T. Koettig

Study of Temperature Wave Propagation in Superfluid Helium Focusing on Radio-Frequency Cavity Cooling

Oscillating Superleak Transducers (OSTs) can be used to localize quenches of superconducting radio-frequency cavities. Local hot spots at the cavity surface initiate temperature waves in the surrounding superfluid helium that acts as cooling fluid at typical temperatures in the range of 1.6 K to 2 K. The temperature wave is characterised by the properties of superfluid helium such as the second sound velocity. For high heat load densities second sound velocities greater than the standard literature values are observed. This fast propagation has been verified in dedicated small scale experiments. Resistors were used to simulate the quench spots under controlled conditions. The three dimensional propagation of second sound is linked to OST signals. The aim of this study is to improve the understanding of the OST signal especially the incident angle dependency. The characterised OSTs are used as a tool for quench localisation on a real size cavity. Their sensitivity as well as the time resolution was proven to be superior to temperature sensors glued to the surface of the cavity.

Heat Extraction From the LHC Main Dipole, Main Quadrupole, and MQXA Superconducting Cables

The forthcoming operation of the CERN Large Hadron Collider (LHC) at 13-14 TeV requires a deep understanding of the heat transfer mechanisms in the most critical superconducting magnets. This is aimed at determining their steady-state quench limits and constitutes an input to compute the magnets stability in transient conditions as well, to prevent beam induced quenches. Heat extraction capability of the LHC Nb-Ti magnets relies on the significant contribution provided by superfluid helium (He II). Due to lack of knowledge of the He II distribution in the cable and in the compressed insulation, experimental investigations are necessary. In this work we present an experimental study aimed at reproducing the thermal behavior of superconducting coils using short length samples. With respect to previous studies, a new instrumentation technique was developed and an in-situ calibration of the thermocouples was performed. The study was conducted on different types of instrumented cables-stack reproducing the main bending dipole (MB), the main quadrupole (MQ), and the MQXA, which is one of the two low-β interaction region quadrupoles. The heat extraction was determined as a function of the cable temperature, of the bath temperature and of the beam loss scenario.

Heat transfer at a sapphire - Indium interface in the 30 mK - 300 mK temperature range

Within the framework of the AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) project a direct measurement of the Earths gravitational acceleration on antihydrogen will be carried out. In order to obtain satisfactory precision of the measurement, the thermal movement of the particles should be reduced. Therefore a Penning trap, which is used to trap antiprotons and create antihydrogen, will be placed on a mixing chamber of an especially designed dilution refrigerator. The trap consists of 10 electrodes, which need to be electrically insulated, but thermally anchored. To ensure that the trap remains at a temperature below 100 mK, the heat transfer at the metallic-dielectric boundary is investigated. A copper - indium - sapphire - indium - copper sandwich setup was mounted on the CERN Cryolab dilution refrigerator. Keeping the mixing chamber at a constant low temperature in the range of 30 mK to 300 mK, steady-state measurements with indium in normal conducting and superconducting states have been performed. Obtained results along with a precise description of our setup are presented.

submitter : Parametric study on the thermal performance of beam screen samples of the High-Luminosity LHC upgrade

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will increase the accelerators luminosity by a factor 10 beyond its original design value, giving rise to more collisions and generating an intense flow of debris. A new beam screen has been designed for the inner triplets that incorporates tungsten alloy blocks to shield the superconducting magnets and the 1.9 K superfluid helium bath from incoming radiation. These screens will operate between 60 K and 80 K and are designed to sustain a nominal head load of 15 Wm−1, over 10 times the nominal heat load for the original LHC design. Their overall new and more complex design requires them and their constituent parts to be characterised from a thermal performance standpoint. In this paper we describe the experimental parametric study carried out on two principal thermal components: a representative sample of the beam screen with a tungsten-based alloy block and thermal link and the supporting structure composed of an assembly of ceramic spheres and titanium springs. Results from both studies are shown and discussed regarding their impact on the baseline considerations for the thermal design of the beam screens.

Final report on the Controlled Cold Helium Spill Test in the LHC tunnel at CERN

The 27 km circumference LHC underground tunnel is a space in which the helium cooled LHC magnets are installed. The vacuum enclosures of the superconducting magnets are protected by over-pressure safety relief devices that open whenever cold helium escapes either from the magnet cold enclosure or from the helium supply headers, into this vacuum enclosure. A 3-m long no stay zone around these devices is defined based on scale model studies, protecting the personnel against cold burns or asphyxia caused by such a helium release event. Recently, several simulation studies have been carried out modelling the propagation of the helium/air mixture, resulting from the opening of such a safety device, along the tunnel. The released helium flows vary in the range between 1 kg/s and 0.1 kg/s. To validate these different simulation studies, real life mock-up tests have been performed inside the LHC tunnel, releasing helium flow rates of 1 kg/s, 0.3 kg/s and 0.1 kg/s. For each test, up to 1000 liters of liquid helium were released under standard operational tunnel conditions. The data recorded include oxygen concentration, temperature and flow speed measurements, and video footage used to assess qualitatively the visibility. These measurements have been made in the up- and downstream directions, with respect to the air ventilation flow, of the spill point.This paper presents the experimental set-up under which these release tests were made, the effects of these releases on the atmospheric tunnel condition as a function of the release flow rate. We discuss the modification to the personnel access conditions to the LHC tunnel that are presently implemented as a result of these tests.