Rohr aus synthetischem Quarzglas für die Herstellung einer Vorform, Verfahren für seine Herstellung in einem Vertikalziehverfahren und Verwendung des Rohres

摘要

Known synthetic quartz glass tubes for the production of a preform have an inner bore with a surface layer produced without using tools in the molten state and an inner zone. The aim of the invention is to provide a tube which does not release any OH groups to the surroundings. For this purpose, the surface layer ( 30 ) has a thickness of 10 mum and an average OH content of not more than 5 ppm by weight and an average surface roughness R_a of not more than 0.1 mum. The inner zone ( 34 ) that starts on the surface layer ( 30 ) and terminates 10 mum before the outer wall has an average OH content of not more than 0.2 ppm by weight. A simple and inexpensive method for producing a quartz tube of the above type is to continuously draw a tube strand from a softened quartz glass mass in a vertical drawing process. A scavenging gas is circulated through the inner bore of the tube, said gas having a water content of less than 100 ppb per weight. The front end of the tube strand ( 19 ) is closed by a flow obstacle ( 26 ) that is permeable the scavenging gas and that reduces the amount of scavenging gas ( 23 ) flowing through.