Hot microplasma in the channel of a solid target induced by a sequence of femtosecond laser pulses

The dynamics of the hard X-ray yield is studied as a function of the laser-shot number, and the maximum temperature of the hot electron plasma component in the channel formed by a sequence of tightly focused near-IR femtosecond laser pulses (τ = 110 fs, λ = 1.24 μm, and I = 10¹⁵–10¹⁶ W/cm²) in air at various pressures (P = 0.01–760.00 Torr) is analyzed. The dependence of the depth of the channel in which the hard X rays are generated with the highest efficiency on the air pressure and the laser focusing is obtained. The electron concentration in air plasma in the laser beam waist is estimated using the spectral shift of the second harmonic radiation that is back reflected from the target channel.