Wenlei Shan

Overview of the East Asia ALMA development program

ALMA has already produced many impressive and scientifically compelling results. However, continuous technical upgrades and development are key for ALMA to continue to lead astronomical research through the 2020-2030 decade and beyond. The East Asia ALMA development program consists of the execution of short term projects, and the planning and initial studies for longer term developments that are essential for future upgrades. We present an overview of all these ongoing East Asia ALMA development projects and upgrade studies, which aim to maintain and even increase the outstanding scientific impact of ALMA in the near future and over the coming decades.

How do we design the interferometric system focused on the analog and digital backend and the correlator for scientifically valuable ALMA developments

ALMA has been demonstrating its exceptional capabilities with unprecedented scientific results achieved over the past six years of operation. To keep ALMA as a leading-edge telescope, it is essential to continue technical upgrades and development of new potential. While our future development programs have already achieved remarkable technological breakthroughs at the level of front-end receivers, we are discussing the upgrades of the analog and digital backend and the correlator. We report the required concept design of the interferometric system focused on these sub-systems to realize new science use cases.

An anomalous peak on intermediate frequency response of superconductor-isolator-superconductor mixers and its effect on mixing performance

In this paper, we mainly investigate the origin and impact of an anomalous peak observed on the intermediate frequency (IF) response of some 600 GHz Nb superconductor-isolator-superconductor (SIS) mixers. The dependences of the IF peak on temperature and magnetic field are studied to verify its Joseph son-junction-like behavior. The cross sections of two SIS junction samples are analyzed by transmission electron microscopy to locate this parasitic Josephson weak link. Five different junction samples are measured to determine the impact of the Josephson weak link on the mixer performance. Finally, an equivalent circuit model is proposed to explain its behavior.

Development of a low-power cryogenic MMIC HEMT amplifier for heterodyne array receiver application

We present in this paper a study of a low-power consumption cryogenic amplifier with GaAs-based HEMT. A two-stage MMIC low noise amplifier for 2.5-4.5 GHz frequency range has been designed, fabricated and measured at a low-power condition with the temperature range from 300 K to 4 K. To design such a cryogenic MMIC amplifier, firstly we extracted the model of the bare-die transistor at cryogenic temperatures fabricated together with the MMIC. The temperature-dependent DC and RF characteristics of the HEMT have been measured. From the approximate noise model based on the DC characteristics, we verified that the HEMTs offer sufficient gain and reasonably noise at a relative lowpower operation condition. Subsequently, we designed a low-power dissipation cryogenic MMIC amplifier utilizing the cryogenic s2p model of the HEMTs biased at the optimal low-power condition. At cryogenic temperature, the GaAsbased amplifier achieves a gain larger than 20dB and a noise temperature as low as 10 K with a total power consumption of 1.2 mW. The low-power amplifiers can be used as first-stage IF amplifiers in a superconductor-insulatorsuperconductor (SIS) receiver, and are especially useful in focal plane arrays with large pixel count because of the merit of the total power consumption.

Technical achievements of the ALMA future receiver development program at the National Astronomical Observatory of Japan

The ALMA telescope has been producing ground-breaking science since 2011, but it is mostly based on front-end and back-end technology from the 2000s. In order to keep ALMA competitive in the coming decade, timely updates are necessary in order to further improve the science output of the telescope. In NAOJ, we have been doing research leading to technological developments which aim to increase the field-of-view of the telescope, and the RF and instantaneous bandwidth for more efficient and accurate spectral surveys. In this contribution, we will describe the most important technical achievements by our group in recent years.

Characterization of the IF noise of a submillimeter SIS receiver

A fitting method is presented here for the accurate characterization of the IF noise contribution of a sub-millimeter SIS receiver. By fitting the mixers IF output power response and junctions IV curve of an SIS mixer without LO pumping, we can obtain the IF noise contribution, the physical temperature of the isolator connected just behind the SIS mixer, the output mismatching of the mixer, and the total gain of the IF chain. Differing from a conventional method, which only uses the normal-state (linear) branch of the junctions IV curve, the method proposed here also includes the nonlinear portion around the gap voltage. The dynamic resistance in this portion is varied dramatically, providing us a good probe to characterize the output mismatching of the mixer, as well as other parameters.

Simulation of Sub-harmonic SIS Mixers with a Full Five-Port Model

The performance of superconductor-insulator-superconductor (SIS) sub-harmonic mixers is thoroughly investigated in this paper. On account of the importance of the harmonic effect, a full five-port model combined with an enhanced Newton solution is applied to calculate the local oscillator (LO) waveform. The mixer performance is studied as functions of principal parameters including the embedding admittances, LO power and bias voltage. The results are compared with those from the quasi five-port model.