The accretion of electrostatic charge in the isolated LISA test masses due to energetic particles in the space environment hinders the drag-free operation of the gravitational inertial sensors. 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 net charging rates of up to +100 elementary charges per second during the solar minimum period, decreasing by half at solar maximum. Charging from sporadic solar events involving energetic protons was also investigated. Other physical processes hitherto overlooked as potential charging mechanisms have 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.
We report on a Monte Carlo simulation of electrostatic charging of the
LISA proof masses by cosmic-ray protons and alpha particles, developed
using the Geant4 toolkit. A positive charging rate of 58+/-17 +e/s
(proton charges per second) was obtained with the minimum Geant4
energy threshold for the production of secondary particles by
electromagnetic processes. This charging rate does not seem to depend
strongly on the tracking of low-energy secondary electrons, and is
some 5 times larger than that found in previous simulations. The
difference is only partly explained by the slightly larger proof mass
considered in this study. This figure is used to place new limits on
the required discharge time of the LISA test masses.
Atmospheric modelling predicts that a window at 200-μm occurs under very dry conditions at high altitude sites. The transmission can reach up to 30 % in the driest conditions, but also exists for as many as 80 nights per year at Mauna Kea. A 200-μm photometer, THUMPER, is currently under construction at Cardiff University for use at the JCMT to exploit this atmospheric window. THUMPER consists of a seven-element hexagonal array of stressed Ge:Ga photoconductors cooled to liquid helium temperature. Initial laboratory testing suggest an NEFD of (formula available in paper)should be possible, under conditions of 0.5-mm pwv. A dichroic splits the beam between SCUBA and THUMPER, allowing simultaneous observations with THUMPER effectively acting as a third SCUBA array. Photometric measurements at 200-μm, in conjunction with SCUBA, will provide valuable information on cold dust sources in the temperature range 10 to 50 K. Since SCUBA fails to sample the peak of the Planck function at these temperatures, it is not possible to differentiate between temperature and density variations across a source using SCUBA data alone. THUMPER will provide these additional data at the same spatial resolution as SCUBA. This will provide an unprecedented combination of wavelength coverage and resolution when imaging sources such as protostars and pre-stellar cores.