1. US R&D activities and plans in SCRF and RF power source efficiency
S. Belomestnykh
Brookhaven National Laboratory and Stony Brook University
FCC Week 2015
March 23-27
Washington, DC

2. US R&D in SCRF and RF power source efficiency
Introduction
This talk is not intended to be a comprehensive overview of the SRF/RF R&D activities in the U.S., but rather a selection of the activities I deem relevant to future energy frontier circular colliders. This means CW operation of large-scale SRF systems to support high beam currents.
I will review:
High Q R&D
Nb3Sn research
Development of HOM-damped SRF cavities
Efforts to improve RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

3. US R&D in SCRF and RF power source efficiency
High Q0 R&D
March 26, 2015
US R&D in SCRF and RF power source efficiency

4. Recent breakthrough results
Capital and operational cost of cryogenic systems is one of the main contributors (along with SRF/RF system) to the total cost of large machines based on SRF technology.
For CW accelerators, the dynamic heat load is dominant, hence improving Q0 at operating gradients (15 – 20 MV/m) can bring significant cost savings.
Recent R&D efforts at Fermilab led to the following breakthroughs:
Nitrogen doping treatment results in record low BCS surface resistance, reduced non-flux residual resistance;
Tuning Meissner effect via fast cooldown leads to no/low contribution from trapped magnetic flux and to the record low residual resistance.
Thanks to LCLS-II at SLAC, nitrogen doping has received a strong technological push to make it production-ready at 1.3 GHz 9-cell cavities. This was accomplished by collaborative efforts at Fermilab, Cornell and JLab.
Initial results from applying nitrogen doping to lower frequency (650 MHz) cavities are very promising for future colliders, which plan to utilize RF frequencies as low as 400 MHz.
More details in Alexander Romanenko’s talk.
March 26, 2015
US R&D in SCRF and RF power source efficiency

5. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

6. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

7. Nitrogen doping of 650 MHz cavities – first results
Applying N doping to 650 MHz (b = 0.9) leads to double Q compared to 120°C bake (standard surface treatment ILC/XFEL), ~7×1010 at 2 K – world record at this frequency
Courtesy of A. Grasselino (Fermilab)
March 26, 2015
US R&D in SCRF and RF power source efficiency

8. US R&D in SCRF and RF power source efficiency
650 MHz cooldown studies
Cooling details have been shown to play an important role in Q retention.
Slow cooling through Tc shown to severely deteriorate Q for 1.3 GHz cavities.
At 650 MHz we find a weaker effect, likely due to smaller impact of trapped flux at lower frequency.
Very promising for Q retention in cryomodule.
A. Romanenko, A. Grassellino, O. Melnychuk, D. A. Sergatskov, J. Appl. Phys. 115, (2014)
A. Romanenko, A. Grassellino, A.Crawford, D. A. Sergatskov, Appl. Phys. Lett. 105, (2014)
Courtesy of A. Grasselino (Fermilab)
March 26, 2015
US R&D in SCRF and RF power source efficiency

9. US R&D in SCRF and RF power source efficiency
Minimizing residual resistance (maximize Q) by avoiding the ambient magnetic flux to be trapped
Same cavity, just cooled differently through 9.2K
Flux expelled efficiently
Flux mostly trapped
Courtesy of A. Romanenko (Fermilab)
March 26, 2015
US R&D in SCRF and RF power source efficiency

10. Utilizing new physics for record high Qs
Combination of nitrogen doping and efficient flux expulsion:
record high Q > 1011 up to 28 MV/m in SRF cavities
Courtesy of A. Romanenko (Fermilab)
A. Romanenko, A. Grassellino, A. C. Crawford, D. A. Sergatskov, and O. Melnychuk, Appl. Phys. Lett. 105, (2014)
March 26, 2015
US R&D in SCRF and RF power source efficiency

11. R&D plans on low frequency cavities
More studies at Fermilab on 650 MHz cavities for PIP-II to fine-tune the recipe.
Initiating a BNL/JLab collaboration on a 400 MHz single cell cavity studies.
Courtesy of R. Rimmer (JLab)
March 26, 2015
US R&D in SCRF and RF power source efficiency

12. New results for Nb3Sn cavities
March 26, 2015
US R&D in SCRF and RF power source efficiency

13. Recent advances in Nb3Sn cavities
Nb3Sn, having critical temperature ~2 times higher than Nb, promises to deliver higher Q and gradients. The Q should be >1010 even at 4.2 K.
However, the material is challenging (brittle), difficult to deal with.
Cornell recently renewed efforts to advance Nb3Sn studies and received very promising results on 1.3 GHz single cell cavities.
The first accelerator cavity made with an alternative superconductor that outperforms Nb at usable gradients.
More details in Matthias Liepe’s talk.
Standard Nb cavity
Nb3Sn-Coated
March 26, 2015
US R&D in SCRF and RF power source efficiency

14. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

15. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

16. Development of HOM-damped cavities
March 26, 2015
US R&D in SCRF and RF power source efficiency

17. Development of HOM-damped cavities at BNL and JLab
BNL and JLab are actively designing future electron ion colliders, eRHIC and MEIC.
Both machines will require HOM-damped SRF cavities operating at frequencies relevant to FCC and CEPC.
March 26, 2015
US R&D in SCRF and RF power source efficiency

18. R&D at Jefferson Lab: MEIC ion ring design concept
0.5 A beam current, MV/m, up to 110 kW per cavity
March 26, 2015
US R&D in SCRF and RF power source efficiency

19. Five-cell cavity with strong HOM damping for eRHIC at BNL
HOM coupler ports
FPC port
A five-cell 704 MHz SRF cavity (BNL3) has been developed at BNL for high current linacs before the eRHIC RF frequency was changed to 422 MHz.
A prototype cavity reached 19.7 MV/m.
Six antenna-type couplers will be attached to the large diameter beam pipes and will provide strong HOM damping while maintaining good fill factor for the linac.
Two HOM filters are currently under consideration: a high pass filter made of lumped elements and a dual-ridge waveguide filter.
March 26, 2015
US R&D in SCRF and RF power source efficiency

20. 2-stage filter HOM coupler and dual-ridge WG HOM coupler
50 Ω transmission line to room temperature
D = 72 mm
More compact than rectangular waveguide.
A very broadband coax-to-waveguide transition was developed.
Better transmission than that of the 2-stage coaxial coupler.
March 26, 2015
US R&D in SCRF and RF power source efficiency

21. US R&D in SCRF and RF power source efficiency
422 MHz HOM-damped cavity
R&D on the 422 MHz BNL4 cavity is in progress.
The RF design is completed with the cavity shape re-optimized from the scaled 704-MHz BNL3 cavity model:
The first HOM of the BNL4 cavity is 15 MHz further away from the fundamental mode.
The BNL4 cavity’s has higher transverse and longitudinal BBU threshold than the scaled BNL3 cavity.
The HOM power for BNL4 is 10% lower than for the scaled BNL3 cavity.
The 3-cell prototype cavity will be ordered in the near future.
March 26, 2015
US R&D in SCRF and RF power source efficiency

22. Efforts to improve RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

23. Efforts to improve efficiency of RF power systems for accelerators
With several new projects relying on large-scale RF installations, there is a renewed interest worldwide to improve efficiency of RF power generation.
There were two talks during RF session on Tuesday: one on high efficiency klystrons and the other on MB-IOTs.
There are also recent results on using magnetrons.
March 26, 2015
US R&D in SCRF and RF power source efficiency

24. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

25. US R&D in SCRF and RF power source efficiency
High-power MB-IOTs
State of the art IOTs are limited to an output RF power of ~100 kW.
To fully realize their advantages – no saturation, efficiency drops slowly at reduced output power – ESS (in collaboration with CERN) has initiated an industrial study to design two Multi-Beam IOTs with a pulsed power of 1.2 MW (3.5 ms, 5% duty factor, 65% efficiency).
Two U.S. companies are involved: CPI as a partner in Thales/CPI Consortium and L3.The contracts were signed in September 2014.
24-month duration, long term testing at CERN.
More details in M. Jensen’s talk.
BNL is initiating R&D on a 844 MHz MB-IOT with an output power of 500 kW CW for eRHIC.
March 26, 2015
US R&D in SCRF and RF power source efficiency

26. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

27. US R&D in SCRF and RF power source efficiency
March 26, 2015
US R&D in SCRF and RF power source efficiency

28. US R&D in SCRF and RF power source efficiency
Magnetrons
There is a renewed interest to using magnetrons as RF power amplifiers.
Magnetrons are inherently efficient devices (~90%), but they are oscillators. However, there is a way to “injection-lock” them.
R&D is underway at JLab, Muons, Inc. and Fermilab.
A concept for a high-power 650 MHz CW magnetron transmitter was developed by Muons, Inc. and Fermilab.
The phase/amplitude stability might be adequate for high intensity frontier SRF linacs (PIP-II), but probably not good enough for storage rings/colliders.
Also, magnetron-type amplifiers (CFAs: Amplitron, Platinotron, Stabilotron) are available at high frequencies, but I am not aware of any devices operating at low frequencies relevant to SRF accelerators.
March 26, 2015
US R&D in SCRF and RF power source efficiency

29. US R&D in SCRF and RF power source efficiency
Courtesy of H. Wang (JLab)
Why?
Space-charge effect in electron bunch forming process in linear motion dominates the efficiency in klystrons. Spent energy deposits in the collector.
Magnetron
forms bunches in spoke-on-hub process in circular motion. Beam-to-RF cavity interaction in multiple passes. Much less wasted energy.
circular devices
linear devices
L kW magnetrons
Original CEBAF
13 kW upgrade
Oven 1.2 kW magnetron
March 26, 2015
US R&D in SCRF and RF power source efficiency

30. JLab R&D on magnetron RF source
Courtesy of H. Wang (JLab)
The potential impact for CEBAF
Capital and operation cost saving for CEBAF SRF Accelerator in RF power:
Low cost of magnetron device;
DC-to-RF efficiency from klystron to magnetron improves from ~35% to ~90%;
3.22 MW of DC power saving;
$1.74 million saving in power bill, if 41 weeks/year of CEBAF in GeV operation.
Technology demonstration for all SRF accelerators in NP/HEP/BES in DOE complexes.
March 26, 2015
US R&D in SCRF and RF power source efficiency

31. JLab R&D on magnetron RF source
Courtesy of H. Wang (JLab)
“First Demonstration and Performance of an Injection Locked CW Magnetron to Phase Control a Superconducting Cavity” was done at JLab in conjunction with Lancaster University, UK, in 2010 [1] with accuracy of 0.95o rms
SBIR Phase I with Muons, Inc. to design system for CEBAF and validate costs.
SBIR Phase I with Muons, Inc. to design optimized 1497 MHz 13 kW magnetron.
References:
[1] A. C. Dexter, G. Burt, R. G. Carter, I. Tahir, H. Wang, K. Davis and R. Rimmer, PRST-AB, 14, (2011).
[2] M. Nuebauer, A.Dudas, R. Rimmer, H. Wang, An Efficient RF Source for JLab, PAC 2013, Pasadena, CA, USA.
[3] H. Wang, T. Plawski, R. Rimmer, A. Dexter, I. Tahir, M. Neubauer, A. Dudas, System Study Using Injection Phase Locked Magnetron as an Alternative Source for Superconducting Radio Frequency Accelerator, IVEC 2014, Monterey, CA.
March 26, 2015
US R&D in SCRF and RF power source efficiency

32. US R&D in SCRF and RF power source efficiency
Experiments with a S-band CW magnetron.
Emulated a wideband feedback system of a dynamic control.
Measured a high frequency phase noise of less than 1° rms.
Also performed experiments with a 2.5 MW pulsed magnetron.
March 26, 2015
US R&D in SCRF and RF power source efficiency

33. US R&D in SCRF and RF power source efficiency
Performed an experiment on a single cell 2.45 GHz SRF cavity on loan from JLab.
Achieved an amplitude dynamic range of 30 dB, amplitude stability of 0.3% rms and phase stability of 0.26° rms.
March 26, 2015
US R&D in SCRF and RF power source efficiency

34. US R&D in SCRF and RF power source efficiency
Summary
There is an active SRF/RF R&D program in the U.S. relevant to future energy frontier circular colliders:
High Q R&D at Fermilab and other laboratories resulted in record quality factors at accelerating gradients used for CW machines. This promises big cost savings in cryogenics, both capital and operating. This treatment will be applied to LSLS-II cavities.
Recent results from single cell Nb3Sn cavities at Cornell promise even bigger savings as Nb3Sn cavities cavities have very high Q even at 4.2 K. However, further R&D is needed to prove robustness of the coating and to develop coating for multi-cell cavities.
Development of HOM-damped SRF cavities is in progress for future EICs at BNL and JLab.
Renewed efforts to improve RF power source efficiency are concentrated in three areas: new concepts of high-efficiency klystrons, MB-IOTs and study of using magnetrons.
March 26, 2015
US R&D in SCRF and RF power source efficiency

35. US R&D in SCRF and RF power source efficiency
Thank you!
March 26, 2015
US R&D in SCRF and RF power source efficiency