Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The Superconducting Quantum Interference Device (SQUID) is one of the most sensitive magnetic field
sensors in the world. These instruments can only be used optimally for geomagnetic research if placed far
from man-made magnetic signals. Moving the SQUID to a remote site leads to several infrastructure-related
problems including construction, power, data connectivity, and cryogenic cooling. This thesis investigates
possible solutions to these problems and develops guidelines for designing future remote SQUID stations.
A remote SQUID observatory typically consists of three structures placed approximately 40 m apart. These
include: the control room, which houses all computers and supporting electronics, the power hut, which contains
a regulated battery bank charged from a solar array that delivers DC power to the rest of the system, and the
SQUID hut itself, which is thermally insulated by cavity walls. The SQUID is placed on an isolation pillar that
decouples it from structural vibrations due to wind and outside temperature
uctuations. The temperature
inside the SQUID hut is also monitored as changes in temperature can result in small deformations of the
SQUID mounting system which lead to changes in the SQUID's orientation. The changes in the orientation
will appear as slow varying magnetic signals on the SQUID output. In principle, it is possible to compensate
for these variations through post-processing.
The SQUID needs to be cryogenically cooled to function. The SANSA SQUID is a High Temperature
Superconductor (HTS) and operates using liquid nitrogen. Immersion cooling is used, as it is the simplest
method, and produces the least amount of mechanical and electrical noise. Over time the liquid nitrogen will
evaporate and needs to be replaced without significantly disrupting SQUID operations. A simple yet effective
pumping scheme was developed that can transfer approximately 1.8 litres of liquid nitrogen every minute from
a refill dewar. Monitoring of the liquid nitrogen level is an important management tool for a remote station, as
refilling will be the primary reason for technicians to visit the site. The monitoring is achieved by placing the
SQUID dewar on a specially designed non-magnetic load cell scale. The scale has been designed to limit the
amount of tilting as the weight changes since this would also change the SQUID orientation.
When a HTS SQUID is cooled in a large magnetic field, such as the Earth's field, Abrikosov vortices are
likely to form in the superconducting material. As these vortices jump between pinning sites in the material,
they increase the 1/f noise of the device and have been found to reduce the stability of the SQUID. Metal shields
can be used to reduce the magnetic field, but are awkward to use and also reduce the magnitude of the signals
of interest. In this thesis, a shielding method using Helmholtz coils is investigated. These coils are relatively
simple and inexpensive to construct and do not attenuate the signals of interest. It was found that by cooling the SQUID in the reduced magnetic field, generated by the Helmholtz coils, the stability of the SQUID can be
improved significantly. / AFRIKAANSE OPSOMMING: Die SQUID is die mees sensitiewe magneetveld sensors in die wêreld. Hierdie instrument kan slegs optimaal
vir geometriese navorsing gebruik word indien dit ver van mensgemaakte magneetvelde opgestel word. Om die
SQUID in 'n afgesonderde area op te stel veroorsaak verskeie probleme met betrekking tot infrastruktuur sover
dit konstruksie, kragvoorsiening, en kriogeniese afkoeling aangaan. Hierdie tesis ondersoek moontlike oplossings
vir die probleme en riglyne te ontwikkel vir die oprigting van toekomstige SQUID stasies.
'n Tipiese afgele SQUID observatorium bestaan gewoonlik uit 3 strukture wat 40m van mekaar opgestel is.
Die beheerkamer bevat al die elektroniese apperaat, die kragkamer bevat 'n stel gereguleerde batterye wat deur
sonpanele helaai word en DS krag verskaf aan die stasie en die SQUID-kamer wat deur middel van spoumure
teen hitte gensoleer is. Die SQUID word op 'n gesoleerde pilaar geplaas om die invloed van vibrasies a.g.v. wind
en wisselende buite temperature te verminder. Die temperatuur binne die SQUID kamer word ook noukeurig
gemonitor aangesien wisseling in temperatuur geringe vervorming van die SQUID se montering kan veroorsaak
wat 'n verandering van die SQUID se orintasie veroorsaak. Hierdie veranderings sal waargeneem word as stadige
varirende sein in die SQUID se lesings. In beginsel is dit moontlik om vir dit te kompenseer deur middel van
naprosessering.
Die SQUID moet kriogenies afgekoel word om te funksioneer. Die SANSA SQUID is 'n Ho Temperatuur
Supergeleier (HTS) en vloeibare stikstof word gebruik vir verkoeling. Afkoeling deur middel van indompeling
word gebruik omdat dit die kleinste hoeveelheid meganiese en elektroniese versteuring veroorsaak. Die vloeibare
stikstof verdamp mettertyd em moet vervang word sonder om die werking van die SQUID merkbaar te onderbreek.
'n Eenvoudige tog effektiewe oorpompstelsel is ontwikkel wat ongeveer 1.8 liter/minuut vloeibare stikstof
vanuit 'n hervullings vakuum
es kan oorpomp. Die meting van die vloeibare stikstof vlak is 'n belangrike aspek
van die instandhouding van 'n afgele stasie aangesien dit die hoof rede sal wees vir tegnici om die perseel te
besoek. Die meting word bewerkstellig deur die plasing van die SQUID se vakuum
es op 'n spesiale ontwerpte,
nie-magnetiese vrag sel skaal. Hierdie skaal is ontwerp om die mate van kanteling te beperk aangesien dit die
orentasie van die SQUID kan benvloed.
Wanneer 'n HTS SQUID binne in 'n groot magnetiese veld afgekoel word, is dit waarskynlik dat Abrikosov
vortekse in die supergeleidende materiaal sal ontstaan. Soos hierdie vortekse rondspring in die materiaal vermeerder
dit die 1/f ruis en daar is gevind dat die stabiliteit van die SQUID nadelig benvloed word. Metaal
skilde kan gebruik word om die invloed van die magneetveld te verminder, maar is ongerie
ik om te gebruik en verminder ook die sterkte van die seine wat waarneem wil word. In hierdie tesis word Helmholtz spoele
ondersoek as 'n afskermings metode. Hierdie spoele is eenvoudig om te vervaardig en verminder nie die sterkte
van waarneembare seine nie. Daar is gevind dat die stabiliteit van die SQUID merkbaar verbeter word deur
afkoeling in 'n lae magnetiese veld soos bewerkstellig deur die Helmholtz spoele.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96817 |
| Date | 03 1900 |
| Creators | Lochner, Emile Tobias |
| Contributors | Fourie, Coenrad J., Gouws, Daniel J., Sanderson, Elda, Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 138 pages : illustrations |
| Rights | Stellenbosch University |
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