Abstract
Work function and surface chemistries of SiC-polished, electropolished, and nitrogen-doped niobium coupons were analyzed before and after plasma processing using a neon-oxygen gas mixture. These studies represent an initial enquiry into the feasibility of applying the plasma processing technique designed at ORNL for the Spallation Neutron Source (SNS) to the nitrogen-doped Nb cavities for the Linac Coherent Light Source (LCLS-II) upgrade. Work function of all measured samples was increased after plasma processing, which indicates the strong potential of the plasma processing technique as a tool for increasing the accelerating gradient of nitrogen-doped cavities.
Highlights
Nitrogen-doped superconducting radio-frequency cavities are being used in the upgrade to the Linac Coherent Light Source (LCLS-II) as Fermi National Accelerator Laboratory (FNAL) has shown that N-doping leads to an increase in the quality factor of niobium (Nb) cavities [1,2,3,4,5,6,7,8]
Oak Ridge National Laboratory (ORNL) has found that oxygen plasma cleans hydrocarbons from the surface of niobium (Nb). Removal of these hydrocarbons from the Nb surface leads to an increase in work function (WF), which in turn leads to an increase in stable accelerating gradient for superconducting radio-frequency (SRF) cavities such as those in use in the Spallation Neutron Source (SNS) at ORNL [9], [10]
Where a = 1.54E6, b=6.53E3, and c=10.4 are constants for niobium, E is the electric field at the cavity surface in MV/m, β is the field enhancement factor, φ is the surface work function in eV, and J is the current density in A/m2
Summary
Nitrogen-doped superconducting radio-frequency cavities are being used in the upgrade to the Linac Coherent Light Source (LCLS-II) as Fermi National Accelerator Laboratory (FNAL) has shown that N-doping leads to an increase in the quality factor of niobium (Nb) cavities [1,2,3,4,5,6,7,8]. Oak Ridge National Laboratory (ORNL) has found that oxygen plasma cleans hydrocarbons from the surface of niobium (Nb). Removal of these hydrocarbons from the Nb surface leads to an increase in work function (WF), which in turn leads to an increase in stable accelerating gradient for superconducting radio-frequency (SRF) cavities such as those in use in the Spallation Neutron Source (SNS) at ORNL [9], [10].
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