Mid and Upper Plane Mirror Steering Mechanisms for MAST-Upgrade
UK
Mega Amp Spherical Tokamak (MAST) – Upgrade at UKAEA’s Culham Campus is an experimental spherical tokamak, recently upgraded to enable higher performance than its predecessor, including longer pulses, increased heating power and a stronger magnetic field, as well as featuring an innovative new plasma exhaust system. MAST-Upgrade is undergoing several enhancements to increase further its capabilities, including a microwave heating and current drive system in the form of Electron Bernstein Wave (EBW) power.
IDOM has developed the Mid and Upper Plane Steering Mechanisms, which very accurately reflect the 1MW microwave beam into the plasma. The mechanisms need to operate with an accuracy of 0.1ᵒ in a vacuum environment and up to 120ºC operational temperature. The mechanism is conceived as one prismatic and two orthogonally oriented rotatory stages nested together to achieve the desired three degrees of freedom mechanism. The transmission of the two rotational degrees of freedom, tip and tilt respectively, is provided by a pulley system composed of three pulleys, two idler and one fixed, and a single cable.
The two idler pulleys rotate around the first rotatory stage while the fixed pulley is attached at the second stage axis to the mirror mounting plate. The cable is clamped to the fixed pulley and threaded through the two idler pulleys to two push-pull motors, one at each end of the cable, in order to be actively actuated in the two angles.
The system works as a differential mechanism. In the mid plane mechanism, the whole tip-tilt pulley system is mounted on a main linear stage that can provide translation movement in the radial direction of the tokamak and an additional rotational degree of freedom.
