Plasma Processing for In-Situ Field Emission Mitigation of Superconducting Radiofrequency (SRF) Cryomodules

Abstract

Field emission (FE) is one of the main limiting factors of superconducting radiofrequency (SRF) cavities operating in accelerators and it occurs whenever contaminants, like dust, metal flakes or even absorbates, are present on the surface of the cavity high electric field region. Field emission reduces the maximum achievable accelerating field and generates free electrons that may interact with the beam, damage or activate the beamline. One practical method that can be used to mitigate this problem is in-situ plasma cleaning, or plasma processing. The development of a processing that can be applied in-situ is extremely advantageous, since it enables the recovery of the cryomodule performance without the need of disassembling the whole cryomodule, which is an extremely expensive and time-consuming process. On the other hand, plasma processing only requires the cryomodule warm-up to room-temperature and the subsequent processing of the contaminated cavities. The entire process is reasonably quick and involves a limited number of personnel. For these reasons we would like to advocate for continuing to invest in the R&D of plasma processing to optimize its applicability in cryomodules and for extending the technique to other frequency ranges and cavities geometries.

Future high energy colliders and options for the U.S.

“…Promising R&D directions are being pursued for increasing gradient, including new superconducting materials, travelling wave cavities, new cell shapes for standing wave structures, cleanroom robotics to reduce field emission, layered superconductor structures, and new impurity doping treatments, as well as more fundamental explorations of the limits of RF superconductivity, such as the use of "slow surface" materials that could prevent dissipation from magnetic flux penetration. For examples of SRF R&D directions, see references [102][103][104][105][106][107][108]. There are many exciting ideas to pursue.…”

RF Accelerator Technology R&D: Report of AF7-rf Topical Group to Snowmass 2021

“…Hence, developing new cleaning methods [24], better cryomodule integration techniques, e.g., by employing robot-assisted assembly, and post-integration mitigation methods remain important R&D topics. White Paper [25] describes recent developments and future perspectives for one of the FE mitigation methods, in situ plasma processing. Plasma processing of SRF cavities was developed originally at Oak Ridge National Laboratory (ORNL), where it was demonstrated that a plasma of mixture of neon and a small percentage of oxygen reduces hydrocarbon-related field emission in the Spallation Neutron Source (SNS) cavities.…”