G. K. Rochester

Detailed calculation of test-mass charging in the LISA mission

Abstract The electrostatic charging of the LISA test masses due to exposure of the spacecraft to energetic particles in the space environment has implications in the design and operation of the gravitational inertial sensors and can affect the quality of the science data. Robust predictions of charging rates and associated stochastic fluctuations are therefore required for the exposure scenarios expected throughout the mission. We report on detailed charging simulations with the Geant4 toolkit, using comprehensive geometry and physics models, for Galactic cosmic-ray protons and helium nuclei. These predict positive charging rates of 50+e/s (elementary charges per second) for solar minimum conditions, decreasing by half at solar maximum, and current fluctuations of up to 30+e/s/Hz 1/2 . Charging from sporadic solar events involving energetic protons was also investigated. Using an event-size distribution model, we conclude that their impact on the LISA science data is manageable. Several physical processes hitherto unexplored as potential charging mechanisms have also been assessed. Significantly, the kinetic emission of very low-energy secondary electrons due to bombardment of the inertial sensors by primary cosmic rays and their secondaries can produce charging currents comparable with the Monte Carlo rates.

Evaluation of disturbances due to test mass charging for LISA

This paper concerns the effects of the build-up of electrical charge on the LISA test masses. Charge accumulates on the isolated test masses due to the bombardment of the spacecraft by galactic cosmic rays and solar particles. This will result in forces on the test masses, due to Coulomb and Lorentz interactions, which will disturb their geodesic motion. The three main disturbances associated with this charge are an increase in the test mass acceleration noise, coupling between the test mass and the spacecraft and the appearance of coherent Fourier components in the measurement bandwidth. These disturbances are estimated using the latest charging rate and noise predictions from GEANT4 for both the LISA mission and the technology demonstration mission, LISA Pathfinder, at different times in the solar cycle. The Coulomb disturbances are evaluated based on a detailed 3D, electrostatic, finite element model and submodels of the LTP sensor. These results are compared with those derived using the customary parallel plate approximation to calculate capacitances, and the accuracy of these approximations is assessed for typical parameter settings. The variation of the magnitude of charging disturbances as different parameters are changed, and the management of such disturbances are discussed.

Description of charging/discharging processes of the LISA sensors

The next generation of gravitational experiments in space is likely to use completely isolated proof-masses. For example, LISA uses proof-masses as mirrors in interferometers for gravitational wave astronomy (Bender et al 1998 Pre-phase A report MPG-233 pp 1–191) and STEP uses proof-masses in Earth orbit for an equivalence principle test (Sumner et al 2003 at press). Nongravitational forces will act on these proof-masses if they become charged, through the action of cosmic rays and solar flare particles for example. This paper examines the consequences of proof-mass charging for LISA, and presents results from using GEANT4 to assess the charging processes. Finally, there is a brief discussion of a means of controlling the charge down to an acceptable level.

CHARGE MANAGEMENT FOR LISA AND LISA PATHFINDER

We present an overview of charge management for LISA, including a review of the problems caused by test mass charging and development of the LISA Pathfinder charge management device.

Unwanted, coherent signals in the LISA bandwidth due to test mass charging

Electrical charges will build up on the LISA test masses due to particle radiation from cosmic rays and from the Sun. This charging can lead to spurious forces both through Lorentz and Coulomb interactions. These effects can give rise not only to acceleration noise, but also to unwanted coherent signals in the LISA bandwidth, due to the time dependence of the amount of charge accumulated on a test mass. The amplitude of these unwanted coherent signals depends on several factors, including the charging rate, the maximum allowed charge, the discharge procedure and a number of design parameters of the sensor itself. This paper estimates the magnitude of such unwanted charging signals based on the current, nominal LTP sensor dimensions and discusses approaches that may be taken to substantially reduce them for LISA.

A preliminary study of a torsion balance based on a spherical superconducting suspension

We present details of the design and construction of a new superconducting, magnetically suspended torsion balance in which the levitation coil and the lift surface of the float lie on the surfaces of concentric spheres. We compare results from calculations of the variation of the inductance with the levitated height and transverse motion of the float with experimental measurements and show that the levitation system is stable. Angular motion of the torsion balance is detected using superconducting pick-up coils whose inductance is modulated by float rotation. The subsequent change in current flowing in the persistent circuit containing the coils is measured using a flux-gate magnetometer. The pick-up coils exert a restoring torque on the float which can be modified by adjusting the persistent current stored. Periods down to 60 s should be obtainable for a current of 2.5 A. Preliminary results of ring-down experiments in He gas at a pressure of 53Pa show that periods of angular oscillations of 24 s with quality factor, Q, of about 200 can be obtained. The moment of inertia of the float is 2 × 10-5 kg m2. The observed period of 24 s indicates that there is an additional restoring torque in the system which may be due to trapped flux. The observed value of Q is consistent with gas damping.

Development Of Silicon Drift Chambers For X-Ray Astronomy

Detectors for x-ray (1-10 keV) astronomy based on silicon drift chamber (SDC) devices offer significant advantages over some types of existing detector and the potential performance of such devices is summarised. Our development program is outlined together with a discussion of device designs which are being pursued. Results are presented from some recently acquired two-dimensional drift devices working at room temperature, and from some large area drift diodes working at low temperature.

On the calculation of spin-coupling forces

Calculations are presented which define the optimum geometry for the spin-coupling experiment (SCE) on STEP. It is shown that the spin source must be composed of two sections in which spin-polarized electrons make different contributions to the net magnetization. Design features of the SCE on STEP are reviewed and the projected sensitivity of a configuration optimized for short-range (1 mm) interactions using a vacuum gap in the spin source is compared with other experiments.

The Charge‐Management System on LISA‐Pathfinder — Status & Outlook for LISA

The electrostatic charging of the test masses in the Lisa Pathfinder inertial sensor has severe implications on the design, operation and noise behaviour of the sensor. The flight model (FM) charge management device (CMD) designed to keep the test masses at a low residual charge is currently being built at Imperial College, London.Detailed simulations and tests of the FM uv‐lamps, the vacuum feedthroughs (F/T) and the distribution of light in the inertial sensor housing have been performed. Simulations show excellent mechanical robustness of the F/Ts and the lamp assembly. Vibration response has been calculated using finite element modelling on the F/T and the lamp assembly. Two lamps have been measured for performance before and after a vibration and shock test to flight acceptance specifications. The lamp performance is found to be unaffected by the vibration load. The distribution of light inside the sensor has been modelled to high accuracy in order to predict discharge rates for various voltage setti...

Low-temperature operation of silicon drift detectors

Two types of drift device, namely photodiodes and position sensitive drift chambers with segmented anode and cathode structures, have been studied at room temperature and below. Leakage current and electron mobility have been investigated at low temperature for the drift photodiodes. Self-triggering has been achieved for the position sensitive drift chambers using 60 keV photons, and differences in arrival time between the prompt trigger signal from the cathode and the delayed anode signal have been studied as a function of drift distance and temperature. The response of the photodiodes when coupled to a CsI scintillator at room temperature has been assessed.