Characterization of deuterium clusters mixed with helium gas for an application in beam-target-fusion experiments

We measured the average deuterium cluster size within a mixture of deuterium clusters and helium gas by detecting Rayleigh scattering signals. The average cluster size from the gas mixture was comparable to that from a pure deuterium gas when the total backing pressure and temperature of the gas mixture were the same as those of the pure deuterium gas. According to these measurements, the average size of deuterium clusters depends on the total pressure and not the partial pressure of deuterium in the gas mixture. To characterize the cluster source size further, a Faraday cup was used to measure the average kinetic energy of the ions resulting from Coulomb explosion of deuterium clusters upon irradiation by an intense ultrashort pulse. The deuterium ions indeed acquired a similar amount of energy from the mixture target, corroborating our measurements of the average cluster size. As the addition of helium atoms did not reduce the resulting ion kinetic energies, the reported results confirm the utility of using a known cluster source for beam-target-fusion experiments by introducing a secondary target gas.

Figure 1

Schematic diagram of the 90° Rayleigh scattering setup for the cluster size measurements.

Figure 2

The RGA measured partial pressures of nitrogen (solid black square), deuterium (hollow blue circle), and ${}^3\mathrm{He}$ (solid red triangle) as a function of time. The gas jet fired at $t=827\mathrm{s}$, and partial pressures of deuterium and ${}^3\mathrm{He}$ peaked at that time. Note that the partial pressure of the nitrogen gas inside the chamber remained largely unchanged.

Figure 3

(a) Density of deuterium as a function of pressure using SESAME Table No. 5265 (solid black line) and an ideal gas EOS (dashed red line) at 87 K. (b) Density of deuterium as a function of temperature using SESAME Table No.5265 (solid black line) and an ideal gas EOS (dashed red line) at a gas jet backing pressure of 54 bar.

Figure 4

(a) Rayleigh scattering signal as a function of the position of the nozzle with a Gaussian fit with a FWHM of 4.6 mm (dashed line). (b) An oscilloscope trace of the Rayleigh scattering signal from deuterium clusters with a backing pressure of 54 bar at 87 K. The pulse valve opened for 1 ms.

Figure 5

(a) Rayleigh scattering signal as a function of the backing pressure of the deuterium gas jet at 87 K, shown with a power-law fit (dashed red line), $P_0^{2.1}$. (b) Average diameter of deuterium clusters at different backing pressures, also shown with a fit calculated using $P_0^{2.1}$ dependence (dashed red line).

Figure 6

(a) Rayleigh scattering signal as a function of the deuterium gas jet temperature at a constant backing pressure of 54 bar, shown with a power-law fit (dashed red line), $1/T_0^{5.7}$. (b) Average diameter of deuterium clusters, with error bars, for different gas jet temperatures at a backing pressure of 54 bar.

Figure 7

(a) Rayleigh scattering signal as a function of the temperature of deuterium plus ${}^4\mathrm{He}$ gas mixture at a constant backing pressure of 54 bar, shown with a power-law fit (dashed red line), $1/T_0^{5.7}$. (b) Average diameter of deuterium clusters in the mixture for different gas jet temperatures at a backing pressure of 54 bar, shown with error bars.

Figure 8

Faraday cup ion TOF data with a mixture target at an incident laser intensity of $2.2\times {10}^{16}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$. The RGA measured the target composition and indicated (partial pressure of deuterium):(partial pressure of ${}^3\mathrm{He}$) = 2:1. The average kinetic energy of deuterium ions was 14.5 ± 1.5 keV on this shot $(k{T}_{\mathrm{TOF}}=9.6\mathrm{keV})$, comparable to the average ion energy reported in Ref. [16] at a similar laser intensity.