Modeling of Beam Loss Induced Quenches in the LHC Main Dipole Magnets

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.

Stability modeling of the LHC Nb-Ti Rutherford cables subjected to beam losses

Luca Bottura, Enrico Felcini

The Large Hadron Collider (LHC) at CERN is being prepared for its full energy exploitation during Run III, i.e. an increase of the beam energy beyond the present 6.5 TeV, targeting the maximum discovery potential attainable. This requires an increase of the operating field of the superconducting dipole and quadrupole magnets, which in turn will result in more demanding working conditions due to a reduction of the operating margin while the energy deposited by particle loss will increase. Beam-induced magnet quenches, i.e. the transition to normal conducting state, will become an increasing concern, because they could affect the availability of the LHC. It is hence very important to understand and be able to predict the quench levels of the main LHC magnets for the required values of current and generated magnetic fields. This information will be used to set accurate operating limits of beam loss, with sufficient but not excessive margin, so to achieve maximal beam delivery to the experiments. In this study we used a one dimensional, multi-strand thermal-electric model to analyze the maximum beam-losses that can be sustained by the LHC magnets, still remaining superconducting. The heat deposition distribution due to the beam losses is given as an input for the stability analysis. 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 energy densities reconstructed from beam-induced MB (Main Bending) dipole quenches during LHC operation at 6.5 TeV. The model was then used to evaluate the stability margin of both MB and MQ (Main Quadrupole) magnets at different beam energies, up to the expected ultimate operating energy of the LHC, 7.5 TeV. The comparison between the quench levels underlines how the increase of beam energy implies a substantial reduction of magnets stability and will require much stricter setting on the allowable beam losses to avoid resistive transitions during operation.

2019

An early separation scheme for the LHC luminosity upgrade

Guido Sterbini

In this thesis we evaluate the potential of the Early Separation Scheme for the Luminosity Upgrade of the Large Hadron Collider (LHC). The main goal of the Early Separation Scheme is to reduce the crossing angle between the proton beams at the collision point in order to increase the luminosity performance of the machine and to alleviate, at the same time, the detrimental effects due to the electromagnetic interaction between the beams. The Early Separation Scheme consists of four dipoles for each of the two high luminosity Interaction Points of the LHC, corresponding to the ATLAS and CMS detectors. Two dipoles out of the four, the so-called D0 dipoles, have to be integrated in the experimental cavern. We show that, working in synergy with an increased beam current and with a stronger final focusing system, the Early Separation Scheme can provide an integrated luminosity of 3000 fb-1 over a period of 6.5 – 7 years with a leveled luminosity of 5.5 1034 cm-2 s-1. These figures are possible thanks to the luminosity leveling by using the crossing angle. We study the impact of the Early Separation Scheme from the beam dynamics point of view by evaluating its linear and non-linear effects. We show, using an analytical approach, that all induced linear effects are negligible. The non-linear effects, namely the electromagnetic interaction between the beams, are considered by means of numerical simulations and dedicated experiments. From the simulation and the experimental results, we have indications about the minimum beam crossing angle that is still compatible with the beam dynamics constraints. From these indications and by considering the different issues related to the integration of the scheme in the detectors, we propose to position the D0 dipole at 14 m from the Interaction Point. The integrated magnetic field required and the space constraints in the detectors imply the use of a superconducting D0 dipole: we optimize its aperture to reduce the power deposited on the magnet's coil by the collision debris. Due to the high power density impinging on the coil and to the 9 T magnetic field required, we propose a magnet cross-section using a Nb3Sn superconducting cable at the temperature of 4.2 K.

EPFL2010

Transient Heat Transfer Model and Stability Analysis of helium-cooled Superconducting Cables

Luca Bottura, Pier Paolo Granieri, Panagiota Xydi

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.