TIDE- Instrument Description

TIDE Instrument Description:

Plate 1 shows a photograph of the TIDE sensor apertures and TOF/detector system, with the cylindrical sensor cover removed. Figure 1 shows an exterior assembly view of the TIDE sensor with all covers in place. The sensor and electronics are integrated into a single unit. Routine access to the sensor components requires removing only the outer cylindrical cover. Routine access to the electronics boards requires removing only the rear panel on the electronics compartment. The outer surfaces of the instrument that are in contact with the plasma are plated with electroless nickel. Optical surfaces within the sensor are treated, for reduced EUV reflectance, with cupric oxide black (Ebanol-C). Other surfaces are polished or bead-blasted aluminum, where scattering of light and particles is not a problem.

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Plate 1. Photograph of TIDE sensor illustrating the orientation of the seven entrance apertures with respect to the semi-cylindrical instrument package, and the layout of assemblies on the sensor baseplate.

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Figure 1. Perspective view of TIDE instrument including its electronics package, showing the orientation of entrance apertures, arming plugs, thermal blankets, purge fittings, connectors, and mounting lugs.

The sensor is designed with a housing which seals tightly to serve as a diffusion barrier against chemical, water vapor, and dust contamination of the sensor interior. TIDE's detectors are sensitive to hydrocarbon contamination and are also strongly hygroscopic. The sealed sensor housing is kept backfilled with gaseous high purity nitrogen. The apertures and other vent openings are fitted with purge covers which are removed before flight. The apertures are covered until orbital operations by a conductively treated Kapton sheet which is retracted onto a double negator spring-actuated spindle at the appropriate time by remote command.

Figure 2 shows the relative location and orientation of the TIDE sensor and the PSI source on the spacecraft, with respective fields of view indicated. The TIDE field of view is a segmented fan coplanar with the spacecraft spin axis and one edge of the field of view is aligned with the spin axis. TIDE's seven polar angle sectors are arranged in a fan spanning 157.5°, as shown in Figures 2 and 3. Each angular sector contains a collimator, electrostatic mirror, and RPA, as shown in Figure 4. The electrostatic mirrors have an off-axis corrected parabolic form and are preceded by collimators that eliminate extraneous particles and photons at the aperture of each sector. Each electrostatic mirror sets an upper energy limit, since ions are only reflected and focused if their energy is below a cutoff set by the mirror potential (and to a lesser extent their incidence angle). Following the mirror is a RPA, similar in operating principle to that of DE-1/RIMS, which independently sets a lower limit on the ion energy. The net result is an electronically-controlled differential energy pass band.

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Figure 2. Two views of the POLAR spacecraft showing TIDE and PSI pointing and mounting layout. Note that one TIDE aperture field of view is contiguous with the spin axis.

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Figure 3. Section view of the TIDE sensor with cutaways at various levels showing the placement of the START detectors and STOP detectors, as well as the angular layout of the seven channels.

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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.

Coupled with each mirror/RPA (M/RPA) system is an EUV-rejection/deflection system and TOF mass analyzer system with electronics similar to that developed for the LOMICS instrument on the CRRES mission [Young, 1989]. After coming to a focus at the entrance to the rejection/deflection section (at chassis ground), ions are first accelerated as they pass through a second aperture at -3.0 kV, then further accelerated to and simultaneously deflected radially inward within the sensor by a UV-rejection/deflection system. The ions are then deflected back outward and accelerated to -12 kV so as to enter the TOF start gate traveling parallel to the direction in which they entered the rejection/deflection system. The ions enter the TOF start gate by passing through thin (<1 mg/cm2) carbon foils. Secondary electrons produced on the foil exit surface are collected at the START micro-channel plates (MCPs) to form a "start" pulse for the TOF electronics. The ions then traverse the TOF region and arrive at the STOP MCPs where a "stop" pulse is generated for the TOF electronics. The delay between start and stop signals is then converted into a voltage which is inversely proportional to velocity at known energy/charge, and therefore can be converted to a digital mass/charge address word (8-bit resolution).

When used in conjunction with a sufficient pre-acceleration potential (-12 kV), the TOF technique has no intrinsic low-energy limit for mass analysis, leading to a straightforward application of TOF to the TIDE investigation. Unlike magnetic mass analysis systems, the TOF technique permits the use of large entrance apertures, since no object slit is required to form a mass spectrum image. The large pre-acceleration potential also allows the use of a very large angular acceptance aperture, so that very large geometric factors become possible using this technique. Similar to the operation of optical systems, the M/RPA optics transforms the TOF analyzer area and solid angle aperture to a still larger area, but smaller angular aperture, which is then presented to the external plasma. The relatively low mass of TOF devices then allows simultaneous measurement in multiple directions of all ionospheric and solar species likely to be encountered by the POLAR spacecraft (viz. H+, He+, He++, O++, O+, and molecular species such as NO+).

TIDE generates very high data rates (~ 105 Bytes per satellite spin period) because of multiple look directions and simultaneous detection of all mass species. Consequently, a powerful onboard data processing unit (DPU) is required. This DPU is based on a pair of SA3300 microprocessors, one to perform onboard computations and data compression, the other to control the instrument and encode the data for telemetering. Further details on each subsystem are given below. The overall functional structure of TIDE is shown in Figure 5. It should be noted that both command and data interfaces to the PSI are managed by the TIDE DPU, so that the PSI has no independent spacecraft interface.

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Figure 5. TIDE functional block diagram, showing the relationships among the principal components.