Measurement of population inversions and gain in carbon fiber plasmas

A CO2 laser (∼0.5 kJ energy, 50–80 ns pulse width) was focused onto the end of an axially oriented, thick (35–350 μ) carbon fiber with or without a magnetic field present along the laser-fiber axis. We present evidence for axial-to-transverse enhancement of the C VI 182 Å (n=3–2) transition, which is correlated with the appearance of a population inversion between levels n=3 and 2. For the B=0 kG, zero field case, the maximum gain length product of kl≊3 (k≊6 cm−1) was measured for a carbon fiber coated with a thin layer of aluminum (for additional radiation cooling). The results are interpreted in terms of fast recombination due mostly to thermal conduction from the plasma to the cold fiber core.