Determination of the Ion Temperature in a High-Energy-Density Plasma Using the Stark Effect

We present the experimental determination of the ion temperature in a neon-puff $Z$ pinch. The diagnostic method is based on the effect of ion coupling on the Stark line shapes. It was found, in a profoundly explicit way, that at stagnation the ion thermal energy is small compared to the imploding-plasma kinetic energy, where most of the latter is converted to hydromotion. The method here described can be applied to other highly nonuniform and transient high-energy-density plasmas.

Figure 1

Sensitivity of Stark broadening to the ion temperature. Results of computer simulations of the Ne x Lyman series in a neon plasma with $N_e={10}^{21}{\mathrm{cm}}^{-3}$ and $T_e=250\mathrm{eV}$ are given (only shown is a spectral range containing $\mathrm{Ly}\text{−}\delta$ and $\mathrm{Ly}\text{−}ϵ$). The spectra are peak normalized.

Figure 2

Ne x $\mathrm{Ly}\text{−}\delta$ Stark width as a function of ${\mathrm{\Gamma }}_{ii}$, assuming three values of the electron density and a constant $T_e=250\mathrm{eV}$. The filled and opaque circles correspond to $T_i=T_e=250\mathrm{eV}$ and $T_i=T_i^D=1000\mathrm{eV}$, respectively. Also shown is the Doppler broadening.

Figure 3

Ly-$\alpha$-satellites and $\mathrm{high}\text{−}n$-line spectra recorded simultaneously at $t=0$, axially resolved across the anode-cathode gap. The positions of the hot spots in all three spectra clearly match. The dashed lines highlight the slice from which the data are analyzed. The $\mathrm{Ly}\text{−}\alpha$ and $\mathrm{high}\text{−}n$ spectra are normalized to the peak intensities of the $\mathrm{Ly}\text{−}\alpha$ singlet satellite and $\mathrm{Ly}\text{−}\delta$, respectively.

Figure 4

Ne ix $\mathrm{Ly}\text{−}\alpha$ satellite spectra: The experimental data are fit with three Voigt profiles. The Gaussian width of the singlet peak gives the total ion kinetic energy $T_i^D=900\pm 200\mathrm{eV}$, and the ratio between the integrated intensities of the two triplet groups gives the electron density $N_e=(6\pm 1)\times {10}^{20}{\mathrm{cm}}^{-3}$ using the CR modeling.

Figure 5

The experimental $\mathrm{high}\text{−}n$ spectra are compared to line shape modeling for two values of $T_i$ assumed: 300 eV and $T_i^D$. $T_i^D=900\mathrm{eV}$, $N_e=6\times {10}^{20}{\mathrm{cm}}^{-3}$, and $T_e=200\mathrm{eV}$ are assumed in both cases. The shaded area designates a spread of modeled spectra for $T_i$ varying between 150 and 450 eV. An enlarged part of the graph is given in the inset.