Oscillation phenomena in argon laser discharges

Abstract

In this thesis instabilities which occur in the plasma tube of an argon ion laser are examined. Coherent oscillations in the discharge potential across the plasma tube are found to be a usual feature of an argon laser. The light output of the laser is shown to be modulated by these oscillations only if the discharge current is modulated. It is found that these oscillations are due to instabilities in the anode zone of the gas discharge. The oscillations occurring within the anode zone of an argon laser have been studied using optical and Langmiuir probe techniques. The oscillations are discussed in terms of a phenomenological model due to Pupp. Many of the observed features of the oscillations can be described in terms of this model. The values of the anode fall that have been observed in the argon laser are substantially less than would be expected on the basis of earlier work on glow discharges in argon. this is discussed and an estimate of the anode fall is made which is closer to the observed value than that predicted by Von Engel's theory of the anode fall. A simple mathematical model of the anode zone is presented and it is shown that under the conditions appropriate to an argon laser the anode fall is not expected to be stable. These instabilities in the anode zone of the plasma tube, while they appear to be a normal feature of the argon laser discharge, do not necessarily modulate the laser light. It is shown that if the power supply for the laser has a high output impedance at the frequency of the oscillations then the modulation of the light may be reduced to a negligible level.