INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY Meas. Sci. Technol. 15 (2004) 918–922 PII: S0957-0233(04)69475-6 Three-component variometer based on a scalar potassium sensor E B Alexandrov 1 , M V Balabas 1 , V N Kulyasov 1 , A E Ivanov 1 , A S Pazgalev 1 , J L Rasson 2 , A K Vershovski 1 and N N Yakobson 1 1 S I Vavilov State Optical Institute, 12 Birzhevaya st., St Petersburg 199034, Russia 2 Centre de Physique du Globe, Institut Royal M´ et´ eorologique, B-5670 Dourbes, Belgium Received 23 September 2003, in final form 12 February 2004 Published 6 April 2004 Online at stacks.iop.org/MST/15/918 (DOI: 10.1088/0957-0233/15/5/020) Abstract A new variometer is developed comprising a fast-response scalar optically pumped potassium magnetometer inside a rotating magnetic field created by a two-dimensional coil system mounted on a quartz frame. The variometer measures three components of the Earth’s field: the total field intensity and two transverse components. The theoretically predicted accuracy of the field component measurement is not worse than 0.1 nT. The noise-limited sensitivity measured in a quiet magnetic field has been proved to be not worse than 25 pT rms at 0.2 s and 30 pT rms at 1 min; comparison with a proton vector magnetometer and a fluxgate magnetometer shows 1.5 nT p–t–p daily deviation. Keywords: geomagnetic instruments, magnetometer, variometer, magnetic field vector, optically pumped potassium magnetometer 1. General principles Here we present yet another implementation of the idea [1–6] of using a scalar magnetic field measuring device for measuring a three-component vector field. As a scalar device we use a potassium M x optically pumped sensor [7, 8] as it is much more sensitive, accurate and (what is most relevant for this application) fast than the proton magnetometer. Originally, the M x magnetometer constitutes a sort of maser, oscillating at a certain frequency ω(H) strictly related to current scalar value H of the ambient magnetic field. This device can be rearranged into a three-component variometer by adding to its construction two orthogonal sets of magnetic coils, producing (a) an additional magnetic field H ⊥ , rotating on a plane perpendicular to the vector of the Earth’s magnetic field H 0 and (b) a slow-changing compensating magnetic field H 1 , lying in the same plane. The scalar sensor positioned exactly at the centre of the coil system measures the total magnetic field H which is the vector sum of H 0 and H ⊥ . Obviously, the scalar value H depends on the mutual orientation of H 0 and H ⊥ ; if H 0 changes direction (a small transverse component H appears), |H(t)| is no longer constant but contains oscillating components with phases and amplitudes depending on the angle between H 0 and H ⊥ . These oscillating components are used as an error signal for a feedback system, producing the small compensating magnetic field H 1 =−H which brings the vector of dc magnetic field back to its initial position; and now from the knowledge of the components of the compensating field H 1 , we know the variation of the transverse components of H 0 (figure 1). The main advantage of this device over all its predecessors, including the proton magnetometer situated in additional magnetic fields with alternating polarity, is its capability of fast continuous measurement limited only by the frequency f of the field H ⊥ rotation (several hundred Hertz, the frequency is limited by the Zeeman potassium resonance line splitting in the Earth’s field). 2. The variometer design The variometer (figure 2) consists of a fast-response scalar magnetometer installed inside a two-dimensional (x–y) coil system, producing the magnetic field H ⊥ of magnitude of about (H 0 /10) rotating on the x–0–y plane around the z-axis with frequency f (where H 0 is the mean magnetic field of the Earth, and f = 362 Hz). Initially the z-axis of the coil system is positioned parallel to the Earth’s field axis, so the output of the scalar magnetometer does not contain any modulation at frequency f . 0957-0233/04/050918+05$30.00 © 2004 IOP Publishing Ltd Printed in the UK 918