The Helium Double-ionization Cell (HDC) measures the approximate number of electrons released during EUV ionization of Helium in a manner similar to the Rare Gas Ionization Cell. The helium gas is cycled into the chamber periodically, and exhausted through the windowless aperture.

The ionization current is measured by two ion collectors, each approximately half the length of the cell, one in front of the other. Since the cell is operated optically thin, knowledge of the absolute gas density is required to complete the data analysis. Ionizing radiation pressure gauges, therefore, are attached to the cell. The Helium Double Ionization Cell has flown on both the Sounding Rocket and the Space Shuttle.