Coherent light generators – Particular pumping means – Electrical
Patent
1990-12-04
1993-06-15
Epps, Georgia Y.
Coherent light generators
Particular pumping means
Electrical
H01S 3038
Patent
active
052205754
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an electrode having the features recited in the prior art part of claim 1.
Gas discharge chambers employed in pulsed gas lasers, such as CO.sub.2 lasers or Excimer lasers, contain two mutually opposite electrodes, which are parallel to the optical axis and between which the electric discharges take place by which the gas enclosed in the gas discharge chamber is excited to emit the laser radiation. In pulsed operation, typical gas lasers may produce pulse powers of a few megawatts and more, the pulse width is typically of an order to 50 ns and the electrode surface area amounts to some 10 cm.sup.2. The highest pulse powers are achieved with Excimer lasers. The gas filling of typical Excimer lasers contains 5 to 10% of an active noble gas, such as krapton, 0.1 to 0.5% of a halogen gas, particularly fluorine or chlorine, and a lightweight buffer gas, such as helium or neon, under a total pressure from 1.5.times.10.sup.5 to 4.times.10.sup.5 N/m.sup.2.
Various requirements are to be met by the electrode in gas discharge chambers of gas lasers. The basic requirement resides in that the electrodes must be particularly suitable for an emission of electrons which permit a diffuse glow discharge, which should spread as uniformly as possible over the electrode surface which is available. In known gas lasers having a gas atmosphere which contains electronegative components, such as oxygen in CO.sub.2 and CO lasers or fluorine and chlorine in Excimer lasers, it is often observed that the gas discharge, in most cases during its late phase, suddenly changes from a glow discharge to an arc discharge. This is due to the fact that electrons made available by the preionizing means become attached to the electronegative atoms or molecules and are then no longer available for the discharge process. If the cathode cannot supply a sufficient number of make-up electrons the discharge will suddenly change. Such arc discharges have numerous disadvantages. In the first place, arc discharges do not contribute to the laser process so that the pulse rate will highly be reduced. Besides, they will increase the electrode erosion. This will result in a roughening of the surface of the electrode and in a change of its profile. On the other hand, a considerable part of the electrode material which has been removed will deposit on the windows of the discharge chamber. The cleaning of the windows cannot be performed in enclosed lasers and is highly expensive in nonenclosed lasers.
Because individual arc discharges cannot entirely be prevented, the tendency of the electrode material to sputter must be as low as possible. Besides, the material is required to have a high electrical conductivity in order to ensure that the voltage pulse which is applied will spread as quickly as possible throughout the surface of the electrode.
Another requirement to be met by the electrode material is that the thermal conductivity should be as high as possible so that an occurrence of hot spots on the electrode at high pulse repetition rates will be prevented. Such hot spots may consist of the roots of arc discharges and may burn in. Finally, chemical reactions on the electrode surface play a significant role. A strong local heating of the gas may result in dissociations and chemical transformations in the gas and may change its properties. For instance in a CO.sub.2 laser which in its gas discharge chamber contains a gas consisting of CO.sub.2 and smaller proportions of nitrogen and helium under a pressure of 1.0.times.10.sup.5 to 1.5.times.10.sup.5 N/m.sup.2 there may be a dissociation of CO.sub.2 with formation of an extremely reactive oxygen, which may react with the material of the electrode or of the chamber walls or may react with the nitrogen contained in the gas filling so that nitrogen oxides are formed, which are electronegative and consume electrodes from the discharge. For this reason the electrodes in a CO.sub.2 laser must be as passive as possible relative to the gases filling the gas discharge chamber so that neither the
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Bosch Wolfgang
Bregel Thomas
Ganz Joachim
Michal Roland
Prein Franz
Doduco GmbH + Dr. Eugen Durrwachter
Eltro Gaselischaft fur Strahlungstechnik
Epps Georgia Y.
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