Kurtis A. Fletcher

Spectrometry of charged particles from inertial-confinement-fusion plasmas

High-resolution spectrometry of charged particles from inertial-confinement-fusion (ICF) experiments has become an important method of studying plasma conditions in laser-compressed capsules. In experiments at the 60-beam OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], utilizing capsules with D2, D3He, DT, or DTH fuel in a shell of plastic, glass, or D2 ice, we now routinely make spectral measurements of primary fusion products (p, D, T, 3He, α), secondary fusion products (p), “knock-on” particles (p, D, T) elastically scattered by primary neutrons, and ions from the shell. Use is made of several types of spectrometers that rely on detection and identification of particles with CR-39 nuclear track detectors in conjunction with magnets and/or special ranging filters. CR-39 is especially useful because of its insensitivity to electromagnetic noise and its ability to distinguish the types and energies of individual particles, as illustrated here by detailed calibrations of its respo...

Core performance and mix in direct-drive spherical implosions with high uniformity

The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)]. Polarization smoothing (PS) with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion (SSD) have been installed on OMEGA. The beam-to-beam power imbalance is ⩽5% rms. Implosions of 20-μm-thick CH shells (15 atm fill) using full beam smoothing (1-THz SSD and PS) have primary neutron yields and fuel areal densities that are ∼70% larger than those driven with 0.35-THz SSD without PS. They also produce ∼35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel–shell mix has been developed. It suggests that there is a “clean” fuel region, surrounded by a mixed region, that acc...

First results from cryogenic target implosions on OMEGA

Initial results from direct-drive spherical cryogenic target implosions on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] are presented. These experiments are part of the scientific base leading to direct-drive ignition implosions planned for the National Ignition Facility (NIF) [W. J. Hogan, E. I. Moses, B. E. Warner et al., Nucl. Fusion 41, 567 (2001)]. Polymer shells (1-mm diam with walls <3 μm) are filled with up to 1000 atm of D2 to provide 100-μm-thick ice layers. The ice layers are smoothed by IR heating with 3.16-μm laser light and are characterized using shadowgraphy. The targets are imploded by a 1-ns square pulse with up to ∼24 kJ of 351-nm laser light at a beam-to-beam rms energy balance of <3% and full-beam smoothing. Results shown include neutron yield, secondary neutron and proton yields, the time of peak neutron emission, and both time-integrated and time-resolved x-ray images of the imploding core. The experimental values are...

Shock-tuned cryogenic-deuterium-tritium implosion performance on Omega

Cryogenic-deuterium-tritium (DT) target compression experiments with low-adiabat (α), multiple-shock drive pulses have been performed on the Omega Laser Facility [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to demonstrate hydrodynamic-equivalent ignition performance. The multiple-shock drive pulse facilitates experimental shock tuning using an established cone-in-shell target platform [T. R. Boehly, R. Betti, T. R. Boehly et al., Phys. Plasmas 16, 056301 (2009)]. These shock-tuned drive pulses have been used to implode cryogenic-DT targets with peak implosion velocities of 3×107 cm/s at peak drive intensities of 8×1014 W/cm2. During a recent series of α∼2 implosions, one of the two necessary conditions for initiating a thermonuclear burn wave in a DT plasma was achieved: an areal density of approximately 300 mg/cm2 was inferred using the magnetic recoil spectrometer [J. A. Frenje, C. K. Li, F. H. Seguin et al., Phys. Plasmas 16, 042704 (2009)]. The other condition—a burn-averaged ion temperature ⟨Ti⟩n of 8–10 keV—cannot be achieved on Omega because of the limited laser energy; the kinetic energy of the imploding shell is insufficient to heat the plasma to these temperatures. A ⟨Ti⟩n of approximately 3.4 keV would be required to demonstrate ignition hydrodynamic equivalence [Betti et al., Phys. Plasmas17, 058102 (2010)]. The ⟨Ti⟩n reached during the recent series of α∼2 implosions was approximately 2 keV, limited primarily by laser-drive and target nonuniformities. Work is underway to improve drive and target symmetry for future experiments.Cryogenic-deuterium-tritium (DT) target compression experiments with low-adiabat (α), multiple-shock drive pulses have been performed on the Omega Laser Facility [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to demonstrate hydrodynamic-equivalent ignition performance. The multiple-shock drive pulse facilitates experimental shock tuning using an established cone-in-shell target platform [T. R. Boehly, R. Betti, T. R. Boehly et al., Phys. Plasmas 16, 056301 (2009)]. These shock-tuned drive pulses have been used to implode cryogenic-DT targets with peak implosion velocities of 3×107 cm/s at peak drive intensities of 8×1014 W/cm2. During a recent series of α∼2 implosions, one of the two necessary conditions for initiating a thermonuclear burn wave in a DT plasma was achieved: an areal density of approximately 300 mg/cm2 was inferred using the magnetic recoil spectrometer [J. A. Frenje, C. K. Li, F. H. Seguin et al., Phys. Plasmas 16, 042704 (2009)]. The other condition—a burn...

Direct-drive inertial confinement fusion research at the Laboratory for Laser Energetics: charting the path to thermonuclear ignition

Significant theoretical and experimental progress continues to be made at the University of Rochesters Laboratory for Laser Energetics (LLE), charting the path to direct-drive inertial confinement fusion (ICF) ignition. Direct drive offers the potential for higher-gain implosions than x-ray drive and is a leading candidate for an inertial fusion energy power plant. LLEs direct-drive ICF ignition target designs for the National Ignition Facility (NIF) are based on hot-spot ignition. A cryogenic target with a spherical DT-ice layer, within or without a foam matrix, enclosed by a thin plastic shell, will be directly irradiated with ~1.5 MJ of laser energy. Cryogenic and plastic/foam (surrogate-cryogenic) targets that are hydrodynamically scaled from these ignition target designs are imploded on the 60-beam, 30 kJ, UV OMEGA laser system to validate the key target physics issues, including energy coupling, hydrodynamic instabilities and implosion symmetry. Prospects for direct-drive ignition on the NIF are extremely favourable, even while it is in its x-ray-drive irradiation configuration, with the development of the polar-direct-drive concept. A high-energy petawatt capability is being constructed at LLE next to the existing 60-beam OMEGA compression facility. This OMEGA EP (extended performance) laser will add two short-pulse, 2.6 kJ beams to the OMEGA laser system to backlight direct-drive ICF implosions and study fast-ignition physics with focused intensities up to 6 × 1020 W cm−2.

High-performance inertial confinement fusion target implosions on OMEGA

The Omega Laser Facility is used to study inertial confinement fusion (ICF) concepts. This paper describes progress in direct-drive central hot-spot (CHS) ICF, shock ignition (SI) and fast ignition (FI) since the 2008 IAEA FEC conference. CHS cryogenic deuterium–tritium (DT) target implosions on OMEGA have produced the highest DT areal densities yet measured in ICF implosions (~300 mg cm−2). Integrated FI experiments have shown a significant increase in neutron yield caused by an appropriately timed high-intensity, high-energy laser pulse.

Stress measurement in sputtered copper films on flexible polyimide substrates

We have investigated the dependence of internal stress on argon pressure for 0.25‐μm copper films sputtered onto a 1‐mil(25‐μm)‐thick polyimide substrate. A dc planar magnetron was used to deposit the copper at a rate of 2 A/s onto the flexible substrate which was held flat by top and bottom edges. The stress was estimated directly from the resulting radius of curvature of the relaxed film and substrate. The critical argon pressure at which a transition from compression to tension occurs was found to be about 2.5 mTorr. In addition, the dependencies of film morphology, resistivity, and optical reflectance on argon pressure were studied. The critical pressures at which resistivity starts to rise and reflectance begins to decrease are very close to the stress transition pressure. In general, the data are consistent with the dependencies of physical properties on pressure observed for other sputtered metals.

A very low energy tandem accelerator

Abstract A small tandem accelerator has been constructed at Triangle Universities Nuclear Laboratory (TUNL), capable of providing up to 250 keV of additional energy to singly charged light ions. Various characteristics of the device, including the beam transmission and the trace space density distribution of the resulting beam, have been measured.

Some Pivotal Thoughts on the Current Balance

The current balance is an excellent device for demonstrating the force on a current-carrying wire. By considering the electrons flowing through the wires and applying some geometrical analysis, we can gain a better understanding of why the wire moves, how the current is distributed in the wires, and why the simplifying assumptions of the force law apply to this realistic situation.

Detection of Charged Particles with Charge Injection Devices

A method for using charge injection devices (CIDs) for detection of high-energy charged particles from inertial-confinement fusion reactions is described. Because of the relatively small depletion region of the CID camera (depletion depth of approximately 7 mum), aluminum foils are placed in front of the device to reduce the energy of the charged particles and maximize the energy deposited in the CID. Simultaneous measurements of (2)H(d,p)(3)H protons with a CID and a surface barrier detector indicate that the CID is an efficient detector of charged fusion products. Tests using high energy alpha particles emitted from a radium-226 source are also reported.