The energy component associated with the cyclotron motion of the beam electrons in the Livermore EBIT-II electron beam ion trap was inferred from measurements of the linear polarization of the K-shell x-ray lines emitted from heliumlike ${\mathrm{Mg}}^{10+}$ ions. The average line polarization was found to be reduced by about 20% from its nominal value. From this it was inferred that the electron cyclotron motion accounted for $190\pm 30\hspace{0.5em}\mathrm{eV}$ of the total electron beam energy. The measured value is in good agreement with the predictions of optical electron beam propagation. It does not agree with the estimates derived from other model assumptions, such as the rigid-rotator model, for determining the size of the energy stored in the electron motion perpendicular to the beam propagation direction.

• Received 31 July 2001

DOI:https://doi.org/10.1103/PhysRevE.64.066408