The Mushroom: A half-sky energetic ion and electron detector

We present a time-of-flight mass spectrometer design for the measurement of ions in the similar to 30keV to 10 MeV range for protons (up to similar to 40 MeV and similar to 150 MeV for He and heavy ions, respectively) and similar to 30 keV to 1 MeV range for electrons, covering half of the sky with 80 apertures. The instrument, known as the "Mushroom," owing to its shape, solves the field of view problem for magnetospheric and heliospheric missions that employ three-axis stabilized spacecraft, yet still require extended angular coverage; the Mushroom is also compatible with a spinning spacecraft. The most important new feature of the Mushroom is the method through which uncomplicated electrostatic optics and clean position sensing combine to permit many apertures to fit into a compact, low-mass sensor head (or wedge), several of which (ideally eight) compose a full instrument. Most of the sensor head's volume is an empty, equipotential region, resulting in the modest 250 g mass of each 10-aperture wedge. The Mushroom is capable of separating ion species across most of its energy range and angular field of view. For example, separation of the neighboring He-3 and He-4 isotopes is excellent; the full width at half maximum mass resolution has been measured to be 0.24 amu to 0.32 amu, respectively. Converting this to a Gaussian width sigma(m) in mass m, this represents a sigma(m)/m mass resolution better than 0.04. This separation is highly desirable for the flight program for which the first Mushroom was built, the Solar Probe Plus mission. More generally, we estimate the mass resolution to be sigma(m)/m approximate to 0.1, but this is energy, mass, and angularly dependent. We also discuss the solid-state detector stack capability, which extends the energy range of protons and helium, with composition, to similar to 100 MeV.