Method and an apparatus for heating glass tubes

A method of heating glass tubes, particularly quartz tubes for the manufacture of optical fibres, by preheating the tube, preferably by means of a gas flame, whereafter the tube is heated by means of microwave energy generated by a microwave generator (1), by introducing the tube axially into a microwave cavity (3) comprising in its end walls (6,7) or end surfaces openings for the tube, characterized in that two electrical fields according to different modes are caused to be formed in said cavity, in that a first of said electrical fields is caused to be formed by a first microwave generator (1) and the second of said electrical fields by a second microwave generator (15) in that the first electrical field is given a field image according to a TM-0n0-mode, preferably according to the TM-020-mode, which mode has a maxima (10) along the central axis (9;c--c) of the cavity and a minima (11,12) on each side of a radially spaced from the central axis (9;c--c), where the electrical field portion located between said minima (11,12) is utilized for heating the wall of said glass tube (8), and in that the second electrical field is given a field image according to a TE-01n-mode, preferably according to the mode TE-011, which field therewith is simultaneous with the superposed upon the field according to the TM-0n0-mode.