The STEREO PLASTIC instrument performs the functions of three sensors and is quite complicated. Issues have been identified with the Entrance System that affect the data quality and instrument operation. These are discussed here. References can be found under the PLASTIC bibliography website.
Entrance System Overview
The entrance system consists of an electrostatic analyzer (ESA), electrostatic deflectors, and three geometrical factors, and is operated by a set of four dynamic high voltage power supplies. The ESA power supply steps the ESA dome through 128 logarithmically spaced voltages that effectively cover ~80 keV/e down to ~0.3 keV/e. The two DEFL power supplies sweep the voltage on the deflectors, which allow the polar angle to be sampled. The main channel geometric factor allows for sufficient counting statistics of heavy solar wind ions. The switch to the smaller geometric channel prevents the saturation of electronics and controls the lifetime fluence on the MCP and SSD detectors. The small channel is the primary channel for solar wind protons and often alphas. The SCHN power supply enables the smaller geometrical factor, while simultaneous electrostatic gating suppresses penetration into the larger main channel. The third geometrical factor channel is used to measure suprathermals.
Entrance System Known Issues
1. Prelaunch: it was found that the design for the main channel geometrical factor was about four times larger than specifications. This affects lifetime fluence. The set point for the switching from main to small channel needed to become more conservative in the early stages of the mission, complicating data analysis of species with an m/q near alphas. (Note: since 2009 this requirement has been relaxed to better retrieve oxygen data.)
2. Prelaunch: during the final pre-delivery calibration of the flight unit, it was found that the entrance system sweeping power supplies were introducing unacceptable noise into the detector section of the instrument, which affected the species resolution (mass, mass-per-charge). With consultation and review of the NASA STEREO Project office, remedial steps were incorporated into the instrument to ameliorate the effects to a level meeting minimum, but not optimum, data analysis requirements. (Due to flight delivery time constraints not all corrections were incorporated.) As a result, extra fitting procedures are needed in the ground data analysis for minor ions from the original ground data plan. The resultant species resolution is reflected in the figures of Galvin et al. (2008).
3. Post launch: Although the 80 to 0.3 keV/e range meets and exceeds the instrument science requirements, the original intention was to cover the 100 to 0.2 keV/e range. The upper value was descoped due to the potential for discharges. The lowest value is not attained because of a small (~20 V) voltage offset in the power supply.
4. Post launch: At the start of each 1-minute ESA sweep the small channel is 'off', and the main channel is open. Upon reaching a set (but commandable) count threshold, the main channel is electrostatically closed and the small channel is enabled. After launch, during the commissioning period, it was found that the behavior in the small channel was not as expected. Extended analysis of in-flight data indicates a major variance in the performance of the small channel instrumental response in geometrical factor, E/Q, and polar and azimuth angular response (Opitz, 2007; Simunac, 2009) from expectations based upon the pre-flight design (Allegrini 2002), entrance system testing and calibrations (Karrer 2007). Beam analysis was performed using the engineering model PLASTIC unit at UNH and with the flight spare entrance system at the University of Bern and the in-flight effect was reproduced. The effects are attributed to insufficient fringe-field control in the aperture section of the Entrance System near the top cap. An extensive in-flight calibration (Simunac, 2009) and post-launch determinations with the engineering model have been performed to retrieve the level 1 proton and alpha science data (level 1 being mission required).
5. Post launch: The increased flux due to item (4) is on the order of 20-50 times above the intended values. This affects the gain on the MCPs (which can be adjusted by the MCP bias) and the lifetime fluence on the MCPs and the SSDs. The increased flux also creates 'accidentals' and 'pileup' in the ion spectra. These last affects are handled in the data analysis as background spectra.
6. Post launch: Due to item (4), the GF is effectively different for each ESA step (hence the need for in-flight recalibration).
7. Post launch: Due to insufficient fringe field determination at the top cap area of the entrance system, there is diversion of ions from one geometrical factor into another. This is handled in the analysis through various background techniques.
Because PLASTIC has the option of single or double coincidence, background is not normally an issue. However, on December 5, 2019, an extremely high rate of background occurred on STA. From the composition data available, the constituents are consistent with outgassing of water products. As of March 2020, this background is ongoing, but declining slowly. The outgassing event has increased the load on various power supplies, resulting in the need to decrease some voltages. Currently there are solar wind proton data (with decreased signal/noise) and suprathermal ion data being obtained. (Note that there will be period of recalibration needed before release of validated data). Recovery is ongoing.
This page was updated 10/2009 (change in switch set point).
This page was updated 03/2020 (background contribution).