Unprecedented quality factors at accelerating gradients up to 45 MVm⁻¹in niobium superconducting resonators via low temperature nitrogen infusion

Abstract

We report the finding of new surface treatments that permit to manipulate the niobium resonator nitrogen content in the first few nanometers in a controlled way, and the resonator fundamental Mattis-Bardeen surface resistance and residual resistance accordingly. In particular, we find surface "infusion" conditions that systematically a) increase the quality factor of these 1.3 GHz superconducting radio frequency (SRF) bulk niobium resonators, up to very high gradients; b) increase the achievable accelerating gradient of the cavity compared to its own baseline with state-of-the-art surface processing. Cavities subject to the new surface process have larger than two times the state of the art Q at 2K for accelerating fields > 35 MV/m. Moreover, very high accelerating gradients ~ 45 MV/m are repeatedly reached, which correspond to peak magnetic surface fields of 190 mT, among the highest measured for bulk niobium cavities. These findings open the opportunity to tailor the surface impurity content distribution to maximize performance in Q and gradients, and have therefore very important implications on future performance and cost of SRF based accelerators. They also help deepen the understanding of the physics of the RF niobium cavity surface.

Ultralow Surface Resistance via Vacuum Heat Treatment of Superconducting Radio-Frequency Cavities

“…Other possible sources of BCS modification include O and C interstitials modifying the mean free path. The higher sensitivity and lower mean free path are consistent with measurements of nitrogen doped/infused cavities [7], [24].s…”

“…To be realizable in applications, new treatments must not just achieve high Q0 and Eacc values, but must do so with high reproducibility. Nitrogen infusion is an example of a treatment that shows extremely promising Q0 and Eacc performance in several tests [7], but has had limited reproducibility in some laboratories [8], [9]. The process starts with a high temperature heat treatment of a cavity in a vacuum furnace at ~800 C, which is expected to dissolve the natural surface oxide layer.…”

“…The furnace temperature is then lowered to 100-200 C, and low pressure nitrogen gas is injected to the furnace. After several hours of the low temperature nitrogen treatment on the post-oxide dissolution surface, SIMS (secondary ion mass spectrometry) measurements show that nitrogen interstitials can be found several tens of nm deep [7]. Unlike most other state-of-the-art cavity preparation methods, no material removal (e.g.…”

Nonlinear dynamics and dissipation of a curvilinear vortex driven by a strong time-dependent Meissner current

“…We show that the LO velocity dependence of the vortex viscosity (1) results in an anomalous decrease of R i (H) with H at strong RF field. A significant decrease of R i (H) with the RF field extending up to H (0.3 − 0.5)H c has been indeed observed by several groups on alloyed Nb cavities [47][48][49][50][51] . One mechanism to this effect can come from the nonlinear response of quasiparticle in the Meissner state [52][53][54] .…”

First demonstration of a cryocooler conduction cooled superconducting radiofrequency cavity operating at practical cw accelerating gradients

“…The unshaded region allows cw operation with the cryocooler. At 4.4 K, conduction cooling produced a modest E acc of ∼3.4 MV/m, Nb 3 Sn coating existing Nb 3 Sn cavity (tested with cryocooler) improved Nb3 Sn cavity (to be tested with cryocooler) Dissipated power vs. temperature for the 650 MHz Nb 3 Sn single cell cavity under study compared with the PT420 cryocooler load curve. The load curve accounts for the 0.8 W heat leak prevailing during measurements.…”