Shielding effects in the laser-generated copper plasma under reduced pressures of He atmosphere

- We report experimental investigations of shielding effect in a laser-induced copper plasma.
- In the irradiance range applied, the ablated mass dependence is non-linear.
- The mass of the sample was determined directly by means of the mass comparator after ablation under various pressures of helium.
- The found threshold irradiance for pronounced plasma shielding is about 2 GW/cm2.
- The assumption of plasma absorption and temperature increase was confirmed using both frontal and side-on spectroscopic observation of Cu I lines.

Irradiation of samples was performed with 6 ns, 1064 nm Nd:YAG laser. For an applied irradiance range (108-1010 W/cm2), the ablation process exhibits non-linear dependance. Ablated mass of the sample was directly determined using 100 ng resolution mass comparator after ablation under various pressures of helium. The ablation rates were dictated by plasma formation mechanisms as well as ambient conditions. However, the surrounding atmosphere did not significantly affect the value of threshold irradiance of about 2 ×109W/cm2 for the onset of ablation mechanism change. This value is additionally verified via spectroscopic information from Cu I lines in the range from 0.4 to 1 μs after the laser pulse. Behaviour of spectral lines was monitored with respect to the laser pulse energy. Plasma diagnostics of axial electron density and excitation temperature distributions was performed under He pressure of 200 Torr. An influence of the possible shielding mechanisms responsible for the plasma absorption is discussed.