Example of cryogenic process simulation using EcosimPro: LHC beam screen cooling circuits

CERN has developed a dedicated library to model helium cryogenic plants with a commercial software called EcosimPro. The aim of such a library is to provide a simple way to model small and large scale cryogenic systems performing dynamic simulations in an acceptable timescale to assist both control and operation teams in the optimal commissioning and operation of cryogenic plants. Moreover, the tool allows users to easily develop models related to their specific components such as cryogenic transfer lines or superconducting magnets. During the last few years, this library has been used to model several CERN cryogenic systems. The models have been used for different purposes, e.g. operator training, virtual commissioning of control systems and control optimization. This paper briefly presents EcosimPro with the cryogenic library developed at CERN and gives an example of modeling the LHC beam screen cooling circuits showing simulation results compared with experimental data.     

0-D thermo-hydraulic approach for predicting pressure and temperature along HELIOS SHe closed loop under pulsed loads

Cryogenic systems for future large superconducting tokamaks (e.g. JT-60SA or ITER) are expected to cope with large pulsed heat loads due to cycling plasma operation. Their superconducting magnets are cooled down with forced flow supercritical helium.The aim of this paper is to verify to what extent a 0-D thermo-hydraulic model can well reproduce in space and time, the variations of pressure and temperature along a supercritical helium closed loop, subjected to pulsed heat loads.A 0-D model has been developed and the paper will present the corresponding equations and assumptions will also be justified. A pulsed heat load tokamak relevant scenario has been tested and the resulting variations of pressure and temperature have been compared with experimental data. The results of the 0-D model demonstrate the relevance of such approach for predicting transient behaviors in response of pulsed heat loads in a closed loop.This simple approach is also a justification to use process modeling codes where dynamics of the cryogenic circuits can be simulated with cryogenics components.     

Development of superconducting and cryogenic technology in the Institute for Technical Physics (ITP) of the Research Center Karlsruhe

Since the early 1970s the Institute for Technical Physics of the Research Center Karlsruhe has been involved in the development of superconductivity for research and industrial applications. A broad program with a focus on the superconducting magnet technology was established to include large magnets for nuclear fusion, high-field magnets for nuclear magnetic resonance spectrometers, ore separation and energy storage magnets. Research and development work was performed in collaborative projects with other national as well as international institutions and industry. The success of these projects has been supported by a broad foundation of engineering science in superconductor development, electrical and cryogenic engineering. Several well known test facilities like TOSKA, STAR, HOMER, MTA along with well equipped laboratories for conductor development, materials at cryogenic temperatures, cryogenic high-voltage engineering have made substantial contributions to in-house, national and international projects. A strong cryogenic infrastructure with two refrigerators and sophisticated cooling circuits from about 4.5 K down to 1.8 K assure the reliable operation of these large facilities. Last but not least, cryogenic research, including vacuum pumps for International Thermonuclear Experimental Reactor, improvements in thermal insulation, cryogenic instrumentation and small on board refrigerators has supported progress in this field. High-temperature superconductivity projects for low AC loss conductors, a 70 kA current lead and a fault current limiter are currently in progress.     

Vacuum performance of a beam screen with charcoal for the LHC long straight sections

The vacuum chamber inside the cryogenic magnets in the LHC Long Straight Sections will have a beam screen at a temperature between 5 and 20 K to protect the cold bore against the synchrotron radiation, electron and ion exposure. The desorbed molecules of H₂ will leave the inner part of the beam screen through the pumping slots on the beam screen and eventually condense on the cryosorber, which is mounted on the shadowed (outer) part of the beam screen for magnets operating at 4.5 K. The design of the experimental set-up, the results of the adsorption capacity measurements for charcoal, the pumping speed and the capture factor of the beam screen with charcoal for a proposed LHC vacuum chamber configuration are described in this paper.     

Effects of injection pressure difference on droplet size distribution and spray cone angle in spray cooling of liquid nitrogen

Spray cooling with liquid nitrogen as the working fluid has been widely employed in a plenty of fields requiring cooling at cryogenic temperature, such as the cryogenic wind tunnels and cooling super-conducting magnets. In this study, we built a liquid nitrogen spray system and experimentally investigated the influence of injection pressure difference on the droplet size distribution and the spray cone angle. The measurements using particle size analyser show increasing the injection pressure difference improves the atomization, as indicated by the homogenization and reduction of the droplet size. The initial spray cone angle is insensitive to the injection pressure difference. However, the far-field spray cone angle decreases dramatically with increasing the injection pressure difference. The results could enrich our knowledge of spray cooling of cryogenic fluids and benefit the design of cryogenic spray cooling systems.     

LHC: The world's largest vacuum systems being operated at CERN

With the successful circulation of beams in the Large Hadron Collider (LHC), its vacuum system becomes the world's largest vacuum system under operation. This system is composed of 54 km of ultra high vacuum (UHV) for the two circulating beams and about 50 km of insulation vacuum around the cryogenic magnets and the liquid helium transfer lines (QRL). The LHC complex is completed by 7 km of high vacuum transfer lines for the injection of beams from the SPS and their dumping.Over the 54 km of UHV beam vacuum, 48 km are at cryogenic temperature (1.9 K), the remaining 6 km are at ambient temperature and use extensively non-evaporable getter (NEG) coatings, a technology that was born and industrialised at CERN.The cryogenic insulation vacuum systems, less demanding technically, impress by their size and volume: 50 km and 15,000 m³. Once cooled at 1.9 K, the cryopumping allows pressure in the 10⁻⁴ Pa range to be attained.     

Flower,a model for the analysis of hydraulic networks and processes

We have developed in the past years a model that describes hydraulic networks that are typical of the cryogenic interconnection of superconducting magnets. The original model, called Flower, was used mostly to provide consistent boundary conditions for the operation of a magnet. The main limitations were associated with the number and nature of modelling elements available, and to the maximum size of the model that could be solved. Here we present an improvement of the model largely relaxing the above limitations by the addition of new modelling elements, such as parallel flow heat exchangers, and by a significant improvement in the numerics of the solver, using sparse matrix storage and solution techniques. We finally show a typical application to the case of a magnet quench in the LHC string.     

Analytical method for the prediction of quench initiation and development in accelerator magnets

The optimal design of the next generation of accelerator magnets calls for a high current density in the superconducting coil, which makes the magnet protection a challenge. Quenches in the high-field magnets for the High Luminosity LHC Upgrade typically develop within tens of ms, and the reaction time needs to be comparable, requiring active firing of heaters or other heat deposition techniques to increase the quench propagation velocity in the magnet. It is important to have a very good understanding of the behavior of a magnet during a quench. Practical scaling laws, and simplified methods, allow quick scans of design and operation parameters, and swift feedback based on experimental results once the magnet is in test. In this paper we describe simplified methods to predict the quench initiation and development in accelerator magnets using active quench protection. We use data from the recent Nb₃Sn model magnets for the High-Luminosity LHC as a benchmark for the method, discussing expected accuracy and the reasons for deviations.     

Stability analysis of the LHC cables

The commissioning and operation of the LHC accelerator calls for better understanding of the stability margin of its superconducting magnets with respect to the perturbation spectrum expected during operation, such as wire motion, AC loss during ramps and in particular beam induced heat depositions. In this paper, the stability analysis of the cables for the LHC dipole for transient heat deposition is reported. The sensitivity of the stability margin to the parameters of the model is investigated and discussed. In particular, the helium content in the cable is shown to have a large impact on stability. In fact, we find that the most important parameter is the heat transfer into helium II and helium I. Furthermore, we quantify the influence of the interstrand thermal and electrical coupling on the cable stability in the several examples reported.     

EAST低温系统超临界氦循环泵的自动控制

为确保EAST超导磁体安全、稳定地进行降温,通过分析超导磁体氦迫流冷却回路并结合低温系统降温操作流程,在低温控制系统中设计并实现了氦循环泵的自动启动和失超处理等顺序控制流程;采用选择性控制结构和改进的增量式PID控制算法,实现了氦循环泵入口压力的自动控制.经过降温实验验证,氦循环泵的自动控制策略满足低温系统的运行要求,在一定程度上提高了系统的自动化程度.     

Magnets for fields above 100 kg

Classifying coils above and below 100 kG does not separate types or methods, but rather indicates the degree of difficulty. It is no longer true that conventional dc fields above 100 kG require larger than existing or contemplated power supplies, and it has been established that superconducting materials will be suitable for fields approaching twice that field. Problem areas, economics, and experience above 100 kG are discussed in three major areas: 1) water-cooled dc magnets, principally those now in operation at the National Magnet Laboratory; 2) pulse magnets covering the continuum from sub-millisecond pulses to the low-duty cycle cryogenic magnet systems; 3) superconducting magnets with emphasis on hybrid systems, combining water-cooled copper magnets and superconducting coils.     

Methods of testing electrical insulating materials for cryogenics

The paper reviews the methods of evaluating materials for cryogenic engineering, both those in use and other methods suggested by the authors. These proposed methods have been verified in practice with superconducting magnets produced in Czechoslovakia. The system of evaluation proved useful. The results of measurements carried out on some selected materials are tabulated.     

Molecular cryosorption properties of porous copper, anodised aluminium and charcoal at temperatures between 10 and 20 K

New types of anodised aluminium, porous copper and charcoal-based materials are being developed as cryosorbing materials and have been studied in collaboration with a number of research institutes. The major aim was to find a suitable cryosorber with a working temperature in the range between 5 and 20 K that could be used in the LHC vacuum chamber inside the superconducting magnets at a temperature of 4.5 K and higher. The design of the experimental set-up, the results of cryosorption capacity measurements for porous copper, anodised aluminium and charcoal-based materials are described in this paper.     

Fracture measurements on insulation for high-field superconducting magnets: Relationship to processing regimes and test temperature

The next European dipole project aims to develop the technology required for the next generation of superconducting dipole magnets for particle physics and other application areas. Development of these magnets is limited by manufacturing issues connected with the insulation systems, and we report work directed towards resolving these issues. In design of superconducting magnets in the past, a lack of knowledge of fracture properties at cryogenic temperatures has lead to the requirement to make customised test pieces simulating regions of high-stress, particularly in the non-metallic insulation. The current work aims to provide fracture properties information to be used in stress calculations, for example by the finite element method, and so allow better optimisation of magnet design. We report early measurements of mode 1 interlaminar fracture toughness on sample insulation materials, focussing on the effects of the heat treatment cycle which forms the niobium-tin superconductor and to which its insulation is subjected.     

Modeling of the very low pressure helium flow in the LHC cryogenic distribution line after a quench

This paper presents a dynamic model of the helium flow in the cryogenic distribution line (QRL) used in the Large Hadron Collider (LHC) at CERN. The study is focused on the return pumping line, which transports gaseous helium at low pressure and temperature over . Our aim is to propose a new real-time model of the QRL while taking into account the non-homogeneous transport phenomena. The flow model is based on 1D Euler equations and considers convection heat transfers, hydrostatic pressure and friction pressure drops. These equations are discretized using a finite difference method based on an upwind scheme. A specific model for the interconnection cells is also proposed. The corresponding simulation results are compared with experimental measurements of a heat wave along the line that results from a quench of a superconducting magnet. Different hypotheses are presented and the influence of specific parameters is discussed.     

Analysis of the quench propagation along Nb3Sn Rutherford cables with the THELMA code. Part I: Geometric and thermal models

The paper describes the new lumped thermal model recently implemented in THELMA code for the coupled electromagnetic–thermal analysis of superconducting cables. A new geometrical model is also presented, which describes the Rutherford cables used for the accelerator magnets. A first validation of these models has been given by the analysis of the quench longitudinal propagation velocity in the Nb₃Sn prototype coil SMC3, built and tested in the frame of the EUCARD project for the development of high field magnets for LHC machine. This paper shows in detail the models, while their application to the quench propagation analysis is presented in a companion paper.     

Superconducting magnets for large accelerators

A significant milestone in the application of superconducting magnets for large accelerators was reached on June 2nd 1983 when the Tevatron at Fermilab transported the first beam in its giant superconducting ring. This machine, which extends over 6 km and contains 1000 superconducting magnets, is the result of 10 years of intensive development work and has opened a new era for high energy physics. Recently, the construction of a similar ring has been started for the HERA collider at DESY, Hamburg (FRG) and development of a 3 TeV synchrotron is going on at Serpukhov (USSR). Even more powerful machines are already being studied such as the SSC in the USA and the LHC collider in the LEP tunnel at CERN. A review of these applications is presented, showing the main technological achievements they represent and the worldwide activity they have given rise to. In addition to accelerator magnets, beam lines and detectors utilize sophisticated superconducting magnets, and are worthy of mention.     

Analytical and finite element investigations of shear/compression test fixtures

Insulation systems are critical components of the international thermonuclear experimental reactor (ITER). They must meet the super conducting magnets design requirements, including mechanical strength under combined shear and compressive stresses at cryogenic temperatures. Past cryogenic magnet systems often relied on woven glass/epoxy materials for insulation. An important point is to find a reliable shear/compression test method for these materials. The present work investigates a commonly used shear/compression setup and aims at measuring the reliability of the obtained test results. Therefore, the stress and failure analysis is performed analytically and numerically using the finite element method. The model is based on woven glass fiber reinforced materials which are subjected to combined shear and compressive stresses as well as to thermal loading, that results from cooling from 293 K to the test temperature of 77 K. A short analytical section shows the problems of common failure criteria which are used to describe the interaction of the shear and compression stresses. The numerical—finite element—section is based on three-dimensional linear elastic finite element models under thermo-mechanical loading. The locations of high stress gradients are investigated using an average stress criterion. Three different model geometries (15°, 45°, and 70°) are analyzed and finally compared with respect to their reliability.     

Systematization of constructional materials based on the level of strain hardening and development of an energy method of determination of the allowable stresses and safety factors. Part 2. Determination of allowable stresses taking into consideration the reserves of strain hardening materials

An energy method of determination of allowable stresses and undetermined strength safety factors and analytically based obtaining of higher values of allowable stresses as the result of taking into consideration reserves of strain hardenability, plasticity, and toughness (crack resistance) materials in addition to their strength properties has been developed. As the result of the proposed method determination of allowable stresses (safety factors), until now based primarily on accumulated engineering experience and traditions, is acquiring a scientific basis and the metallurgical industry, the machine building branches of industry, and construction the capability of using reserves of strength of strain hardening materials and as the result of reducing the metal consumption for production of them, including semifinished products, parts, and structures, without reducing the level of their reliability. The reliability of the method has been confirmed by the results of tests at normal and cryogenic temperatures of large models and full-size welded pressure vessels produced from materials of various types.Translated from Problemy Prochnosti, No. 2, pp. 54–61, February, 1992.     

Simulation of the cabling process for Rutherford cables: An advanced finite element model

In all existing large particle accelerators (Tevatron, HERA, RHIC, LHC) the main superconducting magnets are based on Rutherford cables, which are characterized by having: strands fully transposed with respect to the magnetic field, a significant compaction that assures a large engineering critical current density and a geometry that allows efficient winding of the coils. The Nb₃Sn magnets developed in the framework of the HL-LHC project for improving the luminosity of the Large Hadron Collider (LHC) are also based on Rutherford cables. Due to the characteristics of Nb₃Sn wires, the cabling process has become a crucial step in the magnet manufacturing. During cabling the wires experience large plastic deformations that strongly modify the geometrical dimensions of the sub-elements constituting the superconducting strand. These deformations are particularly severe on the cable edges and can result in a significant reduction of the cable critical current as well as of the Residual Resistivity Ratio (RRR) of the stabilizing copper. In order to understand the main parameters that rule the cabling process and their impact on the cable performance, CERN has developed a 3D Finite Element (FE) model based on the LS-Dyna® software that simulates the whole cabling process. In the paper the model is presented together with a comparison between experimental and numerical results for a copper cable produced at CERN.