Measurement of population inversions and gain in carbon fiber plasmas

A CO/sub 2/ laser (approx.0.5 kJ energy, 70 nsec pulse width) was focussed onto the end of an axially oriented, thick (35 to 350 ..mu..) carbon fiber with or without a magnetic field present along the laser-fiber axis. We present evidence for axial-to-transverse enhancement of the CVI 182A (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 approx. =3 (k approx. =6 cm/sup -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