Theoretical Modeling and Experimental Investigation of the Thermal Performance of the LHC Prototype Lattice Cryostats

This thesis presents the thermal performance of the LHC (Large Hadron Collider) prototype cryostats both in steady-state and in transient conditions. LHC will be built in the 27 km LEP tunnel and will provide proton-proton collisions. It will make use of superconducting magnets operating in static bath of superfluid helium at 1.9 K. The thesis is mainly divided in three parts. The first part cont ains three chapters which present a brief overview of the LHC project. Part 1-Chapter 1 gives a short introduction to the LHC design layout and performance. Part 1-Chapter 2 refers to LHC cryogenic s ystem and describes the general architecture of the cryogenic plants, the temperature levels and the heat loads. The 50 m long LHC prototype half-cell contains one twin-bore quadrupole and four twin-a perture dipoles. In Part 1-Chapter 3 the design and construction of the prototype dipole and quadrupole cryostats are presented. The LHC prototype cryostats have integrated cryogenic lines, while the final LHC cryostats have separate distribution lines. The second part of the thesis illustrates the steady-state cryostat thermal performance beginning with a short description of the heat transfer p rocesses involved in the cryostat thermal budget (Part 2-Chapter 1). An overview of material and helium properties is given in Annexes 2 and 3. The mathematical model used to simulate the cryostat the rmal performance has been validated and the experimental tools used to accomplish this aim are presented in Part 2-Chapter 2. Full-scale prototype cryostats have been designed, constructed and use to assess the cryogenic behaviour. The thermal performance of the cryostats has been investigated both in nominal and in degraded operating conditions and is presented in Part 2-Chapter 3. Screen tempera ture and residual gas pressure have been varied in order to investigate their influence on the total thermal budget. The heat load due to resistive heating in the non-superconducting cable splices h as been analysed and results form electrical measurements have been compared with those from calorimetric measurements. The list of detailed heat loads for the CTM and the string test is given in Anne x 1. The heat interceptions play a very important role in the cryostat thermal performance, since low heat inleaks can be maintained only with a very efficient thermalisation. The main components whic h need to be cooled at intermediate temperatures are the screens, the support posts, the cryogenic valves and the vacuum barrier. The thermal contacts between these components and the cooling pipes ha ve been studied in different cryostats and results of their thermal impedances are presented. Annex 4 describes the basic concept of thermal contacts and gives a few empirical correlations which may b e applied in specific cases. The last chapter in Part 2 presents the potential of an actively cooled "soft" screen with respect to the "floating" insulation system. The efficiency of a soft screen dep ends on the quality of the heat interception and the insulation to the cold mass. test set-ups have been constructed to measure the thermal impedance of shrink-fitted type thermal contacts a nd the conduction through net-type insulating spacers. Test set-up and measurements results are described. The third part explains the cryostat thermal performance in transient modes. Forced flow coo ldown and warmup have been tested in the LHC prototype magnet string and calculated and experimental results are exposed in Part 3-Chapter 1. Natural warmup with and without active pumping on the insu lation system is described in Part 3-Chapter 2 as well as the simulation of accidental loss of insulation vacuum. For each case a one-dimensional non-linear mathematical model has been developed and validated against measured data.