  We describe the direct measurement of the expulsion of a magnetic field from
a plasma driven by heat flow. Using a laser to heat a column of gas within an
applied magnetic field, we isolate Nernst advection and show how it changes the
field over a nanosecond timescale. Reconstruction of the magnetic field map
from proton radiographs demonstrates that the field is advected by heat flow in
advance of the plasma expansion. This changes the dynamics of high energy
density plasmas, in which heat flows and fields are strongly coupled, and may
disrupt magnetised inertial confinement fusion schemes.
