Method of making a glass fiber preform with adjusting a spacing while increasing acceleration of a starting glass powder

A method of manufacturing glass fiber preforms, and in particular optical fiber optical fiber preforms, the method comprising:

• rotating a cylindrical primary preform about its axis;
• moving a plasma torch in translation relative to the preform in a direction parallel to the axis of the preform, the axis of the plasma torch flame not intersecting the axis of the preform and being offset by a certain distance from the axis of the preform;
• and
• inserting glass powder by gravity into the plasma flame from outside the flame;
• the method comprising:
• increasing acceleration of the glass powder inserted under gravity before it penetrates into the plasma flame by means of an accelerator gas inserted into the flow of glass powder advancing under gravity;
• and
• adjusting the offset distance between the axis of the plasma flame and the axis of the preform so that the offset distance between the axes is decreased as the acceleration of the powder is increased.