Direct Heating of a Laser-Imploded Core by Ultraintense Laser-Driven Ions

A novel direct core heating fusion process is introduced, in which a preimploded core is predominantly heated by energetic ions driven by LFEX, an extremely energetic ultrashort pulse laser. Consequently, we have observed the $\mathrm{D}(d,n){}^3\mathrm{He}$-reacted neutrons (DD beam-fusion neutrons) with the yield of $5\times {10}^8\mathrm{n}/4\pi \mathrm{sr}$. Examination of the beam-fusion neutrons verified that the ions directly collide with the core plasma. While the hot electrons heat the whole core volume, the energetic ions deposit their energies locally in the core, forming hot spots for fuel ignition. As evidenced in the spectrum, the process simultaneously excited thermal neutrons with the yield of $6\times {10}^7\mathrm{n}/4\pi \mathrm{sr}$, raising the local core temperature from 0.8 to 1.8 keV. A one-dimensional hydrocode STAR 1D explains the shell implosion dynamics including the beam fusion and thermal fusion initiated by fast deuterons and carbon ions. A two-dimensional collisional particle-in-cell code predicts the core heating due to resistive processes driven by hot electrons, and also the generation of fast ions, which could be an additional heating source when they reach the core. Since the core density is limited to $2\mathrm{g}/{\mathrm{cm}}^3$ in the current experiment, neither hot electrons nor fast ions can efficiently deposit their energy and the neutron yield remains low. In future work, we will achieve the higher core density ($>10\mathrm{g}/{\mathrm{cm}}^3$); then hot electrons could contribute more to the core heating via drag heating. Together with hot electrons, the ion contribution to fast ignition is indispensable for realizing high-gain fusion. By virtue of its core heating and ignition, the proposed scheme can potentially achieve high gain fusion.

Figure 1

(a) A CD shell counterimploded by two GXII beams and a 79° side heated by LFEX. (b) Shell with two holes for LFEX introduction and for preplasma ventilation. (c) The target, lasers, and detectors. (d) X-ray streak image of the shell flow: two GXII beams arrive from the upper and lower sides. The rightmost trace is the core profile without LFEX illumination. Horizontal axis: $1\mathrm{ns}/\text{div}$, vertical axis: $250\mu \mathrm{m}/\text{div}$ (Hamamatsu Photonics C4575-03). (e) Time-integrated x-ray pinhole image without and (f) with LFEX (shot No. 35732).

Figure 2

(a) Neutron and $\gamma$ TOF signals from LS1: Shot No. 35739 of 498-J, GXII without LFEX: the neutron yield is $7\times {10}^5/4\pi \mathrm{sr}$, and (b) Shot No. 35732 of $515\text{−}\mathrm{JGXII}+613\text{−}\mathrm{J}\mathrm{LFEX}$. (c) Neutron energy spectrum, converted from (b). Small peak at 2.45 MeV is a thermal peak with a yield of $6.4\times {10}^7\mathrm{n}/4\pi \mathrm{sr}$. Large peak is a beam-fusion peak with $5\times {10}^8\mathrm{n}/4\pi \mathrm{sr}$. (d) Spectrum from LS2. (e) Neutrons from LS1 (thermal: solid circles; beam: diamonds) and LS2 (thermal+beam: open circles) versus $\mathrm{GXII}+\mathrm{LFEX}$ laser energy $E$. The instrumental and calibration error is 32%. The dashed line is proportional to $E^5$. (f) STAR 1D: beam- and thermal-fusion yields versus LFEX intensity on the target. Solid lines are the beam fusion and dotted are thermal fusion, respectively. Diamonds and circles are the experiments in (e).

Figure 3

(a) Ion spectrum of shot No. 35737 from the CR-39 track detector (a). 71 counts are detected. (b) Spectrum from the detector (b). 231 counts are detected. Both detectors comprise a $20\text{−}\mu \mathrm{m}$-thick aluminum filter and three stacks of $100\text{−}\mu \mathrm{m}$-thick CR-39 films. (c) STAR 1D-simulated temperature profile of the CD plasma, heated by the slowdown of the injected fast deuterons. The initial energy of the deuterons is 0.2, 0.5, 0.8, 1.0, and 2.0 MeV. (d) STAR 1D simulations: 50% absorption of hot electrons heats the core to 1 keV. Fast carbons heat the core surface from 1 keV to 1.8 keV. Core edge and center are 0 and $240\mu \mathrm{m}$, respectively.

Figure 4

(a) PICLS coupled to STAR 1D: bulk electron temperature profile at 2 ps. Core is heated to $\sim 1\mathrm{keV}$. (b) Deuteron energy profile, heated to a few 100 eV at $x\sim 160\mu \mathrm{m}$. (c) 2D electron energy density at 1 ps and (d) deuteron energy density at 2 ps. The $x$ axis is the laser propagation direction. $x=0\mu \mathrm{m}:\mathrm{vacuum};$ $x=100:\mathrm{cutoff};$ $x=200:\mathrm{core}$ center. The $y$ axis is the vertical direction. The CD plasma peak density is $\sim 6\mathrm{g}/{\mathrm{cm}}^3$ and core density $\sim 2\mathrm{g}/{\mathrm{cm}}^3$. Laser is emitted at ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$, 1.5 ps pulse length, $60\mu \mathrm{m}$ focal spot. Assuming the absorbing boundary condition, we detected 675 million particles in $5000\times 5000$ grids and performed up to 2 ps of simulation using 200 cores of AMD processors.