Two-plasmon Decay Research Articles

Utilizing parametric instabilities to diagnose edge density fluctuations in TCV

Abstract Two-plasmon decay instabilities (TPDIs) are a class of parametric instabilities where a gyrotron beam, polarized in X-mode, decays nonlinearly into two upper hybrid waves at approximately half frequency. Here, we present the design of a simple radiometer that was installed at Tokamak á configuration variable to diagnose TPDIs as well as a novel technique that was used to calibrate the radiometer using thermal electron cyclotron emissions. We show examples of measured radiation from TPDIs that occur when the gyrotron beam intersects blob filaments in the scrape-off layer and demonstrate that the frequency separation of the TPDI daughter waves can be used to assess the densities of the blob filaments. This technique could be utilized to diagnose density fluctuations due to both blobs, ELMs, and rotating magnetic islands.

Anomalous hot electron generation from two-plasmon decay instability driven by broadband laser pulses with intensity modulations

We investigate the hot electrons generated from two-plasmon decay (TPD) instability driven by laser pulses with intensity modulated by a frequency Δω m using theoretical and numerical approaches. Our primary focus lies on scenarios where Δω m is on the same order of the TPD growth rate γ 0 ( Δωm∼γ0 ), corresponding to moderate laser frequency bandwidths for TPD mitigation. With Δω m conveniently modeled by a basic two-color scheme of the laser wave fields in fully-kinetic particle-in-cell simulations, we demonstrate that the energies of TPD modes and hot electrons exhibit intermittent evolution at the frequency Δω m , particularly when Δωm∼γ0 . With the dynamic TPD behavior, the overall ratio of hot electron energy to the incident laser energy, fhot , changes significantly with Δω m . While fhot drops notably with increasing Δω m at large Δω m limit as expected, it goes anomalously beyond the hot electron energy ratio for a single-frequency incident laser pulse with the same average intensity when Δω m falls below a specific threshold frequency Δω c . This anomaly arises from the pronounced sensitivity of fhot to variations in laser intensity. We find this threshold frequency Δω c primarily depends on γ 0 and the collisional damping rate of plasma waves, with relatively lower sensitivity to the density scale length. We develop a scaling model characterizing the relation of Δω c and laser plasma conditions, enabling the potential extention of our findings to more complex and realistic scenarios.

Measurements of laser-plasma instabilities in double-cone ignition experiments relevant to the direct-drive conditions at Shenguang-II Upgrade laser facility

Experiments have been performed to investigate laser-plasma instabilities (LPIs) relevant to the direct-drive inertial confinement fusion (ICF) conditions at the Shenguang-II Upgrade (SG-II UP) laser facility during the double-cone ignition (DCI) experimental campaigns, with the overlapped laser intensity ranging from 6×1014Wcm−2 to 1.8×1015Wcm−2 . An overall LPI scenario has been built from the collection and investigation of angularly distributed scattered light. Across broad ranges of laser and plasma parameters, relevant to the direct-drive ICF conditions, the dominance of the stimulated Raman side scattering (SRSS) was confirmed, and its coexistence with the two-plasmon decay (TPD) was also observed. Significant SRSS scattered light was observed across an extremely wide range of emission angles, concentrated at large angles, and demonstrated to be robust throughout the parameter space. Time-resolved spectral measurements show distinctly different SRSS behaviors along different angles and a slightly higher threshold compared to the TPD. The overall SRSS energy loss was measured, indicating a scattered light energy fraction of up to 6%. These results are of crucial importance for the precise assessment of the LPIs in the DCI as well as other laser directly driven ICF schemes.

Spectral characteristics of laser-plasma instabilities with a broadband laser

Abstract Recent experimental progress regarding broadband laser-plasma instabilities (LPIs) showed that a 0.6% laser bandwidth can reduce backscatters of the stimulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS) at normal incidence [Phys. Rev. Lett., 132, 035102 (2024)]. In this paper, we present further discussion on the spectral distributions of the scatters developed by broadband LPIs, in addition to a brief validation of the effectiveness of bandwidth on LPIs mitigation at oblique incidence. SBS backscatter has a small redshift in the broadband case contrary to the blueshift with narrowband laser, which may be explained by the self-cross beam energy transfer between the various frequency components within the bandwidth. SRS backscatter spectrum presents a peak at a longer wavelength in the broadband case compared to the short one in the narrowband case, which is possibly attributed to the mitigation effect of bandwidth on filaments at underdense plasmas. The three-halves harmonic emission (3ω/2) has a one-peak spectral distribution under the broadband condition, which is different from the two-peak distribution under the narrowband condition, and that may be related to the spectral mixing of different frequency components within the bandwidth if the main sources of the two are both two-plasmon decay.

Cascades of Parametric Instabilities in the Tokamak Plasma Edge during Electron Cyclotron Resonance Heating.

We report observations of nonlinear two-plasmon decay instabilities (TPDIs) of a high-power microwave beam, a process similar to half-harmonic generation in optics, during electron cyclotron resonance heating in a tokamak. TPDIs are found to occur regularly in the plasma edge due to wave trapping in density fluctuations for various confinement modes, and the frequencies of both observed daughter waves agree with modeling. Emissions from a cascade of subsequent decays, which indicate a generation of ion Bernstein waves, are correlated with fast-ion generation. This emphasizes the limitations of standard linear microwave propagation models and possibly paves the way for novel microwave applications in plasmas.

Broadband laser driven near-forward scattering light of planar film target

Laser-plasma instability (LPI) is one of the key problems in the ignition process of inertial confinement fusion (ICF), and has been extensively studied in theory, simulation, and experiment for many years. Broadband laser, due to its low temporal coherence, can reduce the effective electric field strength when interacting with plasma and disrupt the phase-matching conditions of LPI, thus an effective approach to solving LPI issues is considered. Current extensive simulation studies indicate that broadband laser can suppress the generation of phenomena such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and two-plasmon decay (TPD) to some extent. There are also a few backward scattering experimental studies, but more experimental researches, such as side-scattering, are still needed. Therefore, based on the broadband second harmonic laser facility “Kunwu”, the experiments are designed for studying the lateral scattering of critical density plasma driven by broadband laser and traditional narrowband laser, and the production of hot electrons as well in this work. The experimental results show that the side SBS spectra and side SRS spectra and portions at different angles excited by broadband lasers with a power density of 1×10<sup>15</sup> W/cm<sup>2</sup> are significantly different from those by narrowband lasers. Further analysis reveals that the overall portion of transverse hot electrons in broadband laser cases is higher than that in narrowband laser case. However, for broadband laser, the portion of SRS at small forward angle and backward angle are significantly lower than that for narrowband laser. Preliminary qualitative analysis suggests that SRS may not be the main mechanism for hot electron generation in this case, and that PDI might play a dominant role in generating hot electrons.

Two-plasmon decay of microwaves in low temperature magnetized laboratory plasmas

Observations from magnetically confined fusion experiments have in the past decades revealed instances of two-plasmon decay instabilities (TPDIs) between injected 2nd harmonic X-mode waves and trapped upper hybrid (UH) waves near half frequency of the injected wave. We present and compare growth rates of TPDIs computed from reduced analytical models and particle-in-cell (PIC) simulations in low-temperature plasmas, emulating conditions found in more accessible controllable laboratory devices. The growth rates obtained from simulations agree with our modeling, confirming that we can model TPDIs using both analytical models and PIC simulations in low-temperature plasmas. Furthermore, the growth rates are on the order of 0.01 ns−1 to 0.1 ns−1 for injected waves with intensity of 103 W/cm2, which indicates that we can expect to see signals of TPDIs in controllable laboratory plasma devices.

Anomalous staged hot-electron acceleration by two-plasmon decay instability in magnetized plasmas.

We present a staged hot-electron acceleration mechanism of the two-plasmon decay (TPD) instability in the transverse magnetic field under the parameters relevant to inertial confinement fusion experiments. After being accelerated by the forward electron plasma wave (FEPW) of TPD, the hot-electrons can be anomalously accelerated again by the backward electron plasma wave (BEPW) of TPD and then obtain higher energy. Moreover, the surfatron acceleration mechanism of TPD in the magnetic field is also confirmed, the electrons trapped by the TPD daughter EPWs are accelerated in the direction along the wave front. Interestingly, the velocity of electrons accelerated by surfing from the FEPW is quite easily close to the BEPW phase velocity, which markedly enhances the efficiency of the staged acceleration. The coexistence of these two acceleration mechanisms leads to a significant increase of energetic electrons generated by TPD in the magnetic field. Meanwhile the EPWs are dissipated, TPD instability is effectively suppressed, and the laser transmission increases.

Parametric instabilities and hot electron generation in the interactions of broadband lasers with inhomogeneous plasmas

The development of parametric instabilities in the interaction of a large-scale inhomogeneous plasma with either a monochromatic or a broadband laser pulse is investigated theoretically and numerically. For a monochromatic laser at an intensity of W cm−2, the development of Stimulated Brillouin Scattering (SBS) in the relatively low density region will obviously dissipate pump laser and hence inhibit the development of Two-Plasmon Decay (TPD) and absolute Stimulated Raman Scattering (SRS) near the quarter-critical density. By using a laser with a moderate fractional bandwidth (∼1.0%) at the same averged intensity, it is found that the laser reflectivity will be greatly reduced since the SBS can be suppressed effectively due to its low linear growth rate. On the contrary, the TPD and absolute SRS are obviously enhanced since the in situ laser intensity near the quarter-critical density becomes stronger in this case. As a result, the hot electron generation due to the TPD and absolute SRS is dramatically enhanced as well. This indicates that the competition between various parametric instabilities in a large-scale inhomogeneous plasma makes it more challenging to simultaneously suppress all kinds of parametric instabilities by using broadband lasers. Particular attention should be paid to the TPD, which not only has a relatively large linear growth rate but also is efficient in generating harmful hot electrons. Increasing the laser bandwidth further, the hot electron generation will be finally reduced as long as the TPD and SRS are also suppressed with a sufficient bandwidth (3%) at the intensity of W cm−2 that may be encountered in some novel ignition schemes such as shock ignition. However, it is worth noting that the laser bandwidth required to mitigate parametric instabilities and hot electron production strongly depends on laser intensity. A moderate laser bandwidth (∼1%) may be sufficient to mitigate both the laser reflectivity and hot electron production for typical ICF target designs operated with laser intensities lower than 1015 W cm−2.

Large-incidence-angle multiple-beam two-plasmon decay instability in inertial confinement fusion

A multi-dimensional code FLAME-MD solving fluid-like laser-plasma instabilities' (LPIs) equations has been developed and is used to study multiple-beam two-plasmon decay (TPD) instability relevant to a laser-entrance-hole window burn-off scenario in inertial confinement fusion (ICF) experiments. It is found that TPD can be collectively driven by multiple beams incident at large incidence angles with respect to the electron density gradient at a very low threshold. The polarization configuration of the laser beams is a key factor determining the way of sharing common daughter electron plasma waves (EPWs). The p-polarized beams arranged on the same incidence cone can collectively drive common EPWs along the cone axis. The common-wave sharing mechanisms among the s-polarized beams are largely dependent on the geometry of the beams and are less robust. The simulation results also show that the p-polarized beams are dominating the multiple-beam TPD processes. The common EPWs along the cone axis can accelerate energetic electrons toward the capsule inside the hohlraum and, therefore, pose a fuel-preheat risk to ICF implosions.

Evolution and hot electron generation of laser–plasma instabilities in direct-drive inertial confinement fusion

A series of 2D in-plane plane wave particle-in-cell simulations find distinctive paths of laser-plasma instability evolution in OMEGA-scale implosions, depending on the initial electron temperature. At low temperatures, two-plasmon decay (TPD) dominates in both initial growth and the steady state. At high temperatures, the initial dominant modes switch to stimulated Raman scattering, but TPD still dominates a steady state characterized by cavitation and Langmuir turbulence. A hot electron scaling is also obtained from the simulations that, when combined with laser/plasma conditions from hydro simulations, can predict hot electron generation in implosions that do not employ smoothing-by-spectral-dispersion (SSD). It also shows that under the same laser/plasma conditions, SSD can reduce hot electron generation.

Suppressing stimulated Raman side-scattering with vector light

Recent observations of stimulated Raman side-scattering (SRSS) in different laser inertial confinement fusion ignition schemes have revealed that there is an underlying risk of SRSS on ignition. In this paper, we propose a method that uses the nonuniform nature of the polarization of vector light to suppress SRSS, and we give an additional threshold condition determined by the parameters of the vector light. For SRSS at 90°, where the scattered electromagnetic wave travels perpendicular to the density profile, the variation in polarization of the pump will change the wave vector of the scattered light, thereby reducing the growth length and preventing the scattered electromagnetic wave from growing. This suppression scheme is verified through three-dimensional particle-in-cell simulations. Our illustrative simulation results demonstrate that for linearly polarized Gaussian light, there is a strong SRSS signal in the 90° direction, whereas for vector light, there is very little SRSS signal, even when the conditions significantly exceed the threshold for SRSS. We also discuss the impact of vector light on stimulated Raman backscattering, collective stimulated Brillouin scattering and two-plasmon decay.

Effects of hydrogen concentration in ablator material on stimulated Raman scattering, two-plasmon decay, and hot electrons for direct-drive inertial confinement fusion

Effects of hydrogen concentration in ablator material on stimulated Raman scattering, two-plasmon decay, and hot electrons for direct-drive inertial confinement fusion

Hot electron preheat in hydrodynamically scaled direct-drive inertial confinement fusion implosions on the NIF and OMEGA

Hot electron preheat has been quantified in warm, directly driven inertial confinement fusion implosions on OMEGA and the National Ignition Facility (NIF), to support hydrodynamic scaling studies. These CH-shell experiments were designed to be hydrodynamically equivalent, spanning a factor of 40 in laser energy and a factor of 3.4 in spatial and temporal scales, while preserving the incident laser intensity of 1015 W/cm2. Experiments with similarly low levels of beam smoothing on OMEGA and NIF show a similar fraction (∼0.2%) of laser energy deposited as hot electron preheat in the unablated shell on both OMEGA and NIF and similar preheat per mass (∼2 kJ/mg), despite the NIF experiments generating a factor of three more hot electrons (∼1.5% of laser energy) than on OMEGA (∼0.5% of laser energy). This is plausibly explained by more absorption of hot electron energy in the ablated CH plasma on NIF due to larger areal density, as well as a smaller solid angle of the imploding shell as viewed from the hot electron generating region due to the hot electrons being produced at a larger standoff distance in lower-density regions by stimulated Raman scattering, in contrast to in higher-density regions by two-plasmon decay on OMEGA. The results indicate that for warm implosions at intensities of around 1015 W/cm2, hydrodynamic equivalence is not violated by hot electron preheat, though for cryogenic implosions, the reduced attenuation of hot electrons in deuterium–tritium plasma will have to be considered.

Stabilization of low-threshold two-plasmon parametric decay instability at oblique incidence of a microwave pump beam onto the plasma

It is shown that with an oblique incidence of a pump microwave in X2-mode electron cyclotron plasma heating (ECRH) experiments, the threshold power of a parametric decay instability (PDI) leading to excitation of two trapped upper hybrid waves increases and the anomalous absorption level decreases. It is proposed to use a small pump beam tilt along the magnetic field to suppress this most dangerous PDI, often observed in the X2-mode ECRH experiments.

Suppressing parametric instabilities in direct-drive inertial-confinement-fusion plasmas using broadband laser light

It has long been recognized that broadband laser light has the potential to control parametric instabilities in inertial-confinement-fusion (ICF) plasmas. Here, we use results from laser-plasma-interaction simulations to estimate the bandwidth requirements for mitigating the three predominant classes of instabilities in direct-drive ICF implosions: cross-beam energy transfer (CBET), two-plasmon decay (TPD), and stimulated Raman scattering (SRS). We find that for frequency-tripled, Nd:glass laser light, a bandwidth of 8.5 THz can significantly increase laser absorption by suppressing CBET, while ∼13 THz is needed to mitigate absolute TPD and SRS on an ignition-scale platform. None of the glass lasers used in contemporary ICF experiments, however, possess a bandwidth greater than 1 THz and reaching larger values requires the use of an auxiliary broadening technique such as optical parametric amplification or stimulated-rotational-Raman scattering. An arguably superior approach is the adoption of an argon-fluoride (ArF) laser as an ICF driver. Besides having a broad bandwidth of ∼10 THz, the ArF laser also possesses the shortest wavelength (193 nm) that can scale to the high energy/power required for ICF—a feature that helps to mitigate parametric instabilities even further. We show that these native properties of ArF laser light are sufficient to eliminate nearly all CBET scattering in a direct-drive target and also raise absolute TPD and SRS thresholds well above those for broadband glass lasers. The effective control of parametric instabilities with broad bandwidth is potentially a “game changer” in ICF because it would enable higher laser intensities and ablation pressures in future target designs.

Effect of overlapping laser beams and density scale length in laser-plasma instability experiments on OMEGA EP

Experiments have been conducted on the OMEGA EP laser facility to study the effect of density scale length and overlapping beam geometry on laser-plasma instabilities near and below the quarter-critical density. Experiments were conducted in both planar geometry (density scale length L n ∼ 190 to 300 μm) and spherical geometry ( L n ∼ 150 μm) with up to four overlapping beams and were designed to have overlapped intensities and density scale lengths comparable to OMEGA spherical experiments, but with many fewer beams. In comparison with previous experiments on OMEGA and National Ignition Facility, it is confirmed that shorter density scale lengths favor the two-plasmon decay (TPD) instability, while longer density scale lengths favor stimulated Raman scattering (SRS). In addition, for experiments at the same scale length and overlapped laser intensity, higher single-beam intensities favor SRS, while a larger number of overlapping beams favor TPD.

Hot electron scaling for two-plasmon decay in ICF plasmas

We present a parametric scaling of hot electron (HE) generation at quarter critical density from the two-plasmon decay process. The study is conducted with the laser plasma simulation environment code, considering Langmuir decay instabilities (LDI) and laser pump depletion in 2D. The parameter scan is conducted as a function of electron temperature, ion–electron temperature ratio, drive strength, and density scale length. The scaling shows an hot electron (HE) conversion fraction up to 40%, HE fluxes up to 6 × 10 14 W / cm 2, and average temperatures in the range of 30 to 100 keV. The electron angular distributions exhibit two main regions: the plasma “bulk,” characterized by homogeneous emission, up to energies of 30 − 60 keV depending on the individual laser–plasma conditions, and a HE tail after ≃ 50 − 60 keV. The mid-energy electrons are homogeneously emitted toward the end of the plasma bulk and acquire energy through electron plasma wave (EPW) Landau damping from Langmuir wave collapse and LDI cascade. The HE tail has electrons emitted in the forward direction and at low divergence, due to turbulence and EPW Landau damping from multi-staged acceleration. Finally, the laser power transmitted through the quarter critical region reaches values from ∼ 80 % down to ∼ 35 % for increasing HE generation, with absorption due to EPW collisional damping in the range of ∼ 10 % − 35 %.

Competition among the two-plasmon decay of backscattered light, filamentation of the electron-plasma wave and side stimulated Raman scattering

Abstract Competition among the two-plasmon decay (TPD) of backscattered light of stimulated Raman scattering (SRS), filamentation of the electron-plasma wave (EPW) and forward side SRS is investigated by two-dimensional particle-in-cell simulations. Our previous work [K. Q. Pan et al., Nucl. Fusion 58, 096035 (2018)] showed that in a plasma with the density near 1/10 of the critical density, the backscattered light would excite the TPD, which results in suppression of the backward SRS. However, this work further shows that when the laser intensity is so high ( $>{10}^{16}$ W/cm2) that the backward SRS cannot be totally suppressed, filamentation of the EPW and forward side SRS will be excited. Then the TPD of the backscattered light only occurs in the early stage and is suppressed in the latter stage. Electron distribution functions further show that trapped-particle-modulation instability should be responsible for filamentation of the EPW. This research can promote the understanding of hot-electron generation and SRS saturation in inertial confinement fusion experiments.

Multibeam laser–plasma interaction at the Gekko XII laser facility in conditions relevant for direct-drive inertial confinement fusion

Abstract Laser–plasma interaction and hot electrons have been characterized in detail in laser irradiation conditions relevant for direct-drive inertial confinement fusion. The experiment was carried out at the Gekko XII laser facility in multibeam planar target geometry at an intensity of approximately $3\times {10}^{15}$ W/cm2. Experimental data suggest that high-energy electrons, with temperatures of 20–50 keV and conversion efficiencies of $\eta <1\%$ , were mainly produced by the damping of electron plasma waves driven by two-plasmon decay (TPD). Stimulated Raman scattering (SRS) is observed in a near-threshold growth regime, producing a reflectivity of approximately $0.01\%$ , and is well described by an analytical model accounting for the convective growth in independent speckles. The experiment reveals that both TPD and SRS are collectively driven by multiple beams, resulting in a more vigorous growth than that driven by single-beam laser intensity.