Calibration System Intro

Calibration Capabilities at Princeton Space Physics Lab

Calibration System

The Princeton Space Physics Calibration System, located at 171 Broadmead, Princeton, NJ, is shown in Figure 1. Residual gas analyzers (RGAs) are installed on both the Main Chamber (4 ft diameter) and Science Chamber to enable routine monitoring of cleanliness. Nominal chamber operating pressures are on the order of 1×10⁻⁸ to 1×10⁻⁹ torr. The setup is designed so that the chamber doors open into a soft-walled ISO 7 (Class 10,000) cleanroom, ensuring minimal contamination during flight instrument installation.

Figure 1. Princeton Space Physics Calibration System.

Beam Source

The ion source used at Princeton is an electron impact ionization source, with the gas supply consisting of a mixture of hydrogen, deuterium, helium, neon, and argon, balanced with nitrogen. Since the pressure in the ion source’s filament region correlates linearly with beam intensity, precise flow control enables a highly stable beam (with intensity variations of <10% over day-long timescales). A Wien filter integrated into the ion optics enables separation of ion species, allowing selection of beams such as protons (H⁺), deuterium (D⁺), molecular hydrogen (H₂⁺), deuterium hydride (DH⁺), helium (He⁺), molecular deuterium (D₂⁺), nitrogen (N⁺, N₂⁺), neon (²⁰Ne⁺, ²²Ne⁺), and argon (Ar⁺) with desired spatial width, energy, and count rate. With residual gases in the system and appropriate Wien filter settings, the source can also provide water group species from ambient water vapor or room air, including oxygen (O⁺), hydroxyl cation (OH⁺), and water (H₂O⁺). The Princeton system can generate stable ion beams with energies ranging from ~100 eV to ≤ 30 keV. The calibration system specifically has a charge-exchange vessel, which is equipped with a precision leak to control Nitrogen gas flow for ion-neutral charge exchange, a turbo backed by a roughing pump, and two knife edges for differential pumping. The precision leak allows the precise control of small gas flows. In this vessel, ion beams will charge-exchange with neutral nitrogen molecules. Nitrogen gas is supplied by a 5N purity 99.999% gas bottle.

Beamline and Beam Monitors

A custom-built Helmholtz coil is mounted on the flight tube and is used either to cancel the Earth's magnetic field—for example, for < 5 keV protons or < 1 keV helium—or as an ion dump to prevent ions from entering the chamber during neutral beam calibrations. The total length of the beamline from the ion source nose to the entry flange of the Main Chamber is 16 feet. Knife-edges placed along the beamline shape the beam such that, by the time it reaches the chamber entry port, it is highly uniform, parallel, and expanded to a diameter of ~60 mm. This beam size can be reduced to any smaller, practical diameter using collimators installed at the entrance flange, such as the 1, 3, 6, and 14 mm diameter collimators for specific calibration purposes. Beam monitors mounted on a two-axis positioning system inside the Main Chamber are used both for beam imaging and flux reference during testing. The diagnostic suite includes a microchannel plate detector (MCP), a carbon-foil absolute beam monitor, a conversion surface absolute beam monitor (Bern ABM), a channel electron multiplier (Bern CEM), and a Faraday cup with retarding potential analyzer capability up to 5 kV bias.

Figure 2. Here shows a CEM, an MCP, a collimator paddle, and the C-foil ABM in a counter-clockwise direction from 12 o'clock.

Control System

The entire system is configured to provide real-time status updates, including valve states, pressures, cryopump temperatures (on both Main Chamber and Science Chamber), turbo speeds (in the beamline and ion source sections), locations of the positioning system (Main Chamber and Science Chamber), and ion source settings. These parameters are recorded at a cadence of ~1 Hz and stored in a locally hosted database. System status information, along with data acquired from the RGAs and beam imagers, will be continuously tracked throughout the calibration campaign, and the complete datasets will be made available to instrument teams.

Flighe Instrument Calibrations

IMAP/SWAPI (Solar Wind and Pickup Ions) instrument

IMAP/IMAP-Lo (low-energy neutral atom imager) instrument