REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D624 (2014)

AXIS: An instrument for imaging Compton radiographs using the Advanced Radiography Capability on the NIFa) G. N. Hall,b) N. Izumi, R. Tommasini, A. C. Carpenter, N. E. Palmer, R. Zacharias, B. Felker, J. P. Holder, F. V. Allen, P. M. Bell, D. Bradley, R. Montesanti, and O. L. Landen Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA

(Presented 4 June 2014; received 3 June 2014; accepted 27 July 2014; published online 27 August 2014) Compton radiography is an important diagnostic for Inertial Confinement Fusion (ICF), as it provides a means to measure the density and asymmetries of the DT fuel in an ICF capsule near the time of peak compression. The AXIS instrument (ARC (Advanced Radiography Capability) X-ray Imaging System) is a gated detector in development for the National Ignition Facility (NIF), and will initially be capable of recording two Compton radiographs during a single NIF shot. The principal reason for the development of AXIS is the requirement for significantly improved detection quantum efficiency (DQE) at high x-ray energies. AXIS will be the detector for Compton radiography driven by the ARC laser, which will be used to produce Bremsstrahlung X-ray backlighter sources over the range of 50 keV–200 keV for this purpose. It is expected that AXIS will be capable of recording these highenergy x-rays with a DQE several times greater than other X-ray cameras at NIF, as well as providing a much larger field of view of the imploded capsule. AXIS will therefore provide an image with larger signal-to-noise that will allow the density and distribution of the compressed DT fuel to be measured with significantly greater accuracy as ICF experiments are tuned for ignition. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892558] I. INTRODUCTION

Radiography of the compressed fuel of an Inertial Confinement Fusion (ICF) capsule requires photon energies in the range 50–200 keV, where opacity is dominated by Compton scattering.1, 2 In the near future, Compton radiography at the National Ignition Facility (NIF) will use the ARC (Advanced Radiography Capability) laser,3 which will ultimately provide eight 1 kJ, 30 ps beams at 1ω to drive multiple, independent X-ray backlighter sources to image experiments driven by the main NIF laser. ARC will drive two 50–200 keV Bremsstrahlung X-ray sources for Compton radiography, and ARC X-ray Imaging System (AXIS) will record the images with a detection quantum efficiency (DQE) several times greater than other framing cameras at the NIF. The design of AXIS and strategies to mitigate noise and to increase the DQE of the detector at high photon energies will be discussed. II. COMPTON RADIOGRAPHY REQUIREMENTS

AXIS is designed to meet the scientific requirements of the Compton radiography platform. This demands two pointprojection radiographs be obtained on a single experiment, allowing study of the dynamics around the time when the fuel is densest, and guaranteeing that a radiograph is always obtained when there is uncertainty in the timing of peak compression. At peak compression, the fuel is 100–150 μm in diameter. For a) Contributed paper, published as part of the Proceedings of the 20th

Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected].

0034-6748/2014/85(11)/11D624/3/$30.00

there to be suitable regions in the radiograph to perform background subtraction (i.e., regions where opacity is zero) and to allow for alignment uncertainties, a 300 μm square field of view of the experiment is required, as shown in Fig. 1(a). The goal is to measure the fuel ρr to an accuracy of 5% for experiments with neutron yields up to 4 × 1014 , requiring a signal-to-noise ratio (S/N) of ≥20. To achieve this, the neutron flux onto the detector must be reduced by placing the detector head as far from the capsule as possible. The maximum distance is limited by the positioning of the backlighter targets: to avoid being hit by unconverted light from the main laser, the two backlighter targets must be mounted directly on the hohlraum, only 6 mm from the capsule center. Pointprojection geometry produces a large magnification of the 300 μm field of view, and the maximum image size (40 × 40 mm) that can be recorded on a single microchannel plate (MCP) occurs when the detector is placed 760 mm from the capsule (the need for gating with MCPs is discussed in Sec. III). In order to position the detector 760 mm from the capsule and capture two 40 × 40 mm radiographs, AXIS is a DIM (diagnostic instrument manipulator) mounted framing camera with two heads, as shown in Fig. 1(b). The resolution of the system is limited by the size of the backlighters, which are a pair of gold wires each illuminated by two ARC beams. Initially, 20 μm diameter wires will be used, limiting the resolution to 20 μm at the capsule. However, the eventual goal is to use 10 μm wires, and so the angular separation of the heads is limited to 5.1◦ to guarantee equivalence of the two radiographs within the resolution limits of the diagnostic, as shown in Fig. 1(c). Half a 10 μm resolution element is magnified to 640 μm, and so the detector need only achieve ∼600 μm of resolution to ensure no

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Hohlraum

(a)

Bac

klig

hte

Background Subtraction region

r ph

Fuel

300µm

(b)

100-150µm

NIF Laser produces LPI X-rays In hohlraum

ARC pulse (30ps) Capsule coasts ~0.6ns

Neutron Time of Flight 14.7ns

oto

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AXIS turns off < 1ns

AXIS turns on in 300ps

50µ µm

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6°

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AXIS head

40.0mm

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32.0

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Head separation 5.1°

FIG. 1. To obtain 2 radiographs, 760 mm standoff and 300 μm field of view (a), AXIS will be a double head detector (b). The head separation is less than the angle subtended by half a 10 μm resolution element on the surface of the compressed fuel, giving the radiographs an equivalent line of sight (c).

degradation. This resolution is obtainable, since it is much greater than the range over which photoelectrons are produced in the MCP system. Simultaneously, there must be no X-ray cross-talk between the two heads. This is achieved by placing a gold collimator around the backlighter entry window as shown in Fig. 2. Backlighter rays pass through the hohlraum windows to their respective AXIS heads, but rays from backlighter 2 emitted in the direction of AXIS head 1 (and vice versa) are blocked by the collimator. The hohlraum, its casing, and additional filters around the exit window are also made of gold, reducing the expected X-ray cross talk to ≈5%. III. NOISE MITIGATION

There are several sources of potential background noise: ∼100 keV X-rays are produced by laser-plasma instabilities To AXIS head 2

Hohlraum

To AXIS head 1

Backlighter rays Hohlraum

300x300µm FOV

0.5mm 0.3mm Collimator window µm Au) (200µ Backlighter 1

Rays from backlighter 2 cannot reach head 1

Backlighter 2

FIG. 2. Slice through a model of the radiography geometry. A collimator window prevents cross-talk between the radiographs. Backlighter rays (red) pass through the windows to their respective AXIS heads, but rays from backlighter 2 emitted towards AXIS head 1 (blue) are blocked, and vice versa.

FIG. 3. Large, time varying sources of background noise can be mitigated with a gated detector. AXIS must turn on after the LPI signal has ended, and off before the neutrons arrive.

(LPI) which occur in gas-filled hohlraums while the main NIF laser is firing. Additionally, neutrons can produce photoelectron events in the detector components, and materials in the NIF chamber can emit neutron-induced gamma rays. These are large, time varying sources of background that can be dramatically reduced by gating the detector. Therefore, AXIS will be gated using MCPs, although it should be noted that the short duration of the ARC pulse (30 ps) provides all the temporal resolution for Compton radiography. Fig. 3 shows how the background can be minimized by turning AXIS on as the capsule is coasting, after the main LPI pulse has ceased, then turning AXIS off after electrons generated by the ARC pulse exit the MCP, before neutrons hit the detector. AXIS will have a turn on/off time of ≈300 ps and 100 keV. DQE takes into account the amplification of noise during the detection process and is defined by DQE = QE/(1+σ 2 /m2 ), where QE is the quantum efficiency, σ is the variance in the final signal, and m is the mean signal.4 DQE increases when more of the MCP volume acts as a photocathode, and for AXIS two approaches to this problem will be considered: First, a single, thick MCP can be used. The overall gain of the system must be ≈1000 in order to produce sufficient signal, but for a single MCP, the fraction of the plate that acts as a photocathode is ∼1/ln(G). This is small for large G (photocathode fraction ≈14% for G ≈ 1000), and so the plate thickness L must be increased to maximize the photocathode volume ∼L/ln(G). A different approach is a variation on the chevron MCP configuration (see Refs. 7, 9, and 10). It is suggested11 that a low-gain and high-gain MCP are stacked in series. In this dual-MCP arrangement, the front MCP operates close to unity gain as a volumetric photocathode (1/ln(G) is large). This produces high DQE, but insufficient gain, and so electrons emerging from the low gain MCP are accelerated across a gap onto the rear MCP which operates at high gain to amplify the signal. However, a trade-off exists between DQE and noise, as the length of time that voltage must be applied to extract the full electron signal tends to be longer for a higher DQE system, resulting in the integration of more background noise. This is illustrated in Fig. 4 which shows the fraction of the ARC electron signal that has emerged from the high gain plate vs. time for two dual plate systems. Photoelectrons are produced throughout the entire volume by the high energy ARC

d

MCP 1: Low gain

ARC photons

c b

Fraction of ARC electron signal leaving gain MCP

1

a

MCP thickness 1: 460µ µm 1: 800µm 2: 460µm 2: 460µm d

d

MCP 2: High gain

Position of initial photoelectron b

AXIS is a framing camera designed specifically for NIF Compton radiography with ARC. It will record two radiographs with an equivalent line of sight, a 300 μm square field of view at the capsule, and 128 × magnification. AXIS will achieve ≥20 signal to noise for neutron yields up to 4 × 1014 , allowing fuel ρr to be measured with an accuracy of 5%. This is achieved by minimizing noise onto the detector through a combination of shielding and time gating, and optimizing the MCP-based detector to achieve high DQE over the range 50–200 keV. ACKNOWLEDGMENTS

Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the (U.S.) Department of Energy (DOE), National Nuclear Security Administration under Contract No. DE-AC52-07NA27344 (LLNL-JRNL-555712). Tommasini et al., Rev. Sci. Instrum. 79, 10E901 (2008). Tommasini et al., Phys. Plasmas 18, 056309 (2011). 3 J. K. Crane et al., J. Phys.: Conf. Ser. 244, 032003 (2010). 4 M. Rabbani et al., J. Opt. Soc. Am. A 4, 895 (1987). 5 J. Veaux et al., Rev. Sci. Instrum. 62, 1562 (1991). 6 J.-L. Miquel et al., Rev. Sci. Instrum. 63, 5097 (1992). 7 J. L. Wiza, Nucl. Instrum. Methods 162, 587 (1979). 8 N. Izumi et al., Rev. Sci. Instrum. 81, 10E515 (2010). 9 W. B. Colson et al., Rev. Sci. Instrum. 44, 1694 (1973). 10 J. E. Bateman, Nucl. Instrum. Methods 144, 537 (1977). 11 N. Izumi et al., “Development of a dual MCP framing camera for high energy x-rays,” Rev. Sci. Instrum. (these proceedings). 2 R.

a 300 380

V. CONCLUSIONS

1 R.

c

0 0

X-rays. When voltage is first applied, the signal produced by photoelectron events in the high gain plate (between surfaces a and b) are the first to emerge, followed by a plateau which represents the time taken for the first electrons from the low gain plate to cross the gap from c to b. Finally, the majority of the electron signal, generated in the low-gain plate, emerges. Testing of an arrangement using an 800 μm or 460 μm thick low gain plate coupled to a 460 μm thick high gain plate (all with 10 μm pore diameter, 12 μm pore spacing) and operated at G≈1000 results in a DQE of 4.3% and 3.2%, respectively11 (at 59 keV), with electron transit times predicted to be ≈800 ps and ≈680 ps. A single 800 μm or 460 μm thick MCP operated at G≈1000 (10 μm pore diameter, 12 μm pore spacing), reduces the transit time to ≈520 ps and ≈300 ps, respectively, but testing at 59 keV shows that DQE falls to 2.6% and 1.4%.11 Further development of the dual-MCP system is ongoing, with the goal of achieving a mean DQE of >5% over the range 50–200 keV. Additional work is required to optimize DQE with respect to electron transit time, and to characterize the effect on DQE of the detection of hard x-rays in both plates of the dual-MCP configuration. Upcoming experiments will also characterize spatial resolution. Some degradation is expected due to the transverse velocity of electrons between the plates,7 but Monte Carlo simulations have shown that the largest point-spread function of any configuration being considered (100 V across a 400 μm gap) has a FWHM