Electrodes
To minimize bulb
contamination the electrodes are made of an extremely pure grade of
Tungsten. The anode and cathode are expensive to produce because of
the grade of material used (in the case of the anode pure tungsten)
and complexity of machining which requires expensive ceramic tools.
The cathode electrode requires thoriated tungsten which adds to the
material processing cost, but is easier to machine. The cathode electrode
heads are brazed onto an electrode shaft made of molybdenum for cost
reasons. The anode shaft material must still be tungsten because of
the higher temperatures seen by the anode.
The anode is made
of pure tungsten and is larger because electrons from the cathode or
smaller electrode are bombarding it. The surface temperature at the
face of the anode is extremely high, over 2000 degrees centigrade;
tungsten has a liquidous temperature of approximately 3000 C and a
melting point of approximately 3410 C.
The smaller, pointed
electrode (cathode) is also made of tungsten doped with Thorium,
which provides more free electrons into the arc. The thoriated tungsten
also facilitates easier starting of the bulb.
Electrodes are machined
then ultrasonically cleaned and de-greased removing any machining lubricants.
The anode electrodes
are heated in a vacuum chamber to approximately 2400 degrees centigrade
for several hours to remove contaminants that would out-gas during
lamp operation.
The cathode electrode
is also heated in a vacuum chamber for cleaning and for thoria activation.
The thoria activation is accomplished by elevating the electrode temperature
for a short period of time. This causes the thorium to migrate to the
cathode surface.
Quartz envelope
The short arc envelope
is made on a large special lathe that heats a piece of quartz tube
to a near molten point using either a hydrogen or propane and oxygen
torches. The glass blower blows into the inside of the tube to form
the desired envelope against a carbon paddle/template.
Normal temperature of
the quartz envelope is between 600 and 700 degrees centigrade during
bulb operation Note the molten or working temperature of quartz is
around 1400 degrees centigrade.
The fill tube on the
envelope is located on the anode side of the envelope keeping it out
of the light path. The hole for the fill tube is either blown or drilled
in the envelope by the glass blower and then a smaller piece of tubing
is fused to the envelope using a smaller hand torch.
Lamp assembly
The electrodes are attached
to molybdenum ribbon and a graded glass seal is performed directly
onto the electrode shaft. This is done because quartz cannot be directly
attached to tungsten. Using the hydrogen or propane and oxygen torch,
the quartz is compression scaled to the molybdenum while rotating in
the lathe under a vacuum to prevent oxidation of the molybdenum. This
also allows the molten quartz collapse around the molybdenum providing
a quartz-to-metal seal.
The lamp needs to be
annealed to remove strain patterns from the assembly process. Typically
it is heated to 1150 C and reduced in controlled temperature slope
to approximately 650 C. A pump pulls a vacuum on the bulb during this
operation to remove contaminants.
Xenon and Mercury-Xenon
gas are added in very precise amounts under a vacuum. The bulb is then
pressurized to approximately 4 atmospheres. In order to seal the lamp
it is cooled with liquid nitrogen to solidify the Xenon gas. The solidified
gas allows a torch to seal the bulb and remove it from the fill station
without the gas escaping. When the bulb is removed from the liquid
nitrogen, the solid xenon turns back into a gas, pressurizing the lamp.
End fittings are then
attached using a precision mechanical fixture and optical comparator.
Lamps are subject to
initial burn-in and voltage/current testing. |
