IEEE TRANSACTIONS zyxwvutsrqponm ON APPLIED SUPERCONDUCTIVITY, VOL. 7, NO. 2, JUNE 1997 3087 A high stability temperature controller based on a superconducting high-T, sensor. M. Lam Chok Sing, E. Lesquey, C. Dolabdjian and D. Robbes GREYC-URA 1526 CNRS, Institut des Sciences de la Matiere et du Rayonnement, Bd Mal Juin, 14050 CAEN Cedex zyxw - FRANCE. Abstract-An analysis of a temperature control scheme we described in a previous article is given in this paper, showing its intrinsic noise limitations. This scheme made good use of YBCO microbridges as temperature sensors. We find that high temperature to voltage transfer coefficient values (>> 1 V/K) are required in order to reach fluctuation levels well below loop$,. Consequently, a new design of the circuit has been made by introducing a current pre-biasing technique of the microbridge and allowing very large offset compensations. A transfer coefficient value of 441 V/K was used to obtain a temperature control with fluctuations less than 1.6 zyxwvutsr pKpP over a 2 hour recording time. Such high stability temperature controllers leading to fluctuation levels in the zyxwvutsr p$ range can prove to be very useful for high-Tc SQUID systems working at 77 K and in high-T, bolometer applications. I. INTRODUCTION 11. THE FORMER TEMPERATURE CONTROLLER AND ITS LIMITATIONS Our former temperature controller has been fully described in [8], except for the noise limitations. Briefly, a superconducting microbridge, operating below its transition temperature, is voltage biased at Vo = -el mV. The periodic switching between these two voltage states is usually made at a frequency fo, typically 1 kHz, so as to operate in the white noise domain of the preamplifier and associated electronics. The output response is the current Io flowing through the microbridge corresponding to the given voltage V,. This current is slightly greater than the critical current I, and I, was shown to have roughly the same temperature dependence as I,. A lock-in detection of I, leads to a quasi-static voltage V, and the latter reflects the temperature dependence of the dlvice once it has been calibrated against a reference thermometer. Using this detecting system and a commercial Proportional Integral (PI) corrector, we showed that the sample holder temperature could be controlled, with peak deviations of 100 pK. We now analyse the origin of the low- frequency noise in these experiments. We define the temperature to voltage transfer parameter for Vs around the operating temperature To as s? = dVs /dT and in Our case Very sensitive bolometers using high-Tc superconductors [ 11 have been obtained, and quite impressive progress has been made [2], [3], [4] in this field. However, it appears that the intrinsic performances of such sensors might in fact be impeded by the temperature fluctuation levels of the cold stage zyxwvutsrqpo [5]. As for magnetic sensors based on directly coupled high-T, SQUIDS, it has also been shown zyxwvutsrq [6], [7], that the effective pick-up loop area is temperature dependent via the London penetration depth. In these cases, the required temperature deviations from the mean value should be about 100 yK, at 77 K. We have previously shown that superconducting high-Tc microbridges [8] can achieve very low intrinsic Noise Equivalent Temperature (NET) values of 10-8-10-7 WdHz and we have devised a way to use them as temperature sensors for a simple commercially available Proportional Integral (PI) corrector. The resulting temperature controller led to temperature deviations of about 100 pK, around the reference value. In this paper, we describe recent improvements of our method that give temperature fluctuation levels in the yK range over a 2 hour period. Part I1 of this paper gives an analysis of the noise limitations of our previous work [8]. In part 111, we provide a method for circumventing these limitations. Finally, part IV describes our new scheme and gives experimental results which support our analysis. x Lecturer at the University of CAEN. Manuscript received August 26. 1996. this was in the range of 1 to 10 V/K. Because the final amplifier used for the demodulated output had an intrinsic low-frequency fluctuation AV, in the range of 0.1 to 1 mVpp, the corresponding fluctuation level as referred to the input was limited to the 100 pK range. A simple way to circumvent this is to achieve higher transfer Sp coefficients without increasing the noise. In our previous scheme, this was not possible, due to the way in which the current Io (typically of the order of a few mA) was being monitored. Calling Io(T,) the current for temperature To, the output voltage Vs is given by : at first order with respect to small temperature variations AT around To. In Eq. (l), R, is the current limiting and sensing resistor in the circuit while zyxwvu Go is the overall gain of the lock- in detector operating at the frequency f,. ST," is the temperature to current transfer coefficient of the superconducting sensor. Eq. (1) implies that the RoGo product is subordinated to a compromise between the highest 1051-8223/97$10.00 zyxwvuts 0 1997 IEEE