Measurement of stopping power of 240 MeV argon ions in partially ionized helium discharge plasma

An energy loss of 240 MeV argon ions in a <e1>Z</e1>-pinch helium plasma has been for the first time observed throughout the entire pinching process. Standard Stark broadening analysis gives an electron density ranging from 4 to 6 × 10¹⁷ cm⁻³ during the pinch. To deduce stopping power from the energy loss, the target thickness of the helium plasma has been evaluated assuming the mean charge of helium based on thermal equilibrium. The observed electron density and the mean charge of helium give a target thickness of 30 ± 3 μg cm⁻² from 1 μs to 1.8 μs after the discharge ignition. The measured stopping power exceeds a tabulated value for cold helium gas by a factor of 2 to 3 around the time of the first pinch. The experimental stopping power is compared with theoretical values calculated using an equation of stopping power for a partially ionized plasma.