Prototype Particle Ion Experiment Ion Electron Sensor (PPIXIES)

This proposal is to build a bench-top prototype of an instrument that can measure the energy spectra and composition of electrons and ions from ~ 30 keV to a few MeV. Need to prove lab performance of instrument in order to give credibility for future Europa-Jupiter System Mission AO.

Low-energy particle detectors of this type are composed of two main parts, a time-of-flight (TOF) section, followed by an energy measurement.  The TOF section consists of a pair of thin foils (Start and Stop) separated by a flight path of typically 6 or more cm.  Secondary electrons generated as the particles pass through the thin foils are electrostatically focused on a MCP, which generates the start and stop pulses that determine the time of flight.  The energy measurement is typically accomplished with one or more silicon solid-state detectors (SSDs).  Electrons are separated from ions by masking off select portions of the SSDs with a thin aluminum foil that stops the low energy ions.

Although something very much like the heritage PEPSSI instrument from APL is base-lined in the EJSM Science and Technology Definition Team report, in several ways this instrument is not suitable for this mission.  The geometry factor and look angle are not optimized for the EJSM science, and the electron separation used will have background issues in the high radiation environment of EJSM.  This leaves the door open for Goddard to have a competitive advantage, by designing a similar instrument specifically for the science and environment of EJSM, instead of rebuilding an instrument designed for a different mission.  Proving that the design has the required resolution will take more than simulation and will almost certainly need fine tuning of actual hardware.

One of the competitive edges for Goddard has come out of the simulations done with a FY11 IRAD.  We have shown that we can move a focused secondary electron distribution by adjusting voltages on the electrostatic system (see Figure 2).  This is very important for a high-radiation environment such as EJSM, both improving the lifetime of the MCPs and increasing the robustness of the overall system.  Showing test results from this prototype hardware, instead of just simulations, will be an important bonus for the EJSM proposal.

Figure 1 (5299.jpg) – a schematic of the Plasma Ion Experiment Ion Electron Sensor (PIXIES) for JGO and JEO.  Primary particles (ions and electrons) come in one of the apertures and travel straight through to the Solid State Detectors (SSDs).  While passing through the two thin foils, secondary electrons are released that are mirrored by the Electrostatic Deflector down to the MCP.