Temporal and spatial evolutions of plasma produced in a nanosecond laser field characterized by langmuir probe diagnostics

Abstract

Based on the temporal and spatial evolutions of the plasma plume, the behaviour of the laser-produced plasma is studied using a wire Langmuir probe. The plasma is generated by focusing a Nd:YAG laser pulse of 30 ns pulse duration, ∼110 mJ energy, and 1064 nm wavelength) on a solid metal (steel-316) target. By conducting experiments at low pressures (1 × 10 3 -5 × 10 −5 mbar), the following results were obtained: (a) At a lower pressure, the rise time of the electron signals is reduced, but the current amplitude is increased for both ions and electrons. (b) Fitting the shock wave model of propagation to the experimental results of the plasma plume expansion within the range 1-7 mm distance from the target shows better agreement at lower pressures. (c) For both electrons and ions, the velocity of the particles exhibited plume splitting (two regimes of velocities, namely fast and slow), which is interpreted as being dominant due to the ambipolar field. In addition, at the centimetre-scale distance from the target and using the time-of-flight method, the electron and ion velocities were determined to be ∼3 × 10 7 and ∼1.2 × 10 6 cm/s, respectively. The electron temperature is in the range 2-29 eV, and the ion density is in the range 10 11 -10 13 cm −3 at milimeter to centimetre distances from the target. KEYWORDS ambipolar field, laser-produced plasma, Longmuir probe, plume splitting, time of flight INTRODUCTIONThe interactions of a nanosecond-pulse laser beam with a solid target generate non-equilibrium and highly transient plasmas.Such plasmas belong to the category of laser-produced plasmas (LPPs), which generally follow photointeractions, ablation, and collisional processes, and lead to plasma plume expansion and thermal equilibrium. The temporal and spatial plume expansion mechanism includes different processes such as shock wave formation, deceleration, diffusion, and clustering. [1,2] Characteristic parameters of the plasma plume, such as temperature and density, show strong spatial and temporal dependences as well as dependence on the laser parameters. Spatio-temporal plume characterization of the plasma requires a variety of diagnostic methods. It is worth mentioning that the investigation of plasma development can be tackled from various angles. Applying different diagnostic techniques not only provides complementary analyses but also paves the way for utilizing the potentiality of various diagnostic systems to their full capacity under different conditions. An electrical probe such as Langmuir probe offers convenience of use and accessibility, and is considered a useful tool for measuring the plasma plume expansion characteristics in highly ionized systems. Nevertheless, interpretation of data using this technique is facilitated by assuming that the probe is located in positions where the probe does not disturb the plasma particle