A heater system comprising: a heater bundle , the heater bundle comprising: a plurality of heater assemblies , more than one of the heater assemblies comprising a plurality of heater units , more than one of the heater units defining at least one independently controlled heating zone ; and a plurality of power conductors electrically connected to the at least one independently controlled heating zone in each of the heater units ; means for determining a temperature in the independently controlled heating zones ; and a power supply device , characterized in that : the plurality of heater units are interchangeable and aligned along a length of the more than one of the heater assemblies ; and the power supply device includes a controller configured to automatically modulate power to the independently controlled heating zones through the power conductors based on the temperature in the independently controlled heating zones such that the independently controlled heating zones are each controlled at a desired power level and such that a desired heat distribution is provided along the length of more than one of the heater assemblies . a heater bundle , the heater bundle comprising: a plurality of heater assemblies , more than one of the heater assemblies comprising a plurality of heater units , more than one of the heater units defining at least one independently controlled heating zone ; and a plurality of power conductors electrically connected to the at least one independently controlled heating zone in each of the heater units ; a plurality of heater assemblies , more than one of the heater assemblies comprising a plurality of heater units , more than one of the heater units defining at least one independently controlled heating zone ; and a plurality of power conductors electrically connected to the at least one independently controlled heating zone in each of the heater units ; means for determining a temperature in the independently controlled heating zones ; and a power supply device , characterized in that : the plurality of heater units are interchangeable and aligned along a length of the more than one of the heater assemblies ; and the power supply device includes a controller configured to automatically modulate power to the independently controlled heating zones through the power conductors based on the temperature in the independently controlled heating zones such that the independently controlled heating zones are each controlled at a desired power level and such that a desired heat distribution is provided along the length of more than one of the heater assemblies . the plurality of heater units are interchangeable and aligned along a length of the more than one of the heater assemblies ; and the power supply device includes a controller configured to automatically modulate power to the independently controlled heating zones through the power conductors based on the temperature in the independently controlled heating zones such that the independently controlled heating zones are each controlled at a desired power level and such that a desired heat distribution is provided along the length of more than one of the heater assemblies . <EOS>