F. H. Seguin

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...

Proton Radiography of Inertial Fusion Implosions

A distinctive way of quantitatively imaging inertial fusion implosions has resulted in the characterization of two different types of electromagnetic configurations and in the measurement of the temporal evolution of capsule size and areal density. Radiography with a pulsed, monoenergetic, isotropic proton source reveals field structures through deflection of proton trajectories, and areal densities are quantified through the energy lost by protons while traversing the plasma. The two field structures consist of (i) many radial filaments with complex striations and bifurcations, permeating the entire field of view, of magnetic field magnitude 60 tesla and (ii) a coherent, centrally directed electric field of order 109 volts per meter, seen in proximity to the capsule surface. Although the mechanism for generating these fields is unclear, their effect on implosion dynamics is potentially consequential.

Charged-Particle Probing of X-ray―Driven Inertial-Fusion Implosions

Ignition Set to Go One aim of the National Ignition Facility is to implode a capsule containing a deuterium-tritium fuel mix and initiate a fusion reaction. With 192 intense laser beams focused into a centimeter-scale cavity, a major challenge has been to create a symmetric implosion and the necessary temperatures within the cavity for ignition to be realized (see the Perspective by Norreys). Glenzer et al. (p. 1228, published online 28 January) now show that these conditions can be met, paving the way for the next step of igniting a fuel-filled capsule. Furthermore, Li et al. (p. 1231, published online 28 January) show how charged particles can be used to characterize and measure the conditions within the imploding capsule. The high energies and temperature realized can also be used to model astrophysical and other extreme energy processes in a laboratory settings. Laser-driven temperatures and implosion symmetry are close to the requirements for inertial-fusion ignition. Measurements of x-ray–driven implosions with charged particles have resulted in the quantitative characterization of critical aspects of indirect-drive inertial fusion. Three types of spontaneous electric fields differing in strength by two orders of magnitude, the largest being nearly one-tenth of the Bohr field, were discovered with time-gated proton radiographic imaging and spectrally resolved proton self-emission. The views of the spatial structure and temporal evolution of both the laser drive in a hohlraum and implosion properties provide essential insight into, and modeling validation of, x-ray–driven implosions.

Impurity injection experiments on the Alcator C tokamak

Transport of trace, non-recycling, injected impurities has been studied on the Alcator C tokamak. Changes of impurity confinement times with varying plasma density, current, toroidal field, majority ion species mass, impurity charge and mass, Zeff, and major and minor radius have been delineated. An empirical scaling is developed from these results and compared with the results of similar transport studies undertaken on other tokamak devices. The agreement is reasonable. A computer model simulating the transport is utilized to compare several models with the empricial results. With the possible exception of low-density, high-Zeff discharges, the transport is not consisten with the predictions of neoclassical theory, but can be well described by simple spreading diffusion with a diffusion coefficient ranging from 1 to 5 × 103 cm2s−1, depending on plasma parameters. This model yields good agreement both with the time histories of single-chord measurements of various ionization states, and with radial soft-X-ray emission profiles. Increased impurity transport with the onset of strong MHD oscillations has also been observed, with the effective diffusion coefficient scaling approximately as (ΔB)4.

Using secondary-proton spectra to study the compression and symmetry of deuterium-filled capsules at OMEGA

With new measurement techniques, high-resolution spectrometry of secondary fusion protons has been used to study compression and symmetry of imploded D2-filled capsules in direct-drive inertial-confinement-fusion experiments at the 60-beam OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Data from target capsules with ∼15 atmospheres of D2 fuel, in CH shells 19–27 μm thick, were acquired with a magnet-based, charged-particle spectrometer and with several new “wedge-range-filter”-based spectrometers incorporating special filters and CR39 nuclear track detectors. Capsules with 19-μm shells, imploded with similar laser energies (∼23 kJ) but different methods of single-beam laser smoothing, were studied and found to show different compression characteristics as indicated by the fuel areal density (determined by the ratio of secondary-proton yield to primary-neutron yield) and the total areal density (determined by the energy loss of protons due to slowing in the fuel and shell). In go...

X-ray computed tomography with 50-μm resolution

Diagnostic-quality x-ray computed tomography (CT) images, with spatial resolution of 0.05–0.1 mm, have now been made of small laboratory animals with a micro-CT scanner incorporating a patented, high-resolution x-ray detector. The images show clearly defined internal organ structure and tumors in intact rodents.

D3He-proton emission imaging for inertial-confinement-fusion experiments (invited)

Proton emission imaging cameras, in combination with proton spectrometers and a proton temporal diagnostic, provide a great deal of information about the spatial structure and time evolution of inertial-confinement fusion capsule implosions. When used with D3He-filled capsules, multiple proton emission imaging cameras measure the spatial distribution of fusion burn, with three-dimensional information about burn symmetry. Simultaneously, multiple spectrometers measure areal density as a function of angle around the imploded capsule. Experiments at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have already proven the utility of this approach. An introduction to the hardware used for penumbral imaging, and algorithms used to create images of the burn region, are provided here along with simple scaling laws relating image resolution and signal-to-noise ratio to characteristics of the cameras and the burn region.

The largest and smallest X-ray computed tomography systems

As far as we know, we have applied X-ray computed tomography (CT) to the largest and the smallest objects inspected to date. The largest object is a section of a Trident C-4 rocket motor, about 2 m in diameter, which was inspected with gamma rays from a 15 MeV linear electron accelerator. Resolution of the reconstructed image is about 2 mm. Cracks 0.08 mm wide are clearly visible in the 1024 by 1024 pixel image, and contrast sensitivity is about two percent. The smallest object is a 2.5 cm diameter chest cavity of a laboratory mouse, inspected with 90 kV X-rays from a rotating anode. Spatial resolution of this image is about 0.05 mm. The heart, esophagus and lung tissue are displayed with exceptional clarity in a 512 by 512 pixel image. These results illustrate the wide range of applicability of CT techniques, and the designs of the two instruments which produced the images illustrate some of the problems which can arise when using CT for non-destructive testing.

Soft x-ray imaging instrument for the Alcator A tokamak

We describe an instrument that images the Alcator A tokamak plasma in the soft x‐ray energy range with high temporal resolution. Radiation from a cross section of the plasma column is detected by an array of seventeen surface barrier detectors which provides 1 cm poloidal spatial resolution over a 16 cm field of view. The instrument’s nominal sensitivity lies in the spectral range from 0.27 to 25 keV; this range includes much of the radiation emitted by the Alcator A plasma, whose core temperature is typically in the 0.6 to 1.1 keV range. The frequency response of the detector‐electronics system extends from dc to 170 kHz. We discuss: (1) some design considerations and instrument calibrations, including the instrument’s measured response to 0.28, 1.5, 8.0, and 17.5 keV x‐rays and its theoretical response to a bremsstrahlung spectrum; (2) the detector‐electronics noise levels under various conditions; (3) the instrument’s response to an electrically modulated x‐ray source; (4) the detector x‐ray response a...

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.