Effects of a static inhomogeneous magnetic field acting on a laser-produced carbon plasma plume

M Favre,E Wyndham,S Reyes,L S Caballero Bendixsen,H M Ruiz,H Bhuyan,F Veloso

doi:10.1063/1.4995527

Abstract

We present time- and space-resolved observations of the dynamics of a laser-produced carbon plasma, propagating in a sub-Tesla inhomogeneous magnetic field, with both, axial and radial field gradients. An Nd:YAG laser pulse, 340 mJ, 3.5 ns, at 1.06 μm, with a fluence of 7 J/cm2, is used to generate the plasma from a solid graphite target, in vacuum. The magnetic field is produced using two coaxial sets of two NeFeB ring magnets, parallel to the laser target surface. The diagnostics include plasma imaging with 50 ns time resolution, spatially resolved optical emission spectroscopy and Faraday cup. Based on our observations, evidence of radial and axial plasma confinement due to magnetic field gradients is presented. Formation of C2 molecules, previously observed in the presence of a low pressure neutral gas background, and enhanced on-axis ion flux, are ascribed to finite Larmor radius effects and reduced radial transport due to the presence of the magnetic field.

Highlights

Despite many investigations on different issues related to laser produced plasmas in the last 50 years, the dynamics and overall properties of freely expanding laser target plasmas generated at characteristic fluences in the 1-100 J/cm[2] range are still the subject of ongoing experimental investigations.[1,2,3,4,5] In this context, the application of either static or transient magnetic fields, parallel or transverse to the forward direction defined by the free expanding laser plasma plume, is known to have a strong effect on plasma dynamics and composition
Several investigations on laser plasma dynamics in sub-Tesla static magnetic fields have been reported, aiming to improve plasma confinement in pulsed laser deposition (PLD) processes,[9,10] to enhance plasma emission in laser induced breakdown spectroscopy (LIBS),[11,12,13] to increase the ion yield and beam current in laser ion sources (LIS),[14] or to get insight in interpreting data related to astrophysical observations,[15,16] among others
In this context we report on a series of experiments aiming to unveil further details of the interaction between the laser produced plasma and a static axial magnetic field

Summary

Introduction

Despite many investigations on different issues related to laser produced plasmas in the last 50 years, the dynamics and overall properties of freely expanding laser target plasmas generated at characteristic fluences in the 1-100 J/cm[2] range are still the subject of ongoing experimental investigations.[1,2,3,4,5] In this context, the application of either static or transient magnetic fields, parallel or transverse to the forward direction defined by the free expanding laser plasma plume, is known to have a strong effect on plasma dynamics and composition. Investigations focused on measurements of plasma parameters and in the identification of magnetic field induced kinetic effects on plasma behavior.[6,7,8] Later, several investigations on laser plasma dynamics in sub-Tesla static magnetic fields have been reported, aiming to improve plasma confinement in pulsed laser deposition (PLD) processes,[9,10] to enhance plasma emission in laser induced breakdown spectroscopy (LIBS),[11,12,13] to increase the ion yield and beam current in laser ion sources (LIS),[14] or to get insight in interpreting data related to astrophysical observations,[15,16] among others. Our results show that in a vacuum background the presence of the magnetic field plays an equivalent role to that of a low pressure neutral background gas, in order to favor the emission of successive plasma fronts and C2 molecules formation, as reported previously in investigations of laser produced carbon plasmas.[4,5,17,18,19,20]

Results

Discussion

Conclusion