Phased plan for the implementation of the time-resolving magnetic recoil spectrometer on the National Ignition Facility (NIF)

The time-resolving magnetic recoil spectrometer (MRSt) is a transformative diagnostic that will be used to measure the time-resolved neutron spectrum from an inertial confinement fusion implosion at the National Ignition Facility (NIF). It uses a CD foil on the outside of the hohlraum to convert fusion neutrons to recoil deuterons. An ion-optical system positioned outside the NIF target chamber energy-disperses and focuses forward-scattered deuterons. A pulse-dilation drift tube (PDDT) subsequently dilates, un-skews, and detects the signal. While the foil and ion-optical system have been designed, the PDDT requires more development before it can be implemented. Therefore, a phased plan is presented that first uses the foil and ion-optical systems with detectors that can be implemented immediately—namely CR-39 and hDISC streak cameras. These detectors will allow the MRSt to be commissioned in an intermediate stage and begin collecting data on a reduced timescale, while the PDDT is developed in parallel. A CR-39 detector will be used in phase 1 for the measurement of the time-integrated neutron spectra with excellent energy-resolution, necessary for the energy calibration of the system. Streak cameras will be used in phase 2 for measurement of the time-resolved spectrum with limited spectral coverage, which is sufficient to diagnose the time-resolved ion temperature. Simulations are presented that predict the performance of the streak camera detector, indicating that it will achieve excellent burn history measurements at current yields, and good time-resolved ion-temperature measurements at yields above 3 × 10 17 . The PDDT will be used for optimal efficiency and resolution in phase 3. deuterons


I. INTRODUCTION
At the National Ignition Facility (NIF), 1 inertial confinement fusion (ICF) implosions are routinely diagnosed using neutron spectroscopy. 2 Current detectors at the NIF are used to measure timeintegrated spectra, from which the time-integrated, burn-weighted ion temperature T i , areal density ρR, and yield Yn are inferred. [3][4][5] However, to fully understand the evolution of an implosion, timeresolved measurements of T i (t), ρR(t), and Yn(t) are needed. The time-resolving magnetic recoil spectrometer (MRSt) is a novel diagnostic that is being implemented at the NIF for time-resolved measurements of the neutron spectrum from an inertial confinement fusion (ICF) implosion, from which T i (t), ρR(t), and Yn(t) will be inferred. 6 Key parameters, such as the burn width, burn

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scitation.org/journal/rsi skewness, burn kurtosis, dT i /dt at bang time (BT), and dρR/dt at BT, will provide valuable insight into the dynamics of the ICF implosion and, in particular, the effects of alpha heating, low-mode asymmetry, and mix, as well as proximity to ignition. 7 MRSt has been identified by the National Diagnostic Working Group as one of the key new diagnostics that will transform how ICF implosions are diagnosed. 8 The MRSt concept, shown in Fig. 1, expands on the existing magnetic recoil spectrometer (MRS). 3 A fraction of the fusion neutrons emitted from an ICF implosion interacts with a deuterated foil mounted on the hohlraum 4 mm away from the implosion, generating recoil deuterons. 9 The forward-scattered deuterons are selected by an aperture located 6 m from the implosion. Deuterons with energies between 10.7 and 14.2 MeV are subsequently energy-dispersed across a 40 cm focal plane by an ion-optical system. 10 The MRSt can be configured to account for different expected yields. 7,10 For low yields, a larger conversion foil and wider entrance will be used to increase the detection efficiency at the expense of time and energy resolution. For high yields, a smaller foil and narrower entrance will be used to improve time and energy resolution at the expense of efficiency. The three configurations that will be used are summarized in Table I. The back-end component of the MRSt system is the detector. The final version of the MRSt will use a pulse-dilation drift tube (PDDT) 11 with a CsI cathode positioned at the focal plane of the ion optics. In the CsI cathode, recoil deuterons will be converted to secondary electrons. A time-and space-varying electric field will dilate and un-skew the secondary electron distribution. The signal will be subsequently amplified by a microchannel plate and detected by an array of anodes. 11 With the PDDT, MRSt will obtain novel measurements that enable studies of alpha heating and different failure modes in an ICF implosion with unprecedented precision. 7 While the conversion foil and ion optics have been designed, 9,10 further research and development is needed to implement the PDDT. Therefore, a phased approach is being planned for the FIG. 1. A schematic layout of the MRSt system. The foil attached to the hohlraum near target chamber center (TCC) will convert fusion neutrons to recoil deuterons. Forward-scattered deuterons will then be selected by an entrance aperture 6 m away and dispersed and focused by an ion-optical system described by Berg et al. 10 The detector will consist of a CR-39 detector in phase 1, streak cameras in phase 2, and a pulse-dilation drift tube (PDDT) in phase 3. 11   I. Properties of the three MRSt configurations, as well as their performance metrics. The resolutions given are the FWHMs of the ion-optic response function, and the ion-optic efficiency is the fraction of neutrons from the implosion that generate a recoil deuteron that passes through the aperture. These numbers do not account for the detector, which may impact both resolution and efficiency.

Configuration
Low-eff. implementation of the MRSt system. In phase 1, a CR-39 detector will be used to obtain a time-integrated spectrum to calibrate the ion optics. In phase 2, two streak cameras will be used to obtain timeresolved Yn(t) and T i (t) measurements. In phase 3, the PDDT will be implemented to obtain high-precision Yn(t), T i (t), and ρR(t).

II. PHASE 1: CR-39 FOR TIME-INTEGRATED MEASUREMENTS
In phase 1, the MRSt will be implemented with a CR-39 detector to calibrate the ion optics. CR-39 is a solid-state detector in which charged particles leave tracks of chemical damage. The diameter of these tracks can be increased by etching the CR-39 in NaOH until they are visible under a microscope. 12 CR-39 has excellent spatial resolution and can cover an arbitrarily large collection area. It also has 100% efficiency for detecting recoil deuterons. This makes it ideal for measuring a time-integrated neutron spectrum. In this phase, the MRSt will be conceptually similar to the existing MRS. 3 This phase will be used to characterize and calibrate the ion optics of the MRSt system. One way to do this is using americium-241 and radium-226 alpha sources. These sources produce alpha particles at energies between 4 and 8 MeV, which can be transported through the ion optics to establish the relationship between particle energy and position at the focal plane. This is much the same as the procedure used to calibrate the charged particle spectrometer at OMEGA. 13

III. PHASE 2: STREAK CAMERAS FOR TIME-RESOLVED MEASUREMENTS
In phase 2, the CR-39 detector will be replaced with two streak cameras. A streak camera is a time-and space-resolving detector that can measure various kinds of radiation. 14,15 The radiation is selected by a narrow slit and then converted to secondary electrons in a cathode. The electrons are subsequently accelerated and focused through an electron-optic streak tube onto a CCD camera, which records them. The time resolution is provided by the electron-optics, which sweep the electron beam in the direction perpendicular to the slit.
Because streak cameras are routinely used in various diagnostics at the NIF, 14 this detector option can be implemented immediately upon the completion of phase 1. This makes streak cameras an excellent temporary detector to provide time-resolved measurements of T i while the PDDT is being developed.

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The hDISC streak camera will be used for the MRSt. 16 It uses a CsI cathode, which is usually used to detect x rays, but will respond similarly to recoil deuterons. 17 The cameras will be positioned with their slits along the focal plane, where the streak cameras' spatial resolution along the slit provides energy resolution of the deuteron spectrum.
Work is ongoing to identify the slit sizes, slit positions, and focal plane tilt angle that optimize the efficiency and resolution of the phase-2 MRSt system. For now, a point design has been identified to characterize the capabilities of the streak camera detector.
The point design utilizes two slits with 26 mm long and 0.4 mm wide active areas. The detection efficiency of each streak camera is limited by the area of the slit relative to the spatial extent of the recoil deuterons incident onto the focal plane, which varies with energy, as illustrated in Fig. 2.
With one slit positioned over the point of vertical focus at 12.0 MeV and the other over the location of the primary peak at 12.5 MeV, the achieved efficiency is 0.12 times the ion-optic efficiency (see Table I). To compensate for this reduction, the highefficiency configuration will be used. The reduction in detection efficiency means that phase-2 MRSt will not diagnose ρR. The ρR must be inferred from the downscattered portion of the spectrum, which produces too few deuterons over too large an area to be measured with appreciable statistics through a streak camera slit.
Synthetic data analysis was used to validate that the phase-2 MRSt can obtain sufficiently precise information about Yn(t) and T i (t), similar to the work described by Kunimune et al. 7 A synthetic neutron spectrum based on the Yn(t), T i (t), and ρR(t) output from a HYDRA simulation was used in this analysis. 18 The primary peak was assumed to be Gaussian, with a width dependent on T i alone. 2 The spectrum was scaled up or down to account for different yield levels. Future analysis will also vary the spectral shape based on different scenarios. To model the response of the foil and ion-opics, Monte Carlo simulations were performed in which recoil deuterons are generated at random locations in the foil, and subsequently allowed to propagate through the ion optics according to a COSY INFINITY model of the MRSt. 19 To model the response of the streak cameras, the image of the slit on the CCD is approximated as a true-size projection moving at a constant speed, assumed to be 5.6 mm/ns. This

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A flat noisy background of 100 ± 10 counts per 25 μm square is added on top of the streak signal. This is a conservative estimate based on the number of fusion neutrons that will collide with the detector during a high-yield shot. This number can be reduced by accounting for shielding.
The Monte Carlo simulations are used to produce synthetic streak images, like the one shown in Fig. 3(a), as well as timecorrected deuteron histograms in time and energy, like the one shown in Fig. 3(b). These histograms are analyzed using a forward-fit technique where perfect knowledge of the instrument response is assumed, and T i (t) and Yn(t) are inferred, as shown in Fig. 3(c). From those curves, key parameters, such as the burn width and dT i /dt at BT, are extracted. By repeating this analysis for many different random seeds and many different yields, the accuracy with which the phase-2 MRSt will measure those parameters is estimated.
As this study neglects important sources of systematic error, such as uncertainty in the instrument response function, it provides a lower-bound on the actual error that can be expected. Analysis of the magnitude of these systematic errors is a work in progress.
The results of this study are shown in Fig. 4 and Table II and compared to the top-level physics requirements for MRSt. With the MRSt in the high-efficiency configuration, the phase-2 system will meet the top-level physics requirements for burn width, burn skewness, and burn kurtosis at current yield levels. It will also help to  Table II, and black lines show their standard deviations. Where the points appear higher than the dashed line, as in the burnwidth plot, it means that the measurement is systematically overestimated (this is due to the nature of the initial guess of the fitting algorithm, which biases the result when the yield is low).

TABLE II.
Measurement capabilities of the phase-2 MRSt in the high-efficiency configuration. Each given value is the error of the measurement at one yield. The error is defined as the root-mean-square of the difference between the synthetic inferences and the true value (essentially the standard deviation of the data points in Fig. 4). The required accuracies are given on the right for reference. constrain the T i and dT i /dt at bang time with modest error bars. When yields are routinely above 3 × 10 17 , the phase-2 MRSt will meet the T i and dT i /dt top-level physics requirements needed to probe the effects of alpha heating, low-mode asymmetry, and mix in ICF implosions at current yields.

IV. PHASE 3: PULSE-DILATION DRIFT TUBE
In the phase-3 implementation of MRSt, a PDDT will be incorporated for optimal resolution and efficiency. The efficiency of the PDDT for detecting recoil deuterons is nearly 100%, 17,20 and its time resolution is 20 ps. 11 Since the phase-3 MRSt will be used to measure the full deuteron spectrum from 10.7 to 14.2 MeV, it will also provide the first time-resolved measurements of ρR(t).
Synthetic data analysis was performed based on the phase-3 MRSt. 7 These data are generated with the medium-efficiency configuration, as the higher efficiency of the PDDT allows us to use

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scitation.org/journal/rsi finer resolution while still obtaining reasonable statistics. The analysis is shown in Fig. 5 and Table III, and it indicates that the phase-3 MRSt system will meet the top-level physics requirements for measurements of burn width, burn skewness, burn kurtosis, T i (t), and ρR(t) at current yields. The PDDT has been conceptually designed and tested, 11,17,[20][21][22] and can be developed in parallel with the phase-1 and phase-2 MRSt operation. Once it is completed, it will replace the streak cameras and serve as the permanent detector for the MRSt.

V. CONCLUSION
The MRSt is a transformative diagnostic tool that will provide the first ever time-resolved measurement of the neutron spectrum generated from an ICF implosion. A phased approach is described for implementation of the system on NIF. In phase 1, a CR-39 detector will be used for measurement of a time-integrated spectrum, which will be used to calibrate the ion optics. In phase 2, two streak cameras will be positioned next to each other to obtain time-resolved measurements of Yn(t) and T i (t). Synthetic data have been generated and analyzed at a variety of yields to predict the performance of the phase-2 system. Finally, in phase 3, a PDDT will be implemented to provide excellent resolution and detection efficiency while also enabling time-resolved measurements of ρR(t). At each phase, the MRSt will advance our understanding of the dynamics that govern an ICF implosion.