Dissipationless microwave radio frequency superconducting interference device (RF-SQUID) multiplexers are attractive for readouts of large format arrays of transition edge sensors (TES). This multiplexer has the potential for readout bandwidth of several GHz and power consumption of ≈0.1 pW per pixel, which are ≈103 and ≈10-4 times as large as those of DC-SQUID-based conventional multiplexers. To evaluate their readout performance for TES photon counters ranging from near infrared to gamma ray, white noise of this multiplexer was experimentally studied at 4 K. The reason for this was, firstly, to avoid the low-frequency fluctuation obvious at around 0.1 K but trivial for the energy resolution of photon counters, and secondly for a feasibility study of readout operation at 4 K for extended applications. To increase resonant Q at 4 K and maintain low-noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1×104≤QI≤2×104 and the square root of spectral density of current noise referred to the SQUID input √SI =31 pA/√Hz, where QI is the internal Q of resonators. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Systematic noise measurement with various microwave readout power PMR and two-directional readout on the complex plane of transmission component of scattering matrix S21 makes the contribution of each noise source obvious. By decreasing these noises to the degree achievable by current technology, we predicted the microwave RF-SQUID multiplexer would exhibit √SI ≤5 pA/√Hz; i.e., close to √SI of state-of-the-art DC-SQUID-based multiplexers.
This paper describes our newly developed compact rotating-sample magnetometer that uses a high-temperature superconductor, superconducting quantum interference device (HTS-SQUID). The system configuration of the developed magnetometer is described. The magnetic signal of pure water was measured, and the signal generated from pure water was successfully detected. In addition, the improvement of the new device was examined, and the signal-to-noise ratio increased by optimizing the measurement system. For practical application of the system, the silica gel moisture content was measured by comparing the magnetic signals obtained from several samples. The magnetic signal intensity increased with the silica gel moisture content. A new moisture content evaluation method using the change in magnetic signal that resulted from changing water content was proposed. Magnetic signal measurements during the magnetic relaxation of magnetic moments were investigated by changing the pick-up coil position. When the magnetic signal from pure water was measured with a time delay of 0.2 s after magnetization, a magnetic signal change could be detected. Therefore, the newly developed system can also measure magnetic relaxation phenomena.
We have developed a low-noise balanced superconductor-insulator-superconductor (SIS) mixer using a waveguide 3-dB 90-degree hybrid coupler over the radio frequency (RF) range of 790-950 GHz and intermediate frequency range of 4-12 GHz at an operating temperature of 4 K. The RF coupler was carefully designed in regards to machining errors so that the entire amplitude imbalance including the mixer gain became smaller. The fabricated coupler was characterized at both room and cryogenic temperature. The receiver noise temperature of the balanced SIS mixer was below 350 K over the entire frequency band and the local oscillator (LO) noise rejection ratio was estimated to be more than 15 dB. This result is equivalent to state-ofthe-art mixer performance at this frequency range.
To calculate the radiation properties of terahertz-wave oscillators using intrinsic Josephson junctions, an analytical solution of the coupled sine-Gordon equation was obtained by applying continuous approximation and a radiation boundary condition in a 3D model for the oscillator mesa. The I-V characteristics, electromagnetic field distributions and bias dependence of radiation power were obtained by the analytical solution. These properties agree well with experimental results obtained from the oscillator fabricated from a Bi2Sr2CaCu2O8+δrectangular mesa with 78 μm wide, 138 μm long and 1.1 μm high. The analytical formulae are useful for designing the oscillator using the intrinsic Josephson junctions for practical applications.
Terahertz emission from BSCCO single-crystal mesa structures has recently attracted considerable attention. Since power fed into the mesa is of the order of 106 W/cm3, a significant self-heating effect appears in I-V characteristics. We measured I-V characteristics of a BSCCO sub-THz oscillator using three cooling methods. When the mesa is cooled directly by liquid helium, the maximum applied voltage is increased up to three times compared with that cooled using a helium flow cryostat. Moreover, radiation characteristics of the BSCCO sub-THz oscillator in liquid helium were investigated using a BSCCO mesa detector. The radiation, identified as TM (2, 0) mode (i.e., the second harmonics of cavity resonance mode along width), is successfully observed.
We have developed a liquid helium recondensing system for MEG used in a laboratory environment. The system has a separable supplying structure using a liquid helium container in order to avoid the noise increase and to simplify the installation. We conducted an unloaded test of the system using a calorimeter which has a vacuum vessel, radiation shield, liquid helium vessel, temperature sensor, heater and pressure sensor. The meter can measure a stand-alone liquid helium recondensation rate by stabilizing the liquid helium in the vessel at saturation temperature using the heater. We found that the stand-alone recondensation rate was 9.7 L/day. We also conducted a performance test loaded with a 100 L liquid helium container, and found that the recondensation rate was 7.9 L/day. To evaluate the system in operation with MEG, we measured a noise in MEG and auditory evoked magnetic fields (AEF). The results showed that a noise increase in MEG was not observed and AEF was not influenced by the operating noise of the system. From these evaluation results, we found that the recondensing system with a separable liquid helium supplying structure was useful for MEG operation.