High Pressure X-Ray Preionized TEMA-CO2 Laser

The construction of a high-pressure (up to 20 atm) transversely excited CO2 laser using transverse X-ray preionization is described. High pressure operation was found to be greatly improved in comparison to UV-preionized systems. Homogeneous discharges have been achieved in the pressure range 5-20 atm, yielding a specific laser output in the order of 35 J/1. PACS: 42.55E, 42.60B Generation and amplification of short optical pulses in the picosecond range as well as continuous tunability over a complete vibrational band demands an overlap of rotational lines of a molecular laser. In the case of an atmospheric CO2 laser, the bandwidth of a rotational line is only about 4 GHz, and there is no overlap. In that case it is not possible to generate or amplify subnanosecond pulses or to obtain continuous tunability. The gain bandwidth of the CO2 laser can be increased by increasing the pressure of the gain medium. The resulting pressure broadening is about 5 GHz/atm. Although from about 5 atm on, adjacent rotational lines start to overlap, it takes about 15 atm to obtain a continuous modulation-free gain spectrum. An additional advantage of these high operating pressures is that the saturation energy also increases, which offers the opportunity to extract extremely high output powers. Operation of high-pressure COz lasers in the selfsustained discharge regime demands a proper preionization technique to obtain a uniform glow discharge. The best known preionization techniques are UV preionization or electron-beam preionization. Both methods, however, have their own shortcomings. Those of UV preionization are mainly caused by the high absorption rate of UV radiation in (laser) gases. In a typical CO z laser, the effective range for UV preionization is limited to about 0.1 m atm [I]. Using a 300 keV e-beam increases this range only by a factor of two. Furthermore, UV preionization is hampered by an increased dissociation of the CO2 molecules and, when dealing with a spark-source, by gas contamination. This decreases lifetime of sealed-off systems significantly. The main limitation of high pressure e-beam preionized or sustained systems is the window. Because the window has to be transparent for electrons of moderate energy (100-300 keV), thin metal foils are used. On the other hand, these foils have to be sufficiently strong (read thick) to separate the low pressure side (i.e., electron gun room) from the high pressure side (laser chamber). This problem can be circumvented by X-ray preionization. Because of the high penetration depth of X-rays, relatively thick windows can be constructed. Medium-energy X-ray sources have been used successfully in several kinds of X-ray preionized lasers during the last few years [24] . So far, only a few TEMA-CO2 lasers have been described [5,6], with operating pressures up to 10 atm. This paper describes the results obtained with a 20 atm transversely excited COg laser system, using X-ray preionization. The results demonstrate the proper operation at these high pressures. 1. Description of the Apparatus The transverse X-ray preionized TEMA-CO 2 laser system is schematically shown in Fig. 1. The X-ray source consists of a cold-cathode e-beam placed in a stainless steel vacuum chamber. The exit window is a 50 gm thick titanium foil which holds the vacuum (pressure range 1-9 x 10-5 mbar). A second, 10 ~tm tantalium foil close to the cathode serves as 38 R.J.M. Bonnie and W. J. Witteman