  We present experimental results that demonstrate - for the first time - the
Zeeman deceleration of helium atoms in the metastable 2^3S_1state. A more than
40% decrease of the kinetic energy of the beam is achieved for deceleration
from 490 m/s to a final velocity of 370 m/s. Metastable atom generation is
achieved with an electron-impact-excitation source whose performance is
enhanced through an additional discharge-type process which we characterize in
detail. Comparison of deceleration data at different electron beam pulse
durations confirms that a matching between the initial particle distribution
and the phase-space acceptance of the decelerator is crucial for the production
of a decelerated packet with a well-defined velocity distribution. The
experimental findings are in good agreement with three-dimensional numerical
particle trajectory simulations.
