A heater that may include an outer housing and an inner tube is provided. The inner tube is in a coaxial relation to and within the outer housing. An inward facing surface of the inner tube defines a volume sufficient to receive a reaction capsule, and the outward facing surface is radially spaced from an inward facing surface of the outer housing sufficient to define a gap. A filler material is disposed within the gap. The filler material responds to pressure such that the filler volume is reduced by less than 5 volume percent at greater than 500 MPa pressure and at greater than 500° C. temperature. One or more heating elements are disposed in the gap. The heating elements are in thermal communication with the inner tube.