Progress on next generation gamma-ray Cherenkov detectors for the National Ignition Facility

Abstract

Fusion reaction history and ablator areal density measurements for Inertial Confinement Fusion experiments at the National Ignition Facility are currently conducted using the Gamma Reaction History diagnostic (GRH_6m). Future Gas Cherenkov Detectors (GCDs) will ultimately provide ∼100x more sensitivity, reduce the effective temporal response from ∼100 to ∼10 ps, and lower the energy threshold from 2.9 to 1.8 MeV, relative to GRH_6m. The first phase toward next generation GCDs consisted of inserting the existing coaxial GCD-3 detector into a reentrant well which puts it within 4 m of the implosion. Reaction history and ablator gamma measurement results from this Phase I are discussed here. These results demonstrate viability for the follow-on Phases of (II) the use of a revolutionary new pulse-dilation photomultiplier tube to improve the effective measurement bandwidth by >10x relative to current PMT technology; and (III) the design of a NIF-specific “Super” GCD which will be informed by the assessment of the radiation background environment within the well described here.

An effort to enhance the threshold contrast index of gas Cherenkov detectors

“…Once E above and E below are selected, the threshold contrast index ζ characterizes the ability of given GCDs to exclude below-threshold interference. According to experimental results reported in the literature [8,9,14], the threshold contrast index of GCDs is about 1000-3500. This means that when the energy of incident electrons is equal to or greater than E above , the Cherenkov signal contributes most of the above-threshold signal S(E above ) while the other signal may be neglected.…”

“…Those GCDs have also been successfully applied for diagnosing bang time, reaction history and ablator areal density measurements in the OMEGA laser-based inertial confinement fusion facility (ICF) and the national ignition facility (NIF). However, the measurement results are not yet completely satisfactory because there appear to be some unexpected tailing phenomena on the falling edge of the waveforms [12][13][14][15]. The primary cause is that the actual performance of the energy threshold response of current GCDs is not ideal so that the interference from secondary gamma rays in GCDs is not cleanly excluded.…”

“…Because the energy of fusion gamma rays is higher than that of most secondary gamma rays (less than 12 MeV generally), it is a very natural choice to exclude interference from secondary gamma rays if considering the energy threshold response characteristic of the Cherenkov mechanism. Therefore, gas Cherenkov detectors (GCDs) with 12 MeV threshold have been designed for detecting fusion gamma rays, and some super GCDs with higher Cherenkov sensitivity or lower Cherenkov threshold energy have also been widely studied and developed in the past few decades [4][5][6][7][8][9][10][11][12][13][14][15]. Those GCDs have also been successfully applied for diagnosing bang time, reaction history and ablator areal density measurements in the OMEGA laser-based inertial confinement fusion facility (ICF) and the national ignition facility (NIF).…”

Nuclear diagnostics for Inertial Confinement Fusion (ICF) plasmas

“…As higher-order moments in the nuclear burn history contain important information about shock reverberations and burn truncation (due to various implosion failure modes), a faster gamma cherenkov detector (GCD) shown in figure 23 was implemented on both OMEGA-60 and the NIF by LANL and collaborators [159]. The GCD is based on the same principle as GRH but with a different geometry and light collection scheme.…”