WG-1; What are the performance limits for low beta resonators?
Conveners:
- Bob Laxdal (TRIUMF)
- Han Li (Uppsala University)
- Kazutaka Ozeki (RIKEN)
There are a variety of significant low to medium beta projects being pursued using non-elliptical TEM mode cavities (QWR, HWR, single and multiple spokes). Projects include FRIB, ESS, RISP, C-ADS and RIKEN. In principal, the performance limits for non-elliptical cavities should be similar to elliptical cavities but the cavities typically operate in a lower frequency range and with unique geometries. Comparing performance across the variants is not as evolved as with 1.3GHz elliptical cavities. The scope of this working group is to discuss the known limitations of TEM mode accelerating cavities. The range of discussion should include:
- Performance results Q, Rs(B) vs Ea, B from various labs
- Experience with heat treatments including doping
- Role of rf frequency
- Multipacting issues
- 4K vs 2K operation
WG-2; How to improve the present specification for bulk Niobium?
Conveners:
- Sam Posen (FNAL)
- Claire Antoine (CEA)
- Akira Yamamoto (CERN/KEK)
Bulk Niobium is at present the most common material for superconducting RF cavities in accelerator applications. Niobium specifications are well established and successfully used for different projects. But recent experience has compelled us to re-evaluate the Niobium material specification in view of advanced technologies like Nitrogen doping and infusion treatments where performance in terms of High Q and/or High Gradient is pushing the limits of the material. The scope of WG-2 is to review the existing Niobium specifications in terms of both High Q and High Gradient performance and discuss the different needs with respect to:
- Optimized specification for low BCS resistance
- Optimized specification for low residual resistance including low trapped flux sensitivity
- Optimized specification for high peak field
- Other specification considerations including thermal conductivity
- Mechanical fabrication methods (deep drawing, hydroforming, extrusion) and the needed mechanical properties.
- Mechanical stability during operation with respect to Lorentz force detuning, microphonics and pressure vessel code.
- Optimized specification in terms of final surface finish with electro polishing or buffered chemical polishing.
- Considerations of Large grain and single crystal applications.
The discussion in the WG-2 should analyze the properties of Niobium and their influence on different applications. Specifically, there is an existing specification that can be used by the vendors that would result in material complying with a reduced pinning requirement. The aim is to give the community information that would help augment the specifications for future projects.
WG-3; High Q and high gradient performance.
Conveners:
- Alex Romanenko (FNAL)
- Marc Wenskat (DESY)
- Kensei Umemori (KEK)
Bulk Niobium is at present the most common material for superconducting RF cavities in accelerator applications. Over the last several years new treatments including High Temperature doping and Low Temperature infusion have been used to reach unprecedented performance of Niobium cavities in terms of High Q and/or High Gradient. Most of the work has concentrated on cavities operating at 1.3GHz driven by CW applications like LCLS-II and future pulsed operation like the ILC. The scope of this working group is to discuss the most recent results with respect to pushing niobium towards Higher Q and Higher Gradient.
Discussions should include:
- High temperature doping results across the laboratories including both cavity results and sample analysis
- Low temperature infusion results across the laboratories including both cavity results and sample analysis.
- New results at other cavity frequencies that could shed light on the core understanding
- Recent additions to our theoretical or technical understanding of either phenomenon.
WG-4; Special applications of SRF cavities
Conveners:
- Jacek Sekutowicz (DESY)
- Ben Hall (HZB)
- Rama Calaga (CERN)
Besides the accelerator cavities for the main linacs, special SRF cavities find application in SRF electron guns, harmonic SRF cavities for bunch lengthening and increased charge density and crab and deflecting mode cavities for bunch rotation (crabbing), RF separation or beam diagnosis.
- In WG-4 the applications and approaches for the different designs of the three types of cavities should be presented.
- Aspects of the cavity fabrication, treatment and operation should be discussed.
- Existing solutions and experience should be exchanged.
- In particular, WG-4 should discuss and identify needed future developments.
The aim of this working group is to address the ongoing issues and difficulties for the three types of cavities.
Hot topic session: Warm vacuum sections between low beta cryomodules
Conveners:
- Hiroyuki Ao (FRIB)
- Marcelo Juni Ferreira (ESS)
- Hiroshi Imao (RIKEN)
Low beta accelerators are designed with normal conducting sections between the cryomodules, hosting focusing and beam diagnostic elements. This session is dedicated to the discussion of practical issues for these machines, reviewing design criteria of warm sections. Participation of vacuum experts should be encouraged, in order to discuss the preparation procedures of these sections with those followed for the SRF components. The large variety of beam diagnostics element required for low beta linacs are not always fully compatible with the application of the full cleaning and high pressure rinsing procedures used in the SRF community. Procedures for limiting particulate production near the cryomodules should be reviewed and compared to the state of the art experience followed by electron accelerators like XFEL or LCLSII. New NEG pumps appeared lately on the market, promising high pumping speeds with a high degree of compatibility for particle-free applications, and some large infrastructures, as the ESS, will employ them as the main pumping method for the warm sections. Latest developments and measurements performed in different laboratories (JLAB, ESS, STFC, …) in this direction should be reported.
