Konstantin Penanen

The Mid-Infrared Instrument for the James Webb Space Telescope, II: Design and Build

The Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST) provides measurements over the wavelength range 5 to 28.5 μm. MIRI has, within a single ‘package’, four key scientific functions: photometric imaging, coronagraphy, single-source low-spectral resolving power (R ∼ 100) spectroscopy, and medium-resolving power (R ∼ 1500 to 3500) integral field spectroscopy. An associated cooler system maintains MIRI at its operating temperature of <6.7 K. This paper describes the driving principles behind the design of MIRI, the primary design parameters, and their realisation in terms of the ‘as-built’ instrument. It also describes the test programme that led to delivery of the tested and calibrated Flight Model to NASA in 2012, and the confirmation after delivery of the key interface requirements.

A novel two-step laser ranging technique for a precision test of the theory of gravity

All powered spacecraft experience residual systematic acceleration due to anisotropy of the thermal radiation pressure and fuel leakage. The residual acceleration limits the accuracy of any test of gravity that relies on the precise determination of the spacecraft trajectory. We describe a novel two-step laser ranging technique, which largely eliminates the effects of non-gravity acceleration sources and enables celestial mechanics checks with unprecedented precision. A passive proof mass is released from the mother spacecraft on a solar system exploration mission. Retro-reflectors attached to the proof mass allow its relative position to the spacecraft to be determined using optical ranging techniques. Meanwhile, the position of the spacecraft relative to the Earth is determined by ranging with a laser transponder. The vector sum of the two is the position, relative to the Earth, of the proof mass, the measurement of which is not affected by the residual accelerations of the mother spacecraft. We also describe the mission concept of the Dark Matter Explorers (DMX), which will demonstrate this technology and will use it to test the hypothesis that dark matter congregates around the sun. This hypothesis implies a small apparent deviation from the inverse square law of gravity, which can be detected by a sensitive experiment. We expect to achieve an acceleration resolution of ∼ 10−14m/s2. DMX will also be sensitive to acceleration towards the galactic center, which has a value of ∼ 10−10m/s2. Since dark matter dominates the galactic acceleration, DMX can also test whether dark matter obeys the equivalence principle to a level of 100 ppm by ranging to several proof masses of different composition from the mother spacecraft.

A cryogen-free ultralow-field superconducting quantum interference device magnetic resonance imaging system

Magnetic resonance imaging (MRI) at microtesla fields using superconducting quantum interference device (SQUID) detection has previously been demonstrated, and advantages have been noted. Although the ultralow-field SQUID MRI technique would not need the heavy superconducting magnet of conventional MRI systems, liquid helium required to cool the low-temperature detector still places a significant burden on its operation. We have built a prototype cryocooler-based SQUID MRI system that does not require a cryogen. The SQUID detector and the superconducting gradiometer were cooled down to 3.7 K and 4.3 K, respectively. We describe the prototype design, characterization, a phantom image, and areas of further improvements needed to bring the imaging performance to parity with conventional MRI systems.

Non-universal Casimir effect in saturated superfluid4He films at Tλ

Measurements of Casimir effects in 4He films in the vicinity of the bulk superfluid transition temperature Tλ have been carried out, where changes in the film thickness and the superfluid density are both monitored as a function of temperature. The Kosterlitz-Thouless superfluid onset temperature in the film is found to occur just as the Casimir dip in the film thickness from critical fluctuations becomes evident. Additionally, a new film-thickening effect is observed precisely at Tλ when the temperature is swept extremely slowly. We propose that this is a non-universal Casimir effect arising from the viscous suppression of second sound modes in the film.