Determination of atomic hydrogen density and fluorescence decay time by 1D resolved, picosecond TALIF in the RAID linear device

Abstract We report on the first 1D resolved two-photon absorption laser-induced fluorescence (TALIF) measurements of atomic hydrogen (H) performed in the Resonant Antenna Ion Device using a picosecond laser system. The results are obtained across a cylindrical plasma column with peak electron density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>18</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mtext>m</mml:mtext> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mtext>eV</mml:mtext> </mml:mrow> </mml:math> on axis, which decrease monotonically to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>⋅</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>17</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mtext>m</mml:mtext> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and 0.75 eV over a scale of a few cm away from the axis. TALIF results in these conditions are compatible with a uniform H density profile across the observed region, with a mean dissociation degree close to 3%. The small variation of H density across regions covering a wide range of plasma parameters suggests that transport processes and wall interactions play an important role in determining the H density profile. The fluorescence decay time is close to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>10</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mtext>ns</mml:mtext> </mml:mrow> </mml:math> at all observed locations, a result compatible with complete substate mixing of the n = 3 states of H.