Gamma bang time analysis at OMEGA

Absolute bang time measurements with the gas Cherenkov detector (GCD) and gamma reaction history (GRH) diagnostic have been performed to high precision at the OMEGA laser facility at the University of Rochester with bang time values for the two diagnostics agreeing to within 5 ps on average. X-ray timing measurements of laser-target coupling were used to calibrate a facility-generated laser timing fiducial with rms spreads in the measured coupling times of 9 ps for both GCD and GRH. Increased fusion yields at the National Ignition Facility (NIF) will allow for improved measurement precision with the GRH easily exceeding NIF system design requirements.     

Experimental study of neutron induced background noise on gated x-ray framing cameras

A temporally gated x-ray framing camera based on a proximity focus microchannel plate is one of the most important diagnostic tools of inertial confinement fusion experiments. However, fusion neutrons produced in imploded capsules interact with structures surrounding the camera and produce background to x-ray signals. To understand the mechanisms of this neutron induced background, we tested several gated x-ray cameras in the presence of 14 MeV neutrons produced at the Omega laser facility. Differences between background levels observed with photographic film readout and charge-coupled-device readout have been studied.     

Development and characterization of sub-100 ps photomultiplier tubes

We describe the evaluation of a microchannel plate (MCP) photomultiplier tube (PMT), incorporating a 3?μm pore MCP and constant voltage anode and cathode gaps. The use of the small pore size results in PMTs with response functions of the order of 85 ps full-width-half-maximum, while the constant electric field across the anode and cathode gaps produces a uniform response function over the entire operating range of the device. The PMT was characterized on a number of facilities and employed on gas Cherenkov detectors fielded on various deuterium tritium fuel (DT) implosions on the Omega Laser Facility at the University of Rochester. The Cherenkov detectors are part of diagnostic development to measure Gamma ray reaction history for DT implosions on the National Ignition Facility.     

Bragg diffraction using a 100 ps 17.5 keV x-ray backlighter and the Bragg diffraction imager

A new diagnostic for measuring Bragg diffraction of petawatt-generated high-energy x rays off a laser-compressed crystal was designed and tested successfully at the Omega EP laser facility on static Mo and Ta (111) oriented single crystal samples using a 17.5 keV Mo?Kα backlighter. The Bragg diffraction imager consists of a heavily shielded enclosure and a precisely positioned beam block attached to the enclosure by an aluminum arm. Fuji image plates are used as the x-ray detectors. The diffraction from Mo and Ta (222) crystal planes was clearly detected with a high signal-to-noise. This technique will be applied to shock- and quasi-isentropically loaded single crystals on the Omega EP laser.     

Computer simulation of the vertex detector of the CLAS12 facility

The CLAS International Collaboration is preparing the CLAS detector for updating in 2013, because the energy of the CEBAF electron accelerator will be raised to 12 GeV. The new facility will be called the CLAS 12. An SVT vertex detector will be one of new units of the CLAS 12. This paper presents the results of computer simulation of the vertex detector using the GEANT4 package. Estimates of the detector counting rate and the expected accuracy of determining coordinates and momenta are obtained. The dependence of the efficiency of operation of the vertex detector on different external conditions is studied.     

Design of a cavity ring-down spectroscopy diagnostic for negative ion rf source SPIDER

The rf source test facility SPIDER will test and optimize the source of the 1 MV neutral beam injection systems for ITER. Cavity ring-down spectroscopy (CRDS) will measure the absolute line-of-sight integrated density of negative (H(-) and D(-)) ions, produced in the extraction region of the source. CRDS takes advantage of the photodetachment process: negative ions are converted to neutral hydrogen atoms by electron stripping through absorption of a photon from a laser. The design of this diagnostic is presented with the corresponding simulation of the expected performance. A prototype operated without plasma has provided CRDS reference signals, design validation, and results concerning the signal-to-noise ratio.     

A technique for measuring the difference of times of arrival of laser beams at a target based on analysis of the X-ray emission from a target on the Iskra-5 facility

A technique for measuring the difference of times of arrival of laser beams at a spherically irradiated target in experiments on the 12-channel Iskra-5 facility is described. This technique is based on detection in test experiments of X rays from a specially designed target with a slit X-ray streak camera. The obtained information is used to perform time locking of laser pulses recorded at the output of the facility’s amplifying stages with a multichannel photochronographic system. The results of recording laser pulses at the output of the amplifying stages obtained in working experiments allow determination of the difference of times of arrival of laser beams at a target with an accuracy no worse than 50 ps.     

A technique for measuring absolute fluxes of line X radiation from laser-produced plasma at the Iskra-5 facility

A technique for measuring absolute fluxes of line X radiation from plasmas produced by a laser in the targets of the Iskra-5 facility is described. Spectrographs based on convex and concave quartz, mica, and KAP crystals have been developed for recording hydrogen-and helium-like X-ray lines of Al, Si, Ar, and Fe. The calibration methods used in the experiments and results from absolute calibrations of crystals, X-ray photographic films, and filters are presented. The technique has been tested at the Iskra-5 facility by exposing Al and Fe targets to second-harmonic laser radiation. Procedures for processing recorded spectrograms taking due account of the calibration data and the results of data processing are presented.     

Dynamic Hohlraums as x-ray sources in high-energy density science

The first demonstration of laser driven dynamic Hohlraums (LDDH) as a spectrally smooth backlighter source for opacity and temperature measurements through absorption spectrometry of materials in local thermodynamic equilibrium at temperatures >150 eV has been made. This is a crucial temperature regime for future astrophysics and ignition fusion experiments at the nearly completed National Ignition Facility (NIF) [E. I. Moses and C. R. Wuest, Fusion Sci. Technol. 47, 314 (2005)] at the Lawrence Livermore National Laboratory. The new backlighter consists of a LDDH filled with either krypton or argon that implodes to create an x-ray flash. The properties of this x-ray flash have been measured in experiments at the Omega laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the Laboratory for Laser Energetics in Rochester, New York, satisfying all requirements imposed by future experiments: (1) the emission spectrum extends to at least 5.5 keV, well above the maximum x-ray energy ( approximately 3.5 keV) obtained from the previously "best" opacity backlighters (uranium M-shell emission backlighters); (2) the spectrum is smooth and featureless (intensity variation <6% rms), allowing absorption spectrometry through experimental samples; (3) the emission source size is sufficiently small (<50 microm) for projection backlighting through future samples; (4) the emission is bright enough (and twice as bright as imploding hydrogen-filled capsules) for gated spectrometer measurements; (5) the emission duration is optimized ( approximately 100 ps) for the current and future generations of spectrometers; and (6) by using only a small number of beams with limited energy and symmetry for the backlighter (10 out of 60 beams in the Omega experiments), the majority of laser beams are left available for heating sample materials to >150 eV.     

Modeling and characterization of a cantilever-based near-field scanning microwave impedance microscope

This paper presents a detailed modeling and characterization of a microfabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes. Using finite-element analysis, we model the tip-sample interaction as small impedance changes between the tip electrode and the ground at our working frequencies near 1 GHz. The equivalent lumped elements of the cantilever can be determined by transmission line simulation of the matching network, which routes the cantilever signals to 50 Omega feed lines. In the microwave electronics, the background common-mode signal is canceled before the amplifier stage so that high sensitivity (below 1 aF capacitance changes) is obtained. Experimental characterization of the microwave microscope was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals can be obtained from different reflection modes of the probe.     

Determination of the temperature of bremsstrahlung photon generated by ultraintense laser using various thickness attenuators

We evaluate the simplified method using the Lambert-Beer law to measure the temperature of bremsstrahlung photon generated by an ultraintense laser. Analytical values are compared to the results of the Monte Carlo calculation of GEANT4 and they agreed very well on the condition of the appropriate distance between the attenuator and the detector. We performed the experiment to measure the temperature of bremsstrahlung x-ray emitted from a metal target irradiated by a Ti:sapphire laser with 76 mJ, 72 fs, 2.2 × 10(18) W∕cm(2). For a Cu target of 30 μm thick, the photon temperature was reasonably determined to be 0.18 MeV, which is in good agreement with previous studies.     

A compact, multiangle electron spectrometer for ultraintense laser-plasma interaction experiments

Experiments on the multiterawatt (MTW) laser at the Laboratory for Laser Energetics will study the effect of the focal-spot shape on the forward acceleration and collimation of electrons. A compact electron spectrometer has been developed to record the energy spectra of electrons ejected in the interaction of the laser at multiple angular locations simultaneously. The modular system with replaceable magnets provides an adjustable energy band, currently 0.2-6 MeV. The detector is an array of imaging plates. The device is designed to operate in the high-noise environment (bremsstrahlung and Compton x rays, gamma rays, and scattered electrons), while being compact enough to fit in the 30 cm radius MTW target chamber. The detector geometry and shielding were optimized with the particle/radiation transport code GEANT4. Calibration was performed with beta sources. The required dynamic range, sensitivity, and resolution were confirmed with initial MTW experimental data.     

Position-sensitive detector of neutrons and nuclear fragments in the kinetic energy range of 10–200 MeV

The design of the position-sensitive neutron detector being developed by the Alikhanov Institute for Theoretical and Experimental Physics is described, and results of the simulation of its properties are presented. Based on the test results of the first prototype, the spatial resolution has been improved, and the detector shape has been optimized. The principal limitation on the positioning accuracy for neutrons is obtained from results of computer simulation in the GEANT4 environment. The expected spatial resolution of the designed detector is <1.5 cm.     

Development of a diagnostic technique based on Cherenkov effect for measurements of fast electrons in fusion devices

A diagnostic technique based on the Cherenkov effect is proposed for detection and characterization of fast (super-thermal and runaway) electrons in fusion devices. The detectors of Cherenkov radiation have been specially designed for measurements in the ISTTOK tokamak. Properties of several materials have been studied to determine the most appropriate one to be used as a radiator of Cherenkov emission in the detector. This technique has enabled the detection of energetic electrons (70 keV and higher) and the determination of their spatial and temporal variations in the ISTTOK discharges. Measurement of hard x-ray emission has also been carried out in experiments for validation of the measuring capabilities of the Cherenkov-type detector and a high correlation was found between the data of both diagnostics. A reasonable agreement was found between experimental data and the results of numerical modeling of the runaway electron generation in ISTTOK.     

A technique for measuring the propagation of a supersonic radiation front in foam via spatially resolved spectral imaging of a tracer layer

We present a technique for measuring the propagation of a supersonic radiation front in low-density foam, where the lack of motion of the objects in its wake makes it difficult to determine its location. We illuminate a thin tracer foil embedded in the foam with a broadband x-ray source, and measure its changing absorption of these x rays as it ionizes. We record both spatial and spectral information of the heated tracer, and thus obtain its ionization state as a function of distance along the front propagation direction. We extrapolate this information to determine the state of the foam and the location of the radiation front. We present the experimental configuration used to test this technique at the Omega laser facility along with experimental results.     

硅片的激光吸收系数修正与弯曲试验

考虑热物性能参数随温度变化的因素,以硅为对象进行激光弯曲模拟和试验,借助APDL语言编写激光弯曲成形的仿真程序,对单脉冲作用过程进行模拟,以得到单点脉冲周期内的温度分布;并采用NiCr/NiSi合金薄膜热电偶对单脉冲作用过程中的温度分布进行测量,对比上述的温度模拟与测量结果,修正硅材料的激光综合吸收系数。采用有限元分析软件实现了硅片的脉冲激光弯曲成形的仿真和模拟,并对多次连续扫描的模拟结果与硅片弯曲试验结果进行对比,验证了仿真程序的有效性,为硅片的激光弯曲成形提供了理论与试验依据。     

Source characterization and modeling development for monoenergetic-proton radiography experiments on OMEGA

A monoenergetic proton source has been characterized and a modeling tool developed for proton radiography experiments at the OMEGA [T. R. Boehly et al., Opt. Comm. 133, 495 (1997)] laser facility. Multiple diagnostics were fielded to measure global isotropy levels in proton fluence and images of the proton source itself provided information on local uniformity relevant to proton radiography experiments. Global fluence uniformity was assessed by multiple yield diagnostics and deviations were calculated to be ～16% and ～26% of the mean for DD and D(3)He fusion protons, respectively. From individual fluence images, it was found that the angular frequencies of ?50 rad(-1) contributed less than a few percent to local nonuniformity levels. A model was constructed using the Geant4 [S. Agostinelli et al., Nuc. Inst. Meth. A 506, 250 (2003)] framework to simulate proton radiography experiments. The simulation implements realistic source parameters and various target geometries. The model was benchmarked with the radiographs of cold-matter targets to within experimental accuracy. To validate the use of this code, the cold-matter approximation for the scattering of fusion protons in plasma is discussed using a typical laser-foil experiment as an example case. It is shown that an analytic cold-matter approximation is accurate to within ?10% of the analytic plasma model in the example scenario.     

Focusing of X-Rays with a Photon Energy of 9.5 keV by an Ellipsoid with a HOPG Crystal

Instruments and Experimental Techniques - The arrangement and results of full-power experiments on the Iskra-5 laser facility (the laser energy is 150 J, the pulse length is 0.3 ns, and the...     

Unfolding core asymmetries with x-ray emission images in symmetry diagnostic experiments

A novel inversion technique is proposed to unfold core asymmetries at the source with x-ray emission images, which were obtained from imploded surrogate capsules in symmetry diagnostic experiments. The axisymmetrical core emission can be expanded as a Fourier series, with Legendre polynomials and spherical Bessel functions as bases concerned with polar angle and radius, respectively. A least-squares estimator is employed to obtain the unknown coefficients from its two-dimensional image data. The unfolded Legendre coefficients can be further used to test modeling of drive asymmetries in hohlraums. This technique is also demonstrated with a proof-of-principle experiment performed on the Shenguang II laser facility [L. Zunqi et al., Chin. J. Lasers B10, 6 (2001)].