KBS: The Kirkpatrick-Báez Focussing System

The KB focusing system of the SCS instrument comprises of two focusing and one deflecting mirror. They have Si substrates and are coated with 50 nm stripes of B₄C to have desired optical properties.

The beam enters through a baffle BFL to define the beam at the entrance of the KBS chamber. The beam is then deflected by three mirrors: a horizontally focussing mirror HFM, a vertical deflection mirror VDM and a vertically focussing mirror VFM.

The focussing mirrors are mounted in benders via clamping arrangements at either end to form a U shape. Arms are attached for actuators to apply moments independently to either end of the mirror for elliptical bending. Coarse motions are principally stepper motors while piezo motors are used for precision motion. Renishaw ‘resolute’ absolute encoders are used throughout. A capacitive sensor is used to monitor the position of the center of the mirror substrate under the bend load.

The vertical deflection mirror is simply a mirror flat, hanging upside-down to orient the beam vertically.

The mirrors are cooled with a Indium/Gallium eutectic bath placed in a slot cut into the surface of the mirror substrate. A nickel-plated OFHC copper cooling blade is immersed into the bath to serve as a heatsink for the substrate. The cooling blade is supported from the mirror mechanism and is not directly attached to the mirror mount plate. Thermoelectric coolers are used to maintain the temperature of the cooling blade at a constant 22°C. The coolers are backed by small heatsinks onto which OFHC copper braids are fit to sink the coolers back to a water-cooled blade. The water-cooled blade is connected directly to the vacuum vessel and is thereby entirely decoupled from the optics support to avoid vibrations. Water is fed to each cooling circuit from outside the vacuum vessel by hoses. The temperature of the optical substrates will be measured by thermocouples mounted on the copper cooling blade in the In-Ga bath.

Functional specifications

FMB supplied functional specification of the entire KB mirror system and a special one for the Optic Mounts.