TIDE- Instrument Description

Mirror/RPA System:
The TIDE electrostatic mirror/RPA system provides differential energy analysis by a two-stage ion selection process. Ions which are too energetic pass through the mirror and subsequently are lost, while those of insufficient energy are reflected by the RPA and excluded from admittance to the TOF analyzer. Thus, ideally, a boxcar-shaped energy pass band is provided which can be adjusted in width and center energy by selection of M/RPA potentials. The actual energy response is asymmetric with a sharp cutoff on the low-energy side (due to the RPA), and a more gradual cutoff on the high- energy end (due to the mirror). Adjustment of the energy response width is used to control overall instrument sensitivity, as described below.

The mirror plays an important role in the achievement of the large TIDE geometric factor, through its focusing geometry, which is similar to the action of focusing lenses and mirrors in optical telescopes. An electrostatic mirror for charged particles consists of a biased surface suspended behind a grounded grid, and the penetration depth is in general non-negligible relative to mirror dimensions. Focusing is accomplished through the implementation of a mirror geometry which differs slightly from parabolic in a functionally-significant way. The shape is derived from the requirement that incoming ions following parallel trajectories are directed through a common focal line. This focusing action translates a large collection area and small solid angle at the entrance aperture, to a smaller collection area/larger solid angle at the entrance to the TOF section. The large solid angle acceptance at the TOF entrance is attributable to the large attractive potential encountered by ions upon entry to that section.

The RPA is a planar device consisting of four grids, as illustrated in Figure 4. The entrance and exit grids are connected to spacecraft chassis ground. The two retarding grids are mounted on opposite sides of a single conducting plate, in close proximity to entrance and exit grids, respectively. Together, they are biased at the commandable voltage VRPA. No electron suppression is included or required in view of the large negative potential separating the TIDE detection system from the RPA.

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Figure 4. Section view of the TIDE sensor showing the optics path through the mirror, RPA, immersion lens, UV rejection deflector, START foils, and STOP detectors for a single channel, with rays plotted for parameters which completely fill the instrument aperture in space, angle, and energy.

The energy resolution of the RPA system is ultimately limited by the angular spread of the ions delivered to it by the mirror system. This spread is inversely related to the breadth of the energy passband defined by the mirror and RPA, which reduces the fraction of the mirror that is active as well as the range of energies that can pass through the system.

The electrical biasing scheme employed in this system is as follows: VRPA is commanded with a 12-bit word through a digital-to-analog converter (DAC) providing a range of 0-300 V with 0.073 V resolution. Vm is commanded through an 8-bit DAC which is referenced to the output of the RPA DAC. In this way, the ratio (Vm/Vrpa) is directly commanded and can be controlled more precisely at the low end of the energy range than with independent commanding for both Vm and VRPA. This directly controls TIDE sensitivity, because the energy pass band (as well as the effective area and solid angle) is directly related to this ratio. The practical range over which TIDE's sensitivity can be varied is approximately two orders of magnitude, as described in the section below on test results.