  Creating magnetized relativistic pair plasma in the laboratory would enable
the exploration of unique plasma physics relevant to some of the most energetic
events in the universe. As a step towards a laboratory pair plasma, we have
demonstrated effective confinement of multi-$\mathrm{MeV}$ electrons inside a
pulsed-power-driven $13$ $\mathrm{T}$ magnetic mirror field with a mirror ratio
of $2.6$. The confinement is diagnosed by measuring the axial and radial losses
with magnetic spectrometers. The loss spectra are consistent with $\leq 2.5$
$\mathrm{MeV}$ electrons confined in the mirror for $\sim 1$ $\mathrm{ns}$.
With a source of $10^{12}$ electron-positron pairs at comparable energies, this
magnetic mirror would confine a relativistic pair plasma with Lorentz factor
$\gamma \sim 6$ and magnetization $\sigma \sim 40$.
