Ion kinetic effects in the overtaking and coalescence of plasma shocks

Abstract The overtaking and coalescence of plasma shocks are important scientific topics in inertial confinement fusion design. We present the comparative study of the impact of ion kinetic effects on the overtaking and coalescence of two shocks in deuterium (D) plasmas of various densities with both hybrid fluid-particle-in-cell simulations and hydrodynamic simulations. It is found that the energetic ions escaping from 2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mrow> <mml:mi>nd</mml:mi> </mml:mrow> </mml:mrow> </mml:mrow> </mml:math> shock can prevent the compression of the fuel layer, by accelerating and heating upstream plasmas. In the strong-collision case ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">D</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mi>th</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>30</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:mi>mg</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>cc</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> ), the coalescent density is reduced by about 8% due to the decreased shock Mach number resulting from energetic ion deposition in the upstream region. In the weak-collision case ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">D</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mi>th</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ), the conversion efficiency of ion kinetic energy to internal energy is inhibited due to the increased ion mean free path, and the coalescent pressure is reduced by about 23%. In addition, the fast ions with a velocity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>v</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> c can drift further with an approximately constant velocity. This mechanism offers a potential explanation for persistently overestimated fuel compression in design simulations of layered implosions. These results also provide a new dataset valuable for benchmarking radiation hydrodynamic models and improving understanding of shock physics.