Development of Testing Methods for Absolute Magnetometers and Some Test Results of the Overhauser Magnetometer POS-1 Jean L. Rasson (Royal Meteorology Institute, B-5670, Dourbes, Belgium), Vladimir A. Sapunov (QM Lab. of USTU, Mira str., 19, Ekaterinburg, 620002, Russia) Proton magnetometers are well known as precise instruments for the measurement of total magnetic field. Such position is based on the proton gyromagnetic ratio, a fundamental physical constant. In this report we discuss some general metrological aspects of the proton and Overhauser magnetometers which can be used for development and exploitation. This discussion is stimulated by researches of the new Overhauser magnetometer POS-1 that is intended for magnetic observatories and fieldwork. The photos show two designs of the POS-1 namely a variant with a flexible cable and a hard-staff design. An example of the recording of geomagnetic variations in polar latitudes (Yakutia) is also shown. 60816.2 60816.0 60815.8 60815.6 60815.4 60815.2 20:20:00 20:30:00 20:40:00 20:50:00 21:00:00 21:10:00 21:20:00 The manufacturer of the POS-1 magnetometer declares a high measurement sensitivity (up to 0.01nT at 3 s cycling rate or 0.1nT at 1 s) and an absolute accuracy of up to ±0.5nT. To check these parameters, tests in the RMI magnetic field standard were carried out. For low fields around 27 µt we have a noise level lower than 0.05nT, while for fields around 60µT the noise level is below 0.01nT. 275
On the other hand, we found a deficiency in the magnetic cleanliness checking methods at the manufacturer materials processing stage, which resulted in absolute and heading errors. Three cylindrical Overhauser sensors were tested. The orientation dependence of measurements for the sensor head rotation around the cylinder axis was measured. One sensor has an orientation dependence of ±0.5nT. Two faulty sensors have shown dependence of about ±3nT. Addressing this last problem, we made comparisons of several methods for magnetic impurity testing. A flux-gate magnetometer was found to be the most successful at the stage of sensor element control and preliminary check. The photo shows the RMI magnetic field standard and testing of a harmful field by the flux-gate magnetometer FLM1/B (0,1nT sensitivity). The flux-gate magnetometer is set-up with the sensitive axis perpendicular to the geomagnetic field and inside the vertical plane (magnetic meridian plane). The flux-gate output is then essentially zero except for the variations due the magnetic inclination time changes. The Overhauser sensor under test (SUT) is approached with the sensor axis horizontal and perpendicular to the magnetic meridian till the cylinder surface is at 1 cm from the flux-gate extremity. Then the SUT is rotated around its axis and the readings of the fluxgate observed. We look for a sinusoidal signal in function of the rotation angle. The peak-topeak (p-p) sinus amplitude reading in nt gives the magnetic signal perturbing the cleanliness. For the POS-1#24 the p-p sinus amplitude was 3.0nT. For the POS-1#23 the p-p sinus amplitude was 3.3nT. As the + and - peaks are identified we put marks on the cylindrical surface of the sensor head. Actually it turns out that the peaks are separated by almost 180 degrees, showing dipolar behaviour. The strongest p-p amplitude is at the centre of the sensor. The second stage of the general check of the POS-1 performs a comparison with a reference device and allows absolute error measurement. Comparisons were performed at 30, 50 and 70 µt. The SUT was each time measured in two positions: once with the perturbing dipole parallel (3 readings) and once anti-parallel (3 readings) to the stabilizer magnetic field vector. A self-oscillating optically pumped potassium vapour (39K) magnetometer stabilizes 276
and controls the current in the coil system (Rasson 1996). The sensor position is adjusted at a previously determined gradient-free spot (H=760mm V=128mm) with the connector towards West. The free field value at the time of the experiment was 48144nT. Stab. set-point 30000nT 50000nT 70000nT SUT: POS-1#23 30002.5 50001.7 70000.2 Parallel 30002.5 50001.6 70000.2 30002.4 Antiparallel 29998.4 29998.2 29998.6 50001.7 49997.7 49997.4 49997.6 70000.2 69996.1 69996.3 69996.0 Mean (par-antipar) 4.1 4.1 4.1 Similar results were obtained for the POS-1#24 where the value of mean (par-antipar) was: 4.8, 5.3 and 4.8 nt. Thus the faulty POS-1 magnetometers have an orientation error around the sensor axis of 4.1 nt for POS1-#23 and 4.8 nt for POS-1#24, confirming the magnitude of the earlier measurement with the flux-gate (take into account the different distances). The defective sensors were disassembled and it was established that a magnetic impurity in the cast plastic holder of the Faraday screen caused the pollution. It is indicative that a magnetic beam balance used by the manufacturer for the impurity control did not find out those significant effects notwithstanding the balance s capability to find out the diamagnetism of liquids. Obviously the volumetric distribution of impurity played an essential role here. The replacement of the impure holders resulted in errors below ±0.5nT. Similar researches and results were obtained in the laboratory magnetic field standard (four-layer magnetic screen and slender solenoid). The photo shows QMLab magnetic standard with the geological survey POS-1 under testing. Unfortunately, the QM laboratory employees did not find the defects described above at the 277
stage of manufacture. On the one hand it is caused by changes brought in the sensor design under observatory requirement and on the other hand by defect of laboratory technique. We observed additional deviations at the manufacture stage but it was explained by the influence of the polarization field on the screen magnetic shell and not by the magnetic impurity presence. Due to joint efforts the technique of magnetic cleanliness test progressed. In particular the RMI transferred a flux-gate magnetometer FLM1/B to QM laboratory. This has permitted to carry out a number of interesting experiments described below. It is established that the magnetic field uniformity of the magnetic screen does not allow to supervise orientation errors better than ±0.5nT. From this point of view it is necessary to carry out metrological researches under systems similar to the RMI magnetic standard (coil system with active stabilisation). In the second stage of research we investigated the reason causing residual orientation error. The value of the error was inexplicable as the magnetic cleanliness of the sensor head by methods described above was established. An hypothesis about a thermal origin of this error was put forward earlier. Within the framework of the submitted results the convincing proofs were found. The main element of the Overhauser sensor of POS-1 magnetometer is the high-frequency resonator inside which there is a vacuum-processed glass ampoule with working substance. The high-frequency resonator is made as two copper coaxial pipes whose ends are connected by the crosspiece and capacitors. The general view is shown in the figure below. The ampoule has small size (diameter of 30, length about 70 mm). When a continuous highfrequency power of about 1 Watt is applied, that causes a temperature increase of about ten degrees. The basic heat-generating elements are the discrete capacitors that cause a temperature gradient as shown approximately in the figure. Due to couples which are formed in the connecting-points of the capacitors with the outside shell, thermocurrents are generated due to the heating by the high-frequency power. The manufacturer of ampoules knows about this property and consequently declares some orientation error. This problem is caused partially by a small irregularity in the capacitor positions that does not allow to cancel completely out the thermogenerated magnetic fields. Some ways were used to research this effect. In particular, we have applied the flux-gate magnetometer to register the perturbing magnetic fields. The experiment was carried out in the above-mentioned magnetic screen at a magnetic field of about 0-10 nt. The flux-gate sensor was set-up parallel to the ampoule at a distance of 5 mm. The difference of measurements was registered when the high-frequency power was switched on and off. The 278
figure shows the result of measurements (nt) at angular rotation of the batch production Overhauser ampoule. Thus the ordinary POS-1 magnetometers have an orientation error around the sensor axis confirming the magnitude of this result. Similar experiments were made with a new ampoule design, which has shown magnitude up to 0.1nT. However, the new ampoule is more difficult to manufacture. The second experiment was aimed at the research of the integrated effect and revelation of the absolute errors caused only by the thermal effect. The laboratory field standard and usual POS-1 magnetometer was involved. To increase the heating effect, an Overhauser ampoule was powered from an external rather powerful HF-generator (up to 3 W). The time behaviour of the total field measurement from the beginning of the cyclic measurements was registered. The HF-generator worked continuously. The maximal magnitude for the usual ampoule was in a range from 0.2nT up to 2nT at the various sensor orientations in relation to the standard stable field of 20000nT. The time decay of this process was about 40 seconds. It is interesting to note that the curve has a small maximum caused by reduction of the temperature gradient after transient (we apologise for the absence of a figure for technical reasons). The new design of the Overhauser ampoule has shown a general change of no more than 0,5nT at a power of 3 W and 0.2nT at the standard power used in the POS-1. Thus, the effect of thermal current generation in the conducting shell of the Overhauser sensor HF-resonator results in absolute errors and must be taken into account to meet of highest absolute accuracy. For the POS-1 magnetometer we offer to carry out a calibration of 279
orientation dependence for example by the RMI reference equipment with marks of minimal absolute error directions. In summary it is necessary to note that the considered themes are applicable to all types of absolute magnetometers and that this research is of interest to others in the field. Dedicated magnetic field standards and careful investigations of all measurement conditions of Overhauser magnetometers are necessary in order to reach absolute accuracies of the order of 0.1nT. Rasson J.L. (1996). Coil system for the magnetic full field stabiliser, in Proceedings of the VIth Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing (JL Rasson Ed.), Publ. Sci. et Techn. No 003, Institut Royal Meteorologique de Belgique, Brussels p87-93. 280