We present new spectral ( FPI and long-slit ) data on the Eastern optical filament of the well known radionebula W50 associated with SS433 .
We find that on sub-parsec scales different emission lines are emitted by different regions with evidently different physical conditions .
Kinematical properties of the ionized gas show evidence for moderately high ( V \sim 100 ~ { } km~ { } s ^ { -1 } \ / ) supersonic motions .
[ \ion OIII ] \lambda 5007 emission is found to be multi-component and differs from lower-excitation [ \ion SII ] \lambda 6717 line both in spatial and kinematical properties .
Indirect evidence for very low characteristic densities of the gas ( n \sim 0.1 ~ { } cm ^ { -3 } \ / ) is found .
We propose radiative ( possibly incomplete ) shock waves in low-density , moderately high metallicity gas as the most probable candidate for the power source of the optical filament .
Apparent nitrogen over-abundance is better understood if the location of W50 in the Galaxy is taken into account .