Process for producing polycrystalline silicon

A process for producing polycrystalline silicon, comprising:

• introducing a reaction gas comprising a silicon-containing component and hydrogen into a reactor containing at least one heated filament rod onto which polycrystalline silicon is deposited, wherein the reactor has an interior surrounded by a reactor wall, and at least one sightglass tube having an internal diameter D secured to an orifice in the reactor wall, the sightglass tube having a reactor side end open to the reactor interior, and having an opposing sightglass end containing a sightglass having an inner surface facing the reactor interior and an outer surface facing away from the reactor interior, wherein the sightglass tube has a first plurality of holes arranged in one or more rows in a wall of the sightglass tube proximate the sightglass, the first plurality of holes being holes in the wall of the sightglass tube which run parallel to the inner surface of the sightglass and which are separated from the inner surface of the sightglass by an axial distance S1_n between the sightglass inner surface and a first row of holes of the first plurality of holes, a further plurality of holes located in the sightglass tube wall between the reactor interior and the first plurality of holes, the further plurality of holes being at an angle to a bore axis of the sightglass tube away from the inner surface of the sightglass and toward the reactor interior, wherein a ratio D/S1_n between the sightglass tube internal diameter D and the axial distance S1_n is greater than 1 and less than 40, introducing a first purge gas stream M1 through the first plurality of holes in a direction parallel to the inner surface of the sightglass, and introducing a further purge gas stream M2 through the further plurality of holes in the direction of the reactor interior, the purge gas streams M1 and M2 supplied during the period over which deposits can be deposited on the sightglass.