N. Sinenian

The response of CR-39 nuclear track detector to 1–9 MeV protons

The response of CR-39 nuclear track detector (TasTrak(®)) to protons in the energy range of 0.92-9.28 MeV has been studied. Previous studies of the CR-39 response to protons have been extended by examining the piece-to-piece variability in addition to the effects of etch time and etchant temperature; it is shown that the shape of the CR-39 response curve to protons can vary from piece-to-piece. Effects due to the age of CR-39 have also been studied using 5.5 MeV alpha particles over a 5-year period. Track diameters were found to degrade with the age of the CR-39 itself rather than the age of the tracks, consistent with previous studies utilizing different CR-39 over shorter time periods.

Initial cone-in-shell fast-ignition experiments on OMEGAa)

Fast ignition is a two-step inertial confinement fusion concept where megaelectron volt electrons ignite the compressed core of an imploded fuel capsule driven by a relatively low-implosion velocity. Initial surrogate cone-in-shell, fast-ignitor experiments using a highly shaped driver pulse to assemble a dense core in front of the cone tip were performed on the OMEGA/OMEGA EP Laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997); L. J. Waxer et al., Opt. Photonics News 16, 30 (2005)]. With optimal timing, the OMEGA EP pulse produced up to ∼1.4 × 107 additional neutrons which is a factor of ∼4 more neutrons than without short-pulse heating. Shock-breakout measurements performed with the same targets and drive conditions demonstrate an intact cone tip at the time when the additional neutrons are produced. Velocity interferometer system for any reflector measurements show that x-rays from the shell’s coronal plasma preheat the inner cone wall of thin-walled Au cones, while the thick-walled cones that are...

Calibration of a Thomson parabola ion spectrometer and Fujifilm imaging plate detectors for protons, deuterons, and alpha particles

A Thomson parabola ion spectrometer has been designed for use at the Multiterawatt (MTW) laser facility at the Laboratory for Laser Energetics (LLE) at the University of Rochester. This device uses parallel electric and magnetic fields to deflect particles of a given mass-to-charge ratio onto parabolic curves on the detector plane. Once calibrated, the position of the ions on the detector plane can be used to determine the particle energy. The position dispersion of both the electric and magnetic fields of the Thomson parabola was measured using monoenergetic proton and alpha particle beams from the SUNY Geneseo 1.7 MV tandem Pelletron accelerator. The sensitivity of Fujifilm BAS-TR imaging plates, used as a detector in the Thomson parabola, was also measured as a function of the incident particle energy over the range from 0.6 MeV to 3.4 MeV for protons and deuterons and from 0.9 MeV to 5.4 MeV for alpha particles. The device was used to measure the energy spectrum of laser-produced protons at MTW.

A novel particle time of flight diagnostic for measurements of shock- and compression-bang times in D3He and DT implosions at the NIF

The particle-time-of-flight (pTOF) diagnostic, fielded alongside a wedge range-filter (WRF) proton spectrometer, will provide an absolute timing for the shock-burn weighted ρR measurements that will validate the modeling of implosion dynamics at the National Ignition Facility (NIF). In the first phase of the project, pTOF has recorded accurate bang times in cryogenic DT, DT exploding pusher, and D(3)He implosions using DD or DT neutrons with an accuracy better than ±70 ps. In the second phase of the project, a deflecting magnet will be incorporated into the pTOF design for simultaneous measurements of shock- and compression-bang times in D(3)He-filled surrogate implosions using D(3)He protons and DD-neutrons, respectively.

Charged-particle spectroscopy for diagnosing shock ρR and strength in NIF implosions

The compact Wedge Range Filter (WRF) proton spectrometer was developed for OMEGA and transferred to the National Ignition Facility (NIF) as a National Ignition Campaign diagnostic. The WRF measures the spectrum of protons from D-(3)He reactions in tuning-campaign implosions containing D and (3)He gas; in this work we report on the first proton spectroscopy measurement on the NIF using WRFs. The energy downshift of the 14.7-MeV proton is directly related to the total ρR through the plasma stopping power. Additionally, the shock proton yield is measured, which is a metric of the final merged shock strength.

Advances in compact proton spectrometers for inertial-confinement fusion and plasma nuclear science.

Compact wedge-range-filter proton spectrometers cover proton energies ∼3-20 MeV. They have been used at the OMEGA laser facility for more than a decade for measuring spectra of primary D(3)He protons in D(3)He implosions, secondary D(3)He protons in DD implosions, and ablator protons in DT implosions; they are now being used also at the National Ignition Facility. The spectra are used to determine proton yields, shell areal density at shock-bang time and compression-bang time, fuel areal density, and implosion symmetry. There have been changes in fabrication and in analysis algorithms, resulting in a wider energy range, better accuracy and precision, and better robustness for survivability with indirect-drive inertial-confinement-fusion experiments.

Time evolution of filamentation and self-generated fields in the coronae of directly driven inertial-confinement fusion capsules

Time-gated radiography with monoenergetic 15-MeV protons, 3-MeV protons, and 4-MeV alpha particles has revealed a rich and complex evolution of electromagnetic field structures in and around imploding, directly driven inertial-confinement fusion (ICF) targets at the OMEGA laser facility. Plastic-shell capsules and solid plastic spheres were imaged during and after irradiation with ICF-relevant laser drive (up to 6 × 1014 W/cm2). Radial filaments appeared while the laser was on; they filled, and were frozen into, the out-flowing corona, persisting until well after the end of the laser drive. Data from specially designed experiments indicate that the filaments were not generated by two-plasmon-decay instabilities or by Rayleigh-Taylor instabilities associated with shell acceleration. Before the onset of visible filamentation, quasi-spherical field structures appeared outside the capsule in the images in a form that suggests outgoing shells of net positive charge. We conjecture that these discrete shells are...

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.

High-resolution Thomson parabola for ion analysis

A new, versatile Thomson parabola ion energy (TPIE) analyzer has been designed, constructed, and used at the OMEGA-EP facility. Laser-accelerated multi-MeV ions from hemispherical C targets are transmitted through a W pinhole into a multi-kG magnetic field and subsequently through a parallel electric field of up to 25 kV/cm. The ion drift region has a user-selected length of 10, 50, or 80 cm. With the highest fields, 400-MeV C(6+) and C(5+) may be resolved. TPIE is ten-inch manipulator (TIM)-mounted at OMEGA-EP and can be used opposite either of the EP ps beams. The instrument runs on pressure-interlocked 15-Vdc power available in EP TIM carts. Flux control derives from the insertion depth into the target chamber and the user-selected pinhole dimensions. The detector consists of CR39 backed by an image plate. A fully relativistic simulation code for calculating ion trajectories was employed for design optimization. Excellent agreement of code predictions with the actual ion positions on the detectors is observed. Through pit counting of carbon-ion tracks in CR39, it is shown that conversion efficiency of laser light to energetic carbon ions exceeds ~5% for these targets.

The coincidence counting technique for orders of magnitude background reduction in data obtained with the magnetic recoil spectrometer at OMEGA and the NIF

A magnetic recoil spectrometer (MRS) has been built and successfully used at OMEGA for measurements of down-scattered neutrons (DS-n), from which an areal density in both warm-capsule and cryogenic-DT implosions have been inferred. Another MRS is currently being commissioned on the National Ignition Facility (NIF) for diagnosing low-yield tritium-hydrogen-deuterium implosions and high-yield DT implosions. As CR-39 detectors are used in the MRS, the principal sources of background are neutron-induced tracks and intrinsic tracks (defects in the CR-39). The coincidence counting technique was developed to reduce these types of background tracks to the required level for the DS-n measurements at OMEGA and the NIF. Using this technique, it has been demonstrated that the number of background tracks is reduced by a couple of orders of magnitude, which exceeds the requirement for the DS-n measurements at both facilities.