The Mu2e experiment will measure the charged-lepton flavor violating (CLFV) neutrino-less conversion of a negative muon into an electron in the field of a nucleus. The conversion process results in a monochromatic electron with an energy of 104.97 MeV, slightly below the muon rest mass. The goal of the experiment is to improve the previous measurement by four orders of magnitude and reach of a SES (single event sensitivity) of 3 x 10^{-17} on the conversion rate, a 90% CL of 8 x 10^{-17}, and a 5sigma discovery reach at 2 x 10^{-16}$. The experiment goal is obtained with a intense pulsed negative muon beam. The pulsed beam is essential to reducing backgrounds. The other essential element is a sophisticated magnetic system composed of three consecutive solenoids to form the muon beam. Mu2e will use an aluminum target and examine ~10^{18} stopped muons in three years of running. The Mu2e experiment is under design and construction at the Fermilab Muon Campus. The experiment will begin data-taking near the end of 2020 with 3 years of running from 2021 to 2023. Upgrades to other materials than aluminum are already planned. This article is written specifically for younger researchers to bridge the gap between conference presentations and detailed design reports, and examines issues not covered in the former without the details of the latter.

The Mu2e Experiment

We report on an effort to improve the performance of superconducting radiofrequency cavities by the use of heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is significantly decreased, and we find an unexpected reduction in the BCS resistance. Together these result in extremely high quality factor values at relatively large accelerating fields Eacc ~20 MV/m: Q0 of 3-4x10^11 at <1.5 K and Q0 ~5x10^10 at 2.0 K. In one cavity, measurements of surface resistance versus temperature showed an extremely small residual resistance of just 0.63+/-0.06 nOhms at 16 MV/m. SIMS measurements confirm that the oxide was significantly dissociated, but they also show the presence of nitrogen after heat treatment. We also present studies of surface oxidation via exposure to air and to water, as well as the effects of very light surface removal via HF rinse. The possibilities for applications and the planned future development are discussed.

Ultra-Low Surface Resistance via Vacuum Heat Treatment of Superconducting Radiofrequency Cavities

We report on an effort to improve the performance of superconducting radiofrequency cavities by using heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is decreased significantly, and we find an unexpected reduction in the Bardeen-Cooper-Schrieffer (BCS) resistance. Together these result in extremely high-quality factor values at relatively large accelerating fields ${E}_{\mathrm{acc}}$ \ensuremath{\sim} 20 MV/m: ${Q}_{0}$ = 3--4 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ at 1.5 K and ${Q}_{0}$ \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{10}$ at 2.0 K. In one cavity, measurements of surface resistance versus temperature showed an extremely small residual resistance of just 0.63 \ifmmode\pm\else\textpm\fi{} 0.06 n\ensuremath{\Omega} at 16 MV/m. Secondary ion mass spectrometry measurements confirm that the oxide is dissociated significantly, but they also show the presence of nitrogen after heat treatment. We also present studies of surface oxidation via exposure to air and to water, as well as the effects of very light surface removal via HF rinse. The study is performed on 1.3 GHz cavities, but the effect may be extendable to other frequencies as well. The possibilities for applications and the planned future development are discussed.

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

A total of ten jacketed single-spoke resonators type 1 (SSR1) have been fabricated for Fermilab’ injection experiment (PIP2IT). PIP2IT is a test bed for Fermilab’s future accelerator named proton improvement plan II that is currently under development. SSR1 cavities operate at 325 MHz to accelerate a proton beam at a relative (to speed of light) velocity ( $\beta = 0.22$ ). In this paper, we present Fermilab’s experience in developing those spoke resonators starting from the design and analysis phase, to fabrication and extensive testing to qualify cavities for cryomodule assembly.

Development of Low $\beta $ Single-Spoke Resonators for the Front End of the Proton Improvement Plan-II at Fermilab

The completion of the PIP-II project and its superconducting linear accelerator will provide up to 1.2 MW of beam power to the LBNF/DUNE facility for neutrino physics. It will also be able to produce high-power beams directly from the linac that can be used for lower-energy particle physics experiments as well, such as directing beam toward the Muon Campus at Fermilab for example. Any further significant upgrade of the beam power to DUNE, however, will be impeded by the limitations of the present Booster synchrotron at the facility. To increase the power to DUNE by a factor of two would require a new accelerator arrangement to feed the Main Injector that does not include the Booster. In what follows, a path toward upgrading the Fermilab accelerator complex to bring the beam power for DUNE to 2.4 MW is presented, using a new rapid-cycling synchrotron plus an energy upgrade to the PIP-II linac. The path includes the ability to instigate a new lower-energy, very high-power beam delivery system for experiments that can address much of the science program presented by the Booster Replacement Science Working Group. It also allows for the future possibility to go beyond 2.4 MW up to roughly 4 MW from the Main Injector.

An Upgrade Path for the Fermilab Accelerator Complex

The Proton Improvement Plan-II (PIP-II) encompasses a set of upgrades and improvements to the Fermilab accelerator complex aimed at supporting a world-leading neutrino program over the next several decades. PIP-II is an integral part of the strategic plan for U.S. High Energy Physics as described in the Particle Physics Project Prioritization Panel (P5) report of May 2014 [1] and formalized through the Mission Need Statement approved in November 2015. As an immediate goal, PIP-II is focused on upgrades to the Fermilab accelerator complex capable of providing proton beam power in excess of 1 MW on target at the initiation of the Long Baseline Neutrino Facility/Deep Underground Neutrino Experiment (LBNF/DUNE) program [2], currently anticipated for the mid- 2020s. PIP-II is a part of a longer-term goal of establishing a high-intensity proton facility that is unique within the world, ultimately leading to multi-MW capabilities at Fermilab....

/pdf/the-pip-ii-conceptual-design-report-1rokge6bm3.pdf

O. Melnychuk

Papers

The PIP-II Conceptual Design Report