The effect of laser irradiance on the charge loss in expanding tungsten plasma

• The influence of laser irradiance on the critical distance was investigated.
• The critical distance was found to be in the range of 2.2–2.8 cm for the range laser irradiance 3–10 × 108 W/cm2.
• Change state and intensity of tungsten ions increases with the laser irradiance.
• The increase of critical distance with laser irradiance is attributed to rise of ions charge state in the plasma.

The charge loss in tungsten plasma produced by 1064 nm wavelength Nd:YAG laser was investigated by a compact ion collector. In particular the influence of laser irradiance on the critical distance, beyond which charge state of ions are frozen, was studied. For the range of laser irradiance, 3–10 × 108 W/cm2, we found critical distance in the range of 2.2–2.8 cm. The charge state distribution of tungsten ions in far expansion zone measured with the help of time-of-flight ion energy analyzer showed that maximum attainable change state and intensity of highly charged ions increases with the laser irradiance. The increase of critical distance with laser irradiance is attributed to rise of ions charge state in the plasma. Highly charged ions are more energetic as compared to low charge state ions, consequently they recombine at larger distances from the target.