Session T08: Accelerator Technologies

22 GeV CEBAF with Novel FFA Design

Extending the energy reach of CEBAF up to 22 GeV within the existing tunnel is being explored. Proposed energy upgrade can be achieved by increasing the number of recirculations, while using the existing CEBAF SRF cavity system. Presented scheme is based on an exciting new approach to acceleratie electrons efficiently with multiple passes in a single FFA (Fixed Field Alternating Gradient) beam line. Encouraged by recent success of the CBETA Test Accelerator, a proposal was formulated raise CEBAF energy by replacing the highest-energy arcs with Fixed Field Alternating Gradient (FFA) arcs. The new pair of arcs configured with FFA lattice would support simultaneous transport of additional 6 passes with energies spanning a factor of two, using the non-scaling FFA principle implemented with Halbach-derived permanent magnets - a novel magnet technology that significantly saves energy and lowers operating costs. One of the challenges of the multi-pass (11) linac optics is to provide uniform focusing in a vast range of energies, using fixed field lattice. Here, we propose a triplet lattice scaled up with increasing momentum along the linac. This would provide a stable periodic solution covering energy ratio of 1:33. The current CEBAF configured with a 123 MeV injector, makes optical matching in the first linac virtually impossible due to extremely high energy span ratio (1:175). Therefore, we envision replacement of the current injector with a 650 MeV 3-pass recirculating injector based on the existing LERF facility. Finally, the 22 GeV CEBAF would promise to deliver in 10-passes a beam with normalized emittance of 80 mm·mrad and with a relative energy spread of 1.5 × 10^-3. Further recirculation beyond 22 GeV is limited by large, 974 MeV per electron, energy loss due to synchrotron radiation.   

Insights from NOvA Test Beam Proton Studies

The NOvA (NUMI Off-Axis electron neutrino Appearance) Experiment is a long-baseline accelerator neutrino experiment at Fermilab, comprising a 300-ton Near Detector (ND) near the beam source and a 14-kton Far Detector (FD) in Ash River, Minnesota, 810 km away. Both detectors are 14 mrad off-axis from the neutrino beam. The experiment aims to determine the neutrino mass ordering, make precise measurent of neutrino mixing  parameters, and investigate potential CP violation in the lepton sector by studying (anti) neutrino flavor oscillations.

Systematic uncertainities heavily influence the measurement of oscillation parameters. To adress this, the NOvA Test Beam program was established to ensure a better understanding of detector response to charged particles (electrons, protons, pions, and kaons). The NOvA Test Beam program used a scaled-down 30-ton version equipped with both FD and ND technologies to take data at Fermilab's Test Beam Facility from 2019-2021. Analysis of this data is currently ongoing. In this presentation, I will discuss my exploration of reducing uncertainites using Test Beam protons, presenting some preliminary results.   

Distributed Coupling Linac for Efficient Acceleration of High Charge Electron Bunches

Future colliders will require injector linacs to accelerate large electron bunches over a wide range of energies. For example the Electron Ion Collider requires a pre-injector linac from 4 MeV up to 400 MeV over 35 m [1]. Currently this linac is being designed with 3 m long traveling wave structures, which provide a gradient of 16 MV/m. We propose the use of a 1 m distributed coupling design as a potential alternative and future upgrade path to this design. Distributed coupling allows power to be fed into each cavity directly via a waveguide manifold, avoiding on-axis coupling [2]. A distributed coupling structure at S-band was designed to optimize for shunt impedance and large aperture size. This design provides greater efficiency, thereby lowering the number of klystrons required to power the full linac. In addition, particle tracking analysis shows that this linac maintains lower emittance as bunch charge increases to 14 nC and wakefields become more prevalent. We present the design of this distributed coupling structure, as well as cold test data and plans for higher power tests to verify on the structure’s real world performance.

[1] F. Willeke, “Electron ion collider conceptual design report 2021,” tech. rep., United States, 2021.

[2] S. Tantawi, M. Nasr, Z. Li, C. Limborg, and P. Borchard, “Design and demonstration of a distributed-coupling linear accelerator structure,” Phys. Rev. Accel. Beams, vol. 23, p. 092001, Sep 2020.   

Resonant Faraday rotation measurements in a potassium vapor cell

The Faraday effect, a magneto-optic rotation induced by a circularly birefringent medium, manifests itself as a rotation in the plane of polarization as light traverses through it. This investigation centers on applying the resonant Faraday effect, for probing the magnitude of external magnetic fields.

For this study, a tunable diode laser operating at a wavelength near the potassium D2 line (766.7 nm) is employed. A potassium vapor cell is positioned inside a uniform magnetic field (approximately 5-10 G) generated by a pair of Helmholtz coils. Using PEM ellipsometry, a basic polarizer-analyzer optics setup, and modulation techniques, we extract changes in polarization as the laser beam interacts with Zeeman-shifted potassium atoms—typically measured less than 100 millidegrees. A temperature stabilization of the potassium vapor cell within 0.1°C keeps the vapor density stable enough to measure the Faraday rotation angle and therefore the magnitude of the external field with high precision.

The primary objective of this study is to explore the potential application of the resonant Faraday effect in probing the polarization dynamics of helium-3 within hybrid Spin Exchange Optical Pumping (SEOP) cells. Progress on the status of the experiment will be presented.   

R&D of Low Gain Avalanche Diode (LGAD) with large area

Low Gain Avalanche Diode (LGAD) has high-precision time performance, and the time resolution can reach 30 ps. The LGAD with a size of 1.3 mm×1.3 mm was used for the High Granularity Timing Detector (HGTD) in the upgrade of ATLAS and CMS on High-Luminosity Large Hadron Collider (HL-LHC). Compared with the HL-LHC, the future electron-positron collider (such as ILC, CEPC, FCC-ee and STCF) will have fewer charged particles in the final states. Large area LGAD will save electronics channels. This presentation will show the test results of the large area LGAD sensors, including beta source test, picosecond laser test, collected charge, signal-to-noise ratio, time resolution, etc.