The Kinetically Dominated Quasar 3C~418
  The existence of quasars that are kinetically dominated, where the jet
kinetic luminosity, $Q$, is larger than the total (IR to X-ray) thermal
luminosity of the accretion flow, $L_{\rm{bol}}$, provides a strong constraint
on the fundamental physics of relativistic jet formation. Since quasars have
high values of $L_{\rm{bol}}$ by definition, only $\sim 10$ kinetically
dominated quasars (with $\overline{Q}/L_{\rm{bol}}>1$) have been found, where
$\overline{Q}$ is the long term time averaged jet power. We use low frequency
(151 MHz$-$1.66 GHz) observations of the quasar 3C\,418 to determine
$\overline{Q}\approx 5.5 \pm 1.3 \times 10^{46} \rm{ergs~s^{-1}}$. Analysis of
the rest frame ultraviolet spectrum indicates that this equates to $0.57 \pm
0.28$ times the Eddington luminosity of the central supermassive black hole and
$\overline{Q}/L_{\rm{bol}} \approx 4.8 \pm 3.1$, making 3C\,418 one of the most
kinetically dominated quasars found to date. It is shown that this maximal
$\overline{Q}/L_{\rm{bol}}$ is consistent with models of magnetically arrested
accretion of jet production in which the jet production reproduces the observed
trend of a decrement in the extreme ultraviolet continuum as the jet power
increases. This maximal condition corresponds to an almost complete saturation
of the inner accretion flow with vertical large scale magnetic flux (maximum
saturation).
