  Circular Rydberg states with $n=70$ have been prepared in helium using a
modified version of the crossed-fields method. This approach to the preparation
of high-$n$ circular Rydberg states overcomes limitations of the standard
crossed-fields method which arise at this, and higher, values of $n$. The
experiments were performed with atoms traveling in pulsed supersonic beams that
were initially laser photoexcited from the metastable 1s2s$\,^3$S$_1$ level to
the 1s73s$\,^3$S$_1$ level by resonance-enhanced two-color two-photon
excitation in a magnetic field of 16.154 G. These excited atoms were then
polarized using a perpendicular electric field of 0.844~V/cm, and transferred
by a pulse of microwave radiation to the state that, when adiabatically
depolarized, evolves into the $n=70$ circular state in zero electric field. The
excited atoms were detected by state-selective electric field ionization. Each
step of the circular state preparation process was validated by comparison with
the calculated atomic energy level structure in the perpendicular electric and
magnetic fields used. Of the atoms initially excited to the 1s73s$\,^3$S$_1$
level, $\sim80$\% were transferred to the $n=70$ circular state. At these high
values of $n$, $\Delta n = 1$ circular-to-circular Rydberg state transitions
occur at frequencies below 20 GHz. Consequently, atoms in these states, and the
circular state preparation process presented here, are well suited to hybrid
cavity QED experiments with Rydberg atoms and superconducting microwave
circuits.
