Off-line Commissioning of the St. Benedict Radiofrequency Quadrupole Cooler-Buncher

TL;DR

The paper documents the off-line commissioning of the St. Benedict RFQ cooler-buncher, a key component for preparing low-emittance ion bunches to be injected into a measurement Paul trap for β-ν angular-correlation studies. It details the dual-region RFQ design, with separate cooling and bunching sections, and the RF/DC circuitry, timing, and gas-handling strategies used to optimize transport, cooling, and bunching. Critical results include a 50 ns FWHM bunch length, 93(1)% bunching efficiency, and a 20.0(5) s trapping lifetime for $^{39}$K$^{+}$, establishing robust baselines for online operation and future mass- or charge-state dependent studies. These commissioning outcomes inform on-line deployment, anticipate space-charge effects at higher beam intensities, and motivate subsequent measurements of energy spread and gas purity in the final setup.

Abstract

The St. Benedict ion trapping system, which aims to measure the $β-ν$ angular correlation parameter in superallowed-mixed mirror transitions, is under construction at the University of Notre Dame. These measurements will provide much-needed data to improve the accuracy of the $V_{ud}$ element of the CKM matrix. One of the major components of this system is the radio frequency quadrupole cooler-buncher, which is necessary to create low-emittance ion bunches for injection into the measurement Paul trap. The off-line commissioning of the cooler-buncher, using a potassium ion source, determined that the device could produce cooled ion bunches characterized by a 50-ns full-width-half-maximum time width. The commissioning results also determined the trapping efficiency to be 93(1)$\%$ and the trapping half-life to be 20.0(5) s.

Figures (18)

• Figure 1: CAD drawing depicting the cross section of the St. Benedict cooler-buncher. Sections A, in yellow, show the RF circuitry mounted to the lid of the cooler-buncher chamber. Separate circuits for the cooling and bunching sections allow for the application of different RF amplitudes and frequencies to each section. Section B, in green, shows the tower feeding gas into the RFQ cooling region with a bypass to the outer region. Section C, in red, shows the injection/ejection optics. Section D, in orange, shows the flared RFQ rods. Section E, in blue, shows the cooling region. The 11 light-colored tooth pattern rectangles of both ends of the section E represent the PEEK baffles holding the helium and creating the pressure differential. Section F, in magenta, shows the bunching region.
• Figure 2: A zoomed-in view of the upstream side of the RFQ cooler-buncher. The three injection electrodes, namely the 1) lens, 2) hyperbola, and 3) cone, are highlighted here in green. The 4) flared RFQ segment A and the 5) RFQ segment B and C are in cyan. The 6) Stethoscopic RF connection is also marked. The location of the cross-section cut of Fig. \ref{['fig::crosscutinjection']} is indicated here as well.
• Figure 3: Cross-cut of Fig. \ref{['fig::injectionOptics']} with a slight tilt showing electrodes 1) B and 2) C, as well as the 3) turning cross-cut between them and the 4) RF backbone. The application of the opposite RF phase is also shown schematically. The 5) PEEK piece serving as differential-pumping barrier enveloped the electrode structure. Helium gas is injected through the 6) bellow above the cooling section.
• Figure 4: The potential scheme for cooling and accumulating ions in the bunching section, shown in red, and the potential scheme for ejecting an ion bunch from the bunching region, in blue and indicated by the arrows. The two schemes are implemented by changing potentials on electrodes I and K, as labeled above.
• Figure 5: A rough schematic showing cross sectional views of the electrodes of the cooling region depicting the segmentation in six different spots to create a smooth drop in potential along the beam axis.