Compensation of phase drifts caused by ambient humidity, temperature and pressure changes for continuously operating interferometers

TL;DR

This paper presents a cost-effective, real-time phase compensation method for interferometers affected by environmental changes, improving measurement accuracy in fusion experiments and other applications.

Contribution

A novel, inexpensive, and easily retro-fitted phase compensation scheme that corrects environmental phase drifts in continuous interferometry measurements.

Findings

Successfully applied to Wendelstein 7-X density interferometer

Cost around 100 EUR for implementation

Effective in real-time correction of phase drifts

Abstract

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.