  Superconducting diode effects have recently attracted much attention for
their potential applications in superconducting logic circuits. Several
mechanisms such as magneto-chiral effects, finite momentum Cooper pairing,
asymmetric edge currents have been proposed to give rise to a supercurrent
diode effect in different materials. In this work, we establish the presence of
a large intrinsic Josephson diode effect in a type-II Dirac semimetal
$1T-PtTe_2$ facilitated by its helical spin-momentum locking and distinguish it
from other extrinsic effects. The magnitude of the Josephson diode effect is
shown to be directly correlated to the large second-harmonic component of the
supercurrent that is induced by the significant contribution of the topological
spin-momentum locked states that promote coherent Andreev processes in the
junction. We denote such junctions, where the relative phase between the two
harmonics corresponding to charge transfers of $2e$ and $4e$ can be tuned by a
magnetic field, as second order ${\phi}_0$-junctions. The direct correspondence
between the second harmonic supercurrent component and the diode effect in
$1T-PtTe_2$ junctions makes topological semimetals with high transparency an
ideal platform to study and implement the Josephson diode effect, while also
enabling further research on higher order supercurrent transport in Josephson
junctions.
