Method and apparatus for suppressing flow instabilities in an optical fiber draw system

A furnace system, comprising:

• a muffle defining a furnace cavity;
• a lower heater coupled to the muffle and configured to create a hot zone within the furnace cavity having a temperature of about 1900° C. or greater;
• an upper muffle extension positioned above the muffle and defining a handle cavity;
• a downfeed handle positioned within the handle cavity such that a gap is defined between an outer surface of the downfeed handle and an inner surface of the upper muffle extension, the gap having a length as measured from the outer surface of the downfeed handle to the inner surface of the upper muffle extension of about 4 cm or less;
• an upper heater thermally coupled to the upper muffle extension and configured to heat the gap to a temperature in a range from about 600° C. to about 1800° C.;
• and
• a gas screen positioned in the upper muffle extension and configured to inject a process gas into the handle cavity.