TIDE- Instrument Description

TIDE Operations:
The performance of the TIDE/PSI system has been extensively simulated by 2.5-dimensional (i.e., axially symmetric potentials) numerical raytracing computations and tested using a full scale prototype instrument. Significant iterations of the design were made and tested in this way, resulting in a thorough understanding of the electrostatic optics system. The final design has been explored with several million trajectories, resulting in a characterization of the angular and energy response over the full range of sensitivities for which the instrument can be programmed. The observed response of the instrument agrees well with the numerically computed response in most areas.

Figure 11 illustrates the simulated effective response width in energy, azimuth angle, polar angle, and effective area, as functions of the mirror ratio Rm = Vm/VRPA. The response in spacecraft polar angle is essentially independent of mirror ratio. The spacecraft azimuth response is slowly varying, but does drop off by a significant factor at the low end of the total aperture range. The energy passband and the effective area Aeps each vary nearly a decade over the range plotted, reflecting the varying separation between the mirror and RPA energy cutoffs. At low mirror ratios, only the uppermost part of the mirror is active, where particle incidence is more grazing. The simulation results make no accounting of grid losses, edge-effect losses, detection efficiencies for the foils or MCPs, or START-STOP coincidence probabilities and the effective area is therefore unrealistically high. End-to-end optics efficiencies are expected to be on the order of 10% (applicable to non-mass-analyzed events on the START detectors), while the combination of foil conversion and START-STOP coincidence efficiencies is expected to introduce a similar multiplicative efficiency factor.

Click image for larger version.
Figure 11. The simulated and measured response of the TIDE instrument as functions of the ratio of Mirror to RPA potentials. The geometric factor (GF [cm2 sr eV/eV]) is the product of the energy bandpass (dE/E), the azimuthal response (dAz[radians]), the polar angle response (dP [radians]), and the effective area (Aeps [cm2]). G (meas.) is the observed geometric factor for TOF coincidence (mass-analyzed) events [cm2 sr eV/eV], and dAz (meas.) is the observed effective width of the azimuth angle response [radians].

TIDE has been extensively tested and calibrated prior to delivery for integration on the POLAR spacecraft. Also shown in Figure 11 are the measured mirror ratio dependencies of the geometric factor and azimuth angle response width. An example of the data upon which these curves are based is shown in Figure 12. The calibration approach is to measure the response of the instrument to a monoenergetic and monodirectional ion beam with a cross sectional area sufficient to completely illuminate one aperture. The geometric factor is then obtained from these data by integration. In Figure 12, the response is plotted versus energy (or RPA voltage) and spacecraft azimuth angle, for two values of Rm. Maximum azimuth response is ~ 6 degrees, while the energy response width is ~ 50% for Rm = 1, and 25% for Rm = 0.6. Polar angle response width (not shown) is ~ 18 degrees, independent of Rm. The measured response is generally in agreement with the simulations. A notable exception is the azimuth response, which is approximately 65% of its simulated value, owing most likely to slightly non-optimal potentials within the (UV) rejection/deflection system.

The calibration to date has not been definitive with regard to absolute effective area and geometric factor, for the following reasons: First, the MCPs used during the initial calibration were engineering plates with long test histories. Second, the test setup was designed for angular response scanning and the beam current measurement was located somewhat upstream of the entrance aperture. Based upon the simulations and the data in hand, it is anticipated that the non-mass analyzed effective area will be ~ 1 cm**2, and that the mass-analyzed effective area will be ~ 0.1 cm**2.

Click image for larger version.
Figure 12. Measured response of a TIDE aperture versus RPA voltage and azimuth angle, for a 25 eV beam.

Figure 13 illustrates a TIDE mass spectrum constructed by superposition of spectra from tests done with the beam source dominated by four different species, simulating a multi-constituent ion beam. The main point here is to illustrate the modest mass resolution for which TIDE has been designed in order to maximize its overall sensitivity to the major ion species known to be of interest in the terrestrial magnetosphere and ionosphere. [Note: In these tests, the TOF analysis was suffering from an electronic problem which both reduces the overall signal/noise and introduces artifactual peaks in the range of Channel 1-50. It is believed, based on testing with engineering model equipment, that this problem has been largely corrected. Operations during refurbishment and on orbit are expected to reflect significantly improved performance in this area.]

Click image for larger version.
Figure 13. TIDE mass spectral response formed by superposing the response to laboratory beams generated from four different gas leaks.

The instrument response function has been used to compute the expected instrument response to hypothetical drifting bi-Maxwellian distribution functions with parameters covering the full range of those expected during flight operations. This procedure provides an end-to-end test for the flight and ground software which is used to store, process, and analyze the TIDE data, as well as a means of designing operating modes intended to resolve special features of the data.

Of particular interest is the evaluation of the adequacy of TIDE to resolve the features of interest in the very high altitude plasma, while maintaining the capability to observe the denser plasma of the inner magnetosphere. Figure 14 illustrates the results for two cases of interest, including a very high Mach number polar wind case and a hot upwelling ion (bi-Maxwellian) case. It may be seen that distribution functions deduced from the TIDE data are expected to be both qualitatively recognizable, as well as quantitatively accurate.

Click image for larger version.
Figure 14. Unsmoothed TIDE response to a polar wind distribution (upper panel: V// = 21 km/s, Tperp = 0.15 eV) and to a bi-Maxwellian model of an upwelling ion distribution (lower panel: T// = 3 eV, Tperp). Sampling resolution corresponds to the normal operating mode for TIDE in flight.