Abstract:
The 53.73 meter long Duke free electron
laser (FEL) cavity consists of two concave
mirrors with radius of curvature longer than
27 meters. A proper radius of curvature is
designed to achieve an optimal and stable
operation of the FEL. This requires accurate
measurements of the cavity mirror's radius of
curvature before its initial installation.
Subsequent radius of curvature measurements
are performed to ensure no significant
deformation of the mirror occurs after a
period of extensive use. A direct measurement
based upon the geometric optics principles
has been used at DFELL for years. Recently,
we have significantly upgraded this
measurement apparatus by utilizing a HeNe
laser as the light source and a straight wire
with a proper size as the object. In this
paper we describe the details of the
measurement setup and report the benefits of
the recent upgrades. In addition, we report
the improved data analysis technique and
results of recent long radius of curvature
measurements.