A furnace system for suppressing process gas flow instabilities during drawing of an optical preform in the production of an optical fiber, the furnace system comprising: a muffle defining a furnace cavity ; an upper muffle extension positioned above the furnace cavity ; a downfeed handle for supporting the optical fiber preform as it is drawn into the furnace cavity ; an upper heater thermally coupled to the upper muffle extension ; a process gas supply positioned in the upper muffle extension ; and a lower heater coupled to the muffle and configured to create a hot zone within the furnace cavity , which hot zone has a temperature of about 1700 °C or greater, for example of from about 1800 °C to about 2000 °C, for example of about 1700 °C, 1800 °C, 1900 °C, or about 2000 °C, or any range having any two of these values as endpoints, in which hot zone viscosity of the optical fiber preform is decreased, wherein the upper heater is configured to heat a furnace system area above the hot zone , to reduce the temperature difference between the furnace system area above the hot zone and the hot zone, wherein the downfeed handle is positioned within the upper muffle extension such that a gap is defined between an outer surface of the downfeed handle and an inner surface of the upper muffle extension , wherein the downfeed handle further comprises a boule , wherein the boule is configured to support the optical fiber preform and is drawable with the optical fiber perform into the furnace cavity and through the upper muffle extension , wherein an exterior surface of the boule is flush with the outer surface of the downfeed handle - , whereby the gap remains constant as the boule is drawn through the upper muffle extension into the furnace cavity . a muffle defining a furnace cavity ; an upper muffle extension positioned above the furnace cavity ; a downfeed handle for supporting the optical fiber preform as it is drawn into the furnace cavity ; an upper heater thermally coupled to the upper muffle extension ; a process gas supply positioned in the upper muffle extension ; and a lower heater coupled to the muffle and configured to create a hot zone within the furnace cavity , which hot zone has a temperature of about 1700 °C or greater, for example of from about 1800 °C to about 2000 °C, for example of about 1700 °C, 1800 °C, 1900 °C, or about 2000 °C, or any range having any two of these values as endpoints, in which hot zone viscosity of the optical fiber preform is decreased, wherein the upper heater is configured to heat a furnace system area above the hot zone , to reduce the temperature difference between the furnace system area above the hot zone and the hot zone, wherein the downfeed handle is positioned within the upper muffle extension such that a gap is defined between an outer surface of the downfeed handle and an inner surface of the upper muffle extension , wherein the downfeed handle further comprises a boule , wherein the boule is configured to support the optical fiber preform and is drawable with the optical fiber perform into the furnace cavity and through the upper muffle extension , wherein an exterior surface of the boule is flush with the outer surface of the downfeed handle - , whereby the gap remains constant as the boule is drawn through the upper muffle extension into the furnace cavity . <EOS>