Sasha Horney

Abstract:

Superfluid liquid ⁴He consists of superfluid and normal components. The superfluid component has zero viscosity and zero entropy, therefore flows without dissipation. The normal component carries heat and can produce a high velocity flow which leads to very high thermal conductivity. In neutron scattering and muon spectroscopy experiments with powder samples, 4He gas is often used as an exchange medium to thermalise the sample. Samples are usually cooled using a dilution refrigerator with installed capillaries which admit ⁴He into the experimental volume. Below 2 K the inner surface of the capillary is covered with a superfluid ⁴He film, which can thermally link stages of the dilution refrigerator with a catastrophic effect on its performance. In our experiment, we utilised a 0.1% mixture of ³He in ⁴He to disrupt the superfluid helium film flow and, as a result, significantly reduced the parasitic thermal flow to the experimental can. The addition of a small amount of ³He into ⁴He allows a significant improvement of thermal performance when using this technique in neutron scattering experiments at very low temperatures.

Abstract:

During 2023, examination of the action dependence of sextupolar resonance driving terms (RDT) in the LHC at injection, as measured with an AC-dipole, demonstrated that a robust measurement of the RDTs could still be achieved even with very small amplitude kicks, typically used for linear optics studies. Consequently, analysis of optics measurements from 2022 to 2024 during the LHC energy ramp allowed a first measurement of the sextupole resonance evolution. A large asymmetry was observed between the two LHC beams, with the clockwise circulating beam (LHCB1) being significantly worse than the counter-clockwise circulating beam (LHCB2), and a clear increase in the RDT strength during the ramp was observed. During 2024 commissioning, a first attempt was made to correct the 𝑓1020 RDT of LHCB1 at injection. Results are presented in this report.

Abstract:

Operation of the Large Hadron Collider (LHC) requires strong octupolar magnetic fields to suppress coherent beam instabilities. The amplitude detuning that is generated by these octupolar magnetic fields brings the tune of individual particles close to harmful resonances, which are mostly driven by the octupolar fields themselves. In 2023, new optics were deployed in the LHC at injection with optimized betatronic phase advances to minimize the resonances from the octupolar fields without affecting the amplitude detuning. This paper reports on the optics design, commissioning, and lifetime measurements performed to validate the optics.

Abstract:

The invention of the ³He/⁴He dilution refrigerator opened a new chapter in experimental ultra-low temperature physics. Dilution refrigerators became essential for providing ultra-low temperature environments for nuclear demagnetisation experiments, superconducting-qubit quantum processors and highly sensitive bolometers used in fundamental physics experiments. Development of dilution refrigeration technology requires thorough understanding of the quantum mechanical processes that take place in liquid helium at ultra-low temperatures. For decades the quantum fluids research community provided valuable information to engineers and designers involved in the development of advanced dilution refrigerators. However, the lack of methods that allow the measurement of physical parameters of liquid helium during the operation of a dilution refrigerator was hindering development of the technology. Here we show direct imaging of an operational dilution refrigerator using neutron radiography. This allows direct observation of the dilution process in ³He/⁴He mixtures and opens an opportunity for direct measurement of the ³He concentration. We observe the refrigerator behaviour in different regimes, such as continuous circulation and single shot, and show that our method allows investigation of various failure modes. Our results demonstrate that neutron imaging applied to the study of dilution refrigeration processes can provide essential information for developers of ultra-low temperature systems. We expect that neutron imaging will become instrumental in the research and development of advanced dilution refrigerators.