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The power deposited in dispersion suppressor (DS) magnets downstream of the LHC betatron cleaning insertion is governed by off-momentum protons which predominantly originate from single-diffractive interactions in primary collimators. With higher beam energy and intensities anticipated in future operation, these clustered proton losses could possibly induce magnet quenches during periods of short beam lifetime. In this paper, we present FLUKA simulations for nominal 7 TeV operation, comparing the existing layout with alternative layouts where selected DS dipoles are substituted by pairs of shorter higher-field magnets and a collimator. Power densities predicted for different collimator settings are compared against present estimates of quench limits. Further, the expected reduction factor due to DS collimators is evaluated., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

In this thesis work we investigate the heat transfer through the electrical insulation of superconducting cables cooled by superfluid helium. The cable insulation constitutes the most severe barrier for heat extraction from the superconducting magnets of the CERN Large Hadron Collider (LHC). We performed an experimental analysis, a theoretical modeling and a fundamental research to characterize the present LHC insulation and to develop new ideas of thermally enhanced insulations. The outcome of these studies allowed to determine the thermal stability of the magnets for the LHC and its future upgrades. An innovative measurement technique was developed to experimentally analyze the heat transfer between the cables and the superfluid helium bath. It allowed to describe the LHC coil behavior using the real cable structure, an appropriate thermometry and controlling the applied pressure. We developed a new thermally enhanced insulation scheme based on an increased porosity to superfluid helium. It aims at withstanding large heat loads, as needed for the High Luminosity LHC upgrade (HL-LHC). Experimental measurements of the new insulation showed a major improvement of heat extraction compared to the present state-of-the-art used in the LHC. We developed a theoretical heat transfer model quantitatively explaining the experimental results of the LHC and HL-LHC insulations. We identified the heat extraction mechanisms, mainly occurring through superfluid helium micro-channels. The average micro-channels dimensions were estimated to vary between few and dozens of micrometers, depending on the insulation scheme. In the model we considered the known laws describing the dynamic regimes of superfluid helium. However such laws were never demonstrated to be valid in the narrow channels typical of the cable insulation. We developed a new experimental device to investigate heat transport through microchannels. Micro-fabrication techniques were used to etch the channels down to a depth of 16 micrometers. We measured the classical laminar and turbulent regimes, thus demonstrating the validity of such heat transport laws independently from the channel geometrical shape and size down to these dimensions. To conclude, we used the obtained experimental and theoretical heat transfer results to determine the LHC and HL-LHC magnets stability. We estimated their quench margin and compared it to the beam induced heat deposit. The resulting difference allows a safe operation of the magnets.

CERN is performing a systematic analysis of the interconnecting bus bars of the Large Hadron Collider (LHC) main magnets. Their thermal, electrical, mechanical and hydraulic performances are addressed. In the frame of these studies, the heat transfer between the main superconducting (SC) bus bars and the surrounding He bath is investigated. It represents a key parameter in the comprehension of the bus bars stability and quench propagation mechanisms, thus crucial for the analysis of the 2008 incident which was triggered by a defective bus bars joint. This paper reports on the experimental tests and relative analysis aiming at describing the thermal behavior of the LHC main bus bars.

The full energy exploitation of the Large Hadron Collider (LHC), a planned increase of the beam energy beyond the present 6.5 TeV, will result in more demanding working conditions for the superconducting dipoles and quadrupoles operating in the machine. It is hence crucial to analyze, understand, and predict the quench levels of these magnets for the required values of current and generated magnetic fields. A one-dimensional multi-strand electro-thermal model has been developed to analyze the effect of beam-losses heat deposition. Critical elements of the model are the ability to capture heat and current distribution among strands, and heat transfer to the superfluid helium bath. The computational model has been benchmarked against experimental values of LHC quench limits measured at 6.5 TeV for the Main Bending dipole magnets.

Superconducting magnets submitted to large heat loads, as the low-β quadrupoles for the LHC luminosity upgrade, need the development of new concepts of cable electrical insulation featuring a He-II porous wrapping scheme. This paper reports and discusses recent results of dedicated heat transfer measurements performed on different variants of such schemes, with emphasis on the heat transfer enhancements achievable with respect to the state-of-the-art insulation used for the main LHC magnets.

The commissioning and the exploitation of the LHC require a good knowledge of the stability margins of the superconducting magnets with respect to beam induced heat depositions. Previous studies showed that simple numerical models are suitable to carry out stability calculations of multi-strands cables, and highlighted the relevance of the heat transfer model with the surrounding helium. In this paper we present a systematic scan of the stability margin of all types of LHC cables working at 1.9 K against transient heat depositions. We specifically discuss the dependence of the stability margin on the parameters of the model, which provide an estimate of the uncertainty of the values quoted. The stability margin calculations have been performed using a zero-dimensional (0-D) numerical model, and a cooling model taking into account the relevant helium phases which may appear during a stability experiment: it includes Kapitza thermal resistance in superfluid He, boundary layer formation and heat transfer in He I, and considers the transition from nucleating boiling to film boiling during He gas formation.

The operation of the Large Hadron Collider calls for a thorough analysis of the thermo-electric behavior of the 13 kA superconducting bus-bars connecting its dipole and quadrupole main magnets. This presentation reports a synthesis of the work performed jointly by researchers and students at CERN and at the University of Bologna as a contribution to the understanding of the LHC incident occurred on September 19th 2008. This work is complementary to the analyses carried out at CERN by the MPE group. The aim of the work is to analyze the stability of the interconnections as far as the quality of manufacturing, operating conditions and protection system parameters are concerned. A first part of the work is devoted to the development of a numerical model suitable for the analysis of the faulty interconnections. The main type of defect analyzed is the lack of solder among the superconducting cable and the copper stabilizer components at the interface between bus bar and splice. The evaluation of the critical defect length limiting the maximum safe current for powering the magnets without risk of thermal runaway is provided, as a function of the RRR of cable and stabilizer, decay time constant of the LHC circuit, spatial distribution of the defect and cooling conditions. A second part of the work is related to the modeling of the heat transfer mechanism between the main superconducting bus bar and the surrounding helium bath. This study is aimed to analyze a set of experimental measurements on the heat transfer coefficient of the main bending dipole bus bars performed at CERN. The final part of the work consists in a preliminary analysis of the heat transfer mechanisms involved in the stability experiments of defective interconnections performed in the FRESCA facility.

The operation of Nb–Ti superconducting magnets in He II relies on superfluidity to overcome the severe thermal barrier represented by the cable electrical insulation. In wrapped cable insulations, like those used for the main magnets of the Large Hadron Collider (LHC) particle accelerator, the micro-channels network created by the insulation wrappings allows to efficiently transfer the heat deposited or generated in the cable to the He bath. In this paper, available experimental data of heat transfer through polyimide electrical insulation schemes are analyzed. A steady-state thermal model is developed to describe the insulation of the LHC main dipole magnets and the Enhanced Insulation proposed for the High Luminosity LHC upgrade (HLLHC), according to the relevant geometric parameters. The model is based on the coupled mechanisms of heat transfer through the bulk of the dielectric insulation and through micro-channels between the insulation tapes. A good agreement is found between calculations and tests performed at different applied pressures and heating configurations. The model allows identifying the heat fluxes in the cable cross-section as well as the dimensions of the micro-channels. These dimensions are confirmed by microscope images of the two insulations schemes.

This memorandum provides alternate estimates of the critical additional resistance Radd of the 13 kA superconducting bus bars interconnections (IC), both for the LHC main bending (MB) dipole and main quadrupole (MQ) magnets. The calculations are performed using the 1-D thermo-electrical model described in [1], based on the definition of transverse local heat transfer coefficient towards the cooling helium bath established from the analysis of short sample tests performed in 2009 and 2010. Details on the model and its validation are not discussed here. The most pessimistic (adiabatic), most optimistic (full cooling) and most likely (partial cooling) critical additional resistances are provided, depending on the bus-bar and cable RRR, dump time constant, and space distribution of the defect, for beam energy between 3.5 and 4.5 TeV.

The stability is a key issue to design superconducting magnets and to ensure their proper exploitation. This is the case of the imminent operation of the Large Hadron Collider (LHC) accelerator, which requires a good knowledge of the stability margin of its magnets with respect to beam induced thermal perturbations. In this paper a thermal model to predict the response of a superconducting cable to a heat deposition is presented. The model includes a detailed description of its most relevant parameter: the heat transfer between the strands of the cable and the surrounding helium. The different helium phases which may appear during a thermal transient are considered, i.e. He II, He I with formation of a thermal boundary layer, nucleate boiling, film boiling and gaseous phase, as well as the transitions among them. The dependence of the stability margin on the parameters of the model is discussed. Finally, the stability characterization of the LHC cables is presented with respect to the duration of the perturbation (ranging from tens of microseconds to tens of seconds) and to the magnets operating parameters.

In this paper we discuss results of a one-dimensional numerical model to simulate electrical and thermal transients in the superconducting cables for the LHC machine, with specific reference to the analysis of quench due to the heat released by beam losses. Two models have been developed, one based on the analysis of the behavior of a single strand, whereas the other accounts for all the strands in the multi-strand cable. As a first step the stability margin is computed considering the single strand subjected to a variable magnetic field and heat deposit along its length. The impact of the field non-uniformity is assessed by com- parison with computations performed assuming a uniform field in the cable cross section. The results of this model are compared to those obtained with the multi-strand model. The numerical results are discussed in terms of current and heat redistribution between strands, and stability margin of the cable.