Schematic of the generation of self-generated and fields by laser-matter interactions, described by the Faraday equation [Eq. (1)] combined with a simplified version of the generalized Ohm’s law (Refs. 1 and 3–8).
(a) Spectra and (b) time history of charged fusion products from an exploding pusher target at OMEGA containing fuel. Note that the particle energies are slightly upshifted from their birth energies, due to the charging of the capsule during the 1 ns square laser pulse illumination (Ref. 12).
Basic configuration of the monoenergetic charged particle radiography setup. Sample subjects are shown in Fig. 4. In some cases, a single backlighter is used to image two subjects located at 90° or 180° relative to the backlighter; each subject has its own detector. Typical system dimensions are 1 cm from backlighter to subject and 30 cm from backlighter to detector.
Some of the subjects used in radiography experiments, with the configuration shown in Fig. 3 and the imaging particles incident from the left. The images that result from subjects (a)–(d), with the particles placed normally in relation to the foil, are referred to as “face-on,” because they show structure in the plane of the foil. Images of these subjects are also recorded with the hohlraum oriented parallel to the imaging particles, showing the “end-on” structure. In subjects (a)–(c) and (g), the dots (green) are mesh grids.
Illustration of a laser-plasma interaction on a CH foil, together with a measured radiography image formed by 15 MeV protons divided into beamlets by a metal mesh (the darker areas indicate more protons) (Ref. 6). simulations (Refs. 26–29) indicate that face-on radiography will be sensitive only to the field, while side-on radiography will be sensitive only to the field. This allows and fields to be measured separately.
proton radiographs showing the evolution of fields in a plasma/field bubble formed by the interaction of a laser beam with a CH foil, as described in the text and Ref. 7. The arrangement of the foil with respect to the imaging system is shown in Figs. 3 and 4(a).
Measured time evolution of the maximum , as described in Ref. 7. In this case, the blue curve is the time history of the laser intensity.
Radiographs (a) generated when two laser beams were incident on a CH foil, showing how magnetic reconnection results in diminished field energy where two plasma bubbles surrounded by magnetic fields collide. (b) shows displacement vectors of the beamlets. Arrays of displacement amplitudes are shown as images in (c); each pixel represents one beamlet, with a value proportional to displacement. The lineouts of (c) (along the red arrow) provide quantitative measurement of at the foil location (d).
Proton fluence distributions in radiographs of (a) a -radius spherical plastic capsule with attached Au cone, before and during implosion (Ref. 14) and (b) spherical capsules before and during implosion (Ref. 15). In all cases, the 1-ns-long laser drive began at 0 ns. The plastic shells were thick in (a) and thick in (b). The linear feature in the upper left corner of each image of a spherical capsule is the mounting stalk.
Radial profiles of the proton fluence images measured at and 1.9 ns. Comparatively, a fluence peak occurs in the image centers during the early stages of implosion, indicating a “focusing” of imaging protons there, while in contrast, the fluence is extremely low or defocused at the image centers at later times. Note that the different level of the proton fluence outside the capsules is due to the variations from the backlighter proton yields.
Radial fields estimated from experimental measurements (open circles) and from LILAC simulations (solid circles) vs implosion times. Horizontal error bars represent uncertainties in backlighter burn time. The differences between simulation and data may result from effects of proton scattering.
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