Pre-Phase A Report - Lisa - Nasa

3.2 The inertial sensor 73
sufficient to easily control the proof-mass motion and attitude, in particular when the
drag-free control is not fully operational, at the beginning of the experiment for instance.
A trade-off between these requirements could be obtained with a specific configuration of
the loop electronics that introduces varying control stiffnesses according to the applied
acceleration.
3.2.6 Proof-mass charge control
Charge control of the proof-mass is crucial if spurious electromagnetic forces are to be
kept at a level such that they do not compromise the science goals. As noted earlier the
nominal requirement is to limit the free charge on the proof mass to < 2×10 −14 Cinthe
presence of a continuous charging rate of 10 −17 to 10 −18 C/s [91]. This charge limit comes
from considering the Lorentz force acceleration noise induced in an unshielded charged
proof-mass interacting with the interplanetary magnetic field and it is explained in more
detail in Section 4.2.4. It is conceivable that a factor of up to 100 might be recovered by
shielding provided by the metallic vacuum enclosure around the sensor. Shielding beyond
that level will be very difficult, given the need for access holes in various positions, and
moreover, noise caused by electrostatic interactions of the free charge with the surrounding
electrode structure then becomes significant anyway (see Section 4.2.4).
The baseline charge control technique involves illuminating both the proof mass and its
surrounding electrodes with ultra-violet light to release photoelectrons from both surfaces,
and then to use bias voltages on the electrodes to control the nett transfer of charge. This
technique has been demonstrated already for GP-B [92], albeit at a somewhat higher
level of charge (10−9 C) and much coarser control authority than those required for LISA.
Laboratory tests using 2.5 W low-power mercury discharge lamps of two different types (rf
discharge for GP-B and dc electric discharge for ROSAT [93]) have shown that sufficient
photoelectric emission can be achieved from gold surfaces in both direct and reflected
illumination. The low magnetic susceptibility of the Au/Pt proof-mass alloy unfortunately
results in a particularly high work function and it may be that a gold coating is necessary
on the proof-mass.
The level of bias voltage required to effect sufficient charge control depends on the detailed
electrode geometry, the gaps to be used and the mode of introducing the UV. In the
current scheme, which offers the simplest mechanical solution, the UV will be introduced
onto the central sections of two opposite tranverse faces of the proof-mass. The facing
electrodes will then be illuminated by reflection. This scheme gives a bipolar discharging
capability. However with our current understanding of the charging mechanisms the
belief is that the proof-mass will charge positively and hence the bias voltage will need to
work against the inherent tendancy of this implementation. An alternative scheme which
would avoid the need for significant bias voltages could be to illuminate the electrode and
proof mass with quasi-independently controllable UV fluxes. This is mechanically more
challenging. Laboratory tests are planned to investigate these options further.
There are a number of possible operational modes for the charge control system. One is to
allow the UV to illuminate the proof mass continuously and only rely on the bias voltages
to control the charge. The bias voltage itself would be derived from measurements of
the charge using a dither voltage applied in the transverse direction at a frequency above
Corrected version 2.08 3-3-1999 9:33