Imperial College London
Our role has been in identifying, quantifying and subsequently finding a technical solution to mitigate against one of the key problematic issues for missions in space using isolated proof-masses. This is charge build up on the proof-masses giving rise to spurious electrostatic forces leading to induced noise due to the stochastic nature of the charging process. We have worked on all aspects of the charge management issues.
The fact that the proof-masses will become charged through the action of cosmic-rays was first realised by the Imperial team in connection with the STEP mission. In order to quantify the charging rate for LISA (and LISAPathfinder) it was necessary to implement a full spacecraft geometry within GEANT4 and track all particle interactions, following secondaries, (tertiaries, …) until their individual energies had fallen below typical ionisation potentials. This meant tracking particles over a 9-orders of magnitude energy range from GeV to eV. A typical cosmic ray event is shown in Figure 1. The results from that work both scoped the level of charging expected and established the noise statistic associated with the stochastic process.
Discharging of the proof masses can be done either through in an intermittent mode, allowing the charge to build up to some limit and then rapidly neutralising it, or in a continuous low-level trickle mode designed to just compensate for cosmic-ray charging. The two options each have their good and bad points which were assessed in a trade-off study. The intermediate discharge mode allows periods of science data taking with no added charge induced noise. However it allows the systematic build-up of electrostatic forces and puts gaps in the data. These will introduce problematic artefacts in the data. On the other hand the continuous trickle mode provides longer contiguous data segments but adds a small amount of additional charge induced noise. The baseline is a continuous trickle during science mode but with a capability to do a more rapid discharge when needed (such as just after uncaging and after a large solar flare).
Cosmic-rays are not the only particle species capable of charging the proof-masses. Solar flare events released higher energy particles will also have an effect. This has been, and is still being, studied in some detail and involves a good deal of data-mining through archive data to extract information on very low level flare particles which have not been well studied so far.
The technical implementation of a charge control system relies on controlled photoelectric emission from surfaces (proof-mass and its local surroundings) and controlled charge transport to achieve bipolar discharge capability. For LISAPathfinder this will use low-pressure Hg lamps to produce the UV photons needed. Such lamps have already been flown by the Imperial Group on the ROSAT satellite. For LISA/NGO we are likely to change to a more modern technology which is now becoming available based on UV light-emitting diodes. Imperial is leading an industrial team developing this with funding from ESA. The LISAPathfinder UV lamp assembly is shown in Figure 2.
The head of the Imperial group, Prof. Tim Sumner is a co-Investigator on the LISAPathfinder mission, was a member of the LISA International Science Team and is now a Study Team member NGO. He also serves on the ESA Space Science Advisory Committee.
The group is part of the High Energy Physics Group within the Physics Department at Imperial College London.
Contacts points are:
Prof. Tim Sumner (group leader)
Dr. Peter Wass (Research Associate)