Digital SQUID Magnetometer Development for Geophysics Applications Validated in Low Noise Environment
A prototype digital version of the widely used Superconducting Quantum Interference Device (SQUID) magnetometer has been tested in real conditions of operation in July 2007 in the low noise underground facility of Rustrel France (Laboratoire Souterrain a Bas Bruit, LSBB) Geophysical studies that require a high magnetic field dynamic range can benefit from this digital device that makes use of the Rapid Single Flux Quantum (RSFQ) technique to achieve fast on chip electronic feedback at cryogenic temperature. First measurements have shown that the on chip digital processing of sensor signals, benefiting of all the advantages of digital technique, is a viable solution at cryogenic temperature. Received June 24, 2008. Accepted July 12, 2008. Reference number ST37; Category 4. Based on a talk presented at the Microcooler Workshop of the "S Pulse" EU Project, April 2008. I. CONTEXT OF DEVELOPMENT During the 2002 2006 period, the 6 FrameWork Program of the European Community funded the project "High Tc digital SQUID Sensor for NDE in Unshielded Environment" "DigiSQUID" under the "Competitive and Sustainable Growth" Programme. It advanced the development of second order gradiometer digital SQUIDs making use of a high critical temperature superconductor (HTS) technology and operating at the temperature of about 30 kelvin. In some fields of application where portability is required, such as the Non destructive Evaluation (NDE) for example, a temperature of operation around 30 K is acceptable. Magnetometers and gradiometers fabricated in HTS technology at the University of Twente, Netherlands, have been studied and developed for use with custom made cryocoolers. To validate the design of digital devices used, a low critical temperature (LTS) equivalent has been also studied at the University of Technology of Ilmenau (IUT) in Germany. This LTS version used the ISO9001 certified mature fabrication technology employing 1 kA/cm current density niobium/aluminum oxide/niobium Josephson junctions available at the Fluxonics Foundry located at the Institute of Photonics Technology (IPHT) in Jena, Germany [1] Several generations of devices were fabricated during the 2004 2007 period at the Fluxonics Foundry. Testing of these at the University of Savoie, France, resulted in step by step improvements aiming towards a viable magnetometer with a high magnetic field dynamic range. Ultimate tests have been performed in real conditions to identify all issues related to the operation of the digital device. For this purpose, we decided to operate the digital SQUID in the low noise LSBB underground facility of Rustrel in French Provence [2] This site, located in a remote place with low human activity (500 meters below the ground level under the Plateau d'Albion) is the former French nuclear terrestrial launch control room. It has been refurbished for scientific purposes and is currently dedicated to miscellaneous scientific experiments in different fields of physics, ranging from the detection of IEEE/CSC ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM, No. 5, July 2008 Page 2 of 6 dark matter in astroparticle physics, or detection of earthquakes in geophysics, to the tests of reliability of semiconductor field programmable gate array (FPGA) circuits for industrial validation. This last research activity is crucial for the development of reconfigurable electronic circuits that are robust in the long term, especially for applications in space. The issues that are currently encountered in the semiconductor field will arise in the near future for superconductive electronics circuits as well, due to their ultimate speed performance and sensitivity, which also means sensitivity to unwanted perturbations. In the present case, as the digital SQUID is mainly a magnetic field sensor, the LSBB environment offers three interesting testing features: there is no environmental magnetic noise at the level of sensitivity of the digital SQUID, since the ambient magnetic noise in the electromagnetically shielded 1250 m chamber of LSBB has been measured not to exceed 3 fTHz above the 10 Hz frequency range; in the chamber, an analog SQUID system is installed for continuous monitoring of magnetic signatures of earthquakes; it can thus provide a direct comparison of digital and analog devices; higher magnetic field dynamic range, beyond the saturation of the analog SQUID sensor by external events, is needed to keep track of the absolute value of magnetic field in the case of magnetic field disturbances by phenomena of no interest. The continuous tracking of the absolute value of local magnetic field is of high scientific interest for geophysicists. II. PRINCIPLE OF OPERATION The pickup loop that detects the magnetic field is connected to a balanced current comparator made of two Josephson junctions, as shown in Figure 1. The comparator triggers when the sum of two currents, that of the clock Single Flux Quantum (SFQ) pulse and that induced by the detected magnetic field, exceeds a certain threshold, DI. When it happens, a new SFQ pulse is generated in the digital SQUID circuitry and transformed into a Non Return to Zero (NRZ) signal by a RSFQ flip flop cell. At the same time, an intrinsic feedback occurs in the pickup loop where the total flux is modified by one flux quantum, resetting the loop. Indeed, a fundamental property of a superconducting loop, due to the macroscopic quantum nature of superconductivity, requires that the magnetic flux in the loop be quantized. (a) (b) Fig 1. (a) Block diagram of the digital SQUID magnetometer. The balanced comparator is composed of two identical Josephson junctions; (b) photo of the device (courtesy of IPHT Jena) the pickup loop is seen at the bottom while the top central part connected to the loop is the balanced comparator. The top left part is the clock generator while the signal propagation and pulse shaping circuit is located to the right of the comparator. IEEE/CSC ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM, No. 5, July 2008 Page 3 of 6 The generated SFQ pulse is due to the switching of one Josephson junction of the balanced comparator. The pulse carries one magnetic flux quantum Ph0 (Ph0 h/2e 2.07 10 T.m 2.07 mV ps 2.07 pH mA) Since it leaves the pickup loop, a current redistribution takes place in the loop to account for the loss of one magnetic flux quantum; this is due to the dynamics of the Josephson junction coupled to its environment. Hence, it plays naturally the role of a current feedback loop in semiconductor electronics. The signal is then transmitted from the cryogenic to room temperature stage and amplified before being processed. In the final step, a computer reconstructs the magnetic field waveform from the digital data. This digital SQUID type is a delta modulator and can either see positive changes of magnetic flux (it sends then positive SFQ pulses) or negative changes (negative SFQ pulses are sent in this case) The detailed explanation of the principle of operation and main properties of the digital SQUID can be found in [3] The first experimental confirmation of simulations is described in [4] The main asset of this rather simple digital SQUID, which only included 11 Josephson junctions, is its intrinsic very high quantized dynamic range, directly related to its slew rate defined by the clock frequency driving the comparator sampling. The main drawback is the intrinsic flux resolution of one magnetic flux quantum Ph0, resulting from the digital nature of its operation principle. It is several orders of magnitude lower than for the analog SQUIDs. A solution to overcome this drawback is to make use of a multi stage version of the present design, which has been described in [5] In this case, the price to pay is an increased complexity of the RSFQ processing unit, and a much larger number of Josephson junctions on the chip. An alternative is to combine one digital and one analog dc SQUID into a hybrid device to take advantage of the ultimate sensitivity of analog SQUID in combination with the high dynamic range and speed of the digital SQUID. This solution has been studied in [6] First experimental proof of principle has been validated in 2007 and is described in [7] III. RECENT ACHIEVEMENTS The SQUID characterization including sensitivity, dynamic range and the overnight stability was performed at the University of Savoie. A further step was the testing in real conditions at LSBB. For the last generation of devices, the detected peak to peak amplitude of 2810 Ph0 has been achieved at 0.1 Hz corresponding to a peak to peak field of 11.8 mT. The measured flux noise level was 220 mPh0.Hz. The noise spectrum is shown in Figure 2. Fig 2. Measured flux noise spectral density of the digital SQUID magnetometer in unshielded conditions. No external field is applied (only the Earth's magnetic field is present) The clock frequency is 2 kHz. The duration of measurements is 128 seconds. IEEE/CSC ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM, No. 5, July 2008 Page 4 of 6 This performance corresponds to a dynamic range of 76 dB (around 12 bits) and to a field quantization step of 4.2 nT. Stability has been observed for several hours. All details can be found in [7] This device has also been characterized as a current comparator by coupling inductively the pickup loop with another loop fed by a controlled current. In this case a much higher peak to peak amplitude of 14800 Ph0 has been achieved, corresponding to a current resolution of 1.14 mA of the balanced comparator. The discrepancy between the performance of the digital SQUID as a magnetometer and in the presence of an inductively coupled magnetic field, localized only in the pickup loop region, is attributed to the perturbation of the digital RSFQ part of the device by the magnetic field applied in the magnetometer mode. We acted to solve this problem. IV. MEASUREMENTS IN LSBB ENVIRONMENT To reveal the potential problems occurring in real co