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Description
This study quantifies the cooldown time, post-lock thermal stabilization, and steady-state operating temperature of the Einstein Telescope cryogenic test mass subject to up to 0.5W of mirror heating. The thermal conductance of the small flexures dominates the behavior. Using flexures made of isotopically pure 28Si significantly reduces cooldown time, operating temperature under optical load, and thermal noise. This design exploits dissipation dilution as the pendulum mode stores most oscillation energy gravitationally, and elastic energy stored in the flexures is very small, keeping the pendulum thermal noise low. The heat is extracted from the upper mass of the mirror suspensions using deeply annealed, 6N aluminum, solid-wire heat links which are also used to feed in suspension control voltages.