Electrodeless fluorescent lamp with bifilar coil and faraday shield

Abstract

An electrodeless fluorescent lamp and fixture is disclosed which operates at radio frequencies and contains a bifilar coil to reduce RF voltage between the plasma and the coil and a metallic cylinder (10) to remove heat from a said bifilar coil. The bifilar coil consists of two windings. The primary (induction) winding (6) is used to generate RF electrical azimuthal field in the bulb volume needed to maintain the inductively-coupled RF plasma. The second (bifilar) winding (18) has essentially the same number of turns and is wound on the inductive winding (6), but in the direction opposite to that of the primary (inductive) winding. The RF current flowing in the inductive winding (6) induces an RF voltage of the opposite polarity in the bifilar winding (18), so two adjacent turns of both windings have equal (or nearly equal) but of opposite sign RF potentials with respect to the plasma. This results in the mutual 'cancellation' of capacitive RF electric fields induced by both windings in the plasma and in a sheath formed between the plasma and the cavity walls. The reduction of the electric field in turn results in the lowering of a direct current voltage across the sheath thereby lowering the energy of ions which are accelerated in this sheath coating. The lowering of ion energy reduces the damage caused by ions and leads to improved maintenance and a longer life lamp.