Ships with a ferromagnetic hull distort Earth’s magnetic field in such a way that magnetic mines can detect them. On-board degaussing systems compensate for the magnetic signature, but the copper coils take up valuable space and cause losses. High-temperature superconductive (HTS) coils are a potential alternative because they don't have Ohmic losses. However, research is lacking on the application of HTS to degaussing. The objective of this thesis is to design an adequate HTS degaussing power supply.

A modelling framework is developed to quantify the magnetic signature and to estimate the degaussing parameters. Analytical and numerical modelling techniques are compared where the numerical provides the most accurate results but takes longer to calculate. Both are verified with an experimental test setup where the (reduced) magnetic signature was measured. A second test setup was built to demonstrate the performance and feasibility of HTS degaussing coils. The HTS coils are placed in a closed-loop vacuum-insulated cryostat filled with sub-cooled liquid nitrogen. The magnetic signature is measured in a static and dynamic case where the behaviour of the HTS coils is similar to that of the copper coils.

The high current-carrying capability of HTS has the advantage that fewer turns are required. However, a larger current density results in more power source and current lead losses. Both problems are solved using a cryo-cooled converter with low on-state resistance MOSFETs. A comparative study between four different topologies has been conducted, concluding that an H-bridge converter with cryo-cooled MOSFETs and a DC capacitor bank inside the cryostat can reduce the amount of losses by a factor 23 compared to a conventional converter. A cryogenic H-bridge converter with ten parallel MOSFETs per switching leg and an HTS load was built and tested. The converter produces a current of 50 A with an input voltage of 3.5 V and a duty cycle of only 0.025. The parasitic inductances cause unwanted effects regarding oscillations above a switching frequency of around 100 Hz because of the low PCB resistances.

A concern exists that a peak in the frequency spectrum can be detected in the magnetic signature around converter's switching frequency. Five switching frequency modulation schemes were tested to reduce this peak. A model of a full-sized frigate estimates the effect of the switching frequency on the magnetic signature. Of the tested schemes, random lead-lag shows the best results.

In this research, a framework is created where the magnetic signature and the degaussing parameters can be estimated. It was shown that HTS can be used to replace copper degaussing coils. Several converter topologies were compared, and it was shown that cryo-cooled MOSFETs can increase performance drastically. A cryogenic converter was built and tested successfully. A variety of switching frequency modulation schemes were tested, and it was found that random switching frequency modulation can be used to reduce the switching ripple in the magnetic signature.@en

A degaussing system can be used to reduce the detectability of the magnetic signature of a ship. Commonly, a degaussing system consists of a set of onboard copper coils that produce a magnetic field to compensate for the magnetic signature. High-temperature superconductive degaussing coils are considered an alternative to copper degaussing coils because of a reduction in energy losses, weight, volume, and costs. The losses of a high-temperature superconductor (HTS) degaussing system can be reduced even further by powering it with a cryocooled converter with parallel mosfets. A low-duty cycle and smaller current leads can be used. These solutions eliminate most of the power source losses. This article investigates such a cryocooled converter. The effect of the low switching frequency on the converter performance is tested. A prototype that can operate at cryogenic temperatures was built. The converter powers an HTS coil. It was found that a load current of 50 A can be achieved with a duty cycle of just 0.025 at an input voltage of 3.5 V while still meeting the requirement of a maximum current ripple of 0.5%. At a switching frequency higher than 100 Hz, the converter's performance deteriorates. Also, oscillations were observed in the circuit. This is a problem due to the low blocking voltage of the mosfets. The parasitic inductances in the circuit have a high impact on the performance because the resistance in the circuit is very low.

@en

The magnetic modelling and experimental validation of a superconducting degaussing system for maritime vessels is discussed. Degaussing coils compensate for the distortion in the earths' magnetic field by the magnetized steel hull of a ship, thus rendering it 'invisible' for magnetic field sensors. Whereas typical power requirements with copper coils are of the order of 100 kW, a ReBCO HTS degaussing system in principle allows to reduce this by an order of magnitude. In order to validate such efficiency estimates and to demonstrate the required hardware, a table-top test setup was realized with magnetic ship steel. The vessel-imitating cylindrical demonstrator is equipped with six degaussing coils, grouped in three sets that act in two different directions, with each set consisting of one copper and one ReBCO coil, the latter one equipped with a sub-cooled forced-flow liquid nitrogen system. Static magnetic field measurements are reported and compared to both analytical and numeric finite element models. The results illustrate how even relatively simple analytical models can be used as a powerful tool to extrapolate design parameters and thus to predict the power requirements of large-scale degaussing systems.

@en

Detection of the magnetic signature of ships can be avoided by using a degaussing system; a set of on-board copper coils that compensates for the magnetic signature. High temperature superconductors (HTS) are currently investigated as a replacement for copper degaussing coils. By using HTS, we have to deal with higher currents and therefore with higher power supply losses. Also, large current leads are needed which introduces extra losses. This paper investigates different possible solutions to minimize these losses. Four H-bridge-based MOSFET topologies are presented that were designed to reduce the power supply and current lead losses. The first topology uses an H-bridge configuration so that the degaussing current can freewheel through the low-resistance MOSFETs. The second topology places the H-bridge inside the cryostat so that the current leads can be made smaller. The third topology includes a smoothing capacitor in the cryostat so that the current leads and input current are even smaller. The fourth topology uses a transformer so that the current leads can be eliminated. Measurements were done to determine the MOSFETs and capacitor performance in liquid nitrogen. The simulated losses of the four topologies are compared to determine the most energy-efficient option for supplying current to the HTS coils. It was found that by submerging multiple parallel MOSFETs in liquid nitrogen, the on-state resistance is decreased and the current supply can be made more efficient. Also, by placing a smoothing capacitor inside the cryostat, the current lead losses can be minimized significantly. The benefits of using a transformer do not outweigh the transformer losses.

@en

Ships can avoid to be detected by magnetic mines by reducing their magnetic signature with degaussing coils. Degaussing currents are provided by switched mode power supplies which impose a current ripple on top of the degaussing current. The ripple might be visible in the magnetic signature which would increase the detectability of the ship. A way to reduce the ripple in the magnetic field is to use a switching modulation scheme in the degaussing power supplies. In this paper, a magnetic model of a ship with degaussing coils is described. It is used to find the magnitude of the ripple in the magnetic signature. Also the effect of reducing the current ripple by frequency modulation is investigated. Several modulation schemes are modelled. It is found that the ripple in the magnetic signature is often, but not always, negligible due to attenuation by the ship’s hull. For low frequency switching applications, like high temperature superconductor degaussing systems, the ripple is visible in the magnetic signature. It is found that switching frequency modulation is a very effective technique to reduce the ripple of degaussing currents. Of the tested schemes, random lead lag and random switching frequency are the most effective.@en

Due to their permeability, merchant and naval vessels distort Earth’s magnetic field which leaves a magnetic signature. These anomalies in the magnetic field may be detected by mines leaving the vessel exposed. To avoid detection by magnetic sensors, the vessels magnetic signature can be reduced in several ways. One of them is degaussing: a technique where a set of on-board coils produces a magnetic field to cancel the magnetic signature. Modelling studies have shown that the performance of a degaussing system can be improved by replacing the copper with high temperature superconductor (HTS) coils in terms of volume, weight and energy efficiency. This study aims to compare the degaussing functionality of copper and HTS windings. A table-top demonstration model is presented which is equipped with both copper and HTS degaussing coils. The demonstrator is prepared by removing the permanent magnetisation. The two degaussing topologies are compared in terms of functionality, efficiency, controllability and mutual inductances.@en

In order to avoid detection by sea mines, the magnetic signature of merchant and naval vessels can be reduced by running a current through a set of on-board copper coils. This process is called degaussing. Studies have shown that the volume, weight and energy losses of a degaussing system can be reduced by replacing the copper coils with high temperature superconductive (HTS) coils. Moreover, since the technology and production of HTS has matured and the material is highly available, the use of HTS for degaussing coils is a serious option. As a preliminary study towards an HTS degaussing test setup, this paper presents the design of a table-top demonstration with copper degaussing coils. The goal of the demonstration is to measure the magnetic signature and the magnetic signature reduction of a cylindrical object. The design choices of the test setup and the measuring system are discussed. The magnetic signature of the table-top model is calculated as well as the optimal placement of the degaussing coils and the optimal degaussing currents. These results are compared with measurements of the magnetic flux density around the demonstrator.@en