Abstract: Hot-electron bolometer devices, used successfully in low noise heterodyne mixing at frequencies up to 2.5 THz, have been analyzed. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, is used to model pumped IV curves and understand the physical conditions during the mixing process. We argue that the mixing is predominantly due to the strongly temperature dependent local resistivity of the NbN. Experimentally we identify the origins of different transition temperatures in a real HEB device, suggesting the importance of the intrinsic resistive transition of the superconducting bridge in the modeling.

Abstract: NbN phonon cooled HEB’s are one of the most promising bolometer mixer technologies for (near) future (space) applications. Their performance is usually quantified by mea- suring the receiver noise temperature at a given IF frequency, usually around 1 – 2 GHz. However, for any real applications it is vital that one fully knows all the relevant properties of the mixer, including LO power, stability, direct detection, gain bandwidth and noise bandwidth, not only the noise temperature at low IF frequencies. To this aim we have measured all these parameters at the optimal operating point of one single, small volume quasioptical NbN HEB mixer. We find a minimum noise temperature of 900 K at 1.46 THz. We observe a direct detection effect indicated by a change in bias current when changing from a 300 K hot load to a 77 K cold load. Due to this effect we overestimate the noise temperature by about 22% using a 300 K hot load and a 77 K cold load. The LO power needed to reach the optimal operating point is 80 nW at the receiver lens front, 59 nW inside the NbN bridge. However, using the isothermal technique we find a power absorbed in the NbN bridge of 25 nW, a difference of about a factor 2. We obtain a gain bandwidth of 2.3 GHz and a noise bandwidth of 4 GHz. The system Allan time is about 1 sec. in a 50 MHz spectral bandwidth and a deviation from white noise integration (governed by the radiometer equation) occurs at 0.2 sec., which implies a maximum integration time of a few seconds in a 1 MHz bandwidth spectrometer.

Abstract: Millimetron Space Observatory (MSO) is mission addressed to creation a space cryogenic telescope with aperture about 10-m [1]. Such telescope will allow scientific community to have an astronomical instrument with enormous sensitivity and angular resolution in the submillimeter and far-infrared wavelength ranges. We plan to install at the telescope several FIR and sub-millimeter scientific instruments, which will enable high-resolution imaging and spectroscopy observations with unprecedented sensitivity. At the same time, MSO will enable observations with an extremely high angular resolution (up to 0.1×10 -6 arcsec) as an element of a ground-space very long baseline interferometry system (S-VLBI). Thereby the observatory will contribute breakthrough capability into solution a number of cosmology and fundamental astrophysics questions about the origin and evolution of our Universe, galaxies, stars and other objects [2]. The MSO is divided into two parts: the payload module and the bus module. Due to the complexity of the payload module, most of the recent years of work are focused on it. This module includes an antenna of the telescope, scientific receivers, functional and service systems and a high-gain radio system for transmitting scientific data to Earth. The primary mirror of the telescope will be deployable and consist from of a 3-m aperture central part surrounded by 24 deployable petals. The concept of petals deployment is based on the successfully launched and currently working Radioastron project [3]. The surface accuracy of the deployable 10-m primary mirror of Radioastron achieves about 1 mm in space conditions. The telescope of MSO would have much better surface accuracy – less than 10 μm (rms). In order to achieve this we plan to use an active surface control system based on a wave front sensing. This system will be periodically employed to correct inaccuracies in the positions of the panels caused by different factors. A combination of a high modulus carbon fiber reinforced plastic (CFRP) and a cyanate ester resin as a binder provides a lightweight structure with low moisture absorption, high thermal stability and high stiffness. This combination has been chosen for the material of the primary mirror of telescope and many parts of it. The panels are mounted on the back support structure (Fig. 1) made from CFRP via precision cryogenic actuators. To achieve the required sensitivity of the telescope in the submm/FIR we need to cool antenna down to the temperature less than 10K (goal). It may be possible to do this on-orbit only by a combination of effective radiation cooling and additional active mechanical cooling. A cold space antenna requires minimization and stability of external thermal radiation. This is one of the reasons why MSO will be placed into orbit around the second Earth-Sun Lagrange point (L2). The MSO antenna into L2 will be cooled passively to a temperature about 30 – 60K by a suite of the deployable multi-layer V-groove shields. The following steps to reduce the temperature of the antenna are based on active reducing the thermal loads applied to it. Active mechanical cooling is based on existing close cycling space mechanical coolers. In this work, we will focus on the progress in the development of payload module.