For the purpose of studying the mechanism of lowering ignition voltage in the Gao-type He-Ne laser, three kinds of laser tubes (the traditional type , the Gao-type I and Gao-type II , see Fig. 1 .) have been made and ignition voltage measurements were performed in several gases (He, Ne, He-Ne, Ar, Kr, H2 and N2). The experimental results showed that the ignition voltages for Gao-types are fairly lower than that of the traditioni one, but this effect persists only within a certain range of gas pressure. As the gas pressure inereases to a certain value P0, which depends on the gas kind, the lowering effects vanish. Figure 1 shows schematically the three kinds oflaser tubes, while Figure 2 shows the dependences ofbreakdown voltages on gas pressures. It is seen that the lowering effect tends to zero as gas pressure increases. A theoretical explanation for the experimental results is also given.
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