Electron measurements are provided by the ELS sensor for CAPS, which is a hemi- spherical top-hat electrostatic analyser (Young et al., 1998). A top-hat consists of two conducting, hemispherical plates which are mounted concentrically, with a small gap between them, and a top cap/circular collimator. An illustration of the generic geometry of a hemispherical top-hat electrostatic analyser is shown in Fig- ure 3.5. In the case of an electron detector like ELS, the inner hemisphere is given a positive voltage with respect to the outer hemisphere, which is grounded.
3.1. The Cassini-Huygens Mission 106
(a) (b)
Figure 3.5: (a) An illustration of a top-hat electrostatic analyser (Paschmann and Daly, 1998). The upper part of the diagram shows a cross section of the analyser taken through a plane containing the rotational axis of symmetry (marked by the dashed line labelled AA). The top-hat consists of two conductive hemispheres mounted concentrically with a small gap between them. There is also a collimator section at the top-hat entry aperture which consists of parallel circular plates. The azimuthal acceptance angle for the collimator is shown by the thin grey lines and labelled ∆ψ. Particles which do not have arrival paths lying within the fan bounded by these lines do not reach the detector plane. Particles that enter the aperture are rotated though 90◦as they pass between the hemispheres and strike the detector (indicated by the grey rectangles). The bold arrows show the direction of particle trajectories at the entry aperture and the exit of the nested hemispheres. The lower part of the diagram shows the detector in plan view (i.e., looking down the axis AA). The detector in this diagram is divided into equal sectors, each associated with a different polar angle zone labelled ∆θ . (b) An illustration of the same top-hat electrostatic analyser with particle trajectories shown with thick black lines (Paschmann and Daly, 1998). The lower part of the diagram shows a projection of particle trajectories onto the plane containing the detector. Parallel beams arriving from three different polar angles are shown, the small, inner circle shows the entrance to the outer hemisphere though which the beams must pass. Particle beams are effectively focussed by the top-hat for all polar angles.
3.1. The Cassini-Huygens Mission 107 Electrons enter the outer hemisphere though the collimating parallel plates which only allow electrons from a finite azimuthal angle to enter the aperture. In the case of the Cassini ELS this azimuthal angle is 5.2◦ and the polar angle fan covers 160◦(unlike in Figure 3.5 which has a full 360◦polar fan). The polar angle is measured in the plane whose normal is parallel to the top-hat symmetry axis, la- belled AA in Figure 3.5. Electrons approaching the aperture with trajectories that lie outside the aperture acceptance angle will strike one of the plates (N.B. ELS has a baffle system designed to reduce background arising from secondary photoelec- trons and internal scattering). Electrons that enter the analyser are then deflected by the voltage applied between the two hemispherical plates. For a given potential difference, only electrons with a specific energy will be deflected in such a way that the trajectory passes though the gap between the two plates and impacts the detec- tor. Electrons with energies that are too low (high) will impact the inner hemisphere (outer hemisphere or collimator). Thus by varying the voltage applied to the inner hemisphere, electrons with different energies can be observed.
Electrons that have the required energy to pass between the two hemispheres will strike the detector. In the case of ELS, the detector is divided into 8 polar an- gle sectors comprising of micro-channel plates (MCPs) mounted above 8 anodes. This provides an instantaneous field-of-view (FOV) of 5.2◦in the azimuthal direc- tion and ±80◦ in elevation (20◦ per anode) (Linder et al., 1998). MCPs consist of thousands of microscopic glass lined pores orientated so that incident electrons will hit the walls of one of these pores (ELS makes use of a chevron pair of MCPs to achieve this). When an electron impacts the MCP wall, one or more secondary electrons are emitted. This process continues, resulting in an avalanche of electrons which then strike the surface of the anode. The surface of the anode becomes nega- tively charged and a count is registered in the detector electronics when a sufficient potential difference has been reached.
3.1. The Cassini-Huygens Mission 108 In order to observe over a range of electron energies, the voltage between the ELS hemispheres is ’swept’ through a number of discrete voltage steps. ELS has an energy/charge response range of ∼ 0.5 - 27,000 eV/e (Linder et al., 1998). This is divided into 63 energy steps with a 31.25ms sampling time per energy level and a total cadence of 2s. However, the time and/or energy resolution of the data were sometimes reduced by summing over several energy sweeps and/or pairing energy bins if CAPS ELS was assigned low telemetry priority (Young et al., 2004; Arridge et al., 2009). For example, during the Earth flyby, the temporal cadence was limited to ∼ 10s (Rymer et al., 2001).
Figure 3.6: An approximate sketch of the physical blockages of the full CAPS FOV [Young et al., 2004]. The acronyms are as follows: HGA, High Gain Antenna; FPP, Fields and Particles Pallet; IMS, Ion Mass Spectrometer; LEMMS, Low Energy Magnetospheric Mea- surement Subsystem; RTG, Radioisotope Thermionic Generator. This figure is only approx- imate as it actually shows the FOV of the CAPS ion instrument mounted directly alongside the ELS. Also not included in the diagram are the multi-layer insulation blankets cover- ing all spacecraft surfaces, adding a further 5cm thickness, or the presence of the Huygens probe mounted in the bottom left corner.
The ELS is mounted with the other CAPS sensors, on a rotating platform driven by a motor actuator, that is able to sweep through ±104◦ in the ELS az- imuthal direction in 208s (Young et al., 2004). The full ELS FOV is shown in Figure 3.6. Although, it should be noted that this is the maximum actuation range and it is not always implemented. For example, during the Earth flyby, the actuator only sweeps through 120◦ and during the Jupiter encounter the actuator was fixed
3.1. The Cassini-Huygens Mission 109 at 0◦ for extended periods in the solar wind (Rymer, 2004). It should also be noted that other instruments on-board the Cassini spacecraft intrude upon the CAPS ELS FOV (see Young et al., 2004, for more details), in particular, the Huygens probe was not deployed until 2004. Hence, the usable FOV of ELS is reduced further as the obstructed regions must be removed from analysis. Finally, when using CAPS ELS data, the background count rate resulting from contamination via Cassini’s radiation sources must also be taken into account. An investigation by Arridge et al. (2009) produced a model for this energy-independent, look-direction and time-dependent background count rate which can be used to subtract the background level from the ELS data. A summary of the key properties of CAPS ELS are shown in Table 3.1.