Resistive transition and protection of LHC superconducting cables and magnets

Superconductivity and superfluidity are macroscopic quantum-effects that are used in technology. One of the most important applications of superconductivity is the design of strong magnets, which guide particles at very high energies in circular accelerators. In the Large Hadron Collider (LHC), which is being constructed at the European Laboratory for Particle Physics (CERN) close to Geneva, magnets wound with conventional superconductors are cooled with superfluid helium to access even higher magnetic field strengths. The resistive transition from the superconducting to the normal-conducting state (known as a quench) can be characterised by mechanical, electrodynamic and thermodynamic processes. Due to the high amount of stored magnetic energy, a quench can potentially cause damage in superconducting elements by overheating or excessive voltages. A detailed description of the related mechanisms is needed to understand the quench process better and to design a reliable protection system. This requires analytical and more importantly numerical models, which include the heat generation of the superconductor, cooling by helium, the thermodynamic propagation of the normal-conducting zone, as well as the impact of induced eddy currents. In the framework of this thesis, a new numerical algorithm has been developed. The improvements and advancements made in the quench modelling are explained in this thesis. It also includes detailed analyses and simulation studies of the quench processes in LHC superconducting cables and magnets. The LHC protection system that has been optimised by the outcome of this thesis is presented. The results and consequences of the performed analyses and simulations are summarised

Angkor Underground - Applying GPR to analyse the diachronic structure of a great urban complex

This thesis is based on surveys of ground-penetrating radar (GPR) conducted at Angkor, Cambodia. The appraisal of preceding remote sensing surveys led to selective ground based prospection for archaeological objects of interest on different scales. The successive relocation of the political and religious centre from the 9th to the 14th century has left a palimpsest landscape that reaches from small artificial habitation mounds, masonry monuments and their enclosures, to the extensive water management network of channels and earthworks that covered large parts of the floodplain between the Kulen Hills and Lake Tonle Sap. To make efficient use of the technique, the GPR survey had to be adjusted to those dimensions. The area-covering grid method was chosen for small scale surveys on habitation patterns, production sites and cemeteries, testing potential and limits in the application. A major factor in the measuring and processing of data was the floodplain geology of predominantly clayey sand and an environment prone to inundation that provided varying signal penetration depths depending on either compact or soft soil. For the larger scales, GPR was used in combination with GPS, GIS and remote sensing data sets. The concept of spatial configuration of monuments in and outside of enclosures led the search for remains of missing laterite and sandstone structures. A survey in the centre of Angkor Wat revealed the outline of six towers as part of a potential quincunx formation. They were further analysed by excavations to establish a preliminary construction history of the area. Surveys inside the peripheral enclosures of Chau Srei Vibol, Banteay Sra and Prasat Komnap showed evidence of demolished structures, some of it possibly from the Angkorian period. For questions concerning the functioning of a water management system in the Angkorian floodplain, GPR profiles in search for infrastructure were conducted alongside and over the embankments of the giant reservoirs. Evidence of outlets in the central areas of the eastern embankments of all four baray at Angkor confirmed them being part of the network. On the largest scale, GPR transects were run across parts of the floodplain to investigate the network of canals and earthworks that had been mapped by remote sensing. Obstacles, profiles and grids as well as the detected anomalies were integrated into a geo-referenced GIS database. Potential connections between centres and temples were integrated at areas where associated and previously mapped earthworks discontinued. Anomalies associated to the water management features were classified according to their characteristics and potential function as former artificial and natural channels, moats, ponds as well as masonry remains, and analysed with regard to archaeological maps and available remote sensing data. Newly acquired high resolution satellite radar (TerraSAR-X) data was used to evaluate a potential relation between water saturation and anomalies. The complete dataset was analysed for a complementation of archaeological maps and with the intent to separate features of the artificial canal network of Angkor from the natural landscape and the original distribution of rivers

Angkor Underground - Applying GPR to analyse the diachronic structure of a great urban complex

This thesis is based on surveys of ground-penetrating radar (GPR) conducted at Angkor, Cambodia. The appraisal of preceding remote sensing surveys led to selective ground based prospection for archaeological objects of interest on different scales. The successive relocation of the political and religious centre from the 9th to the 14th century has left a palimpsest landscape that reaches from small artificial habitation mounds, masonry monuments and their enclosures, to the extensive water management network of channels and earthworks that covered large parts of the floodplain between the Kulen Hills and Lake Tonle Sap. To make efficient use of the technique, the GPR survey had to be adjusted to those dimensions. The area-covering grid method was chosen for small scale surveys on habitation patterns, production sites and cemeteries, testing potential and limits in the application. A major factor in the measuring and processing of data was the floodplain geology of predominantly clayey sand and an environment prone to inundation that provided varying signal penetration depths depending on either compact or soft soil. For the larger scales, GPR was used in combination with GPS, GIS and remote sensing data sets. The concept of spatial configuration of monuments in and outside of enclosures led the search for remains of missing laterite and sandstone structures. A survey in the centre of Angkor Wat revealed the outline of six towers as part of a potential quincunx formation. They were further analysed by excavations to establish a preliminary construction history of the area. Surveys inside the peripheral enclosures of Chau Srei Vibol, Banteay Sra and Prasat Komnap showed evidence of demolished structures, some of it possibly from the Angkorian period. For questions concerning the functioning of a water management system in the Angkorian floodplain, GPR profiles in search for infrastructure were conducted alongside and over the embankments of the giant reservoirs. Evidence of outlets in the central areas of the eastern embankments of all four baray at Angkor confirmed them being part of the network. On the largest scale, GPR transects were run across parts of the floodplain to investigate the network of canals and earthworks that had been mapped by remote sensing. Obstacles, profiles and grids as well as the detected anomalies were integrated into a geo-referenced GIS database. Potential connections between centres and temples were integrated at areas where associated and previously mapped earthworks discontinued. Anomalies associated to the water management features were classified according to their characteristics and potential function as former artificial and natural channels, moats, ponds as well as masonry remains, and analysed with regard to archaeological maps and available remote sensing data. Newly acquired high resolution satellite radar (TerraSAR-X) data was used to evaluate a potential relation between water saturation and anomalies. The complete dataset was analysed for a complementation of archaeological maps and with the intent to separate features of the artificial canal network of Angkor from the natural landscape and the original distribution of rivers

Quench Heater Studies for the LHC Magnets

About 2000 LHC (CERN's Large Hadron Collider) superconducting magnets will be protected with quench heaters against development of excessive voltage and overheating after a resistive transition. The quench heater strips are powered by capacitor bank discharge power supplies. The strips are made of stainless steel partially plated with copper to reduce their resistance and to allow for the connection of quench heaters in series. The strips are embedded in between two polyimide foils. The initial power density and the current decay time determine the quench heater effectiveness. Since only one type of heater power supply will be available, the copper plating cycle is adapted for the various magnet types to keep the resistance of the heater circuit constant. Different quench heater designs have been tested on various prototype magnets to optimise the copper-plating cycle and the electric insulation of the heater strip. This paper summarises the experimental results and computations that allowed to finalise the heater strip layout for all LHC magnets

Partition-dependent framing effects in lab and field prediction markets

Many psychology experiments show that individually judged probabilities of the same event can vary depending on the partition of the state space (a framing effect called "partition-dependence"). We show that these biases transfer to competitive prediction markets in which multiple informed traders are provided economic incentives to bet on their beliefs about events. We report results of a short controlled lab study, a longer field experiment (betting on the NBA playoffs and the FIFA World Cup), and naturally-occurring trading in macro-economic derivatives. The combined evidence suggests that partition-dependence can exist and persist in lab and field prediction markets

Modelling of the Quench Process for the Optimisation of the Design and Protection of Superconducting Busbars for the LHC

The superconducting busbars powering the LHC magnets are highly stabilised with copper to reduce the probability of a quench starting in a busbar and to avoid excessive temperatures after a quench during current discharge. In order to determine the required copper stabilisation and the parameters of the protection system a finite difference program has been developed. The program numerically approximates the heat balance equation and evaluates the temperature profile after a quench as a function of time and space. The approach emphasises the modelling of heat transfer into helium. The evaluation of the temperature includes the entire quench process, i.e., the time for quench detection and the current decay

Review of quench simulations for the protection of LHC main dipole magnets

The simulation program QUABER [1] allows studying the quench process of superconducting magnets for the LHC. The performance of the protection system of the LHC main dipole magnets was simulated under various parameter dependencies at different magnet excitation currents. This simulation study was motivated to complement measurement results in order to help preparing and understanding experiments of the quench propagation and magnet protection. The influence of the quench propagation velocity and the time for a quench propagation between adjacent turns was studied. The different copper plating cycles of the quench heater strips were simulated. Experimental measurement results [2] were used to calibrate the input parameters. The performance of the protection system for various quench detection thresholds was investigated and different failure modes of the system were considered. The maximum voltages and values of the quench load are discussed. The values given are obtained using conservatively chosen parameters. The quench back effect is modelled at high currents by quenching the entire inner layer at a certain time after the quench start. The temperature evaluation is based on adiabatic calculation

Quench simulations for superconducting elements in the LHC accelerator

The design of he protection system for he superconducting elements in an accel- erator such as the Large Hadron Collider (LHC),now under construction at CERN, requires a detailed understanding of the hermo-hydraulic and electrodynamic pro- cesses during a quench.A numerical program (SPQR -Simulation Program for Quench Research)has been developed o evaluate temperature and voltage dis ri- butions during a quench as a func ion of space and ime.The quench process is simulated by approximating the heat balance equation with the finite di fference method in presence of variable cooling and powering conditions.The simulation predicts quench propagation along a superconducting cable,forced quenching with heaters,impact of eddy curren s induced by a magnetic field change,and heat trans- fer hrough an insulation layer in o helium,an adjacen conductor or other material. The simulation studies allowed a better understanding of experimental quench data and were used for determining the adequate dimensioning and protection of the highly stabilised superconducting cables for connecting magnets (busbars),opti- mising the quench heater s rip layou for the main magnets,and s udying quench back by induced eddy curren s in he superconductor.After the in roduction of the theoretical approach,some applications of he simulation model for the LHC dipole and corrector magnets are presented and he outcome of he s udies is compared with experimental data

Quench Propagation and Heating in the Superconducting 600 A Auxiliary Busbars of the LHC

In the Large Hadron Collider (LHC) at CERN 22 km of flexible superconducting cable, the auxiliary busbar cable, will conduct currents of up to 600 A to a large number of corrector magnets distributed throughout the accelerator. A prototype cable with 42 active conductors underwent several experiments to measure the hot spot temperature and the quench propagation velocity as a function of the current. The former was evaluated for various energy extraction scenarios as they are foreseen for the LHC corrector circuits. The experimental results and the heat flow simulations show that the quench behavior in this busbar prototype is strongly influenced by the heat flow through the insulation material (polyimide) into the helium bath, leading to stable configurations above the critical temperature Tc for currents between 250 A and 500 A. Special attention was paid to the study of discontinuities in the wires, like feed-throughs, where the wire is not immersed in liquid helium, and joints, where the wire cross-section is increased. The experiments and simulations led to a thorough understanding of the quench process in the wires of the prototype cable, which resulted in guidelines for the design, the use and the installation of the cable in the LHC

NON-DESTRUCTIVE GPR ANALYSIS OF THREATENED ANGKORIAN CERAMIC KILNS AT BANGKONG, SIEM REAP, CAMBODIA

Pedestrian surveys have identified a large number of 9th century ceramic kilns to the north of the early Angkorian centre of Hariharalaya. Recent development in this area has put this archaeological site under threat. With a large number of kilns already destroyed before analysis, it was necessary to identify the sites for protection from further damage.Since the defined areas showed little evidence on the surface, GPR surveys were conducted to find out what additional subsurface features could be used for classification. Using a 250MHz antenna system, the surveys were conducted over four kilns in varying state of preservation. The analysis included one definite and partly exposed kiln, and three potential kilns. Of the latter, one had been completely bulldozed, one partly destroyed and another one had remained undisturbed. The geophysical surveys shows the possibilities to better define the extent of the kiln mound. 3D visualization of the data displays depth and extent of the fire box and ware chamber of this type of Khmer kiln, providing the opportunity to better outline the area to be protected. The survey serves as a successful sample case for the systematic non-destructive mapping of known and newly discovered kiln sites