E1 plasma experiment aboard the Helios solar probes consists of four independent instruments designed to investigate the solar wind plasma.
By measuring the velocity distribution functions of the different kinds of particles, all important hydrodynamic parameters of the solar wind plasma can be derived.
Three instruments (I1a, I1b, and I3) analyze the positive components (protons and heavier ions with energy-per-charge values from 0.155 to 15.32 kV) of the solar wind.
Two of them allow for an angular resolution in both directions of incidence. The experiment employed a combination of a hemispherical, a quadrispherical, and a sinusoidally-shaped electrostatic analyzer.

All detectors were mounted normal to the spin axis.

One instrument (I2) measures electrons in the energy range from 0.5 the 1660 eV with one-dimensional angular resolution
using a hemispherical electrostatic analyzer.

For more detailed information see p. 226 of Raumfahrtforschung, v. 19, n. 5, 1975.