Engineering architecture of the neutron Time-of-Flight (nToF) diagnostic suite at the National Ignition Facility

Abstract

This paper describes the engineering architecture and function of the neutron Time-of-Flight (nToF) diagnostic suite installed on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). These instruments provide key measures of neutron yield, ion temperature, drift velocity, neutron bang-time, and neutron downscatter ratio.Currently, there are five nToFs on three collimated lines-of-site (LOS) from 18m to 27m from Target Chamber Center, and three positioned 4.5m from TCC, within the NIF Target Chamber but outside the vacuum and confinement boundary by use of re-entrant wells on three other LOS.NIF nToFs measure the time history and equivalent energy spectrum of reaction generated neutrons from a NIF experiment. Neutrons are transduced to electrical signals, which are then carried either by coaxial or Mach-Zehnder transmission systems that feed divider assemblies and fiducially timed digitizing oscilloscopes outside the NIF Target Bay (TB) radiation shield wall.One method of transduction employs a two-stage process wherein a neutron is converted to scintillation photons in hydrogen doped plastic (20x40mm) or bibenzyl crystals (280x1050mm), which are subsequently converted to an electrical signal via a photomultiplier tube or a photo-diode.An alternative approach uses a single-stage conversion of neutrons-to-electrons by use of a thin (0.25 to 2 mm) Chemical Vapor Deposition Diamond (CVDD) disc (2 to 24mm radius) under high voltage bias. In comparison to the scintillator method, CVDDs have fast rise and decay times (<ns), have very low residual tails, are insensitive to shot gammas, and are less sensitive to the neutron signal of interest.

Readout of transient signal based on optical phase modulation

“…The readout electronics such as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) uses the intensity modulation technology to modulate the detector signal to the laser, then transfer the analog signal of photomultiplier tube (PMT) through the 162 meter optical fiber [4]. The Neutron Time of Flight (nToF) diagnostic system on the National Ignition Facility (NIF) in the United States uses the optical intensity modulation technology to transmit the detector signal to digital devices dozens of meters away [5]. The measurement accuracy of optical intensity modulated is not high, because both the internal modulation scheme and the external modulation scheme are susceptible to environmental noise [6].…”

Implementation of the foil-on-hohlraum technique for the magnetic recoil spectrometer for time-resolved neutron measurements at the National Ignition Facility

“…Time-integrated diagnostics, such as neutron time-offlight (nTOF) spectrometers 1,2 and the Magnetic Recoil Spectrometer (MRS), [3][4][5] have routinely measured the yield, downscatter ratio (DSR), and ion temperature (T ion ) in deuteriumtritium (DT) implosions at the National Ignition Facility (NIF). 6 These diagnostics are crucial to the facility; however, in order to assess how the fuel assembly and nuclear burn evolve during the implosion, time-resolved neutron measurements are the key.…”

Signal and background considerations for the MRSt on the National Ignition Facility (NIF)

“…The current suite of neutron diagnostics on the National Ignition Facility (NIF) 1 has been used extensively for measurements of the time-integrated neutron spectrum, from which burn-averaged values of areal density (ρR), yield (Y n ), and apparent ion temperature (T i ) in an Inertial Confinement Fusion (ICF) have been determined. [2][3][4][5][6][7][8][9] Although these diagnostics have been essential for guiding the ignition experiments toward the regime of burning plasmas, it has become clear that new, transformational neutron diagnostics are required to provide detailed information about the time evolution of the fuel assembly, hot core ion temperature, nuclear burn, and alpha-particle heating. As discussed by Frenje et al, 10 this information can be obtained simultaneously with the next-generation magnetic recoil spectrometer (MRS), called MRSt for time-resolved measurements of the ICF neutron spectrum.…”