what is the ac rack chiller heat exchanger thermal control

These are the THERMAL UTILITIES that keep the tool and its subsystems at stable temperature. The AC RACK is the electrical power/cooling infrastructure for the RF/electronics. The CHILLER circulates temperature-controlled coolant. The HEAT EXCHANGER transfers heat between the coolant and the process/equipment. WHY THEY MATTER: thermal stability of the electronics and the chamber drive process stability — if the chiller drifts or a heat exchanger degrades, you get THERMAL DRIFT, which shows up as process/film drift (Lesson 60). The key line: 'The chiller and heat exchanger are the tool's thermostat — when thermal control drifts, the process drifts with it, so I include them in a drift diagnosis.' Your round-2 history asked these (AC Rack, Chiller, Heat Exchanger) — know they're part of thermal stability, not just named components.