Mathematical formulation to predict the harmonics of the superconducting Large Hadron Collider magnets : III. Precycle ramp rate effects and magnet characterization

The Large Hadron Collider (LHC) at CERN is equipped with a feed-forward control system known as the field description for the LHC (FiDeL) which is designed to predict the magnetic field and its multipoles, hence reducing the burden on beam based feedback. FiDeL consists of a physical and empirical parametric field model based on magnetic measurements at warm and in cryogenic conditions. It is particularly critical during beam injection when the field decays and at the beginning of acceleration when the field snaps back. It is known that the decay amplitude is largely affected by the powering history of the magnet, particularly by the precycle flattop current and duration and the preinjection preparation duration. Recently, we have collected data that quantify the dependence of the decay amplitude on the precycle ramp rate. This paper presents the results of the measurements performed to investigate this effect, and the method included in FiDeL to model the precycle dependence.With this complete picture of dynamic changes, we finally discuss the effect on the data taken at nominally constant field, along the magnet loadline. We show that a correction for dynamic changes is required for adequate magnet characterization.peer-reviewe

Pre-cycle selection for the superconducting main magnets of the Large Hadron Collider

Pre-cycles for setting up the main magnets of the Large Hadron Collider are necessary for ensuring field reproducibility and low field-decay rates at injection. In this paper we propose standard pre-cycles for the main magnets of the LHC. We study the influence of the pre-cycle parameters on the field decay at injection by two different models. One already proven model is semi-empirical based on magnetic measurements of the magnets. The other is a new network based model of a Rutherford cable which directly calculates the current redistribution and associated magnetization change in the cable strands. The pre-cycle to be used may depend on the history of the machine or may have to be changed because of unforeseen phenomena in the machine. The choice of a new pre-cycle on the basis of magnetic measurements alone is a lengthy process. We confirm the usefulness of the network based model as a tool for selecting new pre-cycles, including decay-blocking degaussing pre-cycles, and compare with magnetic measurements.peer-reviewe

Pre-cycle selection for the superconducting main magnets of the Large Hadron Collider

Pre-cycles for setting up the main magnets of the Large Hadron Collider are necessary for ensuring field reproducibility and low field-decay rates at injection. In this paper we propose standard pre-cycles for the main magnets of the LHC. We study the influence of the pre-cycle parameters on the field decay at injection by two different models. One already proven model is semi-empirical based on magnetic measurements of the magnets. The other is a new network based model of a Rutherford cable which directly calculates the current redistribution and associated magnetization change in the cable strands. The pre-cycle to be used may depend on the history of the machine or may have to be changed because of unforeseen phenomena in the machine. The choice of a new pre-cycle on the basis of magnetic measurements alone is a lengthy process. We confirm the usefulness of the network based model as a tool for selecting new pre-cycles, including decay-blocking degaussing pre-cycles, and compare with magnetic measurements.peer-reviewe

Hysteresis Effects of MCBX Magnets on the LHC Operation in Collision

The Large Hadron Collider beams are brought into collision by superconducting orbit corrector magnets which generate the parallel separation and crossing angles at the interaction points during the different cycle phases. Unfortunately, the magnetic field errors that result from hysteresis effects in the operation region of these magnets lead to unwanted orbit perturbations. In a previous paper, it has been shown that these effects are within the perturbations coming from beam-beam interactions for the MCBC and the MCBY magnets but are significant in the case of the MCBX magnets. This paper presents a refined model of their field in the frame of the Field Description for the LHC (FiDeL); the results obtained from new magnetic measurements in cold conditions to test the model; the powering mechanism employed to maximize their field reproducibility; and the impact the modelling error is predicted to have on the LHC orbit in phase 1

Hysteresis effects of MCBX magnets on the LHC operation in collision

The Large Hadron Collider beams are brought into collision by superconducting orbit corrector magnets which generate the parallel separation and crossing angles at the interaction points during the different cycle phases. Unfortunately, the magnetic field errors that result from hysteresis effects in the operation region of these magnets lead to unwanted orbit perturbations. In a previous paper, it has been shown that these effects are within the perturbations coming from beam-beam interactions for the MCBC and the MCBY magnets but are significant in the case of the MCBX magnets. This paper presents a refined model of their field in the frame of the Field Description for the LHC (FiDeL); the results obtained from new magnetic measurements in cold conditions to test the model; the powering mechanism employed to maximize their field reproducibility; and the impact the modelling error is predicted to have on the LHC orbit in phase 1.peer-reviewe

Measurement and modeling of magnetic hysteresis in the LHC superconducting correctors

The Large Hadron Collider, now under construction at CERN, relies heavily on superconducting magnets for its optics layout: besides the main magnets, almost all the correcting magnets are superconducting. Along with clear advantages, this brings about complications due to the effects of persistent currents in the superconducting filaments. Corrector magnets that trim key beam parameters or compensate field errors of the main magnets (among others those due to hysteresis), are in their turn hysteretic. In this paper we present the measured magnetic hysteresis and its possible influence on accelerator operation, with particular reference to realtime compensation of dynamic effects in the main magnets, and reproducibility issues between runs. The modeling strategy as a function of the required accuracy is discussed, and two examples are presented.peer-reviewe

Parametric field modeling for the LHC main magnets in operating conditions

The first beam injections and current ramps in the LHC will require a prediction of the settings of the magnet current as well as the main correctors. For this reason we are developing a parametric model of the magnetic field generated by the LHC magnets that will provide the field dependence on current, ramp-rate, time, and history. The model of the field is fitted on magnetic field measurements performed during the acceptance tests of the magnets before their installation in the machine. In this paper we summarize the different steps necessary to select the relevant data and identify the parameters: the data extraction, the filtering and the validation of the measurements, and the fitting procedure that is used to obtain the parameters from the experimental results. The main result reported is a summary of the value of the parameters obtained with the above procedure, and describing the behavior of the magnetic field in the LHC main dipoles and quadrupoles.peer-reviewe

Measurement and effects of the magnetic hysteresis on the LHC crossing angle and separation bumps

The superconducting orbit corrector magnets (MCBC, MCBY and MCBX) in the Large Hadron Collider (LHC) at CERN will be used to generate parallel separation and crossing angles at the interaction points during the different phases that will bring the LHC beams into collision. However, the field errors generated by the inherent hysteresis in the operation region of the orbit correctors may lead to unwanted orbit perturbations that could have a critical effect on luminosity. This paper presents the results obtained from dedicated cryogenic measurements on the orbit correctors and the resulting simulations performed to quantify the impact of the hysteresis on the LHC orbit.peer-reviewe

Dependence of the static and dynamic field quality of the LHC superconducting dipole magnets on the pre-cycle ramp rate

The allowed multipoles in the Large Hadron Collider (LHC) superconducting dipole magnets decay whilst on a constant current plateau. It is known that the decay amplitude is largely affected by the powering history of the magnet, and particularly by the pre-cycle flat top current and duration and the pre-injection preparation duration. Recently, it was observed that the decay amplitude is also highly dependent on the pre-cycle ramp rate, which has an indirect effect also on the sample of data taken at constant field along the magnet loadlines. This is an important consideration to be included in the Field Description for the LHC (FiDeL), to cope with the difference between the test procedure followed for series tests and the expected cycles during the machine operation. This paper presents the results of the measurements performed to investigate this phenomenon and describes the method included in FiDeL to represent this dependence.peer-reviewe

A demonstration experiment for the main field tracking and the sextupole and decapole compensation in the LHC main magnets

A dedicated measurement campaign was set up to test the FiDeL concept and its LSA implementation. The test was performed by demonstrating the tracking of B1 and B2 for two LHC main dipoles and one LHC main quadrupole. It also included the compensation of the b3 and b5 harmonics in the dipole magnets using the sextupole and decapole corrector magnets. In this report we present the techniques developed to power the magnets for these tests during the current ramps; the instrumentation and data acquisition setup used to perform the tracking experiments; the calibration procedure and data corrections employed; and finally the main results obtained.peer-reviewe