Experimental study of hot electron generation in shock ignition relevant high-intensity regime with large scale hot plasmas

Abstract

In the shock ignition (SI) laser fusion scheme, hot electrons generated by the laser spike pulse can either preheat the fuel or strengthen the ignition shock, depending on the hot electron characteristics. We conducted a planar target experiment on the OMEGA-EP laser facility and characterized the temperature and total energy of hot electrons generated from a kilojoule-class 100-ps infrared (IR) or a 1-ns ultraviolet (UV) laser interacting with a large (Ln∼330−450 μm) and hot (Te∼1−2 keV) coronal plasma at the SI-relevant intensities (∼1016 W/cm2). The IR laser converts ∼2.5% energy into hot electrons with Thot∼ 60–90 keV, while the UV laser couples 0.8% ± 0.7% energy into Thot=27 ± 9 keV hot electrons. The IR-produced hot electrons yield five times higher Cu Kα emission than the UV case, confirming the higher electron conversion efficiency with the IR laser. The low energy conversion from the UV laser to hot electrons may be due to the refraction of the off-normal incident laser in the large coronal plasma. These findings are the first comparisons of hot electron generation between the IR and UV pulses at kilojoule scales in SI-relevant large-scale plasmas. The findings may expand the SI design space to include IR lasers as the possible spike lasers.