A capsule assembly for an ultra-high pressure furnace comprises a containment tube defining a central longitudinal axis, a chamber suitable for accommodating a reaction assembly, a proximate end heater assembly and a distal end heater assembly, and a side heater assembly. When assembled, the chamber and the side heater assembly are contained within the containment tube and arranged longitudinallybetween the proximate and distal end heater assemblies. Each end heater assembly comprises a respective conduction volume forming a respective electrical path through the end heat assembly. The side heater assembly electrically connects the respective conducting volumes to each other, and heat is produced in the chamber in response to an electric current flowing through the side heater assembly and the conducting volumes. At least the proximate end heater assembly comprises a first insulation component including an outer insulation volume. The conducting volume of at least the proximate end heater assembly includes an inner conducting volume, and the inner conducting volume is laterally spaced apart from the containment tube by the outer insulation volume.