. W. Wuensch, rf development meeting 25-3-2015 Power Gradient

4. W. Wuensch, rf development meeting 25-3-2015

5. 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.

6. 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. RF in (Slide S. Tantawi, my arrow) Same surface resistance change!

7. W. Wuensch, rf development meeting 25-3-2015

8. HEP Budget Briefing, Mar 6 2015 8 Understanding the Physics of High Gradients has Established the Limits of Normal Conducting Copper Structures Cu@45K Hard CuAg#1 Soft Cu Hard CuAg#2 Cu@45K Hard CuAg#1 Soft Cu Hard CuAg#2 Hard 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%

9. W. Wuensch, rf development meeting 25-3-2015 We didn’t measure breakdown rate and quote “maximum.” From memory was probably around 10 -2

10. W. Wuensch, rf development meeting 25-3-2015 50 K 300 K Esurface [MV/m] Log(BDR) E 30 +20% +70% (being near BDR=1 field worries me)

11. W. Wuensch, rf development meeting 25-3-2015 1kHz dc pulser T 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.

12. 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.