  High sensitivity observations of Halpha polarization of 30 flares of
different sizes and disk positions are reported. Both filter and spectrographic
techniques have been used. The ZIMPOL system eliminates spurious polarizations
due to seeing and flat-field effects. We didn't find any clear linear
polarization signature above our sensitivity level which was usually better
than 0.1%. The observations include an X17.1 flare with gamma-ray lines
reported by the RHESSI satellite. These results cast serious doubts on previous
claims of linear polarization at the one percent level and more, attributed to
impact polarization. The absence of linear polarization limits the anisotropy
of energetic protons in the H$\alpha$ emitting region. The likely causes are
isotropization by collisions with neutrals in the chromosphere and defocusing
by the converging magnetic field.
