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A new in-situ plasma processing technique is being developed at the Spallation Neutron Source (SNS) to improve the performance of the cavities in operation. The technique utilizes a low-density reactive oxygen plasma at room-temperature to remove top-surface hydrocarbons. The plasma processing technique increases the work function of the cavity surface and reduces the overall amount of vacuum and electron activity during cavity operation; in particular it increases the field-emission onset, which enables cavity operation at higher accelerating gradients. Experimental evidence also suggests that the SEY of the Nb surface decreases after plasma processing which helps mitigating multipacting issues. In this article, the main developments and results from the plasma processing R&D are presented and experimental results for in-situ plasma processing of dressed cavities in the SNS horizontal test apparatus are discussed. 2. FIELD EMISSION AND END-GROUP THERMAL INSTABILITY LIMITING THE ACCELERATING GRADIENTS IN THE SNS LINAC Field emission in superconducting radio-frequency (SRF) cavities is a well-known limiting factor for operation at high accelerating gradients [1-3]. Beyond certain electric field thresholds, the electrons from the metal surface of the cavity have a non-negligible probability of tunneling out. The field emitted electrons are accelerated by the stored electromagnetic fields in the cavity and subsequently deposit their energy by collision with the cavity radio-frequency (RF) surface leading to vacuum activity, increase of the surface temperature and Bremsstrahlung radiation. If the deposited energy-density is larger than the cooling capacity it can also lead to thermal breakdown of the superconductivity.

The Spallation Neutron Source (SNS) makes use of superconducting cavities for the acceleration of negative H ions in the main linac. Two types of 6-cell Niobium cavities are used in the superconducting portion of the linac: 33 β=0.61 cavities and 48 β=0.81 cavities. Each cavity is powered via a coaxial fundamental power coupler (FPC) of a simple yet robust design. The electromagnetic design of the main components of that coupler has been modeled and some of those properties have been measured experimentally. Modeling includes impedance matching of the window and of the waveguide to coaxial doorknob transition; coupling of the coupler fields to the cavity fields; and multipacting behavior of the coaxial line and window. Various aspects of design, simulation, and testing on the coupler and cavity are presented.