Ion-temperature measurements of direct-drive implosions using a neutron time-of-flight detector at the SG-II upgrade facility

摘要

For inertial confinement fusion (ICF) studies, a hotspot's ion temperature is a key parameter for assessing implosion performance. Ion temperature not only characterises the energy generated by the central hotspot but also reflects parameters such as the asymmetry and velocity of the implosion. Herein, we present an ion-temperature diagnostic technique for the ICF implosion hotspot based on the neutron time-of-flight method. We use a neutron time-of-flight spectrometer comprising a large-size (18 cm in diameter; 10 cm in thickness) plastic scintillator and a largecalibre (13.3 cm in diameter) fast photomultiplier tube (PMT). The total system uncertainty of this spectrometer is only 13.6 when the DD neutron yield is 3.1 × 10~7 neutrons per shot and the ion temperature is 1.6 keV. We can use this system to determine the ion temperature when the DD neutron yield is as small as 2.5 × 10~6 neutrons per shot and the ion temperature is as low as 0.8 keV. We describe a time-of-flight spectrum-analysis method based on deconvolution and low-pass filtering, and we propose a method for determining the optimal filter cutoff frequency by finding the trough of the neutron-energy spectrum. Despite the low neutron yields and low ion temperatures of the direct-drive ICF implosion experiments at the Shenguang-II (SG-II) upgrade facility, we have successfully used this diagnostic technique for the first time to obtain the ion temperature of the implosion hotspot.