Collective Thomson scattering (TS) is a powerful technique to accurately diagnose the parameters of laser-produced plasmas. However, when the scattering parameter α, which is inverse to the product of electron Debye length and wavenumber of plasma fluctuations, is significantly larger than 1, which of the conditions is easily satisfied in the experiment, the accuracy of electron density measurement is usually poor just with the single-wavenumber ion-acoustic wave feature of the TS spectrum. This situation can be greatly improved in the imaging TS experiment because the signal intensity is proportional to electron density. A novel algorithm is developed and validated for the data analysis of imaging TS through the combination of signal intensity and spectral profile. The results show that we can obtain the electron density with high confidence, as well as other plasma parameters like electron and ion temperatures.
I. INTRODUCTION
Collective Thomson scattering (TS) is one of the most powerful diagnostics of under-dense plasmas in the field of high-energy-density physics.1,2 With various experimental setups, time-integrated or time-resolved Thomson scattering spectra from different plasma positions are obtained, and plasma parameters are thus inferred.3–9 In the early experiments, the states of laser-produced plasmas at different spatial positions are usually measured shot-by-shot. In order to minimize the uncertainties due to shot-to-shot fluctuations, the imaging Thomson scattering (iTS) technique is developed,10,11 which is able to offer a space-resolved diagnosis of plasma parameters along the probe beam in a single laser shot. At present, iTS is widely used and has become a significant research method for numerous physical issues such as heat transportation,10,12–14 plasma shocks,15,16 ion species fraction,17 self-generated magnetic field,18 collisional absorption,19 etc.
In collective Thomson scattering diagnostics, plasma states are mainly inferred via fitting the spectra with theoretical ones. When the scattering parameter, i.e., the reciprocal of the product of wave number and electron Debye length, is significantly larger than one, the ion-acoustic wave (IAW) feature of the spectrum weakly depends on electron density, leading to a large fitting uncertainty of electron density.20 In order to enhance the measurement accuracy of electron density, one way is to detect the electron plasma wave feature of collective Thomson scattering. Unfortunately, for the coronal plasma in laser fusion, the scattered intensity of the electron plasma wave feature is two or three orders lower than that of the ion-acoustic feature1 and, therefore, is easily corrupted with noises and stray signals. Hence, it is common practice to focus solely on the measurement of ion-acoustic features in many cases. Several methods are developed to improve measurement accuracy, such as absolute calibration by Rayleigh scattering,12 dual-wavelength TS,21 and dual-angle TS.20,22
Since iTS offers the dependence of signal intensity as well as spectral profile on spatial position along the probe beam and the signal intensity is proportional to electron density, it is natural to expect that electron density could be accurately inferred from the iTS spectrum even if only ion-acoustic wave features are detected. In this article, we develop an algorithm to improve the retrieval accuracy of plasma parameters from ion-acoustic wave features of iTS by combining the spectral profile and intensity distribution. To our best knowledge, this is the first algorithm for iTS. In the fitting procedure, of course, preliminary knowledge of electron density distribution has to be assumed. It is a fortune that, in many cases, the electron density distribution is roughly known. For example, when a solid plate target is irradiated with a laser pulse, the spatial distributions of electron density along the target normal can be approximately described with an exponential23 or double-exponential24 function. With reasonable assumptions on the electron density distribution and the aid of radiation hydro simulation, we show that the fitting uncertainties of plasma parameters can be greatly improved with the newly developed algorithm, even in the region of scattering parameters significantly larger than one.
II. ALGORITHM AND VALIDATION
A. Algorithm
(a) The configuration of Thomson scattering in simulation and (b) the scattering vector.
(a) The configuration of Thomson scattering in simulation and (b) the scattering vector.
(a) Pseudo-color image of spectrum-integral form factor as a function of scattering parameter α and quotient ZTe/Ti for a fully ionized CH plasma. (b) Integral form factor as a function of electron density when Te = Ti = 1 keV. (c) Integral form factor as a function of electron temperature when ne = 8 × 1020 cm−3 and Te = Ti.
(a) Pseudo-color image of spectrum-integral form factor as a function of scattering parameter α and quotient ZTe/Ti for a fully ionized CH plasma. (b) Integral form factor as a function of electron density when Te = Ti = 1 keV. (c) Integral form factor as a function of electron temperature when ne = 8 × 1020 cm−3 and Te = Ti.
B. Numerical validation
We perform a numerical experiment to validate our method. With the radiation-hydrodynamic simulation code FLASH,28 we simulate laser ablation processes relevant to the actual experiment29 and generate a synthetic iTS graph of IAW. A probe beam at a wavelength of 263 nm (4ω) with an energy of 60 J is used for Thomson scattering. The probe beam has a 100 μm focal diameter and a pulse length of 3.5 ns. Four 1500 J heater beams at 351 nm (4ω) irradiate a spherical crown plastic target uniformly for 2.5 ns while the probe beam enters along the normal direction of the target, as shown in Fig. 1(a). The wave vectors of incident light, scattered light, and ion acoustic waves are illustrated in Fig. 1(b). A typical synthetic iTS graph is shown in Fig. 3(a), where the coordinate z denotes the distance from the target surface. A spectral resolution of 0.7 Å and a spatial resolution of 90 μm are convolved, and a white noise with a standard deviation of 20% is added to the spectra via the Monte Carlo method. We treat the artificial data as an experimental result.
(a) The synthetic spatial-resolved Thomson scattering spectra from ion-acoustic waves. (b) The scattered spectrum at z = 400 μm, where the electron density is 8.09 × 1020 cm−3, the electron temperature is 886 eV, and the ion temperature is 784 eV. The corresponding positions of ωi − 5ωia/2 and ωi + 5ωia/2 are labeled by arrows.
(a) The synthetic spatial-resolved Thomson scattering spectra from ion-acoustic waves. (b) The scattered spectrum at z = 400 μm, where the electron density is 8.09 × 1020 cm−3, the electron temperature is 886 eV, and the ion temperature is 784 eV. The corresponding positions of ωi − 5ωia/2 and ωi + 5ωia/2 are labeled by arrows.
The chi-square values (a) , (b) , and (c) χ1χ2 vs the first two arguments in the fitting of the synthetic spectra show that the smaller the value, the less deviation there is between the spectra and the fitting data. The values are normalized from the minimum.
The chi-square values (a) , (b) , and (c) χ1χ2 vs the first two arguments in the fitting of the synthetic spectra show that the smaller the value, the less deviation there is between the spectra and the fitting data. The values are normalized from the minimum.
The comparison of the three forms of χ2 in terms of (a) diagnostic results and (b) fitting uncertainty of electron density.
The comparison of the three forms of χ2 in terms of (a) diagnostic results and (b) fitting uncertainty of electron density.
Besides electron density, other plasma parameters are also calculated in the process of fitting the IAW spectra. The comparisons between the inferred results and the simulation values are shown in Figs. 7(a)–7(c). As seen in this figure, the inferred plasma parameters are in excellent agreement with the results from hydro simulations, which reveals that the model of Eq. (10) is reasonable. Figure 7(d) shows the scattering parameter α calculated with the plasma parameters of the simulation. Notice that the fitting uncertainty of Te is smaller when α is larger and becomes greater with the decrease of α, although the uncertainty of ne is smaller. For example, comparing the fitting results at z = 300 and 700 μm, the uncertainty of ne varies from 22 to 15%, while that of Te varies from 1.1 to 2.3%. As revealed in the introduction, when the scattering parameter α is much larger than 1, the separation of two resonance peaks in the IAW spectrum is almost only dependent on temperature, and as α decreases, the influence of ne gradually becomes significant. If we just fit the IAW spectra at each spatial point, the precision will be very poor, as shown in Figs. 7(a)–7(c). The error bars of dot-fitting are calculated in a similar way, where χ2 is treated as a function of ne, Te, and Ti at each spatial point. The numerical experiment indicates that our algorithm can be applied to laser-plasma diagnostics and improve both precision and uncertainty with the IAW component of iTS. Because we are dealing with solid targets, we take the double-exponential function as the prior electron density distribution. We believe that in other cases, as long as the appropriate density distribution form is selected, the diagnostic accuracy can also be improved.
The (a) electron density, (b) electron temperature, and (c) ion temperature calculated via our statistical fitting method (blue lines) and fitting at each space point (green dots) are compared to the original simulation results. We also present (d) the spatial distribution of the scattering parameter α as a reference for the fitting uncertainty.
The (a) electron density, (b) electron temperature, and (c) ion temperature calculated via our statistical fitting method (blue lines) and fitting at each space point (green dots) are compared to the original simulation results. We also present (d) the spatial distribution of the scattering parameter α as a reference for the fitting uncertainty.
Figures 7(b) and 7(c) show that the fitting uncertainties of electron and ion temperatures are surprisingly low, which may be attributed to our using exactly the same Thomson scattering model in both generating and analyzing the spectra. There are several factors that might distort the signal in real experiments:
The effect of non-Maxwellian distribution and collision. It is worth pointing out that the scattering model we use is collisionless and quasi-thermal equilibrium. When the heating beams keep irradiating, the TS spectra near the target exhibit abnormal shapes. With strong inverse bremsstrahlung heating and thermal conduction, the electron distribution function may significantly deviate from quasi-thermal equilibrium,30 rendering our Thomson scattering model unsuitable.31,32 Recently, many theoretical33,34 and experimental35 works about the collective Thomson scattering of non-equilibrium plasma have been carried out. It is necessary to modify the model when diagnosing plasmas in regions with large gradients.
The effect of absorption. The absorption effect will influence the intensity distribution. Although we take the absorption of incident light into account in Eq. (4), the absorption rate of the scattered light when it leaves plasma is unknown because we do not diagnose the lateral distribution of the plasma parameters. Under ideal circumstances, we can use Gaussian density and constant temperature to estimate the lateral properties. However, in more general cases, we recommend lowering the absorption rate with a higher-frequency probe and analyzing areas where the absorption is not obvious.
Errors in signal acquisition. Noises can introduce errors in the calculation of the scattered intensity. The uniform background (like bremsstrahlung) can be easily figured out from the area without signal, so what is important is to shield irrelevant signals like stray lights in the optical path.
Here we choose the double-exponential function as the expression for ne(z) because predecessors’ research shows that this function can well describe ne(z) in the condition of a spherical crown target. Figure 8 shows the fitting result of ne(z) under different assumptive expressions, including exponential, double-exponential, and triple-exponential (adding one more exponential term). The result of the exponential distribution indicates that an improper assumption will lead to a large deviation. We believe that our analysis method can be applied to other configurations as long as the appropriate forms of ne(z) are selected. If a diagnosis under some new configuration is needed and the form of ne(z) is unknown, we think asymptotic approximation by a system of functions is feasible. The fitting results as well as the convergent χ2 values of double-exponential and triple-exponential distributions are similar, as shown in Fig. 8, indicating that the first few terms are sufficient to describe ne for a suitable function system. The selection of the system of functions can be roughly judged from the integral scattering intensity distribution curve because it is strongly dependent on ne(z). Besides, radiation-hydrodynamic simulation also has guiding significance.
The diagnostic results of different pre-determined analytical expressions of ne(z), including exponential (green), double-exponential (red), and triple-exponential (blue) functions.
The diagnostic results of different pre-determined analytical expressions of ne(z), including exponential (green), double-exponential (red), and triple-exponential (blue) functions.
III. CONCLUSION
Based on the imaging Thomson scattering (iTS) technique, we develop a novel data processing algorithm. Taking into account the spatial distribution information of the scattered light intensity, we propose a solution to the quest for electron density diagnostics with only an imaged IAW scattered component. The algorithm is validated by fitting synthetic Thomson scattering data generated from radiation hydrodynamic simulations. The fitting uncertainty of electron density is around 20%, and that of electron temperature is within 3%. Compared with the method of fitting IAW spectra point by point, the novel algorithm can be applied to infer plasma parameters with much lower uncertainties and, therefore, should be useful to analyze iTS results to accurately and reliably obtain the spatial distributions of plasma parameters.
ACKNOWLEDGMENTS
This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA25010200) and the National Key R&D Project (Grant No. 2023YFA160005602).
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Yi-fan Liu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Peng Yuan: Conceptualization (equal); Data curation (equal); Investigation (lead); Methodology (equal); Visualization (supporting); Writing – review & editing (supporting). Tao Tao: Formal analysis (equal); Investigation (equal); Software (equal). Yao-yuan Liu: Conceptualization (equal); Methodology (supporting); Software (lead). Xin-yan Li: Data curation (supporting); Investigation (equal). Jun Li: Funding acquisition (equal); Writing – review & editing (equal). Jian Zheng: Conceptualization (equal); Funding acquisition (lead); Methodology (equal); Project administration (lead); Resources (lead); Software (lead); Validation (equal); Writing – original draft (equal); Writing – review & editing (supporting).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.