Aerospace and Electronic Systems - April 2019 - 20

The CubeSat Mission to Study Solar Particles

Figure 2.
(A) ESA assembly. (B) Collimator assembly. (C) Principle of operation of SIS. The energy analyzer selects ions in a large E/q range with
a single voltage setting on the outer electrode. The ions are separated across the MCP detector area according to their incident angle and
E/q. (D) Mechanical model of SIS with electronics boards.

approaching IP shock up to $24 h ahead of time [9].
CuSP/SIS complements HSO-L1 observations and measures temporal variations (<5 min) of the intensities,
energy spectra, and angular distributions of $3-70 keV/q
ST ions escaping ahead of approaching IP shocks. This
allows us to identify properties that could increase the
lead-time for predicting the arrival of geo-effective IP
disturbances on future Heliophysics or dedicated space
weather satellites (Table 1: Objective 2b). Table 1 summarizes the CuSP science objectives and provides a traceability matrix for the mission.

C U SP PAYLOAD
SUPRATHERMAL ION SENSOR
SIS utilizes a novel electrostatic analyzer (ESA) whose
performance characteristics have been tested and verified
with a laboratory prototype [10], [11] and an engineering
model (see Figure 2). SIS provides the spectral and
angular distributions of $3-70 keV/q ST ions at $1 min
resolution (see "Conclusion") and measures key signatures of ST ion populations present in the IP medium near
Earth orbit, including ST ions from solar and IP activity.
The SIS concept and mechanical model are shown in
Figure 2(C) and (D). An ion (red trajectory) enters the
ESA through a cylindrical entrance grid and is accelerated
by the electric field created by the potential difference
between the inner (at ground potential) and outer (at VESA
potential) electrodes. An ion from the appropriate incident
direction and energy per charge (E/q) passes through the
ESA and the collimator to reach the detector, a positionsensitive MCP. The innovative, unusual shape (wide
opening and exit apertures with a narrow gap in the middle) of the ESA serves as a spectrograph and allows ions
20

in a large E/q range to be measured simultaneously with a
single VESA setting. For example, for each voltage step,
high E/q ions hit the detector closer to the axis of symmetry while low E/q ions hit the detector further away from
the axis of symmetry. This enables SIS to cover the entire
energy range of $3-70 keV/q using only 5 voltage settings and thereby increase the duty cycle (accumulation
time) for each E/q compared to conventional ESA-based
sensors (e.g., Wind/Suprathermal Ion Composition Sensor, or STICS, uses 32 steps to cover $3-270 keV/q).
Since the minimum gap between the electrodes is $1 mm,
the ion's E/q and incident direction are tightly related. At
a given E/q, only ions within a small incident angle range
($20 ) are permitted to enter the detector subsystem. Similarly, for a given angle of incidence, ions with only a narrow range of E/q ($26-34% at full-width-half-maximum,
or FWHM) reach the detector. Thus, the impact location
on the detector identifies the ion's E/q and incident angle.
Figure 3(A) shows Simion simulations [12] of the
impact location of ions on the MCP with an outline of
32 anode pixels corresponding to 5 rings Â 6 sectors and
2 background pixels ("Bgnd 1" and "Bgnd 2"). The color
of the dots corresponds to the ion's E/q, which is ordered
radially. The collimator [see Figure 2(B)] has two functions: to filter out ions with trajectories further away from
the axis of symmetry, and to block some of the ultraviolet
(UV) radiation while still letting ions through. The collimator is based on previous designs used in many space flight
instruments, e.g., Solar Wind Ion Composition Spectrometer
(SWICS) on Ulysses and ACE [13], Interstellar Boundary
Explorer (IBEX)-Lo [14], and IBEX-Hi [15]. It consists of a
stack of 18 thin plates with coaligned holes. Only ions with
the desired arrival directions make it through the collimator
while other ions are stopped when they hit a plate. This
design has three advantages over a similar monolithic collimator with holes: the thin plates limit the area where ions

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Aerospace and Electronic Systems - April 2019

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