A Cost-Efficient Approach to Payload Thermal Control on the Imaging X-ray Polarimetry Explorer (IXPE)

In its first year of on-orbit operations, the Imaging X-ray Polarimetry Explorer (IXPE) has made numerous polarimetric observations of astronomical X-ray sources, providing a new dimension of data with which to understand our universe. IXPE measures the linear polarization angle of each incoming X-ray photon in an imaged scene, allowing study of both point and extended source phenomenon. Each of IXPE's three X-ray telescopes consists of a grazing incidence angle X-ray mirror module and an imaging gas pixel detector at a 4-meter focal length. During the design phase, thermal modeling revealed that simple thermostatic heater control would be insufficient to achieve low gradients across the concentric shells of IXPE's mirror modules and the structural elements that ensure telescope alignment. Although traditional instrument electronics with proportional-integral-derivative (PID) controllers were considered, the project selected an alternative solution to achieve the requisite temperature control while avoiding the cost and complexity of additional payload electronics. IXPE implemented a proportional heater controller with a pulse-width-modulated (PWM) output using the spacecraft bus flight software and standard power switches. Pre-flight demonstrations during the observatory thermal vacuum test exhibited excellent payload thermal performance but created challenges for spacecraft bus voltage regulation. These challenges were overcome through frequency separation between the charge control algorithm and the heater controller. This paper examines the benefits, challenges, and lessons learned for this implementation, as well as provides recommendations for future implementations of this cost saving strategy.