  We report the polarized optical light curve of a gamma-ray burst afterglow
obtained using the RoboPol instrument. Observations began 655 seconds after the
initial burst of gamma-rays from GRB131030A, and continued uninterrupted for 2
hours. The afterglow displayed a low, constant fractional linear polarization
of $p = (2.1 \pm 1.6)\,\%$ throughout, which is similar to the interstellar
polarization measured on nearby stars. The optical brightness decay is
consistent with a forward-shock propagating in a medium of constant density,
and the low polarization fraction indicates a disordered magnetic field in the
shock front. This supports the idea that the magnetic field is amplified by
plasma instabilities on the shock front. These plasma instabilities produce
strong magnetic fields with random directions on scales much smaller than the
total observable region of the shock, and the resulting randomly-oriented
polarization vectors sum to produce a low net polarization over the total
observable region of the shock.
