Experimental study on plasma characteristics in the extraction region of a high-power RF negative ion source based on electrostatic probe

Abstract The plasma characteristics of the extraction region in high-power RF negative ion source have a significant impact on the production and extraction of negative hydrogen ions. This study utilized electrostatic probe to investigate the effect of RF power, source pressure, magnetic filter field and bias voltage on the plasma parameters of the extraction region (without cesium), and also studied the variation of plasma parameters with position and time. The results indicate that as RF power increases, the plasma density in the extraction region significantly rises, but it also leads to an increase in the electron temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) of the extraction region (which increases the loss of negative hydrogen ions); increasing the source pressure can effectively increase the electron density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>N</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) in the extraction region and reduce <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> as expected; increasing the magnetic filter field can effectively reduce the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> in the extraction region, but after the plasma grid current exceeds 1900 A, the change in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> is not significant; although increasement of the bias voltage can effectively suppress the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>N</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> in the extraction region, it causes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> to rise; moreover, the distribution of plasma parameters in the extraction region is relatively uniform; in addition, the variation of plasma parameters with time is not obvious. These findings not only deepen the understanding of the physical mechanisms of plasma behavior in the extraction region of negative ion sources, but also provide important theoretical and experimental bases for the optimization of high-power RF negative ion sources.