Isochoric Heating of Solid-Density Matter with an Ultrafast Proton Beam

A new technique is described for the isochoric heating (i.e., heating at constant volume) of matter to high energy-density plasma states () on a picosecond time scale (). An intense, collimated, ultrashort-pulse beam of protons—generated by a high-intensity laser pulse—is used to isochorically heat a solid density material to a temperature of several eV. The duration of heating is shorter than the time scale for significant hydrodynamic expansion to occur; hence the material is heated to a solid density warm dense plasma state. Using spherically shaped laser targets, a focused proton beam is produced and used to heat a smaller volume to over 20 eV. The technique described of ultrafast proton heating provides a unique method for creating isochorically heated high-energy density plasma states.

Figure 1

(a) Experimental setup for flat and focusing target geometries. Each target consists of a flat or hemispherical $10\mu \mathrm{m}$ thick Al target irradiated by the laser, and a flat $10\mu \mathrm{m}$ thick Al foil to be heated by the protons. (b) Corresponding streak camera images showing space- and time-resolved thermal emission at 570 nm from the rear side of the proton-heated foil. The streak camera images an $800\mu \mathrm{m}$ spatial region with a 1 ns temporal window.

Figure 2

Interferogram of focusing target shot taken 180 ps after incidence of the main pulse. The enlarged image on the right shows an approximately $50\mu \mathrm{m}$ region of expanding plasma originating from the rear surface of the proton heated foil.

Figure 3

Comparison of time-dependent experimental (blue) and simulated (red) emission intensities for a hemispherical target shot. The experimental curve is a temporal line out spatially integrated over the $46\mu \mathrm{m}$ FWHM of the signal. The simulated curve is a LASNEX calculation of the 570 nm emission from a 23 eV solid density Al plasma.

Figure 4

Particle-in-cell calculation of the electric field density at 3.4 ps for a $5\times {10}^{18}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$ intensity laser pulse incident on a $250\mu \mathrm{m}$ diameter, $10\mu \mathrm{m}$ thick hemispherical Al shell.