NOVELTY - The heater (100) has an inner tube member with two regions, an outer surface region and an inner surface region, and two heating elements placed respectively within the two regions. A cylindrical structure is provided by the inner tube member, the elements and the thickness of insulating material to form an incompressible sandwiched structure, where the cylindrical structure is free from voids and gaps. An inner portion of the cylindrical structure and an outer portion of the cylindrical structure are electrically isolated from the elements.    USE - Heater for use in an inner region of a high pressure apparatus for processing materials in supercritical fluids at high pressure and high temperature to grow crystals of gallium nitride, aluminum nitride, indium nitride, indium gallium nitride, aluminum gallium nitride, and aluminum indium gallium nitride for manufacturing of bulk or patterned substrate to manufacture an optical or electronic device (all claimed). Can also be used for laser, LED, photo detector, solar cell and transistor.    ADVANTAGE - The end portions of the heater comprise a relatively higher winding density relative to the middle portions of the heater, so that the controlling of the power density allows compensation of a higher heat loss rate at the ends relative to the region between the ends, thus ensuring that the temperature distribution is uniform over the length of the heater, and hence improving the performance of the high pressure apparatus for crystal growth in an effective manner. The heater maintains the temperature in a relatively uniform manner within the axially-spaced hot zones with a smooth transition in the temperature between adjacent zones, so that the pitch is selected to prevent, minimize, or eliminate wall nucleation during the high pressure crystal growth process.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) an apparatus for processing materials    (2) a method for forming crystalline material.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a heating device.    Heater (100)    Grooves (101, 103)    Tube (105)