11D621-2
Baker et al.
Rev. Sci. Instrum. 85, 11D621 (2014)
FIG. 1. Experimental/simulated setup radiographing a shock tube to measure the mix width between an accelerated nickel foam/CRF interface. The halfraum is driven from below with 6 kJ of laser energy in a 1 ns flat top pulse.
1.4 mm long by 1.1 mm diameter cylinder of CRF foam at a density of 50 mg/cc. The OMEGA laser drives the halfraum with a 1 ns flat top pulse and ∼6 kJ of energy. Several aspects of the experiment have been tested using the geometry shown in Fig. 1. This includes testing of the backlighter spectrum obtained with nickel, zinc, and the manufacturing and testing of a cosputtered nickel/zinc backlighter. Nickel and zinc were cosputtered onto a CH foil to a thickness of 1.6 μm and with a final concentration of 45.5% Zn and 54.5% Ni. These backlighters were then tested on the OMEGA laser where they were driven with three beams of the OMEGA laser with a 600 ps flat top pulse. The intensity on the backlighter foil was approximately 2 × 1015 W/cm2 . The spectrum was measured using the Henway crystal spectrometer and is shown below in Fig. 2. The nickel filter, shown in Fig. 3, was manufactured by laser cutting the troughs from a 15 μm thick nickel foil. The foil was designed to have 500 μm wide bars and troughs. The transmission through the 15 μm thick bars on the nickel picket fence filter is predicted to be 50% for the Ni Heα emission, 7.77 keV, and 2.5% for the Zn Heα emission, 8.95 keV, thereby attenuating the Zn Heα emission a factor of 20 more than the Ni Heα emission.
FIG. 3. Manufactured nickel picket fence filter. The width and trough were designed to be 500 μm wide (25.9 μm in the object plane) and the nickel filter was 15 μm in thickness.
foam and the low density CRF which drives the RM instability. The interface then experiences a deceleration at later times which drives the RT instability. The simulations conducted for this article were two-dimensional and as such the ridge of nickel foam was actually modelled as a 400 μm diameter cylinder. Simulated radiographs, using the experimentally measured spectra in Fig. 2, were taken 18.7 ns after the onset of the laser energy at a time when the nickel interface had been driven ∼780 μm into the low density CRF. The simulated radiographs did not include the effect of mixing across the interface. Mixing would result in a decrease in signal to the right of the interface, furthest from the halfraum, and an increase in signal to the left, closest to the halfraum. The decrease would be enhanced in the Zn image and the increase would be enhanced in the Ni image. A lineout through the simulation in the center of the cylinder was then used to construct what would be expected in the experiment radiographing the 400 μm wide ridge. The simulated radiograph positions the backlighter 1.25 cm from the center of the shock tube and the detector plane 22.86 cm from the center of the shock tube for an overall magnification of 19.3. The resultant radiographs from the simulation are shown in Fig. 4. In
III. SIMULATED RADIOGRAPHS
In the simulations, the peak temperature inside the hohlraum reaches ∼220 eV. The resultant x-ray ablation of the CH ablator drives a strong shock, Mach number ∼ 30, into the nickel foam and across the interface between the nickel
FIG. 2. Backlighter x-ray spectrum driven on the OMEGA laser.
FIG. 4. Simulated radiographs through the nickel foam at 18.7 ns from the onset of the lasers in the halfraum. Figures 4(a) and 4(b) represent radiographs through the nickel foam using the spectrum from the zinc backlighter and the nickel backlighter, respectively. Figure 4(c) then represents the radiograph using the composite spectrum passing through the nickel picket fence filter.
11D621-3
Baker et al.
Rev. Sci. Instrum. 85, 11D621 (2014)
the bar region of the filter quantitatively reproduces the radiograph of the nickel foam by the nickel backlighter and once that is determined it can be used in the lineout of the trough region to determine the radiograph of the nickel foam by the zinc backlighter. By iteratively accounting for the two spectra even closer agreement can be achieved. When mixing occurs between materials of similar atomic number the dual color backlighter allows one to infer the concentrations in the mix region from the two radiographs under the same experimental conditions. V. SUMMARY FIG. 5. Lineouts through the simulated radiographs.
particular, Figs. 4(a) and 4(b) represent radiographs through the nickel foam using the spectrum from the zinc backlighter and the nickel backlighter, respectively, shown in Fig. 2. Figure 4(c) then represents the radiograph using the composite spectrum passing through the nickel picket fence filter. In particular, the radiograph in Fig. 4(c) was constructed by taking the zinc and nickel radiographs and adding them together in the trough region of the picket fence filter and in the bar region multiplying the two radiographs by their respective transmission through the 15 μm thick nickel filter, 50% for the nickel and 2.5% for the zinc, and adding those signals together.
In this article, a radiography technique was introduced which allows two backlighter energies to be used on the same experiment to diagnose the mix region between materials. This was accomplished by using a cosputtered backlighter to generate multiple Heα emission lines and then to demultiplex the signal on the detector using a picket fence filter. Simulations were performed of an accelerated nickel foam with the subsequent simulated radiographs that would be obtained with a Ni Heα backlighter, a Zn Heα backlighter, and a cosputtered Ni/Zn Heα backlighter. In the absence of noise it was shown that the nickel picket fence filter in conjunction with the cosputtered Ni/Zn source enabled a deconvolution of the radiographs from both the Zn Heα and Ni Heα sources.
IV. DISCUSSION
ACKNOWLEDGMENTS
The nickel picket fence filter enables the decoupling of the two spectra taken with the same line-of-sight and on the same shot. In that way differences that arise due to target property variations or drive changes between experiments do not have to be accounted for. Lineouts from the radiographs in Fig. 4 are shown in Fig. 5. The solid black line represents a horizontal lineout through a trough region of the radiograph shown in Fig. 4(c). The thin light gray line represents the difference between the lineout of the trough region minus the lineout of the bar region with the latter lineout divided by the transmission through the 15 μm thick nickel bar, 0.5. This is compared with the dashed gray line representing the lineout through the nickel foam with the zinc backlighter. The dark gray solid line represents a lineout through the bar region of the radiograph and this is compared with the dark dashed gray line representing the transmission of the 15 μm thick nickel bar multiplied by the lineout through the nickel foam with the nickel backlighter. The lineouts illustrate that the lineout in
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. We would like to acknowledge the OMEGA operations staff for their help with the experiments that were conducted and General Atomics for making the picket fence filter. 1 L.
Rayleigh, Proc. London Math. Soc. 14, 170 (1883). I. Taylor, Proc. R. Soc. London, Ser. A 201, 192 (1950). 3 S. Chandrasekhar, Hydrodynamic and Hydromagnetic Stability (Clarendon, Oxford, 1961), Chap. X. 4 R. D. Richtmyer, Commun. Pure Appl. Math. 13, 297 (1960). 5 E. E. Meshkov, Fluid Dyn. 4, 101 (1969). 6 C. Cherfils, S. G. Glendinning, D. Galmiche, B. A. Remington, A. L. Richard, S. Haan, R. Wallace, N. Dague, and D. H. Kalantar, Phys. Rev. Lett. 83, 5507 (1999). 7 D. R. Farley, T. A. Peyser, L. M. Logory, S. D. Murray, and E. W. Burke, Phys. Plasmas 6, 4304 (1999). 8 J. Kane, D. Arnett, B. A. Remington, S. G. Glendinning, R. Wallace, A. Rubenchik and B. A. Fryxell, Astrophys. J. 478, L75, (1997). 9 O. L. Landen et al., Rev. Sci. Instrum. 72, 627 (2001). 2 G.
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D622 (2014)
A novel solution to the gated x-ray detector gain droop problema) J. A. Oertelb) and T. N. Archuleta Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
(Presented 2 June 2014; received 20 May 2014; accepted 2 August 2014; published online 21 August 2014) Microchannel plate (MCP), microstrip transmission line based, gated x-ray detectors used at the premier ICF laser facilities have a drop in gain as a function of mircostrip length that can be greater than 50% over 40 mm. These losses are due to ohmic losses in a microstrip coating that is less than the optimum electrical skin depth. The electrical skin depth for a copper transmission line at 3 GHz is 1.2 μm while the standard microstrip coating thickness is roughly half a single skin depth. Simply increasing the copper coating thickness would begin filling the MCP pores and limit the number of secondary electrons created in the MCP. The current coating thickness represents a compromise between gain and ohmic loss. We suggest a novel solution to the loss problem by overcoating the copper transmission line with five electrical skin depths (∼6 μm) of Beryllium. Beryllium is reasonably transparent to x-rays above 800 eV and would improve the carrier current on the transmission line. The net result should be an optically flat photocathode response with almost no measurable loss in voltage along the transmission line. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4893006] I. INTRODUCTION
The gated x-ray framing cameras used by the global Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP) community are time-dependent, twodimensional imagers.1 These instruments are important to researchers who require many sequential image frames, with adjustable temporal resolutions from 40 ps to several nanoseconds, spatial resolution of 5 μm (image magnified), spectral sensitivity from 0.2–10 keV, several orders of magnitude dynamic range, and centimeter size image areas. Although fundamentally similar to the original cameras developed over 20 years ago,2 the individual components that make up these instruments have progressively evolved. Primarily used at large laser facilities and required to operate in vacuum, these cameras are either mounted on a vacuum flange on the side of a vacuum target chamber or loaded into an insertable mechanism and translated inside the chamber. The gated x-ray framing cameras used in ICF and HEDP are designed around Microchannel Plate (MCP) detectors. First demonstrated3 in the early 1960s, the MCP has been used to detect a wide band of the electromagnetic spectrum. In the 1970s, MCPs gained notoriety for use in night vision goggles for military applications. The spike in applications motivated researchers to study the fundamental properties of MCPs and the basic proximity-focusing scheme.4, 5 It was not until the 1980s that researchers began routinely using MCP’s to detect and image x-rays, thus opening the possibilities for diagnostics in many scientific fields, including Astrophysics and Plasma Physics.6 In 2005, LLNL and the National Ignition Facility (NIF) collaborated with Los Alamos National Laboratory (LANL) to construct the next 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)/11D622/3/$30.00
generation gated x-ray imager for the NIF facility.1 This instrument exploited all the technology advancements to date to include computer control, modern phosphor screens, and custom CCD coupled cameras. Most recently, Benedetti et al. have made significant advances in the understanding and mitigation of cross-talk.7 Despite the significant incremental advances these workhorse instruments have achieved, they still have a fundamental shortcoming in that the gain along the length of the image plane microstrip decreases. This issue can be particularly challenging for researchers who are analyzing spectroscopic data with varying line intensities and ratios. II. GAIN DEGENERATION AS A FUNCTION OF MICROSTRIP LENGTH PROBLEM
The current microstrip based detectors have an effective drop in gain as a function of microstrip length that can be greater than 50% over 40 mm (see Figures 1 and 2). This is a result of the fact that the voltage drops along the microstrip due to the resistance of the strip and that the gain of the MCP (L/D-40) is strongly nonlinear and goes as G∼V20 . Attenuation of a microstrip is primarily caused by two loss components: conductor loss and dielectric loss.8 The dielectric loss is normally very small compared to the conductor loss for dielectric substrates.9 The resistivity and the conductor loss in a given microstrip increases the thinner the conductor becomes, governed eventually by the electrical skin depth for a given pulse frequency. Skin depth δ is a measure of how far electrical conduction takes place in a conductor and is a function of frequency f, bulk resistivity ρ, and relative permeability μ. Where the resistivity and permeability are constants unique to the microstip material. For copper, bulk resistivity is 1.69 × 10−8 -m and permeability is 1 (see Eq. (1)): ρ . (1) δ= πf u
85, 11D622-1
© 2014 AIP Publishing LLC
11D622-2
J. A. Oertel and T. N. Archuleta
Rev. Sci. Instrum. 85, 11D622 (2014)
pletely fill the MCP pores and limit the number of secondary electrons created in the MCP. The current coating thickness of 0.5 μm of copper over-coated by 0.1 μm of gold is currently a compromise. III. A NOVEL SOLUTION TO THE GAIN DROOP ISSUE
FIG. 1. Flatfield exposure demonstrating gain as a function of strip length. Data taken at Trident laser facility with GXD2, 130 J, 2 w, 2.4 ns square pulse, 200 um spot on Au disk. Pulse moves from right to left in figure.
Microstrip loss increases with the square root of the frequency due to the skin depth effect. As the frequency increases the energy moving in the trace is forced to the outer perimeter by the large magnetic field present in the higher frequency fields. Furthermore, as the frequency increases, the skin depth effect layer of current becomes thinner. As the current becomes more and more confined to the conductor surface, the resistance increases. At 3.0 GHz (appropriate for the risetime of the gating pulse), the calculated electrical skin depth for a copper transmission line is 1.2 μm while typical MCP microstrip coatings are standardized at 0.6 μm. Simply increasing the copper coating thickness to the recommended five electrical skin depths10 to reduce losses to ∼99% would begin to, or com-
A reasonable technological step forward to reduce the gain droop problem would be to overcoat the copper transmission line with ∼6 μm of a material that is reasonably transparent to low energy x-rays and has low electrical resistivity. It would also be beneficial if the back (ground plane) had additional material to improve return currents, but care must be taken here not to block the electrons from exiting the MCP. There are several thin foil materials that are good electrical conductors, but Beryllium is a logical choice because it has practical transmission to x-rays above 500 eV and has a very low electrical resistivity (4 × 10−7 -m). Using a material like Beryllium would allow transmission of x-rays above 500 eV, permitting adequate production of secondary electrons in the MCP and would also greatly improve the carrier current on the transmission line. The net result of adding 6 μm of Beryllium would be an optically flat photocathode response with almost no measurable loss in voltage along the transmission line. The obvious downside to adding a thin metallic coating is the loss of sensitivity to x-rays below 500 eV. IV. EXPERIMENTAL VERIFICATION
The best way to add 6 μm of Beryllium over the existing 0.6 μm copper and gold would be to use a Chemical Vapor Deposition (CVD) process. Depositing Beryllium with a CVD process would insure a uniform controlled monolithic layer without possibility of flatness or delamination problems. As a initial, proof-of-principle electrical test, we physically clamped a 12.7 μm Beryllium foil to one of the four standard gated x-ray imager MCP microstrips and compared the results to the nearest uncoated microstrip neighbor. Using a Time Domain Reflectometer (TDR) that measures impedance as a function of conductor length, we observed the typical ∼1 impedance change over 40 mm in the strip without the Beryllium foil and a virtually flat impedance profile for the coated strip (see Figure 3). With the encouraging TDR results, we plan to advance to the next step of CVD coating a Beryllium surrogate material on a MCP substrate. Since Beryllium is considered a toxic material we plan to start with Aluminum as an initial proof of concept. While the Aluminum will cut out much of the lower energy x-rays, an experiment verifying the concept can be easily accomplished at the LANL Trident laser facility using flat Titanium disk targets. V. SUMMARY
FIG. 2. Lineout of strip 3 from flatfield exposure above.
We report on a novel technique to solve the issue of ohmic microstrip loss in a MCP based gated x-ray detector. By simply CVD coating 6 μm of Beryllium over the existing copper/gold microstrip coating, preliminary measurements
11D622-3
J. A. Oertel and T. N. Archuleta
Rev. Sci. Instrum. 85, 11D622 (2014)
ACKNOWLEDGMENTS
Special thanks to the dedicated staff at LANL’s Diagnostic and Systems Engineering Team who helped support these measurements. This work was conducted under the auspices of the (US) Department of Energy by Los Alamos National Laboratory under Contract No. DE-AC52-06NA25396. 1 J.
A. Oertel et al., Rev. Sci. Instrum. 77, 10E308 (2006). D. Kilkenny, Laser Part. Beams 9, 49 (1991). 3 W. C. Wiley and C. F. Hendee, IRE Trans. Nucl. Sci. NS-9, 103 (1962). 4 J. Wiza, “Microchannel plate detectors,” Nucl. Instrum. Methods 162(1–3), 587–601 (1979). 5 A. W. Woodhead and G. Eschard, “Microchannel plates and their applications,” Acta Electron. 14(2), 181–200, (1971). 6 M. Katayama et al., “Multiframe x-ray imaging for temporally and spatially resolved measurements of imploding inertial confinement fusion targets,” Rev. Sci. Instrum. 62, 124 (1991). 7 L. R. Benedetti et al. “Crosstalk in x-ray framing cameras: Effect on voltage, gain, and timing,” Rev. Sci. Instrum. 83, 10E135 (2012). 8 S. F. Kahn et al. “Methods for characterizing x-ray detectors for use at the National Ignition Facility,” Rev. Sci. Instrum. 83, 10E118 (2012). 9 E. J. Denlinger, “Losses of microstrip lines,” IEEE Trans. Microwave Theory Tech. MTT-28(6), 513 (1980). 10 H. W. Deng et al., Prog. Electromagn. Res. M 9, 1–8 (2009). 2 J.
FIG. 3. TDR comparison with and without Beryllium foil on MCP microstrip. Note the improvement in the MCP section of the TDR trace by adding the Beryllium foil.
indicate the photocathode response will be flat and predicable.
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D623 (2014)
Development of a dual MCP framing camera for high energy x-raysa) N. Izumi,1,b) G. N. Hall,1 A. C. Carpenter,1 F. V. Allen,1 J. G. Cruz,1 B. Felker,1 D. Hargrove,1 J. Holder,1 J. D. Kilkenny,2 A. Lumbard,1 R. Montesanti,1 N. E. Palmer,1 K. Piston,1 G. Stone,1 M. Thao,1 R. Vern,1 R. Zacharias,1 O. L. Landen,1 R. Tommasini,1 D. K. Bradley,1 and P. M. Bell1 1 2
Lawrence Livermore National Laboratory, Livermore, California 94550, USA General Atomics, San Diego, California 92121, USA
(Presented 4 June 2014; received 2 June 2014; accepted 18 July 2014; published online 25 August 2014) Recently developed diagnostic techniques at LLNL require recording backlit images of extremely dense imploded plasmas using hard x-rays, and demand the detector to be sensitive to photons with energies higher than 50 keV [R. Tommasini et al., Phys. Phys. Plasmas 18, 056309 (2011); G. N. Hall et al., “AXIS: An instrument for imaging Compton radiographs using ARC on the NIF,” Rev. Sci. Instrum. (these proceedings)]. To increase the sensitivity in the high energy region, we propose to use a combination of two MCPs. The first MCP is operated in a low gain regime and works as a thick photocathode, and the second MCP works as a high gain electron multiplier. We tested the concept of this dual MCP configuration and succeeded in obtaining a detective quantum efficiency of 4.5% for 59 keV x-rays, 3 times larger than with a single plate of the thickness typically used in NIF framing cameras. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891712] I. INTRODUCTION
X-ray framing cameras based on proximity-focused micro-channel plates (MCP) have been playing an important role as diagnostics of inertial confinement fusion experiments for several decades.1–3 Most of the current x-ray framing cameras consist of a single MCP, a phosphor, and a recording device (e.g., CCD or photographic films). This configuration is successful for imaging x-rays with energies below 20 keV. In this single MCP configuration, the MCP works as both the photocathode and the electron multiplier. To have enough optical output from the phosphor, the MCP has to be operated with electron multiplication gains of 500–2000. When the MCP is operated in this high gain regime, x-rays detected in the shallow layer experience high electron-multiplication and dominate the output, while x-rays detected deep inside have a limited contribution. Due to this large gain differential, the detective quantum efficiency (DQE) for volumetrically absorbed photons (above 20 keV) is severely reduced.4 One approach to mitigate this DQE reduction is to separate the photocathode from the electron multiplier. We propose to stack two MCPs and operate the first one in low gain regime, as a thick photocathode, and use the second MCP as the electron multiplier, to achieve enough optical output from the phosphor. II. DQE AND NOISE FACTOR
the obtainable image when the output of the single detection event has small variance. For detectors having broad pulseheight distribution (PHD), DQE is the quantity of interest.5 The definition of the DQE is SN Ro 2 DQE ≡ , (1) SN Ri where SNRo and SNRi are signal-to-noise ratio at output and input of the detector, respectively. When the statistical behavior of the input photons follows Poisson statistics, the expected signal-to-noise ratio of the image can be evaluated as SN Ro =
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)/11D623/3/$30.00
DQE × Ni ,
(2)
where Ni is the number of incident photons per resolution element. Another useful measure of the loss of the available information caused by the statistical fluctuation of the avalanche process is the noise factor (NF) defined as6 NF = 1 +
σ2 , ξ 2
(3)
where ξ is the mean and σ is the standard deviation of the PHD. When noise from other sources (e.g., multiplicative noise or statistical fluctuation of background exposure) is small, the DQE can be expressed as7
Quantum efficiency (QE), which is defined as the number of detected events per number of incident quanta, is a commonly used metric for estimating the statistical noise of a) Contributed paper, published as part of the Proceedings of the 20th
DQE =
QE . NF
(4)
III. EXPECTED PHD OF SINGLE MCP
The statistical uncertainty of an avalanche multiplication process is usually modeled by Pólya or Furry statistics.8 When an avalanche stream is started from a single electron
85, 11D623-1
© 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
Baker et al.
Rev. Sci. Instrum. 85, 11D621 (2014)
FIG. 1. Experimental/simulated setup radiographing a shock tube to measure the mix width between an accelerated nickel foam/CRF interface. The halfraum is driven from below with 6 kJ of laser energy in a 1 ns flat top pulse.
1.4 mm long by 1.1 mm diameter cylinder of CRF foam at a density of 50 mg/cc. The OMEGA laser drives the halfraum with a 1 ns flat top pulse and ∼6 kJ of energy. Several aspects of the experiment have been tested using the geometry shown in Fig. 1. This includes testing of the backlighter spectrum obtained with nickel, zinc, and the manufacturing and testing of a cosputtered nickel/zinc backlighter. Nickel and zinc were cosputtered onto a CH foil to a thickness of 1.6 μm and with a final concentration of 45.5% Zn and 54.5% Ni. These backlighters were then tested on the OMEGA laser where they were driven with three beams of the OMEGA laser with a 600 ps flat top pulse. The intensity on the backlighter foil was approximately 2 × 1015 W/cm2 . The spectrum was measured using the Henway crystal spectrometer and is shown below in Fig. 2. The nickel filter, shown in Fig. 3, was manufactured by laser cutting the troughs from a 15 μm thick nickel foil. The foil was designed to have 500 μm wide bars and troughs. The transmission through the 15 μm thick bars on the nickel picket fence filter is predicted to be 50% for the Ni Heα emission, 7.77 keV, and 2.5% for the Zn Heα emission, 8.95 keV, thereby attenuating the Zn Heα emission a factor of 20 more than the Ni Heα emission.
FIG. 3. Manufactured nickel picket fence filter. The width and trough were designed to be 500 μm wide (25.9 μm in the object plane) and the nickel filter was 15 μm in thickness.
foam and the low density CRF which drives the RM instability. The interface then experiences a deceleration at later times which drives the RT instability. The simulations conducted for this article were two-dimensional and as such the ridge of nickel foam was actually modelled as a 400 μm diameter cylinder. Simulated radiographs, using the experimentally measured spectra in Fig. 2, were taken 18.7 ns after the onset of the laser energy at a time when the nickel interface had been driven ∼780 μm into the low density CRF. The simulated radiographs did not include the effect of mixing across the interface. Mixing would result in a decrease in signal to the right of the interface, furthest from the halfraum, and an increase in signal to the left, closest to the halfraum. The decrease would be enhanced in the Zn image and the increase would be enhanced in the Ni image. A lineout through the simulation in the center of the cylinder was then used to construct what would be expected in the experiment radiographing the 400 μm wide ridge. The simulated radiograph positions the backlighter 1.25 cm from the center of the shock tube and the detector plane 22.86 cm from the center of the shock tube for an overall magnification of 19.3. The resultant radiographs from the simulation are shown in Fig. 4. In
III. SIMULATED RADIOGRAPHS
In the simulations, the peak temperature inside the hohlraum reaches ∼220 eV. The resultant x-ray ablation of the CH ablator drives a strong shock, Mach number ∼ 30, into the nickel foam and across the interface between the nickel
FIG. 2. Backlighter x-ray spectrum driven on the OMEGA laser.
FIG. 4. Simulated radiographs through the nickel foam at 18.7 ns from the onset of the lasers in the halfraum. Figures 4(a) and 4(b) represent radiographs through the nickel foam using the spectrum from the zinc backlighter and the nickel backlighter, respectively. Figure 4(c) then represents the radiograph using the composite spectrum passing through the nickel picket fence filter.
11D621-3
Baker et al.
Rev. Sci. Instrum. 85, 11D621 (2014)
the bar region of the filter quantitatively reproduces the radiograph of the nickel foam by the nickel backlighter and once that is determined it can be used in the lineout of the trough region to determine the radiograph of the nickel foam by the zinc backlighter. By iteratively accounting for the two spectra even closer agreement can be achieved. When mixing occurs between materials of similar atomic number the dual color backlighter allows one to infer the concentrations in the mix region from the two radiographs under the same experimental conditions. V. SUMMARY FIG. 5. Lineouts through the simulated radiographs.
particular, Figs. 4(a) and 4(b) represent radiographs through the nickel foam using the spectrum from the zinc backlighter and the nickel backlighter, respectively, shown in Fig. 2. Figure 4(c) then represents the radiograph using the composite spectrum passing through the nickel picket fence filter. In particular, the radiograph in Fig. 4(c) was constructed by taking the zinc and nickel radiographs and adding them together in the trough region of the picket fence filter and in the bar region multiplying the two radiographs by their respective transmission through the 15 μm thick nickel filter, 50% for the nickel and 2.5% for the zinc, and adding those signals together.
In this article, a radiography technique was introduced which allows two backlighter energies to be used on the same experiment to diagnose the mix region between materials. This was accomplished by using a cosputtered backlighter to generate multiple Heα emission lines and then to demultiplex the signal on the detector using a picket fence filter. Simulations were performed of an accelerated nickel foam with the subsequent simulated radiographs that would be obtained with a Ni Heα backlighter, a Zn Heα backlighter, and a cosputtered Ni/Zn Heα backlighter. In the absence of noise it was shown that the nickel picket fence filter in conjunction with the cosputtered Ni/Zn source enabled a deconvolution of the radiographs from both the Zn Heα and Ni Heα sources.
IV. DISCUSSION
ACKNOWLEDGMENTS
The nickel picket fence filter enables the decoupling of the two spectra taken with the same line-of-sight and on the same shot. In that way differences that arise due to target property variations or drive changes between experiments do not have to be accounted for. Lineouts from the radiographs in Fig. 4 are shown in Fig. 5. The solid black line represents a horizontal lineout through a trough region of the radiograph shown in Fig. 4(c). The thin light gray line represents the difference between the lineout of the trough region minus the lineout of the bar region with the latter lineout divided by the transmission through the 15 μm thick nickel bar, 0.5. This is compared with the dashed gray line representing the lineout through the nickel foam with the zinc backlighter. The dark gray solid line represents a lineout through the bar region of the radiograph and this is compared with the dark dashed gray line representing the transmission of the 15 μm thick nickel bar multiplied by the lineout through the nickel foam with the nickel backlighter. The lineouts illustrate that the lineout in
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. We would like to acknowledge the OMEGA operations staff for their help with the experiments that were conducted and General Atomics for making the picket fence filter. 1 L.
Rayleigh, Proc. London Math. Soc. 14, 170 (1883). I. Taylor, Proc. R. Soc. London, Ser. A 201, 192 (1950). 3 S. Chandrasekhar, Hydrodynamic and Hydromagnetic Stability (Clarendon, Oxford, 1961), Chap. X. 4 R. D. Richtmyer, Commun. Pure Appl. Math. 13, 297 (1960). 5 E. E. Meshkov, Fluid Dyn. 4, 101 (1969). 6 C. Cherfils, S. G. Glendinning, D. Galmiche, B. A. Remington, A. L. Richard, S. Haan, R. Wallace, N. Dague, and D. H. Kalantar, Phys. Rev. Lett. 83, 5507 (1999). 7 D. R. Farley, T. A. Peyser, L. M. Logory, S. D. Murray, and E. W. Burke, Phys. Plasmas 6, 4304 (1999). 8 J. Kane, D. Arnett, B. A. Remington, S. G. Glendinning, R. Wallace, A. Rubenchik and B. A. Fryxell, Astrophys. J. 478, L75, (1997). 9 O. L. Landen et al., Rev. Sci. Instrum. 72, 627 (2001). 2 G.
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D622 (2014)
A novel solution to the gated x-ray detector gain droop problema) J. A. Oertelb) and T. N. Archuleta Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
(Presented 2 June 2014; received 20 May 2014; accepted 2 August 2014; published online 21 August 2014) Microchannel plate (MCP), microstrip transmission line based, gated x-ray detectors used at the premier ICF laser facilities have a drop in gain as a function of mircostrip length that can be greater than 50% over 40 mm. These losses are due to ohmic losses in a microstrip coating that is less than the optimum electrical skin depth. The electrical skin depth for a copper transmission line at 3 GHz is 1.2 μm while the standard microstrip coating thickness is roughly half a single skin depth. Simply increasing the copper coating thickness would begin filling the MCP pores and limit the number of secondary electrons created in the MCP. The current coating thickness represents a compromise between gain and ohmic loss. We suggest a novel solution to the loss problem by overcoating the copper transmission line with five electrical skin depths (∼6 μm) of Beryllium. Beryllium is reasonably transparent to x-rays above 800 eV and would improve the carrier current on the transmission line. The net result should be an optically flat photocathode response with almost no measurable loss in voltage along the transmission line. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4893006] I. INTRODUCTION
The gated x-ray framing cameras used by the global Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP) community are time-dependent, twodimensional imagers.1 These instruments are important to researchers who require many sequential image frames, with adjustable temporal resolutions from 40 ps to several nanoseconds, spatial resolution of 5 μm (image magnified), spectral sensitivity from 0.2–10 keV, several orders of magnitude dynamic range, and centimeter size image areas. Although fundamentally similar to the original cameras developed over 20 years ago,2 the individual components that make up these instruments have progressively evolved. Primarily used at large laser facilities and required to operate in vacuum, these cameras are either mounted on a vacuum flange on the side of a vacuum target chamber or loaded into an insertable mechanism and translated inside the chamber. The gated x-ray framing cameras used in ICF and HEDP are designed around Microchannel Plate (MCP) detectors. First demonstrated3 in the early 1960s, the MCP has been used to detect a wide band of the electromagnetic spectrum. In the 1970s, MCPs gained notoriety for use in night vision goggles for military applications. The spike in applications motivated researchers to study the fundamental properties of MCPs and the basic proximity-focusing scheme.4, 5 It was not until the 1980s that researchers began routinely using MCP’s to detect and image x-rays, thus opening the possibilities for diagnostics in many scientific fields, including Astrophysics and Plasma Physics.6 In 2005, LLNL and the National Ignition Facility (NIF) collaborated with Los Alamos National Laboratory (LANL) to construct the next 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)/11D622/3/$30.00
generation gated x-ray imager for the NIF facility.1 This instrument exploited all the technology advancements to date to include computer control, modern phosphor screens, and custom CCD coupled cameras. Most recently, Benedetti et al. have made significant advances in the understanding and mitigation of cross-talk.7 Despite the significant incremental advances these workhorse instruments have achieved, they still have a fundamental shortcoming in that the gain along the length of the image plane microstrip decreases. This issue can be particularly challenging for researchers who are analyzing spectroscopic data with varying line intensities and ratios. II. GAIN DEGENERATION AS A FUNCTION OF MICROSTRIP LENGTH PROBLEM
The current microstrip based detectors have an effective drop in gain as a function of microstrip length that can be greater than 50% over 40 mm (see Figures 1 and 2). This is a result of the fact that the voltage drops along the microstrip due to the resistance of the strip and that the gain of the MCP (L/D-40) is strongly nonlinear and goes as G∼V20 . Attenuation of a microstrip is primarily caused by two loss components: conductor loss and dielectric loss.8 The dielectric loss is normally very small compared to the conductor loss for dielectric substrates.9 The resistivity and the conductor loss in a given microstrip increases the thinner the conductor becomes, governed eventually by the electrical skin depth for a given pulse frequency. Skin depth δ is a measure of how far electrical conduction takes place in a conductor and is a function of frequency f, bulk resistivity ρ, and relative permeability μ. Where the resistivity and permeability are constants unique to the microstip material. For copper, bulk resistivity is 1.69 × 10−8 -m and permeability is 1 (see Eq. (1)): ρ . (1) δ= πf u
85, 11D622-1
© 2014 AIP Publishing LLC
11D622-2
J. A. Oertel and T. N. Archuleta
Rev. Sci. Instrum. 85, 11D622 (2014)
pletely fill the MCP pores and limit the number of secondary electrons created in the MCP. The current coating thickness of 0.5 μm of copper over-coated by 0.1 μm of gold is currently a compromise. III. A NOVEL SOLUTION TO THE GAIN DROOP ISSUE
FIG. 1. Flatfield exposure demonstrating gain as a function of strip length. Data taken at Trident laser facility with GXD2, 130 J, 2 w, 2.4 ns square pulse, 200 um spot on Au disk. Pulse moves from right to left in figure.
Microstrip loss increases with the square root of the frequency due to the skin depth effect. As the frequency increases the energy moving in the trace is forced to the outer perimeter by the large magnetic field present in the higher frequency fields. Furthermore, as the frequency increases, the skin depth effect layer of current becomes thinner. As the current becomes more and more confined to the conductor surface, the resistance increases. At 3.0 GHz (appropriate for the risetime of the gating pulse), the calculated electrical skin depth for a copper transmission line is 1.2 μm while typical MCP microstrip coatings are standardized at 0.6 μm. Simply increasing the copper coating thickness to the recommended five electrical skin depths10 to reduce losses to ∼99% would begin to, or com-
A reasonable technological step forward to reduce the gain droop problem would be to overcoat the copper transmission line with ∼6 μm of a material that is reasonably transparent to low energy x-rays and has low electrical resistivity. It would also be beneficial if the back (ground plane) had additional material to improve return currents, but care must be taken here not to block the electrons from exiting the MCP. There are several thin foil materials that are good electrical conductors, but Beryllium is a logical choice because it has practical transmission to x-rays above 500 eV and has a very low electrical resistivity (4 × 10−7 -m). Using a material like Beryllium would allow transmission of x-rays above 500 eV, permitting adequate production of secondary electrons in the MCP and would also greatly improve the carrier current on the transmission line. The net result of adding 6 μm of Beryllium would be an optically flat photocathode response with almost no measurable loss in voltage along the transmission line. The obvious downside to adding a thin metallic coating is the loss of sensitivity to x-rays below 500 eV. IV. EXPERIMENTAL VERIFICATION
The best way to add 6 μm of Beryllium over the existing 0.6 μm copper and gold would be to use a Chemical Vapor Deposition (CVD) process. Depositing Beryllium with a CVD process would insure a uniform controlled monolithic layer without possibility of flatness or delamination problems. As a initial, proof-of-principle electrical test, we physically clamped a 12.7 μm Beryllium foil to one of the four standard gated x-ray imager MCP microstrips and compared the results to the nearest uncoated microstrip neighbor. Using a Time Domain Reflectometer (TDR) that measures impedance as a function of conductor length, we observed the typical ∼1 impedance change over 40 mm in the strip without the Beryllium foil and a virtually flat impedance profile for the coated strip (see Figure 3). With the encouraging TDR results, we plan to advance to the next step of CVD coating a Beryllium surrogate material on a MCP substrate. Since Beryllium is considered a toxic material we plan to start with Aluminum as an initial proof of concept. While the Aluminum will cut out much of the lower energy x-rays, an experiment verifying the concept can be easily accomplished at the LANL Trident laser facility using flat Titanium disk targets. V. SUMMARY
FIG. 2. Lineout of strip 3 from flatfield exposure above.
We report on a novel technique to solve the issue of ohmic microstrip loss in a MCP based gated x-ray detector. By simply CVD coating 6 μm of Beryllium over the existing copper/gold microstrip coating, preliminary measurements
11D622-3
J. A. Oertel and T. N. Archuleta
Rev. Sci. Instrum. 85, 11D622 (2014)
ACKNOWLEDGMENTS
Special thanks to the dedicated staff at LANL’s Diagnostic and Systems Engineering Team who helped support these measurements. This work was conducted under the auspices of the (US) Department of Energy by Los Alamos National Laboratory under Contract No. DE-AC52-06NA25396. 1 J.
A. Oertel et al., Rev. Sci. Instrum. 77, 10E308 (2006). D. Kilkenny, Laser Part. Beams 9, 49 (1991). 3 W. C. Wiley and C. F. Hendee, IRE Trans. Nucl. Sci. NS-9, 103 (1962). 4 J. Wiza, “Microchannel plate detectors,” Nucl. Instrum. Methods 162(1–3), 587–601 (1979). 5 A. W. Woodhead and G. Eschard, “Microchannel plates and their applications,” Acta Electron. 14(2), 181–200, (1971). 6 M. Katayama et al., “Multiframe x-ray imaging for temporally and spatially resolved measurements of imploding inertial confinement fusion targets,” Rev. Sci. Instrum. 62, 124 (1991). 7 L. R. Benedetti et al. “Crosstalk in x-ray framing cameras: Effect on voltage, gain, and timing,” Rev. Sci. Instrum. 83, 10E135 (2012). 8 S. F. Kahn et al. “Methods for characterizing x-ray detectors for use at the National Ignition Facility,” Rev. Sci. Instrum. 83, 10E118 (2012). 9 E. J. Denlinger, “Losses of microstrip lines,” IEEE Trans. Microwave Theory Tech. MTT-28(6), 513 (1980). 10 H. W. Deng et al., Prog. Electromagn. Res. M 9, 1–8 (2009). 2 J.
FIG. 3. TDR comparison with and without Beryllium foil on MCP microstrip. Note the improvement in the MCP section of the TDR trace by adding the Beryllium foil.
indicate the photocathode response will be flat and predicable.
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D623 (2014)
Development of a dual MCP framing camera for high energy x-raysa) N. Izumi,1,b) G. N. Hall,1 A. C. Carpenter,1 F. V. Allen,1 J. G. Cruz,1 B. Felker,1 D. Hargrove,1 J. Holder,1 J. D. Kilkenny,2 A. Lumbard,1 R. Montesanti,1 N. E. Palmer,1 K. Piston,1 G. Stone,1 M. Thao,1 R. Vern,1 R. Zacharias,1 O. L. Landen,1 R. Tommasini,1 D. K. Bradley,1 and P. M. Bell1 1 2
Lawrence Livermore National Laboratory, Livermore, California 94550, USA General Atomics, San Diego, California 92121, USA
(Presented 4 June 2014; received 2 June 2014; accepted 18 July 2014; published online 25 August 2014) Recently developed diagnostic techniques at LLNL require recording backlit images of extremely dense imploded plasmas using hard x-rays, and demand the detector to be sensitive to photons with energies higher than 50 keV [R. Tommasini et al., Phys. Phys. Plasmas 18, 056309 (2011); G. N. Hall et al., “AXIS: An instrument for imaging Compton radiographs using ARC on the NIF,” Rev. Sci. Instrum. (these proceedings)]. To increase the sensitivity in the high energy region, we propose to use a combination of two MCPs. The first MCP is operated in a low gain regime and works as a thick photocathode, and the second MCP works as a high gain electron multiplier. We tested the concept of this dual MCP configuration and succeeded in obtaining a detective quantum efficiency of 4.5% for 59 keV x-rays, 3 times larger than with a single plate of the thickness typically used in NIF framing cameras. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891712] I. INTRODUCTION
X-ray framing cameras based on proximity-focused micro-channel plates (MCP) have been playing an important role as diagnostics of inertial confinement fusion experiments for several decades.1–3 Most of the current x-ray framing cameras consist of a single MCP, a phosphor, and a recording device (e.g., CCD or photographic films). This configuration is successful for imaging x-rays with energies below 20 keV. In this single MCP configuration, the MCP works as both the photocathode and the electron multiplier. To have enough optical output from the phosphor, the MCP has to be operated with electron multiplication gains of 500–2000. When the MCP is operated in this high gain regime, x-rays detected in the shallow layer experience high electron-multiplication and dominate the output, while x-rays detected deep inside have a limited contribution. Due to this large gain differential, the detective quantum efficiency (DQE) for volumetrically absorbed photons (above 20 keV) is severely reduced.4 One approach to mitigate this DQE reduction is to separate the photocathode from the electron multiplier. We propose to stack two MCPs and operate the first one in low gain regime, as a thick photocathode, and use the second MCP as the electron multiplier, to achieve enough optical output from the phosphor. II. DQE AND NOISE FACTOR
the obtainable image when the output of the single detection event has small variance. For detectors having broad pulseheight distribution (PHD), DQE is the quantity of interest.5 The definition of the DQE is SN Ro 2 DQE ≡ , (1) SN Ri where SNRo and SNRi are signal-to-noise ratio at output and input of the detector, respectively. When the statistical behavior of the input photons follows Poisson statistics, the expected signal-to-noise ratio of the image can be evaluated as SN Ro =
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)/11D623/3/$30.00
DQE × Ni ,
(2)
where Ni is the number of incident photons per resolution element. Another useful measure of the loss of the available information caused by the statistical fluctuation of the avalanche process is the noise factor (NF) defined as6 NF = 1 +
σ2 , ξ 2
(3)
where ξ is the mean and σ is the standard deviation of the PHD. When noise from other sources (e.g., multiplicative noise or statistical fluctuation of background exposure) is small, the DQE can be expressed as7
Quantum efficiency (QE), which is defined as the number of detected events per number of incident quanta, is a commonly used metric for estimating the statistical noise of a) Contributed paper, published as part of the Proceedings of the 20th
DQE =
QE . NF
(4)
III. EXPECTED PHD OF SINGLE MCP
The statistical uncertainty of an avalanche multiplication process is usually modeled by Pólya or Furry statistics.8 When an avalanche stream is started from a single electron
85, 11D623-1
© 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
