Fig 5. On-axis Gain Curves for one-meter-long tubes of different diameters. These data were taken by measuring the power gain of another low-power laser beam at the center frequency of the P18 transition, 10.57 microns. The laser beam was 5 mm in diameter and was projected down the tube axis. The effective pumping speed was 15 cfm and the gas inlet and outlet port diameters were 3/8 in.