REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D613 (2014)

The NIF x-ray spectrometer calibration campaign at Omegaa) F. Pérez,1 G. E. Kemp,1 S. P. Regan,2 M. A. Barrios,1 J. Pino,1 H. Scott,1 S. Ayers,1 H. Chen,1 J. Emig,1 J. D. Colvin,1 M. Bedzyk,2 M. J. Shoup III,2 A. Agliata,2 B. Yaakobi,2 F. J. Marshall,2 R. A. Hamilton,2 J. Jaquez,3 M. Farrell,3 A. Nikroo,3 and K. B. Fournier1,b) 1

Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, California 94551, USA Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA 3 General Atomics, P.O. Box 85608, San Diego, California 92186, USA 2

(Presented 2 June 2014; received 23 May 2014; accepted 12 July 2014; published online 31 July 2014) The calibration campaign of the National Ignition Facility X-ray Spectrometer (NXS) was carried out at the O MEGA laser facility. Spherically symmetric, laser-driven, millimeter-scale x-ray sources of K-shell and L-shell emission from various mid-Z elements were designed for the 2–18 keV energy range of the NXS. The absolute spectral brightness was measured by two calibrated spectrometers. We compare the measured performance of the target design to radiation hydrodynamics simulations. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891054] The National Ignition Facility (NIF)1 X-ray Spectrometer “NXS” will record calibrated, time-resolved x-ray spectra in the 2–18 keV photon energy range. It consists of singly curved, elliptical Bragg-reflection crystals. The energy range is divided in ten partially overlapping windows, each corresponding to one crystal configuration. For each spectral window, three identical NXS assemblies are built, thus a total of 30 assemblies. On NIF, a streak camera2 will provide temporal resolution between 8 and 160 ps for a time window from 1 to 20 ns. The absolute calibration of the NXS was carried out at the O MEGA laser facility,3 replacing NIF’s streak camera with an image plate (time-integrated measurement). Within two shot days, 26 shots were taken, fielding three NXS assemblies on each shot: all 30 assemblies of NXS have acquired spectra on two shots at least. Simultaneously, two absolutely calibrated x-ray spectrometers “XRS” were fielded in order to cross-calibrate each NXS crystal, as shown in Fig. 1 (left panel). All spectra were recorded on FUJI SR-type image plates.4 Note that the shot-to-shot variation was of the order of a percent: two shots per assembly give sufficient calibration data. The performance of the targets used for the NXS calibration is the main focus of this paper. To generate the x-ray spectra in this calibration campaign, a target design has been carefully developed. As the instruments are fielded around the target in different directions, isotropic x-ray emission was required. This was accomplished with spherical targets, uniformly irradiated by the O MEGA lasers. The calculated irradiation is ∼1015 W/cm2 and its non-uniformity of ±0.5% is shown in Fig. 1 (right panel). It corresponds to the 60 laser beams (1 ns, 0.5 TW square pulses), with phase plates adapted to the 1 mm diameter spheres. To ensure a steady target size a) Contributed paper, published as part of the Proceedings of the 20th Top-

ical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) [email protected]

0034-6748/2014/85(11)/11D613/3/$30.00

(i.e., a non-imploding target), solid glass beads were selected instead of spherical shells. The x-ray spectra required line emission instead of continuum in order to calibrate the spectral range and to provide contrast that can be discriminated against potential background. To this goal, various metals were coated on the bead’s surface. As these metals must all be ablated during the laser irradiation to emit x rays, they were coated as an alloy instead of layers. The ∼1.5 μm thickness of this metallic alloy was determined using the simulations described below, to ensure maximum emission. The ten NXS configurations span ten different x-rayenergy ranges from 2 to 18 keV. A number of metal coatings were selected (Si, Ti, Cr, Ni, Zn, Zr, Mo, and Ag) to produce line emission in this whole range using their K-shell or Lshell lines. To the target fabrication, only two or three metals were coated on a single bead, and four types of targets were fabricated. Working with these constraints, we designed four different alloys, denoted by A, B, C, and D, summarized in Table I. Each alloy is adapted to the x-ray range of two or three different NXS configurations, so that all ten configurations can detect line emission from one of these alloys. To have comparable brightness from the various x-ray lines as predicted by the simulations, the metallic layers have equal parts of each element. The targets were fabricated by General Atomics. The bead radius was measured to 0.6% precision with a microscope. The thickness of the coating and its composition were determined with 7% and 1.5% respective uncertainties using x-ray opacity measurements at the K-edge of each element (or electron-impact K-shell x-ray fluorescence in the case of Si coating). Two sets of simulations, with two different codes, were performed to design and address the performance of each target configuration. Simulations were performed in 1D with spherical geometry using H YDRA ,5 a multiphysics arbitrary Lagrangian-Eulerian (ALE) multi-dimensional radiationhydrodynamics code. In an earlier stage of the design process,

85, 11D613-1

© 2014 AIP Publishing LLC

11D613-2

Pérez et al.

Rev. Sci. Instrum. 85, 11D613 (2014) TABLE II. QEOS input parameters in the H YDRA simulations for the various target compositions.

Target A

B C

D

Coating (%)

δr [μm]

Si/Mo/Ag 41/28/31

1.4

Ti/Cr/Ag 30/35/35 Cr/Ni/Zn 33/33/34 Zn/Zr 48/52

1.7 1.3

1.6

NXS configurations #1 from 1.9 to 2.4 keV #2 from 2.2 to 2.9 keV #3 from 2.6 to 3.7 keV #4 from 3.0 to 4.6 keV #5 from 3.6 to 6.1 keV #6 from 5.9 to 7.4 keV #7 from 6.7 to 8.9 keV #8 from 7.9 to 11.1 keV #9 from 8.9 to 13.7 keV #10 from 10.8 to 18.2 keV

ρ¯ [g/cm3 ]

c¯s [cm/μs]

A B C D

32.17 31.45 27.36 35.20

72.05 70.32 50.76 78.82

4.32 6.73 7.66 6.80

0.46 0.40 0.39 0.34

The H YDRA results match the data better than the early A RES simulations; thus, we will mostly discuss the former. Shown in Fig. 2 is the spatially resolved evolution of the electron temperature Te , radiation energy density  R in the 5.9 − 11.2 keV band and the electron density ne (green) normalized to the critical density nc ≈ 1022 cm−3 , for the target C. Throughout the simulation, we observe a peak Te in the underdense blowoff plasma (∼150 μm from the initial interface) of approximately 3 keV and  R peaked at ∼0.5nc (∼25 μm from the initial interface). Power emitted into this band dies rapidly after the laser turns off. At this time, nearly 90% (by mass) of the coating has become sub-critical indicating that the laser did not completely ablate away the mid-Z layer. Target types B and D performed similarly with ∼70% ablated away by the end of the pulse whereas the coating of target type A had completely been burnt through ∼0.25 ns before the pulse turned off. The H YDRA x-ray spectra are reconstructed from a spherical harmonic decomposition of the IMC photons, recorded as they pass through a diagnostic sphere located at r ≈ 0.5 cm. The simulated laser-to-x-ray conversion efficiency, integrating the spectra from 0 to 20 keV, varies between 45% and 60%. By restricting the integration of the spectra to each

4 (a) 0.0 ns 3 2 1 0

2

T

e

ε

R

n /n e

c

101 100 10−1 10−2 10 2

4 (b) 0.5 ns 3 2 1 0

101 100 10−1 10−2 10

4 (c) 1.0 ns 3 2 1 0

101 100 10−1 10−2 10

4 (d) 1.5 ns 3 2 1 0

101 100 10−1 10−2 10

0

2

2

0.05

0.1

Normalized electron density, ne/nc

TABLE I. Target composition, thicknesses (δr), and corresponding NXS crystal configurations.



Electron temperature, Te [keV]

simulations using the A RES code had also been performed in the same geometry. Targets A−D were modeled according to the experimental design with a 0.1 cm-diameter solid SiO2 bead with the mid-Z coating. In H YDRA, the glass and coatings were treated using the Livermore equation of state (LEOS) tables and the Thomas-Fermi based quotidian equation of state (QEOS6 ) model, respectively.  We used atomic-fraction¯ = weighted atomic numbers Z i fi Zi and atomic weights  7 A¯ = i fi Ai with pressure equilibrated (harmonically av eraged)  mass densities 1/ρ¯ = i fi /ρi and sound speeds 1/c¯s = i fi /cs,i ; a summary of the QEOS parameters for each target is shown in Table II. In A RES, LEOS were used for all elements, simply averaging by atomic fraction. The simulations were performed in the purely Lagrangian formulation and strict mass-matching was observed across the glass/mid-Z interface. The 3ω laser was treated using the 3D ray-tracing algorithm8 in H YDRA, and with a similar approach in A RES. Guided by the model developed by Colvin et al.,9 H YDRA employs the Lee-More electron thermal conductivity formulation10 with a large flux limiter (f = 0.2) to account for non-local electron transport, non-LTE rates with detailed super-configuration atomic models (DCA) from C RETIN,11 and implicit Monte Carlo (IMC) photonics for radiation transport12 with ∼106 photons and ∼480 radiation bins. A RES simply uses the Lee-More model without flux limiter, radiation diffusion with 60 bins, LTE rates, and a multi-group opacity model. It uses C RETIN only to post-process the x-ray spectra.



Radiation energy density (hν = 5.9−11.2 keV), εR [x200 J/cm3]

FIG. 1. Calibration campaign setup. Blue and red lines identify the lines of sight of different diagnostics.

Target

0.15

r [cm] FIG. 2. (a)–(d) H YDRA-simulated profiles of electron temperature Te , radiation energy density  R in the 5.9 − 11.2 keV band, and electron density ne (normalized to the critical density nc ), for target type C.

11D613-3

Pérez et al.

Rev. Sci. Instrum. 85, 11D613 (2014)

FIG. 3. Simulated spectra for targets A–D in J/keV/sr are given for both codes, as well as experimental spectra from the XRS spectrometer. The relative intensities between cases A and D can be different for a given code as they correspond to different targets, densities, ablation rates, etc.

x-ray range of interest (see Table I) for targets A–D, we obtain conversion efficiencies of 19%, 14%, 1.2%, and 0.3%, respectively. Figure 3 provides the four simulated spectra in absolute units corresponding to the four target types, together with a few experimental spectra from the calibrated XRS spectrometer. Line locations and ratios are overall in good agreement with the H YDRA simulations. Note that this comparison is preliminary, since the published photoresponse calibration of the image plates13 may not be sufficient for the photon energies of interest. Results from A RES show poorer agreement, due to the reduced hydrodynamics and atomic physics models employed. Future publications will report the NXS spectra acquired during this campaign, and the corresponding calibration analysis. To this goal, the XRS spectra will be analyzed, especially focusing on the response of the image-plate detectors, which will be precisely characterized in the x-ray range of interest with gamma sources. Overall, we described the target design that led to a successful calibration campaign of the NXS. The simulated spectra were good enough to configure the spectrometers and record a signal within the dynamic range of the detectors. All crystals have acquired spectra and will be cross-calibrated to the absolutely calibrated XRS instrument. The reflectivity and

surface quality of each crystal will then be reported for use on NIF experiments. The authors thank M. Patel and M. Marinak for their contribution to the simulations. This work was performed under the auspices of the (U.S.) Department of Energy (DOE) by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 and supported by the Defense Threat Reduction Agency under IAA 10027-5009 BASIC, “DTRA time-resolved x-ray spectrometer for the National Ignition Facility.” 1 G.

H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004). P. Opachich et al., Rev. Sci. Instrum. 83, 125105 (2012). 3 T. Boehly, Opt. Commun. 133, 495 (1997). 4 B. R. Maddox et al., Rev. Sci. Instrum. 82, 023111 (2011). 5 M. M. Marinak, G. D. Kerbel, N. A. Gentile, O. Jones, D. Munro, S. Pollaine, T. R. Dittrich, and S. W. Haan, Phys. Plasmas 8, 2275 (2001). 6 R. M. More, K. H. Warren, D. A. Young, and G. B. Zimmerman, Phys. Fluids 31, 3059 (1988). 7 P. Sterne, private communication (2013). 8 T. B. Kaiser, Phys. Rev. E 61, 895 (2000). 9 J. D. Colvin, K. B. Fournier, J. Kane, S. Langer, M. J. May, and H. A. Scott, High Energy Density Phys. 7, 263 (2011). 10 Y. T. Lee and R. M. More, Phys. Fluids 27, 1273 (1984). 11 H. Scott and S. Hansen, High Energy Density Phys. 6, 39 (2010). 12 J. A. Fleck, Jr. and J. D. Cummings, Jr., J. Comput. Phys. 8, 313 (1971). 13 A. L. Meadowcroft, C. D. Bentley, and E. N. Stott, Rev. Sci. Instrum. 79, 113102 (2008). 2 Y.

REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D614 (2014)

Using penumbral imaging to measure micrometer size plasma hot spots in Gbar equation of state experiments on the National Ignition Facilitya) B. Bachmann,1,b) A. L. Kritcher,1 L. R. Benedetti,1 R. W. Falcone,2,3 S. Glenn,1 J. Hawreliak,1 N. Izumi,1 D. Kraus,2 O. L. Landen,1 S. Le Pape,1 T. Ma,1 F. Pérez,1 D. Swift,1 and T. Döppner1 1

Lawrence Livermore National Laboratory, Livermore, California 94550, USA Department of Physics, University of California, Berkeley, California 94720, USA 3 Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 2

(Presented 3 June 2014; received 1 June 2014; accepted 14 July 2014; published online 31 July 2014) We have developed an experimental platform for absolute equation of state measurements up to Gbar pressures on the National Ignition Facility (NIF) within the Fundamental Science Program. We use a symmetry-tuned hohlraum drive to launch a spherical shock wave into a solid CH sphere. Streaked radiography is the primary diagnostic to measure the density change at the shock front as the pressure increases towards smaller radii. At shock stagnation in the center of the capsule, we observe a short and bright x-ray self emission from high density (∼50 g/cm3 ) plasma at ∼1 keV. Here, we present results obtained with penumbral imaging which has been carried out to characterize the size of the hot spot emission. This allows extending existing NIF diagnostic capabilities for spatial resolution (currently ∼10 μm) at higher sensitivity. At peak emission we find the hot spot radius to be as small as 5.8 +/− 1 μm, corresponding to a convergence ratio of 200. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891303] I. INTRODUCTION

Gbar pressures occur in inertial confinement fusion (ICF) plasmas and astrophysical objects such as planetary gas giants and brown dwarfs. As such, a goal is to benchmark computational dense matter models at 100 Mbar to 1 Gbar. In order to probe matter at these pressures we developed a platform to conduct equation of state (EOS) experiments on the NIF.1 Gbar pressures are reached by launching a spherical converging shockwave into solid CH. A schematic of the experimental platform is shown in Fig. 1. Here, the 184 incident laser beams on a gold hohlraum generate x-rays, which shock-compress the target to densities up to 50 g/cm3 . Xray radiography is used to measure the speed of the converging shockwave as well as the density and opacity of the CH.1 Applying the Rankine-Hugoniot relations the equation of state for the shocked material can be calculated. X-ray scattering is used to measure the electron velocity distribution due to Doppler broadening and thus infer the electron temperature.2–4 At stagnation of the spherical shockwave we observe a short and bright flash of x-rays, which is generated from high density (∼50 g/cm3 ) plasma at temperatures of about 1 keV (see Sec. II). A detailed understanding of the emission size and strength of such a hot spot is crucial for benchmarking radiation-hydrodynamics simulations in a regime that is not accessible to radiography. This has been accomplished by fielding x-ray penumbral imaging on the NIF. In penumbral imaging the target is imaged over the edge of 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)/11D614/3/$30.00

an aperture (e.g., a pinhole).5 Here we use a pinhole array with pinhole diameters of 100 μm, which is about 10 times larger than the emitting plasma. The penumbra (annular width D3 ) can be detected, e.g., by a gated x-ray detector (GXD).6 It contains the spatial information about the hot spot structure convoluted with the variation of the edge of the aperture D1 and the point-spread-function (PSF) of the detector D2 . Knowing the magnification M of the setup the hot spot diameter D0 can be de-convoluted:  D32 − (M + 1)2 D12 − D22 . (1) D0 = M With this approach self-emitting plasma hot spots extending only a few micrometers can be imaged with high spatial resolution (∼1μm). Another advantage of this approach is the high sensitivity of the measurement with regard to x-ray photon yield. Hence penumbral imaging has been qualified to detect plasma x-ray intensities which are one hundred times lower than in ICF implosions (see Sec. II). At such intensities, a measurement with conventional pinhole imaging (using 10 μm diameter pinholes as routinely employed in ICF implosions on the NIF) would be below the detection threshold of a GXD. In contrast to this, the use of large aperture pinholes (100 μm diameter) yields enough high-energy xray photons (above 5 keV) to measure the penumbra (see Sec. IV). The resulting main limiting factors for penumbral imaging are blurring due to diffraction on the edge of the aperture (1–2 μm in the plane of the detector for the current setup), the available magnification M of the setup (M = 4 for the current setup), and the resolution of the detector (∼40 μm based on the PSF of the GXD) which can be improved by averaging as applied to the data and folded into the standard

85, 11D614-1

© 2014 AIP Publishing LLC

11D614-2

Bachmann et al.

Rev. Sci. Instrum. 85, 11D614 (2014)

FIG. 1. (a) Overview of the Gbar platform at the NIF providing streaked radiography, x-ray scattering, and penumbral imaging. (b) Simulated x-ray spectrum incident on GXD for penumbral imaging. The plasma emission is attenuated by the compressed CH sphere and 2.5 mm of Polyimide film. (c) Capsule design with graded Ge doped ablator and solid CH core.

deviation resulting in the quoted error bars. Further limitations are the knowledge about the structure of the aperture, the photon yield as well as noise of the measurement due to onedimensional convolution7 and background due to hohlraum radiation (for indirectly driven targets). II. RESULTS OF SIMULATED TEMPERATURE AND DENSITY PROFILES

To predict the plasma conditions for the conducted experiments, 1D radiation hydrodynamics simulations have been performed using the HYDRA code.1 The simulated electron temperature and mass density as a function of radius are shown in Fig. 2. These simulations show the evolution of plasma conditions closely around shock stagnation. While mass densities up to 50 g/cm3 are predicted, the electron temperature yields values ∼1 keV in the central hot spot. These temperatures are about a factor 3 lower than found in ICF experiments, which leads to approximately 100 times less emission from the hot spot8 as a result of the scaling of the Bremsstrahlung emission coefficient:9   ne ni Z 2 hν , (2) εBrems ∝ exp − T 1/2 kT where ne , ni , Z, T, h, v, and k are the electron density, ion density, atomic number, electron temperature, Planck’s constant, frequency, and Boltzmann’s constant, respectively. Thus, high-resolution pinhole imaging with 10 μm pinhole diameters does not provide sufficient signal for the employed GXD. As a consequence, penumbral imaging with 100 μm diameter pinholes has been fielded as an alternative to determine the time of shock stagnation tst and to measure the spatial extension of the central hot spot plasma, which is a measure of shock sphericity. III. EXPERIMENTAL SETUP, IMAGE PROCESSING, AND DATA EVALUATION

Penumbral imaging was carried out in the NIF target chamber with an imaging snout from the polar Diagnostic In-

FIG. 2. Pre-shot HYDRA simulations of the electron temperature and mass density around shock stagnation time.

strument Manipulator (DIM) separating the GXD from the pinhole array by 1 m. The distance of the 100 μm diameter pinholes to the target chamber center (TCC) was 250 mm thus yielding a magnification of 4. The x-rays emitted by the hot spot plasma pass the laser entrance hole (LEH) and are imaged through the pinhole pattern onto a GXD with a time resolution of 105 +/− 10 ps FWHM. The x-rays are attenuated by the surrounding compressed CH and a filter assembly. 2.5 mm of Polyimide film was utilized to protect equipment and constrain the incident x-ray spectrum to a range of 8–14 keV (6 keV FWHM). The thus attenuated simulated self-emission spectrum is shown in Fig. 1(b). To extract the information about the penumbra, noise corrected images were treated in five steps: 1. Center detection by Hough transform10 (Fig. 3(a)). 2. Radial integration along radii Ri over an angle of 2π to improve Signal to Noise. (The standard deviation of the average is employed in error propagation calculations contributing to the quoted error bars.) 3. Fitting of the data with an error function. (95% confidence intervals were included in the error propagation calculations) (Fig. 3(b)). 4. Calculation of analytic derivatives of the previously fitted error function to unfold the spatial hot spot structure (source function) (Fig. 3(c)), and 5. De-convolution of the source function with the detector PSF and the characteristic variation of the pinhole edge (see Eq. (1)). The latter was obtained by x-ray radiographs of the pinholes prior to the shot (Fig. 3(d)). This method of data evaluation allows for accurate determination (+/−1 μm) of hot spot radii from a penumbra, which extends only over a few pixels.

11D614-3

Bachmann et al.

FIG. 3. (a) Pinhole image with overlaid circle (Ri ) along which averaging is performed. (b) Fit of the averaged data (+) with an erf. function (solid line) including bounds (dashed lines) obtained by error propagation calculations. (c) Analytical derivative of erf. function fit (solid line). D3 is the width of the penumbra in the detector plane. Shown are also the limits of the derivative resulting from error propagation calculations (dashed lines). (d) X-ray radiograph of a Pinhole taken before the shot showing variations of the edge measured as D1 .

IV. RESULTS AND DISCUSSION

Fig. 4 shows the relative intensity (corrected for signal droop) of measured x-rays as a function of time, thus providing a measure for shock stagnation time tst = 25.26 ns. The analysis of penumbra from each pinhole yields a measurement of the hot spot radius (Fig. 5). These results show the first successful measurement of x-ray self-emission from shock stagnation on the NIF. The smallest hot spot radius happens to appear at tst and yields a value of 5.8 μm +/− 1 μm. This corresponds to a convergence ratio of about 200. The uncertainties include the standard deviation of calculated averages along radii Ri , the 95% confidence intervals of the erffits, as well as the uncertainties induced in the de-convolution with the pinhole aperture and the detector PSF. The rather slow change in measured hot spot radii with time can be explained due to the response time of the GXD. The high voltage pulse travelling over the parallel plate transmission line functions as a moving shutter with a pulse length of ∼100 ps FWHM. For shorter lasting x-ray emissions, resulting hot spot radii smear out in time and thus overestimate the radii at tst and underestimate them before and after tst . Error bars

Rev. Sci. Instrum. 85, 11D614 (2014)

FIG. 5. Evolution of measured hot spot radii.

increase in size at early and late times due to the lower signal measured by the GXD. At tst , where the self-emission is strongest the smallest error bars are present giving the most accurate measurement of the spatial extension of self-emitting plasma in the 8-14 keV range. The error bar at 25.31 ns is increased since the pinhole image was collected at the edge of the GXD and thus only part of the penumbra was evaluated. V. OUTLOOK

Future experiments can be performed with higher magnification leading to improved experimental conditions. This would enable the analysis of asymmetries by averaging the penumbra over smaller angles and thus unfolding the internal hot spot structure. Selective spectral measurements could be performed by additionally filtering part of the GXD sensor. This and a quantitative evaluation of emission intensities can be used to infer spatially resolved electron temperature and density which would be an important next step to benchmark radiation hydrodynamics simulations in a regime that is not accessible to x-ray radiography. ACKNOWLEDGMENTS

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. The authors acknowledge support from LLNL-LDRD Grant Nos. 11-ER-050 and 13-ERD-073 and SSAA Contract No. DEFG52-06NA26212. 1 A.

L. Kritcher et al., High Energy Density Phys. 10, 27 (2014). Döppner et al., J. Phys.: Conf. Ser. 500, 192019 (2014). 3 T. Döppner et al., “Qualification of a high-efficiency, gated spectrometer for X-ray Thomson scattering on the National Ignition Facility,” Rev. Sci. Instrum. (these proceedings). 4 D. Kraus et al., Rev. Sci. Instrum. 85, 11D606 (2014). 5 S. Fujioka et al., Rev. Sci. Instrum. 73, 2588 (2002). 6 S. Glenn et al., Proc. SPIE 8144, 814409 (2011). 7 D. Ress et al., Rev. Sci. Instrum. 66, 579 (1995). 8 G. A. Kyrala et al., Phys. Plasmas 18, 056307 (2011). 9 S. Atzeni and J. Meyer-Ter-Vehn, The Physics of Inertial Fusion (Oxford Science Publications, Clarendon, Oxford, 2004). 10 P. V. C. Hough, U.S. patent US3069654 A (1962). 2 T.

FIG. 4. Time evolution of relative intensities measured with the GXD. The shown curve is close to the GXD response curve, implying a burn width of less than 100 ps.

REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D615 (2014)

Monte-Carlo simulation of noise in hard X-ray Transmission Crystal Spectrometers: Identification of contributors to the background noise and shielding optimizationa) I. Thfoin,1,b) C. Reverdin,1 S. Hulin,2 C. I. Szabo,3 S. Bastiani-Ceccotti,4 D. Batani,2 E. Brambrink,4 M. Koenig,4 A. Duval,1 X. Leboeuf,5 L. Lecherbourg,1 B. Rossé,1 A. Morace,6 J. J. Santos,2 X. Vaisseau,2 C. Fourment,2 L. Giuffrida,2 and M. Nakatsutsumi4 1

CEA, Centre de Saclay, IRFU, F-91191 Gif-sur-Yvette, France CELIA, Université de Bordeaux-CNRS-CEA, F-33405 Talence, France 3 Laboratoire Kastler Brossel, ENS, CNRS, UPMC, 75005 Paris Cedex, France 4 LULI Ecole Polytechnique, CNRS, CEA, UPMC, 91128 Palaiseau, France 5 CEA, Centre de Saclay, IRFU, F-91191 Gif-sur-Yvette, France 6 University of Milano, via Celoria 16, 20133 Milano, Italy 2

(Presented 2 June 2014; received 30 May 2014; accepted 7 July 2014; published online 1 August 2014) Transmission crystal spectrometers (TCS) are used on many laser facilities to record hard X-ray spectra. During experiments, signal recorded on imaging plates is often degraded by a background noise. Monte-Carlo simulations made with the code GEANT4 show that this background noise is mainly generated by diffusion of MeV electrons and very hard X-rays. An experiment, carried out at LULI2000, confirmed that the use of magnets in front of the diagnostic, that bent the electron trajectories, reduces significantly this background. The new spectrometer SPECTIX (Spectromètre PETAL à Cristal en TransmIssion X), built for the LMJ/PETAL facility, will include this optimized shielding. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4890534] I. INTRODUCTION

Along with the development of energetic and powerful lasers delivering hundreds of Joules in 1–10 ps duration came the development of hard X-ray spectrometers for the detection of high-Z Kα lines. The emission of such hard X-rays in the 20–100 keV range is correlated to the acceleration of electrons at relativistic energies by these ultra-intense lasers. Such electrons are responsible for the inner-shell ionization of the target materials. The most efficient system to detect in such a spectral range is the so-called Cauchois geometry spectrometer.1, 2 It is based on the diffraction by thin cylindrically curved Bragg crystals working in transmission. This geometry is perfectly adapted to laser plasma experiments because it is intrinsically shielded against direct exposure of the Imaging Plate (IP) detector.3 It has been developed and used on several laser facilities worldwide.4–7 However, it may happen in some experiments that a very intense and disturbing signal appears. We will show in this paper what the source of this spurious signal is and give some solution to remove, or at least decrease, most of it. II. SIGNATURE OF THE PERTURBATION

It appears in some specific experiments that a spurious signal is detected by Transmission Crystal Spectrometers such as LCS at LULI2000 and TCS on Omega EP (which 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)/11D615/4/$30.00

are twin spectrometers) as shown in Figure 1. In some cases, a large uniform background with a rectangular structure was detected covering the totality or only part of the detector. In other cases, the spurious signal was very intense close to the symmetry axis of the spectrometer and then decreases rapidly. In all cases this background is intense and can compromise the detection of the expected signal as shown in Figure 2 where are presented the energy spectra extracted from the raw images of Figure 1. One can see that the signal level is much lower than the background level. To understand its origin and then define an efficient shielding, we have performed a complete modeling of the spectrometer using Monte-Carlo simulations considering the different noise sources that can come from the target: very hard x-rays and relativistic particles.

III. MONTE-CARLO SIMULATIONS A. Modeling of TCS with GEANT4

GEANT48 is a Monte-Carlo based toolkit which simulates the passage of particles through matter. The TCS/LCS geometry was precisely implemented in the program. Thus, the entire aluminum envelope was implemented as well as the internal lead shielding. Debris shield (Al, 100 μm thick), frontal collimator with a central pinhole, crystal with its holding, lead cross-over slit with an opening of 5 mm, and imaging plates (Fuji BAS-MS IPs) were also simulated (cf. Figure 3). Source particles are sent from the target position (point source, 600 mm from the crystal) inside a cone that covers the entire solid angle of the spectrometer. The output

85, 11D615-1

© 2014 AIP Publishing LLC

Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp

Upgrading optical information of rotating mirror cameras.

To date, rotating mirror (RM) cameras still serve as indispensable imaging equipment for the diagnosis of microsecond transient processes due to their...
888KB Sizes 2 Downloads 6 Views