  Pulsar wind nebulae are formed when outflows of relativistic electrons and
positrons hit the surrounding supernova remnant or interstellar medium at a
shock front. The Vela pulsar wind nebula is powered by a young pulsar
(B0833-45, age 11 kyr) and located inside an extended structure called Vela X,
itself inside the supernova remnant. Previous X-ray observations revealed two
prominent arcs, bisected by a jet and counter jet. Radio maps have shown high
linear polarization of 60 per cent in the outer regions of the nebula. Here we
report X-ray observation of the inner part of the nebula, where polarization
can exceed 60 per cent at the leading edge, which approaches the theoretical
limit of what can be produced by synchrotron emission. We infer that, in
contrast with the case of the supernova remnant, the electrons in the pulsar
wind nebula are accelerated with little or no turbulence in a highly uniform
magnetic field.
