  The screening length in a quark-gluon plasma, the dispersion relations of
thermal gluon self-energy and the quark potential at high temperature are
studied within thermo field dynamics framework. By calculation of the real and
imaginary parts, of the gluon self-energy in one-loop order in thermo field
dynamics, we obtain an expression for the screening length in a quark-gluon
plasma and the dispersion relation between the real and imaginary parts. At
high temperature, using photon exchange between electron-positron in a skeleton
expansion and ladder approximation, the screened Coulomb potential is obtained,
and using one-gluon and two-gluon exchange between a quark-antiquark, we get an
expression for the screened quark potential up to $O(g amplitudes of generic
process taking place in a many body system in equilibrium at temperature T. The
relationship of the scattering and decay amplitudes as calculated in Thermo
Field Dynamics to the conventional techniques is established. It is shown that
in many cases the calculations are relatively easy in TFD.
