  Fusion experiments rely heavily on the measurement of the line-integrated
electron density by interferometry for density feed-back control. In recent
years the discharge length has increased dramatically and is continuing to
rise, resulting in environmentally induced phase drifts to become an
increasingly worrisome subject, since they falsify the interferometer's
measurement of the density. Especially in larger Tokamaks the loss of density
control due to uncontrolled changes in the optical path length can have a
disastrous outcome. The control of environmental parameters in large
diagnostic/experimental halls is costly and sometimes infeasible and in some
cases cannot be retro-fitted to an existing machine. In this report we present
a very cheap (ca. 100 EUR), easily retro-fitted, real-time capable phase
compensation scheme for interferometers measuring dispersive media over long
time scales. The method is not limited to fusion, but can be applied to any
continuously measuring interferometer measuring a dispersive medium. It has
been successfully applied to the Wendelstein 7-X density feed-back
interferometer.
