Saps Buchman

LISA and its in-flight test precursor SMART-2

LISA will be the first space-home gravitational wave observatory. It aims to detect gravitational waves in the 0.1 MHz+1 Hz range from sources including galactic binaries, super-massive black-hole binaries, capture of objects by super-massive black-holes and stochastic background. LISA is an ESA approved Cornerstone Mission foreseen as a joint ESA-NASA endeavour to be launched in 2010-11. The principle of operation of LISA is based on laser ranging of test-masses under pure geodesic motion. Achieving pure geodesic motion at the level requested for LISA, 3×10^(−15) ms^(−2)/√Hz at 0.1 mHz, is considered a challenging technological objective. To reduce the risk, both ESA and NASA are pursuing an in-flight test of the relevant technology. The goal of the test is to demonstrate geodetic motion within one order of magnitude from the LISA performance. ESA has given this test as the primary goal of its technology dedicated mission SMART-2 with a launch in 2006. This paper describes the basics of LISA, its key technologies, and its in-flight precursor test on SMART-2.

Charge measurement and control for the Gravity Probe B gyroscopes

We describe a technique based on photoemission for controlling the charge of the Gravity Probe B electrostatically suspended gyroscopes, and three methods for measuring this charge. Charging is caused by cosmic radiation in orbit and by enhanced field emission in ground testing. Errors induced by disturbing torques require the potential of the gyroscope to be smaller than 15 mV (15 pC) during the space experiment. The disturbing drift rate produced by measuring and controlling the charge in orbit is smaller than 10−13 deg/h, as compared with the 10−11 deg/h systematic drift rate of the gyroscope. The charge control technique is based on ultraviolet photoemission of electrons from both the gyroscope and a charge control electrode on the gyroscope housing. We demonstrate the effectiveness of this method in ground testing and therefore its suitability for the space experiment. Calculations indicate that heating by absorbed photons is, in the worst case, smaller than 1 nW and thus not a problem for the experi...

LED deep UV source for charge management of gravitational reference sensors

Proof mass electrical charge management is an important functionality for the ST-7-LTP technology demonstration flight and for LISA. Photoemission for charge control is accomplished by using deep ultraviolet (UV) light to excite photoelectron emission from an Au alloy. The conventional UV source is a mercury vapour lamp. We propose and demonstrate charge management using a deep UV light emitting diode (LED) source. We have acquired selected AlGaN UV LEDs, characterized their performance and successfully used them to realize charge management. The UV LEDs emit at a 257 nm central wavelength with a bandwidth of ~12 nm. The UV power for a free-space LED is ~120 µW, and after fibre coupling is ~16 µW, more than sufficient for LISA applications. We have directly observed the LED UV light-induced photocurrent response from an Au photocathode and an Au-coated GRS/ST-7 proof mass. We demonstrated fast switching of UV LEDs and associated fast changes in photocurrent. This allows modulation and continuous discharge to meet stringent LISA disturbance reduction requirements. We propose and demonstrate AC charge management outside the gravitational wave signal band. Further, the megahertz bandwidth for UV LED switching allows for up to six orders of magnitude dynamic power range and a number of novel modes of operations. The UV LED based charge management system offers the advantages of small-size, lightweight, fibre-coupled operation with very low power consumption.

A space-based superconducting microwave oscillator clock

Superconducting Cavity Stabilized Oscillators, SCSO, have produced the most stable clocks to date, achieving an Allen variance of 3×10−16 for integration times between 102 and 103 seconds. Cavity frequency variations are mainly caused by acceleration effects due to gravity and vibrations, temperature variations, and fluctuations in the energy stored in the cavity. We describe the status of a project aimed at building an improved cavity system suitable for use on the International Space Station, ISS. Primary experimental applications include the measurement, in conjunction with other types of clocks, of the dependence of fundamental constants on the gravitational potential, gravitational redshift measurements, and the measurement of the anisotropy of the velocity of light. A major secondary application is as a flywheel for the atomic clocks co-located on the ISS.

Modular gravitational reference sensor development

The Modular Gravitational Reference Sensor (MGRS) is targeted as a next generation core instrument for both space gravitational wave detection and an array of other precision gravitational experiments in space. The objectives of the NASA funded program are to gain a system perspective of the MGRS, to develop key component technologies, and to establish important test platforms. Our original program was very aggressive in proposing ten areas of research and development. Significant advancements have been made in these areas, and we have met or exceeded the goals for the program set in 2007-2008. Additionally, we have initiated research projects for innovative technologies beyond the original plan. In this paper we will give a balanced overview of progress in MGRS technologies: the two layer sensing and control scheme, trade-off studies of GRS configurations, multiple optical sensor signal processing, optical displacement and angular sensors, differential optical shadow sensing, diffractive optics, proof mass center of mass and moment of inertia measurement, UV LED charge management, proof mass fabrication, thermal control and sensor development, characterization for various proof mass shapes, and alternative charge manage techniques.

Magnetic flux distribution on a spherical superconducting shell

Abstract We report measurements of flux distributions on superconducting spherical shells in an ambient magnetic field of 0.2±0.1 μG. The aim of these experiments is to minimize the number of flux lines trapped in the superconducting shells, an important requirement for the Gravity Probe B gyroscopes.

Progress in Developing the Modular Gravitational Reference Sensor

Modular Gravitational Reference Sensor (modular GRS) was proposed by the Stanford Team in 2004. In a modular GRS, the laser beam from the remote the sensor does not illuminate the proof mass directly. The internal measurement from the housing to proof mass is separated from the external interferometry. A double‐sided grating further simplifies the structure and may better preserve the measurement precision. We review the recent progress in developing the modular GRS at Stanford. We are developing optical sensors with picometer resolution, capable of operating with a large gap for high precision readout. We have conducted an initial experiment incorporating RF heterodyne detection and thus lowered the optical power compared with direct detection. We have demonstrated sub‐nanoradian sensitivity of a grating angular sensor. We have successfully demonstrated fabrication of localized grating patterns on dielectric and gold surfaces. We have made critical progress in optical measurement of the mass center (MC) ...

Modular Gravitational Reference Sensor: Simplified Architecture to future LISA and BBO

We present the Modular GRS (previously named as Stand-Alone GRS), in which the laser light from the remote spacecraft does not illuminate the proof mass. The modular GRS uses only a single spherical proof mass on each spacecraft and optical, as opposed to capacitive, position sensing. The use of a single sphere as the test mass avoids the issue of cross coupling that is inherent for the cubic proof mass, and allows true drag free flight with no forcing. Together, the modular design, optical sensing and a single spherical proof mass reduce the disturbances and the number of degrees of freedom that must be managed for future LISA and BBO.

Design of a highly stable and uniform thermal test facility for MGRS development

We have designed combined passive and active thermal control system to achieve sub microkelvin temperature stability and uniformity over an optics bench size enclosure, which has an analogous structure to the LISA spacecraft. For the passive control, we have constructed a new thermal enclosure that has a multilayer structure with alternative conducting and insulating layers, which enables the temperature uniformity and ease the burden of the active control. The thermal enclosure becomes an important test facility for Modular Gravitational Reference Sensor (MGRS) development. For the active control, we have developed a model predictive control (MPC) algorithm, which will regulate temperature variations of the proof-mass (PM) down to sub-microkelvin over the LISA science band. The LISA mission requires extremely tight temperature control, which is as low as 30 μK/ over 0.1 mHz to 1 Hz. Both temporal stability and spatial uniformity in temperature must be achieved. Optical path length variations on optical bench must be kept below 40 pm/ over 0.1 mHz to 1 Hz. Temperature gradient across the proof mass housing also must be controlled to reduce differential thermal pressure. Thermal disturbances due to, for example, solar radiation and heat generation from electronics, are expected to be significant disturbance source to the LISA sensitivity requirements. The MGRS will alleviate the thermal requirement due to its wider gap between the proof-mass and the housing wall. However, a thermally stable and uniform environment is highly desirable to achieve more precise science measurement for future space science missions.

Grating Angle Magnification Enhanced Angular and Integrated Sensors for LISA Applications

The Laser Interferometer Space Antenna (LISA) and the Big Bang Observer (BBO) require angular sensing in their gravitational reference sensor (GRS), telescope pointing, and spacecraft control. The conventional angular sensing schemes utilize simple geometric reflections as the sensing mechanism. We propose and demonstrate the use of grating diffraction orders as angular sensing signal beams. The grating angular sensor can be far more sensitive than a simple reflection scheme for two reasons. First, the diffractive angles can vary more than the incident angle when the grating rotates. The grating thus magnifies the variation of the input angle. Second, the cross section of the diffracted beam is compressed by the oblique projection, resulting in a higher energy density. These two favorable effects become more pronounce for a normal incidence beam to diffract at grazing angles. We have conducted a preliminary experiment and demonstrated an angular sensitivity below 10 nanoradians per root hertz over a short working distance, meeting the Space Technology 7 (ST-7) GRS and LISA requirements for proof mass angular sensing. Our proposed grating-based angular measurement does not introduce additional optical elements between the sensed surface and the photodiode. Thus, it eliminates measurement uncertainty due to in-path optics. The proposed grating sensor can be generalized to build an integrated sensor for both angular and displacement sensing.