Confinement-driven enhancement of negative ion production in anode spot plasma

Abstract This study presents a novel application of anode spot plasmas to enhance negative ion production within electronegative gas discharges, specifically focusing on negative hydrogen ions ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>H</mml:mtext> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> ions) in argon-hydrogen mixtures. By confining ions within a double layer structure, our experiments demonstrate a significant increase in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>H</mml:mtext> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> ion density within the anode spot plasma, with electronegativity (defined as the density ratio of negative ions to electrons) increasing by up to 20-fold. This enhancement in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>H</mml:mtext> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> ion production extends beyond conventional volume production mechanisms. We propose a model highlighting the role of the anode spot plasma’s double layer potential in ion confinement, introducing an ‘inflow’ process where negative ions from the surrounding plasma are effectively trapped. Our findings underscore the influence of key parameters, such as pressure and anode size, in optimizing negative ion density via tailored confinement. These insights provide a pathway for advancing negative ion source technology, particularly <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>H</mml:mtext> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> ion sources, by refining ion transport and confinement strategies.