电弧等离子体，离子数密度的测量，双光栅折射

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A special differential interferometer consisting of two gratings was developed for diagnostics of plasma density. Compared with other differential interferometers, our system has an important advantage that the shear distance, shear direction, and fringe width can be adjusted independently, enabling easy control of the parameters. This feature allows precise tuning of the two probe beams in the interferometer for rigorous differential phase diagnosis and more accurate information of the plasma density can be obtained. The double-grating-based differential interferometer was tested for diagnostics of the laser-produced plasma which was generated by focusing a 1 TW/35 fs Ti:sapphire laser pulse in a gas jet with a 100 μm orifice diameter. It was confirmed that our differential interferometer can provide more reliable and accurate plasma density information, especially for plasmas with a high spatial gradient in density.

Talbot-Lau X-ray Deflectometry (TXD) has been developed as an electron density diagnostic for High Energy Density (HED) plasmas. The diagnostic delivers refraction, attenuation, elemental composition, and scatter information from a single-shot Moire image. A Talbot-Lau interferometer was benchmarked using laser-target and X-pinch x-ray backlighters. Grating survival and electron density mapping were demonstrated for: a) 25–60 J, 8–30 ps laser pulses using copper targets and b) X-pinches driven by a ~350kA/350ns generator. X -ray backlighter quality was assessed in order to optimize areal electron density gradient retrieval and electron density mapping. TXD enabled accurate areal electron density detection with high contrast (>25%) and spatial resolution of $\sim 50\mu \mathrm{m}$ in the high-power laser experiments, while a higher spatial resolution $< 27\mu \mathrm{m}$ and lower contrast (<15%) were found in pulsed power experiments, thus demonstrating the potential of TXD as an electron density diagnostic for HED plasmas.

Abstract A multifunctional optical diagnostic system, which includes an interferometer, a refractometer and a multi-frame shadowgraph, has been developed at the Shenguang-II upgrade laser facility to characterize underdense plasmas in experiments of the double-cone ignition scheme of inertial confinement fusion. The system employs a 266 nm laser as the probe to minimize the refraction effect and allows for flexible switching among three modes of the interferometer, refractometer and multi-frame shadowgraph. The multifunctional module comprises a pair of beam splitters that attenuate the laser, shield stray light and configure the multi-frame and interferometric modules. By adjusting the distance and angle between the beam splitters, the system can be easily adjusted and switched between the modes. Diagnostic results demonstrate that the interferometer can reconstruct electron density below 1019 cm–3, while the refractometer can diagnose density approximately up to 1020 cm–3. The multi-frame shadowgraph is used to qualitatively characterize the temporal evolution of plasmas in the cases in which the interferometer and refractometer become ineffective.

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Based on a new technique, a tunable, multi-channel system that covers the Q-band (33-55 GHz) is presented in this article. It has a potential use of the Doppler backscattering system diagnostic that can measure the turbulence radial correlation and the perpendicular velocity of turbulence by changing the incident angle. The system consists primarily of a double-sideband (DSB) modulation and a multiplier, which creates four probing frequencies. The probing frequency enables the simultaneous analysis of the density fluctuations and flows at four distinct radial regions in tokamak plasma. The amplitude of the probing frequency can be adjusted by the initial phase of the intermediate frequency (IF) input from the double-sideband, and the typical flatness is less than 10 dB. The system was tested in the lab with a rotating grating, and the results show that the system can operate in the frequency range of 33-55 GHz with a Q-band multitude and that the power of each channel can be adjusted by the phase of the IF input of DSB.

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Refraction of an optical probe beam by a plasma can be measured with angular filter refractometry (AFR), which produces an image of the beam's 2D spatial profile that contains intensity contours corresponding to curves of constant refraction angle. Further analysis is required to reconstruct the underlying line-integrated electron density. Most prior efforts to calculate density from AFR data have been limited to 1D analysis or forward-fitting techniques. In this paper, we detail the use of a fast-marching Eikonal solver to directly invert AFR data and obtain the full 2D line-integrated electron density. The analysis method is first verified with synthetic data and then applied to experimental measurements of single and colliding plasma plumes collected at the OMEGA EP Laser Facility. The calculated densities agree with 1D results and are shown to be consistent with the original AFR measurements via forward modeling. We also discuss ways to improve the precision of this technique.

During recent years, there has been considerable interest in obtaining spatially localized time resolved density measurements in fusion plasmas. However, the study of such phenomena requires many channels of information on a scale much finer than available with current discrete chordal view multichannel interferometers. These problems can be overcome by imaging an expanded probe beam occupying the entire plasma port crosssection onto a linear detector array[1], thereby significantly reducing the number of optical components and hence the cost and complexity of the system compared with a comparable discrete chord multichannel interferometer. Other more fundamental advantages of the imaging technique include compensation for phase errors due to plasma refraction, whilst the diffraction limited system resolution (typically ≃ 1cm for FIR probe wavelengths) allows the use of many detector channels for high spatial sampling rates. and hence accurate reconstruction of the density profiles.

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In plasma diagnostics using interferometry, the phase shift caused by the plasma in the fringes is extracted to determine the plasma density. The common method to extract the phase shift from the fringes is the fast-Fourier-Transform (FFT), but this technique encounters challenges when dealing with insufficient fringe numbers, spatially varying fringe frequencies, or extremely sharp phase changes. These challenges result in errors and hinder the acquisition of precise phase measurements. To tackle this issue, we introduced the fringe normalization (FN) method. The simulations demonstrated that the FN method extracts accurate phase information, surpassing the capabilities of the FFT method. As a result, this advancement enables more precise plasma diagnostics by mitigating errors that arise during the phase data processing. Furthermore, we improved the code for the inverse matrix Abel inversion to convert phase information into density. The simulation employing this code showed that the developed code provides more accurate values in the analysis of plasmas with a sharp density profile, assisting in electron beam manipulation in laser-plasma acceleration.

Laser-plasma based experiments are always more demanding about the plasma features which need to be generated during the interaction. This is valid for laser-plasma acceleration as well as for inertial confinement fusion experiments. Most of these experiments are moving toward high repetition rate operation regimes, making even more demanding the requests on the plasma sources and the diagnostics to be implemented. Interferometry is one of the most used methods to characterize these sources, since it allows for non-perturbative, single-shot measurements either of the neutral gas or the plasma density. The design of the interferometric setup is non-trivial and needs to be shaped on the actual conditions of the experiment. Similarly, the analysis of the raw data is a complex task, prone to many sources of error and dependent on the manual inputs. In this work, we will present the techniques we are developing for the analysis of the interferograms to measure both the gas and plasma density. We will show the methods, the progress and the problems we encountered in the development of novel routines of analysis based on machine learning. The architectures and the methods to obtain data used for training and testing them will be introduced. The study is ongoing and preliminary results with synthetic data will be presented. The goal is to set up a fast and operator independent diagnostic for the feedback of plasma sources toward high repetition rate experiments.

This study employs the simultaneous phase-shift interferometry (SPSI) system to diagnose laser-induced plasma (LIP) and shock wave (SW). In high-density LIP diagnostics, the Faraday rotation effect causes probe light polarization deflection, rendering traditional fixed-phase-demodulation methods ineffective, the Carré phase-recovery algorithm is adopted and its applicability is verified. Uncertainty analysis and precision verification show that the total phase shift uncertainty is controlled within 0.045 radians, equivalent to a refractive index accuracy of 8.55×10−6, with sensitivity to weak perturbations improved by approximately one order of magnitude compared to conventional carrier-frequency interferometry. Experimental results demonstrate that the SPSI system precisely captures the initial spatiotemporal evolution of LIP and tracks shock waves at varying attenuation levels, exhibiting notable advantages in weak shock wave detection. This research validates the SPSI system’s high sensitivity to transient weak perturbations, offering a valuable diagnostic tool for high-vacuum plasmas, low-pressure shock waves, and stress waves in optical materials.

We present a numerical study on the electron and ion density perturbation in low-temperature plasmas driven by the frequency detuning of two intense laser beams. Our study is performed in the hydrodynamic regime, which becomes applicable when the plasma grating period induced by the beating of the laser beams is greater than the Debye length and collective processes such as plasma oscillations can be excited. Our findings show a resonance in electron density perturbation as the frequency detuning approaches a value consistent with the Bohm–Gross dispersion relation in low- and high-pressure plasmas. We discuss the potential of this resonance as a diagnostic tool for precisely measuring electron temperature and density in low-temperature plasmas through coherent scattering.

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We developed a highly sensitive double-grating interferometer using four probe beams, which can measure one order of magnitude smaller phase shifts compared with other conventional interferometers. To achieve the unprecedented sensitivity, a highly dynamic 16-bit CCD camera was used and the balanced detection technique with four probe beams was employed in the 2-dimensional (2-D) interferometry for the first time. By using this interferometer, we could directly measure a low helium (He) gas density of n ≃ 1 × 1017 cm-3 in a capillary gas cell for the laser-plasma acceleration research, which is almost impossible with other conventional laser interferometers. This interferometer may provide a new tool for applications with extremely small phase shifts in science.

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Laser light scattering systems with volume Bragg grating (VBG) filters, which act as spectral/angular filters, have often been used as a point measurement technique, with spatial resolution as low as a few hundred μm, defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with several μm resolution over a few millimeters along the beam propagation axis. The rejection ring, as determined by the angular acceptance criteria of the filter, is derived analytically, and the use of the ring for 1D laser line rejection is explained. For the example cases presented,i.e., for a focused probe beam waist with a diameter of ∼150 μm, the rejection ring can provide resolution up to several millimeter length along the beam propagation axis for a 1D measurement, which is also tunable. Additionally, methods to further extend the measurable region are proposed and demonstrated, using a collimation lens with a different focal length or using multiple VBG filters. The latter case can minimize the scattering signal loss, without the tradeoff of the solid angle. Such use of multiple VBGs is to extend the measurable region along the beam axis, which differs from the commonly known application of multiple filters, to improve the suppression of elastic interferences. 1D rotational Raman and Thomson scattering measurements are carried out on pulsed and DC discharges to verify this method. The system features compactness, simple implementation, high throughput, and flexibility, to accommodate various experimental conditions.

Mapping individual components of the electric field, E, with high spatial resolution around the plasma sheath remains challenging, as most in situ plasma probes are generally too intrusive for reliable plasma potential measurements. An optically trapped micron-sized particle in a plasma environment represents the smallest, nonintrusive, or minimally intrusive diagnostic tool because it causes minimal disturbance to the plasma environment in which it is suspended. These microparticles can be used to map the electric field in situ with high spatial resolution in a radio frequency (RF) plasma sheath region. In this study, we optically trapped micron-sized single particles and precisely transported a single trapped particle within the cylindrically symmetric capacitively coupled plasma (CCP), over a radial distance of ∼ 0–15 mm, a 2400-fold displacement relative to the particle size, corresponding to a spatial resolution of tens of micrometers. We measured |E| and its spatial distributions in the examined range by analyzing the particle’s trajectory in the plasma after the optical forces were turned off. The radial component of the electric field, |Er|, was measured at multiple locations parallel to the electrode at 6.7 Pa. The |Er| was strongest near the circular electrode edges, reaching 0.478 ± 0.005 V mm−1, and decreased to 0.458 ± 0.001 V mm−1 toward the center of the electrode. We reconstructed the radial plasma potential, yielding a center-to-edge potential decrease of 4.6 V over one centimeter. This ΔV is consistent with reported CCP trends of small mid-plane radial potential gradients and serves as the confining potential for dust crystal and dust cluster studies. These results highlight the use of optical trapping of a single particle as an in situ, nonintrusive microprobe for quantitative mapping of Er(r) and V(r) in RF plasmas.

The advances in Charge Coupled Devices in one hand and the high resolution measurements of holographic technique on the other hand, we have adopted the method of digital real-time holographic interferometry for the diagnostics of high density plasma. The measured values of plasma electron density agree with the measurements from other techniques.

Plasma diagnostics often employ computerized tomography to estimate emissivity profiles from a finite, and often limited, number of line-integrated measurements. Decades of algorithmic refinement have brought considerable improvements, and led to a variety of employed solutions. These often feature an underlying, common structure that is rarely acknowledged or investigated. In this paper, we present a unifying perspective on sparse-view tomographic reconstructions for plasma imaging, highlighting how many inversion approaches reported in the literature can be naturally understood within a Bayesian framework. In this setting, statistical modelling of acquired data leads to a likelihood term, while the assumed properties of the profile to be reconstructed are encoded within a prior term. Together, these terms yield the posterior distribution, which models all the available information on the profile to be reconstructed. We show how credible reconstructions, uncertainty quantification and further statistical quantities of interest can be efficiently obtained from noisy tomographic data by means of a stochastic gradient flow algorithm targeting the posterior. This is demonstrated by application to soft x-ray imaging at the TCV tokamak. We validate the proposed imaging pipeline on a large dataset of generated model phantoms, showing how posterior-based inference can be leveraged to perform principled statistical analysis of quantities of interest. Finally, we address some of the inherent, and thus remaining, limitations of sparse-view tomography. All the computational routines used in this work are made available as open access code.