Plasma Kinetics Issues During the Excitation Phase in an Elemental Copper Vapour Laser: Influence of the "Phantom Current" on the Formation of Laser Output

Experimental studies of the excitation phase in an elemental copper vapour laser immediately before the establishment of lasing action at 510.6nm and 578.2nm have shown that the discharge current can reach 60% of the peak value before any measurable excitation of the discharge plasma is observed [1]. This “phantom current” is observed typically 50-70ns before the appearance of spontaneous emission from Cu I states, and changes in atomic population densities in Cu I, as measured by spatially and time resolved hook spectroscopy. It has been proposed that the phantom current coincides with the acceleration phase of free-electrons remaining from the previous excitation pulse whose energies remain below the threshold for inelastic collisions [1]. However, a number of issues relating to the phenomenon remain unclear. For example, it is not known whether there is any significant energy deposition into the plasma during this period and whether this affects subsequent lasing action and overall efficiency of the laser. To provide further insight, a detailed computer model [2] has been used to simulate the plasma kinetics and lasing behaviour during the excitation phase of the discharge. The calculations have been carried out over multiple excitation/afterglow cycles to yield fully self-consistent results and accurately reproduce the pre-pulse plasma conditions. Results from the model will be compared with experimental data for I-V characteristics, radially and time resolved hook population densities for selected Cu I states, electron densities, and laser pulse intensities. Results from the model indicate that the phantom current can indeed be attributed to the acceleration and drift of the pre-pulse electrons which occurs as the electron temperature is raised to ~2-3eV corresponding to the threshold energies required for excitation of Cu states. The phantom current also coincides with a local minimum of the plasma resistivity which occurs as a result of the complex interplay between the electron temperature and the overall electron-heavy particle collision frequency. Under conditions where the phantom current is less important (ie. reduced pre­pulse electron density), the model suggests that laser pulse energies should increase, although this effect appears to be unrelated to power deposition issues during the period of the phantom current.