  Helium atoms have been prepared in the circular
$|n=55,\ell=54,m_{\ell}=+54\rangle$ Rydberg state using the crossed electric
and magnetic fields method. The atoms, initially travelling in pulsed
supersonic beams, were photoexcited from the metastable $1s2s\,^3S_1$ level to
the outermost, $m_{\ell}=0$ Rydberg-Stark state with $n=55$ in the presence of
a strong electric field and weak perpendicular magnetic field. Following
excitation, the electric field was adiabatically switched off causing the atoms
to evolve into the circular state with $m_{\ell}=+54$ defined with respect to
the magnetic field quantization axis. The circular states were detected by
ramped electric field ionization along the magnetic field axis. The dependence
of the circular state production efficiency on the strength of the excitation
electric field, and the electric-field switch-off time was studied, and
microwave spectroscopy of the circular-to-circular
$|55,54,+54\rangle\rightarrow|56,55,+55\rangle$ transition at $\sim38.5$~GHz
was performed.
