Collisional heating and adiabatic expansion of warm dense matter with intense relativistic electrons

Adiabatic expansion of a warm dense Ti plasma has been observed after isochoric heating of a $100\text{−}\mu \mathrm{m}$-thick Ti foil with an $\sim 100$-ns-long intense relativistic electron bunch at an energy of 19.8 MeV and a current of 1.7 kA. The expansion fits well with the analytical point-source solution. After 10 J is deposited and the plasma rapidly expands out of the warm dense phase, a stable degenerate plasma $(T\sim 1.2\mathrm{eV})$ with $n_e>{10}^{17}{\mathrm{cm}}^{-3}$ is measured for $>100$ ns. This is the first temporal measurement of the generation and adiabatic expansion of a large volume $(3\times {10}^{-4}{\mathrm{cm}}^3)$ of warm dense plasma isochorically heated by intense monochromatic electrons. The suite of diagnostics is presented, which includes time-resolved plasma plume expansion measurements on a single shot, visible spectroscopy measurements of the emission and absorption spectrum, measurements of the beam distribution, and plans for the future.

Figure 1

View of the vacuum test section in which the expanding warm dense plasma is diagnosed. The target location, optical diagnostic layouts, lines of sight, and sample data sets are shown. The imaging station for OTR is shown in red; the imaging station for the interferometer camera, in blue; the fast-framing camera, in green; and the ${15}^{\circ }$ upstream spectrometer, in orange. (The orthogonal visible spectrometer is not shown).

Figure 2

(a) A 10-ns gated OTR image of the beam distribution on the foil, 20 ns after the beginning of the pulse. (b) Integrated intensity and Gaussian fit of the OTR image. Note the enhancement at the peak.

Figure 3

(a) Twenty-nanosecond gated images of the expanding plume; the delay after the beginning of the beam pulse and the shot number are indicated. (b) Integrated intensity and axial extent of the plume images in (a). (c) Plot of $\sigma$ vs the $z$ axis for each time slice in (a). (d) Sample Gaussian fit at 2.1 mm for the 110-ns gate on shot 23127. Note that 0 is the upstream side of the target face.

Figure 4

(a) One hundred–nanosecond gated images of the expanding plume; the delay after the beginning of the pulse is indicated, and the shot number is 23103. (b) One-microsecond gated images of the expanding plume and gate with respect to the beginning of the pulse; shot number 23105 (note the scale differences). Images are shown at log scale to indicate full expansion. (c) Velocity profile of multiple 20-ns (red circles), 100-ns (orange squares), and $1\text{−}\mu \mathrm{s}$ (green diamonds) gated images from the PI-Max512 and Simacon fast-framing cameras. The black line represents the point-source solution from Eq. (1).

Figure 5

Measured Ti-I spectra over a 200-ns gate, indicating (a) emission and (b) absorption for shot 23115 over the 496- to 506-nm band and (c) emission for shot 23127 over the 454- to 466-nm band. The atomic fit calculation is shown in blue for comparison.