C. K. Li

Charged-particle acceleration and energy loss in laser-produced plasmas

Spectral measurements have been made of charged fusion products produced in deuterium + helium-3 filled targets irradiated by the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Comparing the energy shifts of four particle types has allowed two distinct physical processes to be probed: Electrostatic acceleration in the low-density corona and energy loss in the high-density target. When the fusion burn occurred during the laser pulse, particle energy shifts were dominated by acceleration effects. Using a simple model for the accelerating field region, the time history of the target electrostatic potential was found and shown to decay to zero soon after laser irradiation was complete. When the fusion burn occurred after the pulse, particle energy shifts were dominated by energy losses in the target, allowing fundamental charged-particle stopping-power predictions to be tested. The results provide the first experimental verification of the general form of stopping power theories over ...

A neutron spectrometer for precise measurements of DT neutrons from 10 to 18 MeV at OMEGA and the National Ignition Facility

A model independent method to determine fuel 〈ρR〉 is to measure the energy spectrum and yield of elastically scattered primary neutrons in deuterium–tritium (DT) plasmas. As is the case for complementary methods to measure fuel 〈ρR〉 (in particular from knock-on deuterons and tritons [S. Skupsky and S. Kacenjar, J. Appl. Phys. 52, 2608 (1981); C. K. Li et al. (unpublished)]), minimizing the background is critical for successful implementation. To achieve this objective, a novel spectrometer for measurements of neutrons in the energy range 10–18 MeV is proposed. From scattered neutrons (10–13 MeV), the DT fuel 〈ρR〉 will be measured; from primary neutrons (∼14 MeV), the ion temperature and neutron yield will be determined; and from secondary neutrons, in the energy range 12–18 MeV, the fuel 〈ρR〉 in deuterium plasmas will be inferred at the National Ignition Facility. The instrument is based on a magnetic spectrometer with a neutron-to-deuteron (nd) conversion foil for production of deuteron recoils at nearly...

Study of direct-drive, deuterium–tritium gas-filled plastic capsule implosions using nuclear diagnostics at OMEGA

Implosions of direct-drive, deuterium–tritium (DT) gas-filled plastic capsules are studied using nuclear diagnostics at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. In addition to traditional neutron measurements, comprehensive sets of spectra of deuterons, tritons, and protons elastically scattered from the fuel and shell by primary DT neutrons (“knock-on” particles) are, for the first time, obtained and used for characterizing target performance. It is shown with these measurements that, for 15-atm DT capsules with 20-μm CH shells, improvement of target performance is achieved when on-target irradiation nonuniformity is reduced. Specifically, with a two-dimensional (2D) single-color-cycle, 1-THz-bandwidth smoothing by spectral dispersion (SSD), plus polarization smoothing (PS), a primary neutron yield of ∼1×1013, a fuel areal density of ∼15 mg/cm2, and a shell areal density of ∼60 mg/cm2 are obtained; these are, respectively, ∼80%, ∼60%, and ∼35% higher than those achiev...

Target diagnostic system for the national ignition facility (invited)

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x ray, gamma ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating in the high radiation, electromagnetic pulse, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in t...

Direct-drive high-convergence-ratio implosion studies on the OMEGA laser system*

A series of direct-drive implosion experiments, using room-temperature, gas-filled CH targets, are performed on the University of Rochester’s OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The target performance at stagnation and its dependence on beam smoothing and pulse shaping is investigated. Compressed core conditions are diagnosed using x-ray and neutron spectroscopy, and x-ray imaging. The individual beams of OMEGA are smoothed by spectral dispersion in two dimensions (2D SSD) with laser bandwidths up to ∼0.3 THz, with 1 ns square to 2.5 ns shaped pulses. A clear dependence of target performance on pulse shape and beam smoothing is seen, with the target performance (yield, areal density, and shell integrity) improving as SSD bandwidth is applied.

Radiation-hardened x-ray imaging for burning-plasma tokamaks

A special type of vacuum-photodiode detector is being developed for x-ray imaging of plasma in fusion-producing tokamaks such as the international thermonuclear experimental reactor (ITER), where the radiation environment will be too hostile for conventional x-ray detectors. The vacuum photodiode has modest efficiency, but it is intrinsically immune to radiation damage if built in such a manner as to expose only metal components to radiation. A design based on appropriately chosen materials (including high-Z cathodes) and geometries (including a small angle between cathode surface and incident x rays) can provide good signals from the 1–100 keV x rays that are of particular importance for imaging the plasmas in the Joint European Torus (JET) and ITER. It should also provide better rejection of signal distortion and noise due to unwanted detection of neutrons and hard gamma rays than conventional detectors. A prototype design is described, along with performance parameters predicted for JET and ITER. In addition, we show results of laboratory experiments that confirm some of the predicted behaviors of the design.

A study of CR-39 track response to charged particles from NOVA implosions

We have exposed CR-39 track recording material to a number of NOVA implosions. Radiation from the implosion passed through an array of ranging filters, which aided identification of the incident particles and their energies. The etching procedure was calibrated by including a piece of track exposed to DD protons from a small accelerator. For the same shots, we quantitatively compare the DD neutron yield with the DD proton yield determined from the track. In DT implosions, tracks produced by neutron interactions prevent observation of charged-particle tracks that are produced by the processes of knock on, secondary, or tertiary fusion.

A fusion‐product source

A Texas Nuclear Cockcroft–Walton neutron generator was refurbished for use as a general fusion‐product source. This well‐calibrated source is now used routinely for characterizing energetic charged‐particle detectors, for the development of nuclear fusion diagnostics, for studying radiation damage, and for calibrating x‐ray detectors for laboratory and space plasmas. This paper is an overview of the facility. We describe the main accelerator operating systems, the primary fusion reactions studied, and several diagnostics used to characterize the fusion‐product source.

Charged-coupled devices for charged-particle spectroscopy on OMEGA and NOVA

Charge-coupled devices (CCDs) are to be utilized as charged-particle detectors for ρR and implosion symmetry diagnostics on OMEGA and NOVA. Using a large range of MeV charged particles, comprehensive studies have been performed on several back-illuminated CCDs in order to establish their utility for determining particle energy and identity (e.g., H, D, or T). Issues of signal and noise (i.e., especially from neutrons and gammas interacting with the CCDs) are also being investigated.