EFFECT OF ELECTROSTATIC SCREEN ON THE IGNITION CAPACITIVE AND INDUCTIVE DISCHARGES OF LOW PRESSURE

Abstract

Electrostatic shield effects on low pressure inductive and capacitive discharge ignition in the A-type bulb (IDb = 105 mm, Hb = 118 mm) with a reentry cylindrical cavity (ODcav = 36 mm, Hcav = 65 mm) on its axis were studied. Lamps filled with the mixture of mercury vapor, p = 7,5⋅10–2 Torr, and inert gas (Ar) at pressure of 0,3 Torr, were ignited at a frequency of f = 13,56 MHz with the help of an induction coil with 6, 8, and 11 turns and was positioned in the cavity. It was experimentally found that placing the induction coil inside of the electrostatic shield, at 5 mm below the shield’s edge, prevents forming in the plasmawall sheath dc voltage accelerating plasma ions towards to the cavity walls thereby causing degradation of phosphor coating on the vacuum side of cavity wall’s surface. The use of an electrostatic shield was found to increase capacitive discharge ignition voltage to values higher than those of inductive discharge ignition. Lamp optimization approach based on the separation of the areas, where capacitive and inductive discharges are ignited, was implemented in the design and the construction of a novel inductively-coupled lamp. In this lamp, a capacitive discharge was ignited in the expansion area with diameter of 15 mm located on the top of the lamp exhaust tubing. The ignition voltage of the capacitive discharge in the expansion area was lower than that required for inductively-coupled discharge ignition. The novel inductively-coupled low pressure discharge light source employing the electrostatic shield with 6 or more vertical slots was found to have good electrical and optical characteristics. Light output of this source was 5% lower than that of the inductively-coupled low pressure light source operated without electrostatic shield.