The PTB Josephson Arbitrary Waveform Synthesizer (JAWS) enables the generation of arbitrary waveforms up to voltages of 70 mVRMS (199 mVPP) using two Josephson arrays in series containing 4000 Josephson junctions each. The SNS-like double-stacked junctions are based on NbxSi1-x as barrier material. While the JAWS system is typically operated using a Dewar with liquid helium, the operation in a closed-cycle pulse-tube cryocooler at temperatures around 4.2 K range was here investigated and successfully demonstrated. For this purpose a special designed cryoprobe was used to provide high quality pulses to the Josephson arrays.
Pulse-driven Josephson junctions are the basis for one version of an AC Josephson voltage standard which enables the synthesis of arbitrary AC waveforms with extremely pure frequency spectra. Therefore, it is often called Josephson Arbitrary Waveform Synthesizer (JAWS). At present, three national metrology institutes develop and fabricate the required Josephson junction series arrays: NIST, AIST and PTB [1-4].
The operation of Josephson voltage standards is based on the Josephson effect (cf. [
For the application in the JAWS we use SNS-like Josephson junctions based on NbxSi1−x barriers at PTB [5, 6]. The junctions are arranged in double stacks to double the output voltage for a given design. The junctions are embedded into the center line of a 50 Ω coplanar-waveguide (CPW) to ensure high-quality pulse propagation along the array.
Our JAWS setup enables the synthesis of spectrally pure bipolar arbitrary waveforms with a signal-to-noise ratio of up to 120 dBc. As already demonstrated, amplitudes of up to 184 mVRMS (521 mVpp) can be generated for DC-voltages and AC-voltages in a frequency range from 60 Hz to 1 MHz by connecting two arrays in series with 12,000 Josephson junctions in total (see
eration of the arrays in a cryocooler, we characterized these arrays in a Dewar with liquid helium (LHe) at temperature T = 4.2 K: the critical current is Ic ≈ 3.5 mA for each array and the normal resistance is Rn ≈ 4.4 mΩ for a single junction. The arrays are able to generate pure waveforms up to 70 mVRMS (199 mVPP) each with a Ʃ∆-code amplitude of AƩ∆ = 0.8. By mounting the JAWS chip in a specially designed cryoprobe we investigated the opportunity to operate this JAWS array with a pulse tube cryocooler. This operation mode without the need of liquid helium is desired for prospective applications and easy operation of the JAWS.
Recently, Urano et al. operated the Josephson arrays of an optoelectronics JAWS system in a two stage Gifford-MacMahon refrigerator [
The main components of the experimental setup for the cryocooler operated JAWS are the same than for the LHe-operated system explained in detail in [5,9]. The main difference is the adapted cryoprobe. Some details of the setup are briefly described in this section. Two arrays are integrated on a 10 mm × 10 mm silicon chip. The JAWS chip is mounted onto a chip carrier (23 mm × 40 mm) based on Rogers RO30061. The conducting CPW paths of this carrier are made of copper with a 2 µm gold layer on top (without nickel). The CPW lines of this carrier are connected to two PCB-SMA launchers. Two semi-rigid cables (length about 1 m) are connected to these launchers and fed through the inner tube of the cryoprobe to the probe head. The cryoprobe contains eight isolated DC-lines in total to deliver a compensation signal [
The cryocooler is a closed-cycle pulse tube refrigerator (Oxford Instruments Optistat PTR-TL41) with top loading sample assembly. The base temperature of this cryocooler is about 3.5 K with a nominal cooling power of 1 W at 4 K. The advantages of this system are the quite large sample space (diameter 50 mm), the rather short cool down time of about 4 hours and the low vibration of the setup. A photograph of the whole setup (JAWS electronics and cryocooler) is shown in
After mounting the JAWS chip in the cryocooler, the current-voltage characteristics of the two Josephson arrays at 4.2 K are unchanged compared to the measurements in liquid helium: the critical current is Ic ≈ 3.5 mA and the normal resistance is Rn ≈ 4.4 mΩ. As expected, the dc characteristics of the Josephson arrays are not affected by their operation in the cryocooler.
Then, we investigate the behavior under pulse operation. The correct operation of the JAWS is often proven by measuring frequency spectra of simple synthesized waveforms; the JAWS system works well, when all higher harmonics are completely suppressed.
The high noise floor is indicating that the shielding of our setup is not sufficient to suppress the influence of the noisy environment in this laboratory building. By switching off all electrical devices of the JAWS and even the cryocooler (including the compressor) in our laboratory the noise floor was not influenced at all. Therefore the noise seems to be caused by external sources. Unfortunately the cryocooler cannot be moved to a shielded laboratory. At least it was possible to check the mobile JAWS setup and the cryoprobe in such a shielded laboratory in liquid helium. Here, no distortion tones were visible in the noise floor. The operation margins of both
arrays are remaining mainly unchanged when performing the measurements in the cryocooler rather in LHe.
The cryocooler allows the adjustment of different temperatures by controlled heating with a nominal temperature stability of about 5 mK under unloaded conditions. It was possible to achieve operation margins under pulse drive (small but stable: e.g. current margins of about 100 µA) up to temperatures of about 5.6 K (measured at the cryoprobe temperature sensor). At this temperature the critical current was reduced to about 0.8 mA from 3.5 mA at 4.2 K. The operation margins were proportionately reduced too. For temperatures above 5.6 K the higher harmonics were not fully suppressed anymore, indicating that there are no operation margins.
nal of a 3750 Hz synthesized sine wave at an operation temperature of 5.7 K. Although the first harmonic is not fully suppressed at this temperature, a very good signalto-noise ratio of −106 dBc is still achieved.
The results clearly demonstrate that a stable operation of the JAWS system in this cryocooler is possible using our purpose-built cryoprobe also for temperatures above 4.2 K. The temperature stability at the JAWS chip is sufficient to ensure stable operation margins even in pulse mode operation. The cycle frequency of the cold head of about 1 Hz was not visible in the measured signal.
2Ic) of the Josephson array. At a temperature of about 8.4 K the critical current is nearly zero. The shown value of 1 µA is the lower measurement limit of the sourcemeasure-unit used. The jump in the normal resistance at a temperature of about 9.1 K shows the critical temperature of the niobium electrodes of the Josephson junctions. This temperature was calculated using the mean value of our two temperature sensors: cold head and cryoprobe. The JAWS chip is arranged in the middle between these two sensors. Therefore this method is convenient for a first estimation of the “real” chip temperature, even if the temperature difference between these two sensors is about 0.2 K at 4.2 K and 0.04 K at 10 K. During the automated measurement the temperature was controlled to be stable within 0.01 K.
As the JAWS chip contains two Josephson arrays, the temperature dependence of the critical current of one array can be used as a temperature sensor (detector array) for the second array. A more detailed evaluation of the temperature behavior during pulse operation of the sec-
ond array is possible now. For example, we operated the JAWS chip at 4.2 K with a critical current Ic ≈ 3.5 mA. By applying a typical pulse amplitude of 2000 a.u. (for positive and negative pulses) to the second array with a maximum and constant pulse repetition frequency of 15 GHz, the critical current of the detector array dropped to a constant value of Ic = 2.7 mA indicating a chip temperature of 4.4 K. This means, that the temperature was increased about ∆T = 0.2 K during the measurement. The temperature sensor of the cryoprobe showed a temperature difference of only ∆T = 0.02 K. We repeated this measurement for a maximum pulse amplitude of 4000 a.u. and measured a drop of the critical current to Ic = 2.1 mA corresponding to a temperature of 4.6 K, i.e. a temperature difference of ∆T = 0.4 K (cryoprobe temperature sensor: ∆T = 0.05 K). These results clearly indicate that the temperature sensor of the cryoprobe significantly underestimate the temperature increase of the JAWS chip under pulse operation. This is probably caused by the distance to the chip and by the fact that the heat generation on the chip takes place in a limited space with a rather low specific heat capacity. The compensation signal is not influencing the temperature in the system at all.
In the recent paper by Urano et al. [
This paper describes the operation of Josephson arrays of the PTB JAWS system in a closed-cycle pulse tube refrigerator. The temperature stability at the JAWS chip is sufficient to ensure stable operation margins even in pulse mode operation. Using Josephson arrays with 4000 Josephson junctions stable margins were possible for the generation of AC voltages up to 200 mVPP and temperatures of up to 5.6 K.
The authors would like to thank R. Behr, L. Palafox, Th. Weimann, F. Müller, R. Wendisch, B. Egeling, P. Hinze, K. Störr for technical assistance, discussions, and comments.
This work was partly carried out with funding by the European Union within the EMRP JRP SIB59 Q-WAVE. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.