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Target

Blast Shield

3p (Ch. 2)

2p (Ch. 1)

3p (Ch. 1)

2p (Ch. 2)

Crystal Ch. 1 Rear Slit/Filter

Crystal 1

Front Aperture Channel 1

Center Block Crystal Ch. 2 MCP

FIG. 3. The schematic of the SIS with the side cover removed is shown. The instrument body contains a front aperture, center block, and a rear slit to limit the line-of-sight to the image plane to x-rays originating at the backlighter position.

Channel 2 Crystal 2

FIG. 1. The ray trace design of the SIS is shown. Two, symmetric elliptical crystals are positioned such that the 1s-2p Ti absorption features of channel (1/2) probe the heated sample positioned at z = 0 at the same location as the 1s-3p absorption features of channel (2/1).

backlighter along the x-ray line-of-sight resulting in a magnification of ∼20× for experiment on NIF and ∼70× on OMEGA. Pointing offsets of the instrument in the axial direction are realized at the intersection of the 2 channels at a ratio of 2 to 1 (instrument to intersection). In the spectral direction, this ratio is 1 to 1. In both cases, the separation between the crossings is 400 ± 50 μm. In the imaging direction, tolerances are dictated by the magnification of the specific platform. Figure 2 shows a 3D representation of the raytrace of the instrument specifically demonstrating how the spectral, spatial, and temporal data traverse the heated tracer and appear on the 2-strip MCP. In this configuration, the spatial and temporal domains are always coupled. Figure 3 shows a schematic of the SIS with the side cover removed. The SIS has a standoff distance of 650 mm and a total diagnostic envelope that accommodates the Ten Inch Manipulators (TIM) at OMEGA and the Diagnostic Instrument Manipulator (DIM) at NIF. A blast shield filter pack is located behind the front aperture for each channel. A center block prevents any straight through light from reaching the image plane while a 1 mm wide rear slit reduces background from x-ray scattering inside of the instrument. A second filtration pack is located in front of the rear slit. III. CALIBRATION EXPERIMENTS

Calibration experiments were performed on the TRIDENT laser19 where Titanium-Vanadium (Ti-V) foils were Sp ati al/ Te

mp

ora

l

Elliptical ADP(101) Crystal

Spectral

Spectral 2-Strip MCP (detector)

Broadband X-Ray Backlighters Heat Heated Tracer

Elliptical ADP(101) Crystal

FIG. 2. A 3D representation of the instrument ray trace is shown, demonstrating the spectral, spatial, and temporal orientation of the instrument.

driven with 527 nm (2) laser light at ∼250 J with a ∼3 ns square pulse to produce Ti Lyman-alpha (∼4.9 keV), Helium-alpha (∼4.75 keV) and V Helium-alpha (∼5.2 keV). The Ti-V foil was positioned at the crossing locations of the 2 channels of the SIS (the location of the tracer for absorption spectroscopy measurements). Time integrated data were taken using a 50 mm Fuji BAS-MS image plate. Figure 4(a) shows typical time integrated image plate data cropped to match the footprint of the strips of the 2-strip MCP. The figure shows the locations of the Ti Lyα , Heα and the V Heα emission lines on both of the channels. Both channels of the instrument show data filling ∼10 mm of the MCP in the spectral direction, with the data positioned correctly near the axis of the instrument but falling significantly short of filling the total 15 mm of the MCP as in the design. Experiments were also performed using broadband sources in the spectral range of the instrument (CsI and Au) and the data consistently occupied the inner-most 10-mm of each MCP strip. This characteristic is observed in all 6 of the SIS ADP(101) crystals. As a result, the individual channels are unable to measure both the 1s-2p and 1s-3p absorption features as shown in figure 4(b). Lineouts of each crystal indicate a spectral range of ∼4.7–5.3 keV. The regions in yellow show the energy range of the key Ti absorption features. As built, the 1s-2p Ti absorption features are outside the spectral range of the instrument. The crystals can be shimmed to shift the spectral coverage to observe either spectral feature. Each channel can be shimmed to observe either the 1s-2p or 1s-3p absorption feature such that two temperature measurements with an overlapped line of sight can still be achieved. In addition to the shortcoming of the overall reflection of the crystals onto the image plane, the absolute locations of the spectral lines observed during the calibration experiments were not consistent with the locations anticipated from ray tracing the instrument. Figure 4(c) shows the ray trace of the emission spectrum of a 500 eV Ti-V foil simulated using Helios20 and PrismSpect21 for a foil located at z = 0 (left) and z = 50 mm (right) towards the SIS. Compared to the data in Figure 4(a), all three of the spectral emission lines in the z = 0 simulation are located farther away from the axis of the instrument by ∼1 mm. Moving the Ti-V foil 50 mm towards the SIS (z = 50 mm) shows much better agreement with the measurements in Figure 4(a). The discrepancies between the spectral performance of the designed and as-built SIS are still being investigated. The overall build of the instrument has been verified to be

Hager et al.

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Rev. Sci. Instrum. 85, 11D601 (2014)

(a)

Ch. 2 Crystal Edge

Spectral Position (mm)

Ti Heα Ti Lyα V Heα V Heα Ti Lyα Ti Heα

Crystal Edge

Ch. 1

ACKNOWLEDGMENTS

Imaging Position (mm)

(b)

1s-2p Ti Absorption

the same location in space. As built, the instrument’s spectral range is below the design specification and experimental requirements. However, the positioning of the crystals has been modified to use each channel of the instrument for an individual spectral line, still probing the heated tracer at a single location at 2 different spectral energies. Future work will focus on understanding the discrepancies between the as-built and designed instrument to design new crystals that will meet the original design specification.

This work was performed under the auspices of the U.S. Department of Energy by LANL under Contract No. DEAC52-06NA25396.

1s-3p Ti Absorption

Ti Heα Crystal Edge

1 C.

V Heα Crystal Edge

Ti Lyα

Spectral Position (mm)

(c) zTi-V = 0

zTi-V = 50 mm Ti Heα Ti Lyα V Heα

Imaging Position (mm)

FIG. 4. Time-integrated measured spectral data from a laser driven Ti-V foil is shown (a). The data are cropped to the image plane of a 2-strip MCP. The positions of the Ti Heα , Lyα , and V Heα are indicated. Each crystal demonstrates a spectral range of ∼4.7- to ∼5.3-keV (b), which includes the 1s-3p absorption feature but not the 1s-2p. The simulated spectrum from a 500 eV Ti-V foil is shown ray traced through the SIS design for foil locations of z = 0 (left) and z = 50 (right) in Figure 4(c). Compared to the measured spectrum (Ch. 2 of Fig. 4(a)) the z = 0 spectrum shows the Ti and V emission lines ∼ 1 mm farther from the axis of the instrument than measured. By moving the source 50 mm towards the SIS, the ray trace accurately reproduces the measured data.

within specification to < 5 mm precision. The curvature of the crystal substrates have been verified within specifications to ± 100 μm using direct and optical interferometry measurements. Reverse ray tracing of the measured spectrum indicates the crystal curvature would need to be out of specification by over 500 μm to reproduce the measured data. Future experiments will investigate the properties of the crystals using monoenergetic x-rays to probe the 2d spacing and effective x-ray crystal curvature. IV. CONCLUSION

We have shown the design for a split imaging spectrometer for absorption spectroscopy of Ti tracers with temperatures in the 50-200 eV range using elliptically curved ADP(101) crystals. This instrument has 2 symmetric lines of sight that allow spectral lines of different energies to probe the tracer at

A. Back, J. D. Bauer, O. L. Landen, R. E. Turner, B. F. Lasinski, J. H. Hammer, M. D. Rosen, L. J. Suter, and W. H. Hsing, Phys. Rev. Lett. 84, 274 (2000). 2 C. A. Back, J. D. Bauer, J. H. Hammer, B. F. Lasinski, R. E. Turner, P. W. Rambo, O. L. Landen, L. J. Suter, M. D. Rosen, and W. W. Hsing, Phys. Plasmas 7, 2126 (2000). 3 J. M. Taccetti, P. A. Keiter, N. Lanier, K. Mussack, K. Belle, and G. R. Magelssen, Rev. Sci. Instrum. 83, 023506 (2012). 4 T. Afshar-rad, M. Desselberger, M. Dunne, J. Edwards, J. M. Foster, D. Hoarty, M. W. Jones, S. J. Rose, P. A. Rosen, R. Taylor, and O. Willi, Phys. Rev. Lett. 73, 74 (1994). 5 D. Hoarty, A. Iwase, C. Meyer, J. Edwards, and O. Willi, Phys. Rev. Lett. 78, 3322 (1997). 6 J. Massen, G. D. Tsakiris, K. Eidmann, I. B. Földes, T. Löwer, R. Sigel, S. Witkowski, H. Nishimura, T. Endo, H. Shiraga, M. Takagi, Y. Kato, and S. Nakai, Phys. Rev. E 50, 5130 (1994). 7 P. Keiter, M. Gunderson, J. Foster, P. Rosen, A. Comley, M. Taylor, and T. Perry, Phys. Plasmas 15, 056901 (2008). 8 T. R. Boehly, D. L. Brown, R. S. Craxton, R. L. Keck, J. P. Knauer, J. H. Kelly, T. J. Kessler, S. A. Kumpan, S. J. Loucks, S. A. Letzring, F. J. Marshall, R. L. McCrory, S. F. B. Morse, W. Seka, J. M. Soures, and C. P. Verdon, Opt. Commun. 133, 495 (1997). 9 C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers, S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian, M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall, N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nostrand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M. Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K. White, S. T. Yang, and B. M. Van Wonterghem, Appl. Opt. 46, 3276 (2007). 10 D. Hoarty, L. Barringer, C. Vickers, O. Willi, and W. Nazarov, Phys. Rev. Lett. 82, 3070 (1999). 11 D. Hoarty, O. Willi, L. Barringer, C. Vickers, R. Watt, and W. Nazarov, Phys. Plasmas 6, 2171 (1999). 12 O. Willi, L. Barringer, C. Vickers, and D. Hoarty, Astrophys. J. Suppl. Ser. 127, 527 (2000). 13 J. L. Wiza, Nucl. Instrum. Methods 162, 587 (1979). 14 D. G. Stearns, J. D. Wiedwald, W. M. Cook, and R. L. Hanks, Rev. Sci. Instrum. 57, 2455 (1986). 15 D. G. Stearns, J. D. Wiedwald, B. M. Cook, R. L. Hanks, and O. L. Landen, Rev. Sci. Instrum. 60, 363 (1989). 16 D. K. Bradley, P. M. Bell, O. L. Landen, J. D. Kilkenny, and J. Oertel, Rev. Sci. Instrum. 66, 716 (1995). 17 J. A. Oertel, R. Aragonez, T. Archuleta, C. Barnes, L. Casper, V. Fatherley, T. Heinrichs, R. King, D. Landers, F. Lopez, P. Sanchez, G. Sandoval, L. Schrank, P. Walsh, P. Bell, M. Brown, R. Costa, J. Holder, S. Montelongo, and N. Pederson, Rev. Sci. Instrum. 77, 10E308 (2006). 18 A. J. Burek, D. M. Barrus, and R. L. Blake, Astrophys. J. 191, 533 (1974). 19 N. K. Moncur, R. P. Johnson, R. G. Watt, and R. B. Gibson, Appl. Opt. 34, 4274 (1995). 20 J. J. MacFarlane, I. E. Golovkin, and P. R. Woodruff, J. Quant. Spectrosc. Radiat. Transfer 99, 381 (2006). 21 J. J. MacFarlane, I. E. Golovkin, P. R. Woodruff, D. R. Welch, B. V. Oliver, T. A. Mehlhorn, and R. B. Campbell, Proceedings of the Inertial Fusion Sciences and Applications 2003 (IFSA 2003), Monterey, California, (American Nuclear Society, La Grange Park, Illinois, 2004), p. 457.

REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D602 (2014)

A novel femtosecond-gated, high-resolution, frequency-shifted shearing interferometry technique for probing pre-plasma expansion in ultra-intense laser experimentsa) S. Feister,1,2,b) J. A. Nees,2,3 J. T. Morrison,4 K. D. Frische,2 C. Orban,1,2 E. A. Chowdhury,1,5 and W. M. Roquemore6 1

Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA Innovative Scientific Solutions, Inc., Dayton, Ohio 45459, USA 3 Center for Ultra-Fast Optical Science, University of Michigan, Ann Arbor, Michigan 48109, USA 4 Fellow, National Research Council, Washington, D.C. 20001, USA 5 Intense Energy Solutions, LLC., Plain City, Ohio 43064, USA 6 Air Force Research Laboratory, Dayton, Ohio 45433, USA 2

(Presented 2 June 2014; received 5 June 2014; accepted 18 June 2014; published online 17 July 2014) Ultra-intense laser-matter interaction experiments (>1018 W/cm2 ) with dense targets are highly sensitive to the effect of laser “noise” (in the form of pre-pulses) preceding the main ultra-intense pulse. These system-dependent pre-pulses in the nanosecond and/or picosecond regimes are often intense enough to modify the target significantly by ionizing and forming a plasma layer in front of the target before the arrival of the main pulse. Time resolved interferometry offers a robust way to characterize the expanding plasma during this period. We have developed a novel pump-probe interferometry system for an ultra-intense laser experiment that uses two short-pulse amplifiers synchronized by one ultra-fast seed oscillator to achieve 40-fs time resolution over hundreds of nanoseconds, using a variable delay line and other techniques. The first of these amplifiers acts as the pump and delivers maximal energy to the interaction region. The second amplifier is frequency shifted and then frequency doubled to generate the femtosecond probe pulse. After passing through the laser-target interaction region, the probe pulse is split and recombined in a laterally sheared Michelson interferometer. Importantly, the frequency shift in the probe allows strong plasma self-emission at the second harmonic of the pump to be filtered out, allowing plasma expansion near the critical surface and elsewhere to be clearly visible in the interferograms. To aid in the reconstruction of phase dependent imagery from fringe shifts, three separate 120◦ phase-shifted (temporally sheared) interferograms are acquired for each probe delay. Three-phase reconstructions of the electron densities are then inferred by Abel inversion. This interferometric system delivers precise measurements of pre-plasma expansion that can identify the condition of the target at the moment that the ultra-intense pulse arrives. Such measurements are indispensable for correlating laser pre-pulse measurements with instantaneous plasma profiles and for enabling realistic Particle-in-Cell simulations of the ultra-intense laser-matter interaction. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4886955] I. INTRODUCTION

In ultra-intense laser systems, so-called “pre-pulses” are inexorably produced during the laser amplification process, and can pre-ablate the target, changing the condition of the target at the time of arrival of the main ultra-intense laser pulse. The outcome of this main pulse interaction sensitively depends on these pre-ablations.1, 2 The pre-ablation of the target by the pre-pulse can be difficult to predict ab initio using hydrodynamic and particle-in-cell codes in large part due to the wide range of timescales involved in the lasermatter interaction. Relevant pre-pulses occur and their effects evolve on the nanosecond and/or picosecond scale, with consequences to the main laser-plasma interaction occurring on the femtosecond scale.3 Characterizing and controlling these 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) Electronic mail: [email protected] 0034-6748/2014/85(11)/11D602/4/$30.00

pre-plasma conditions is an integral part of ultra-short pulse experiments.4, 5 Interferometry is a well-known technique that can reveal the plasma electron density profile.6 However, the temporal resolution of interferometric measurements, even with state-of-the-art streak camera technology, is typically limited to picosecond timescales and these devices are prohibitively expensive. We present a pump-probe technique with precision timing features that allow interferometric phase reconstruction to occur on timescales of less than 100 fs, in a cost-effective manner, while retaining roughly nine orders of magnitude temporal dynamic range. Femtosecond timing stability between pump and probe beams is achieved through use of a common oscillator. The probe light is used in two ways: shadowgraphy reveals general features,7 while interferometry and an Abel inversion6 recover phase and reveal subtle features of plasma evolution.8 The contamination of shadowgraphs and interferograms by the plasma self-emission (e.g., Refs. 9 and 10) is avoided

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by selective optical filtering and a frequency shift of the probe beam before amplification. These techniques create a unique platform for performing spatially and temporally resolved measurements of the target evolution. We show results from an experiment at the Air Force Research Lab (AFRL) in which a flowing water jet target11, 12 is irradiated by nanosecond-scale and picosecond-scale pre-pulses that precede a 30-fs FWHM ultra-intense (1018 W/cm2 ) interaction. Section II describes the experimental setup, and Sec. III describes how the ultra-short timescale synchronization of the probe and pump pulse is achieved. Section IV describes the frequency shift of the probe pulse and selective filtering. Section V presents some preliminary data of the water jet expansion due to laser-target interactions, including Abel inversion of interferometric data. Section VI states our conclusions and describes how this instrument will be incorporated into future work. II. EXPERIMENTAL OVERVIEW

Interferometry and shadowgraphy are used to characterize plasma expansion in experiments at the AFRL at Wright Patterson Air Force Base in Dayton, OH. An 800 nm, 30-fs FWHM pump beam produces high intensities (1018 W/cm2 , 2.6 μm FWHM spot size) on flowing water jet column targets. To avoid disruption of the high intensity laser light as it propagates to focus and to prevent freezing of the water jet nozzle, the experiment is housed in a vacuum chamber that is held at 20 Torr partial vacuum using a thermocouple gauge solenoid valve feedback loop. Known pre-pulse artifacts of the amplification process pre-ablate the target, creating conditions of interest picoseconds and nanoseconds before the main laser pulse interaction. To interrogate these conditions, a 420 nm, 80-fs FWHM probe beam is passed through the interaction region and subsequently split to image the target for shadowgraphy and interferometry. The pump-probe experimental layout is shown in Fig. 1. The probe beam is frequency-shifted, as discussed in Sec. IV. Distortions to the probe pulse wavefront due to variations in the index of refraction along the line of sight are revealed by interfering the probe pulse with a reference pulse at the same frequency. In this laterally sheared Michelson interferometer setup,13 an image of the interaction region of the water jet is overlapped with a sheared image of the water jet downstream. Interference fringes shift in response to changes in line-ofsight index of refraction, allowing one to “see” ablated liquid and ionized plasma in a way complementary to shadowgraphy. With a high dynamic range timing setup (described next) and selective optical filtering, one can get a sense of the evolution of this ablated liquid and plasma. III. FEMTOSECOND RESOLUTION OVER MICROSECONDS

One commonly used approach to pump/probe experiment is to use two entirely separate femtosecond laser systems, synchronizing the laser oscillators via electronic signals. A typical setup gives picosecond stability between oscillators; achieving femtosecond stability requires advanced electron-

Rev. Sci. Instrum. 85, 11D602 (2014)

FIG. 1. The experiment employs a high-intensity pump and low-intensity probe, relatively timed with better than 40-fs precision. The pump beam (sketched in red) and its pre-pulses irradiate the water jet target, creating a dynamic laser-plasma interaction region. The probe beam (sketched in blue) envelops the interaction region and is then split for target-imaged interferometry and shadowgraphy. Within the laterally sheared Michelson interferometer, a piezoelectric mirror allows for controlled adjustment of phase (mirror translation) and shear (mirror tilt). The shear angle is out of the page, though sketched in the plane of the page for easy visualization. Optical notch filters and an iris (spatial filter) exclude unwanted light such as that from plasma self-emission (see Fig. 2).

ics and a reduction in the output repetition rate.14 Utilizing two pulses from a single oscillator is simpler and results in a more stable relative timing. To achieve femtosecond relative precision in such a setup, the probe and pump beams must be seeded from a common oscillator. In a kHz system, a Pockels cell selects one oscillator pulse every millisecond to be further amplified as the pump pulse, rejecting all other pulses. The oscillator used in the experiment at AFRL produces pulses at 80 MHz, and the rejected pulses are routed into a second Pockels cell that selects a different pulse to be amplified as the probe pulse. By varying the pulse selected, coarse delays can be introduced between the pump and probe on the order of the oscillator rate, 80 MHz or 12.5 ns. After amplification, the probe pulse passes through a delay line. The double-passed delay line allows fine adjustments to the relative pump-probe delay, with < 40 fs resolution over 19 ns. Combining coarse and fine delay techniques results in 40 fs resolution over 10 μs, and potentially even longer times (e.g., 900 μs) if desired. The final resolution of the system is limited by the greater of the delay line resolution (< 40 fs in this setup) or probe pulse duration (80 fs in this setup). The combination of pulse seed selection and delay line adjustment allows roughly nine orders of magnitude temporal dynamic range. Fig. 3 exhibits this dynamic range, showing the pre-plasma expansion before the arrival of the main ultra-intense pulse (upper sequence) and the hydrodynamic response of the water jet over 10 μs of evolution (lower sequence). IV. ELIMINATION OF PLASMA SELF-EMISSION NOISE

Irradiated by the pump beam at 800 nm, the target will naturally emit light at the second harmonic (400 nm),15 which

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Evaluation of two-stage system for neutron measurement aiming at increase in count rate at Japan Atomic Energy Agency-Fusion Neutronics Source.

In order to increase the count rate capability of a neutron detection system as a whole, we propose a multi-stage neutron detection system. Experiment...
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