Speaker
Description
A commercial 3-axis high temperature SQUID system, cooled with liquid nitrogen, is currently operating unshielded as a geomagnetic sensor in an aluminium dewar at SANSA Space Science, South Africa. The collaborative project between South Africa (SANSA), France (LSBB) and Czech Republic (CTU) is aimed at continuous, low-noise geomagnetic space weather measurements. The environment is urban with commercial installations [1]. The vertical SQUID is a M2700 from Cryostar (white noise < 300 fT/√Hz) and the two horizontal SQUIDs are HTM-8 from FZ Jülich (white noise <65 fT/√Hz). The SQUIDs are controlled with Magnicon SEL-1 electronics, and output is digitized with National Instruments 24-bit card. There is provision for a non-magnetic environment, isolation from mains power noise and a low noise reference fluxgate magnetometer, running simultaneously at a distance of 3 m. SQUID measurements under different environmental circumstances reveal significant noise due to interference from the commercial environment – RF and/or magnetic. Also, the manual process of cryogen refilling results in instability of the SQUIDs due to temperature settling in the dewar [2]. Furthermore, between refills we recently observed 5 mHz oscillations of up to 2 nTptp amplitude in the HTM-8 SQUIDs, which are shown to correlate to 6 mKptp temperature variations of the liquid nitrogen [3]. The frequency and amplitude of these oscillations are functions of cryogen level and compromise system operation as a geomagnetic sensor even in the FLL loop. We discuss the investigation of mitigation techniques; the high sensitivity of the HTM-8 SQUIDs to these temperature oscillations is currently unexplained.
[1] E.F. Saunderson, M. Dressler and M. Janosek, “Practical aspects of an HTS SQUID operating as a Geomagnetic sensor”, SCSSM2021, Prague, 2021
[2] F. P. Milliken et al, “The response of high-Tc SQUID magnetometers to small changes in temperature”, Journal of Applied Physics, vol: 82, 6301, 1997
[3] D.J. Blundell and B.W. Ricketson, “The temperature of liquid nitrogen in cryostat dewars”, Cryogenics, January 1979, pp 33-36
