What is STROFIO?

STROFIO (Start from a Rotating FIeld mass spectrOmeter) included in the SERENA instrument package is a mass spectrograph that determines particle mass-per-charge (m/q) by a time-of-flight (TOF) technique. The name comes from the Greek word Strofi, which means “to rotate”: the phase of a rotating electric field “stamps” a start time on the particles’ trajectory, and the detector records the stop time.
STROFIO is the only instrument that can detect all neutral species reaching the spacecraft, not just those with emissions or absorptions at specific wavelengths. In contrast with optical techniques, STROFIO can make measurements with high sensitivity, both in full sunlight and in Mercury’s shadow.
The major STROFIO scientific objectives may be resumed as in the following:
• Chemical and elemental composition of the exosphere and its variability
• Neutral gas density asymmetries
• Surface release processes
• Relationship between the exospheric composition and the surface composition

STROFIO is characterized by a high-sensitivity (0.14 counts/s when the density is 1 particle/cm3). The achieved mass resolution is m/Dm = 60. STROFIO is a novel type of mass spectrometer: the start time is imprinted on the trajectory of the particle by a radio frequency electric field, that bends the trajectory in a given plane, and the stop time is the time when the particle reaches the detector.
In particular, the neutral particles enter into the ionization chamber through the entrance in the ram direction (see the figure). The neutral gas is ionized and accelerated into the mass analyzer. Here the ions experience the effects of an electric field, constant in magnitude, but with direction rotating uniformly in space, in a plane perpendicular to the initial ion velocity, at a frequency f. The trajectory of an ion can hit the detector only if the field points to the detector while the ion traverses the dispersing region. At other times, the ion will simply miss the detector. The time difference between the instant when the particle arrives at the detector and the time when the field was pointing in the appropriate direction is equal to the travel time through the field free region. In the table the major STROFIO characteristics are summarized.

STROFIO mechanical design