QL demonstration lamp

The present lamp was designed and built for demonstration purposes. It features a fully clear QL 85W discharge vessel so as to make the internal antenna and its inductive discharge plainly visible. The lamp is built with an elongated XL lamp base that contains the 27 MHz electronic driver that feeds the antenna. The latter consists of a low-impedance inductor, which couples power to the discharge, and a secondary capacitive electrode whose purpose is to ensure a reliable discharge ignition. Such design is required as the electric field generated by the primary inductive antenna is not high enough to properly ionize its surrounding low-pressure krypton gas volume. While standard QL lamps are provided with two mercury amalgams, i.e., the auxiliary one which releases mercury quickly after ignition and the main one which stabilizes the mercury pressure during operation, the present lamp has only the main indium-bismuth amalgam, located in one of the vessel's exhaust tubes. The omission of the auxiliary amalgam combined with a low RF power input of about 14–20 W, result is a very slow run-up phase which can be observed easily as the discharge's atmosphere changes from krypton to a krypton-mercury mixture.

The mercury vapor pressure is so low at room temperature that the properties of the inductive plasma are dominated by the krypton buffer in the first minute after ignition. During that phase most of the gaseous mercury is ionized due to the high energy of the electrons and the resulting positive mercury ions are pulled towards the negatively charged glass wall of the vessel. This ionic pumping mechanism causes a clearly visible mercury/krypton segregation in the plasma. Because of the close proximity of the central glass cavity with the discharge, mercury vapor accumulates preferentially there, resulting in a distinctive blue glow sheath surrounded by an off-white voluminous discharge. In that regime the plasma ohmic impedance is low and the lamp's power consumption is 16 W only. As the main amalgam heats up, the mercury vapor pressure increases and the plasma then becomes gradually dominated by that element, resulting in an increasing discharge impedance which causes the total power consumption to rise to 24 W. During that transition the appearance of the discharge becomes homogeneous and acquires mercury’s characteristic greenish blue color as the rising density of that element causes the plasma's electron energy and the metal vapor's ionization degree to drop, effectively quenching the ion pumping mechanism.