  The radio nebula W50 is a unique object interacting with the jets of the
microquasar SS433. The SS433/W50 system is a good target for investigating the
energy of cosmic-ray particles accelerated by galactic jets. We report
observations of radio nebula W50 conducted with the NSF's Karl G. Jansky Very
Large Array (VLA) in the L band (1.0 -- 2.0 GHz). We investigate the secular
change of W50 on the basis of the observations in 1984, 1996, and 2017, and
find that most of its structures were stable for 33 years. We revise the upper
limit velocity of the eastern terminal filament by half to 0.023$c$ assuming a
distance of 5.5 kpc. We also analyze the observational data of the Arecibo
Observatory 305-m telescope and identify the HI cavity around W50 in the
velocity range 33.77 km s$^{-1}$ -- 55.85 km s$^{-1}$. From this result, we
estimate the maximum energy of the cosmic-ray protons accelerated by the jet
terminal region to be above 10$^{15.5}$ eV. We also use the luminosity of the
gamma-rays in the range 0.5 -- 10 GeV to estimate the total energy of
accelerated protons below 5.2 $\times$ 10$^{48}$ erg.
