7+(25<2)23(5$7,21 9HUVD3XOVH3RZHU6XLWH6HULHV6HUYLFH0DQXDO
5(9%
4.5 FLASHLAMP POWER CIRCUITS
Refer to the IGBT Driver PCB schematic page 8-19 in Section 8, and the HVPS & Control Circuits
Simplified Block Diagram in topic 4.4.5.
Flashlamps are used as the exciter (pump) mechanism for the lasers. The flashlamps are a gas discharge
device designed to produce intense pulsed radiation (visible, IR and UV) flashes. Its construction consists of a
sealed, xenon filled, linear quartz tube, with a tungsten based cathode on one end and a pure tungsten anode
on the other end. Tube and electrode materials, as well as size and shape, gas composition and pressure, all
contribute to the overall performance of the flashlamp which will not be discussed here.
When used in conjunction with the HVPS, the flashlamps operate in two modes: simmer and discharge. Both
modes generate heat in the lamp which is carried away via closed loop, water to air heat exchange.
In simmer mode, the flashlamp is controlled by the HVPS to simmer, or to maintain a steady state par-
tial ionization in the flashlamp between flashes. This is accomplished by the Simmer Control, as
described earlier, which maintains a low current (150mA) DC arc between electrodes. It is not unusual
to observe voltage fluctuations as great as 50 Volts with a fixed simmer current over a period of sec-
onds. Arc wander in the lamp bore and rapid changes in the positions of arc atteachment on the elec-
trode tips are the cause of this behavior. This normal behavior is seen as “winking” of the lamp
ionization.
In discharge mode, and since the lamp is conducting (in simmer mode), the plasma field is almost
instantaneously developed as soon as the lamp is triggered (IGBT turns on). This period of time is
characterized by rapidly increasing current through the lamp, usually occupying the first 15 to 20% of
the rise time of the current pulse through the lamp. This interval of rapid arc expansion during which
the energy required to stabilize the plasma in the tube is being dissipated along the tube wall and elec-
trodes. Once the arc is stabilized, the remainder of the pulse duration (80 to 85%) consists of slow and
controlled arc growth, and the stabilized plasma discharge occurs at high current. During discharge,
extreme temperatures are developed on both eletrodes and in the quartz tube.
Lamp lifetime is primarily by four factors:
• Degradation of the electrodes (eletrode wear) which results in deposition of chemical and
metalic inside the tube and vlocks light emission from leaving the tube.
• Aging and destruction of the quartz tube typically due to improper handling (finger oil con-
tamination) and overstressing (overheating) the lamp.
• Contamination of the gas, either by leaking seals or tube cracking or by electrode wear.
• Failure of electrode seals, again usually a result of improper handling, flexing lamp leads,
etc.
To Next Page
Loading...
