Plasmon Decay Instability Research Articles

Boosting electron and x-ray emission from laser produced plasmas by doping liquid droplets

In recent studies, it has been shown that under optimal dual pulse irradiation, it is possible to generate anomalous relativistic temperature (>500 keV) electron emission even with mJ/pulse lasers at 1/100th of the relativistic intensity when micrometer droplets of methanol are used as a high repetition rate target. In this paper, we present the results of doping these droplets with CH3I to bring out a 30-fold enhancement in electron emission. Experiments and simulations presented here find a route to enhance electron emission driven by two plasmon decay instability. Improving compact laser plasma based sources of MeV electrons, x-ray emission schemes that use a high repetition rate, and mJ/pulse laser systems is important not only for basic science but also applications of imaging and radiography.

Shaped liquid drops generate MeV temperature electron beams with millijoule class laser

MeV temperature electrons are typically generated at laser intensities of 1018 W cm−2. Their generation at non-relativistic intensities (~1016 W cm−2) with high repetition rate lasers is cardinal for the realization of compact, ultra-fast electron sources. Here we report a technique of dynamic target structuring of micro-droplets using a 1 kHz, 25 fs, millijoule class laser, that uses two collinear laser pulses; the first to create a concave surface in the liquid drop and the second, to dynamically-drive electrostatic plasma waves that accelerate electrons to MeV energies. The acceleration mechanism, identified as two plasmon decay instability, is shown to generate two beams of electrons with hot electron temperature components of 200 keV and 1 MeV, respectively, at an intensity of 4 × 1016 Wcm−2, only. The electron beams are demonstrated to be ideal for single shot high resolution (tens of μm) electron radiography.

Two plasmon decay instability stimulated by large-incidence-angle laser in inertial confinement fusion

The two-plasmon-decay instability (TPD) in the large-incidence-angle regime is studied via fluid and particle-in-cell (PIC) simulations. It is found that the TPD threshold in this regime within a narrow incidence angle range close to 60∘ can be orders of magnitude lower than the normal-incidence threshold. A threshold formula is proposed for this regime and is verified by fluid and PIC simulations. PIC simulations show significant laser absorption and energetic electron generation once TPD is excited. The incident angle and the laser spot size have a significant influence on the TPD in the large-incidence-angle regime. The low threshold and significant hot electron generation in this regime pose potentially unexpected preheating risks to inertial confinement fusion.

Kinetic investigations of nonlinear electrostatic excitations in quantum plasmas.

For plasmas in an extremely high-density state, like stellar cores or compressed fuel in inertial fusion facilities, their behavior turns out to be quite different when compared with those plasmas in interstellar space or magnetic confinement devices. To figureout those differences and uncover the kinetic physics in electrostatic excitations, a quantum kinetic code solving Wigner-Poisson equationshas been developed. Basic plasmon decay, Landau damping, and two-stream instability of extremely high-density plasmas are investigated by using our newly developed code. Numerical simulations show that in the linear region, the dispersion relations of intrinsic modes can be significantly affected by quantum effects, and such simulation results can be well described by the existing analytical theory. Especially in the nonlinear region, since the space-time scale of collective modes of plasmas is comparable to the electron de Broglie wavelength, their couplings produce some new physics: the energy exchange between the electron and the collective mode results in an abnormal oscillation that does not exist in classical plasmas.

Evidence of ‘two plasmon’ decay of energetic particle induced geodesic acoustic mode

Secondary low frequency mode generation by energetic particle induced geodesic acoustic mode (EGAM) observed in LHD experiment is studied using nonlinear gyrokinetic theory. It is found that the EGAM frequency can be significantly higher than local geodesic acoustic mode (GAM) frequency in low collisionality plasmas, and it can decay into two GAMs as its frequency approaches twice GAM frequency, in a process analogous to the well-known two plasmon decay instability. The condition for this process to occur is also discussed.

Hybrid stimulated Raman scattering-two-plasmon decay instability and 3/2 harmonic in steep-gradient femtosecond plasmas.

We numerically study interaction of a very intense (I∼10^{17} to 5×10^{19}W/cm^{2}) femtosecond obliquely incident p-polarized laser pulse with a steep-gradient (L∼λ) plasma, i.e., within the conditions typical for modern experiments. It is shown that the hybrid stimulated Raman scattering-two-plasmon decay instability develops near the quarter-critical density surface and plays the dominant role for the plasma waves' excitation and energy absorption. The plasmons are excited as two wave packets confined near this surface with very wide ≈ω_{0}/c spatial spectra along its normal. Hence, phase-matching conditions for the 3/2 harmonic generation are fulfilled immediately and include the mechanism coming from the high harmonics of plasma waves. This mechanism has been proved experimentally by observing an additional 3/2 harmonic beam.

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

Parametric decay instabilities (PDIs) exciting daughter waves trapped inside a magnetized plasma with a non-monotonic density profile are investigated numerically. The investigation is motivated in particular by observations of low threshold PDI signatures during second harmonic electron cyclotron resonance heating experiments in magnetically confined fusion experiments. We use the particle-in-cell code EPOCH to study conversion of a fast X-mode pump wave into a combination of half frequency X-mode and electron Bernstein waves and identify two regimes where PDIs can excite trapped electrostatic waves. Above the second harmonic upper hybrid (UH) density, a PDI known also as a two plasmon decay (TPD) instability excites a pair of UH waves that we locate in frequency and wavenumber space. At lower densities, a PDI known as stimulated Raman scattering may produce one trapped and one returning X-mode daughter wave with a much slower growth rate than the TPD instability. In both cases, we show that the frequency separation of the daughter waves depends on the density in a predictable manner. With little loss from the decay region, the trapped daughter waves become unstable with respect to secondary parametric instabilities (PIs), leading to distinctly different phases of the UH spectrum. Unlike the primary instability, the secondary PIs are shown to depend on ion dynamics. Furthermore, we observe escaping waves near the 3/2 pump frequency resulting from tertiary PIs in agreement with recently proposed backscattering during magnetically confined fusion experiments.

X-ray spectroscopy of super-intense laser-produced plasmas for the study of nonlinear processes. Comparison with PIC simulations

We present X-ray spectroscopic diagnostics in femto-second laser-driven experiments revealing nonlinear phenomena caused by the strong coupling of the laser radiation with the created plasma. Among those nonlinear phenomena, we found the signatures of the Two Plasmon Decay (TPD) instability in a laser-driven CO2 cluster-based plasma by analyzing the Langmuir dips in the profile of the O VIII Lyε line, caused by the Langmuir waves created at the high laser intensity 3 1018Wcm-2. With similar laser intensities, we reveal also the nonlinear phenomenon of the Second Harmonic Generation (SHG) of the laser frequency by analyzing the nonlinear phenomenon of satellites of Lyman δ and ε lines of Ar XVII. In the case of relativistic laser-plasma interaction we discovered the Parametric Decay Instability (PDI)-induced ion acoustic turbulence produced simultaneously with Langmuir waves via irradiation of thin Si foils by laser intensities of 1021Wcm-2.

Anomalous backscattering and absorption in second harmonic ECRH experiments due to low-threshold two UH plasmon decay instability in blobs

An universal mechanism responsible for excitation of the low-threshold two upper-hybrid (UH) plasmon absolute parametric decay instability of a pump extraordinary microwave is proposed and analysed in detail. A corner-stone of it is the 2D localization of the daughter UH waves in an intensive magnetic field aligned blob/filament and the presence of a finite-width mm-wave pump beam

Modulational instabilities in relativistic pair plasmas

We study the modulational instability of an intense photon beam in a relativistic pair plasma. We use the wave-kinetic description of the photon field and relativistic fluid equations for electrons and positrons. This allows us to consider the influence of the photon spectral distribution and photon recoil effects on the instability threshold and growth rates. The case of very low frequencies modulations, well below plasma frequency, is compared to that of high-frequency modulations corresponding to the plasmon decay instability.

StarDriver: A Flexible Laser Driver for Inertial Confinement Fusion and High Energy Density Physics

We propose a novel method to minimize laser–plasma instabilities and improve laser–plasma coupling by the use of multi-beam laser architecture with a large system frequency bandwidth and many beamlets per unit solid angle. The StarDriver™, laser driver is constructed from 104 to 105 individual lasers, each delivering nominally 100 J in pulses of ~3–30 ns at a nominal wavelength of ~355 nm with better than 3–5 diffraction-limited performance. The beamlets are individually relatively narrowband to facilitate maximum laser performance, but the ensemble of beamlets span a wide frequency range. Currently available laser media enable Δω/ω ~ 2 % at 355 nm with the possibility of system bandwidths approaching 10 % in the future. The many beamlets of StarDriver™ provide optimal asymptotic smoothing for hydrodynamic instabilities (0–1 %), innovative focusing strategies including zooming, and the large bandwidth enables extremely rapid hydrodynamic smoothing times ~30 fs. The distribution of frequencies among the beamlets allows flexibility for fine control of the seeding of the Rayleigh–Taylor instability. The ultra-broad bandwidth combined with the large total k-spectrum of the laser drive in the plasma corona may enable complete suppression of the most problematic laser–plasma instabilities such as stimulated Brillouin backscatter, stimulated Raman scatter, cross-beam energy transfer, and the two plasmon decay instability. StarDriver™ offers potentially superior flexibility in laser drivers for inertial confinement fusion, enabling almost arbitrary sequencing of wavelength, polarization, focus, and fine control of the spatio-temporal properties of the drive in the corona. The highly modular strategy of StarDriver™ should enable an attractive development pathway as well as maximizing overall system efficiency.

Nonlinear development of the two-plasmon decay instability in three dimensions

Most recent experiments on the excitation of the two plasmon-decay (TPD) instability involve a three-dimensional (3D) array of overlapping laser beams. Our recent two dimensional (2D) simulations suggested that Langmuir cavitation and collapse are important nonlinear saturation mechanisms for TPD. There are important quantitative differences in the Langmuir collapse process in 2D and 3D. To address these and other issues, we have developed a 3D Zakharov code. It has been applied to study the evolution of TPD from absolute instabilities (arising from 3D laser geometries) to the nonlinear state (J. Zhang et al., Phys. Rev. Lett. (submitted)). The present paper concentrates on the nonlinear saturated state excited by the collective action of two crossed laser beams with arbitrary polarizations. Remarkable agreement between 3D and 2D simulations is found for several averaged physical quantities when the beams are polarized in their common plane. As in the previous 2D simulations, we find: (a) the collective, initially convectively unstable triad modes dominate after a sub-picosecond burst, (b) Langmuir cavitation and collapse are important nonlinearities, and (c) that the statistics of intense cavitons are characteristic of a Gaussian random process. The 3D steady-state saturated Langmuir energy level is about 30% higher than in 2D. The auto-correlation functions of the Langmuir envelope field and of the low-frequency electron density field yield the spatial shape of the strongest collapsing cavitons which are 3D ellipsoids whose orientation depends on the laser polarizations. This tilting of the caviton's strongest electric field direction away from the normal to the target surface is a major new 3D result. This tilting may deflect the hot electron flux and thereby mitigate target preheat.

Experimental Validation of the Two-Plasmon-Decay Common-Wave Process

The energy in hot electrons produced by the two plasmon decay instability, in planar targets, is measured to be the same when driven by one or two laser beams and significantly reduced with four for a constant overlapped intensity on the OMEGA EP. This is caused by multiple beams sharing the same common electron-plasma wave. A model, consistent with the experimental results, predicts that multiple laser beams can only drive a resonant common two plasmon decay electron-plasma wave in the region of wave numbers bisecting the beams. In this region, the gain is proportional to the overlapped laser beam intensity.

The reduced-description particle-in-cell model for the two plasmon decay instability

Recently, motivated by the resurgent interest in suprathermal electron generation by two plasmon decay (TPD) in direct-drive laser-fusion [J. A. Delettrez et al., Bull. Am. Phys. Soc. 53, 248 (2008); V. A. Smalyuk et al., Phys. Rev. Lett. 100, 185005 (2008); B. Yaakobi et al., Phys. Plasmas 12, 062703 (2005)], the fully kinetic, reduced-description, particle-in-cell (RPIC) methodology has been extended to include TPD. It provides a computationally efficient fully kinetic simulation tool, especially in nonlinear regimes where the Langmuir decay instability (LDI) is a dominant saturation mechanism. This RPIC methodology is an extension of the modeling of laser-plasma instabilities in underdense plasmas reported previously [H. X. Vu, B. Bezzerides, and D. F. DuBois, J. Comput. Phys. 156, 12 (1999)]. The relationship between RPIC and the extended Zakharov model previously used for TPD [D. F. DuBois, D. A. Russell, and H. A. Rose, Phys. Rev. Lett. 74, 3983 (1995)] is explored theoretically and tested in simulations. The modification of the electron velocity distribution—in particular, the generation of hot electrons—as calculated in RPIC leads to weakening of the wave turbulence excited by TPD compared to the Zakharov model predictions but the locations in wave vector space of important spectral features, e.g., arising from the LDI, of the nonlinear wave fluctuations are exactly the same in the two approaches. New results involving two oblique, overlapping laser beams, a common geometrical feature in direct-drive schemes, are presented. The two laser beams can cooperatively excite common primary Langmuir waves which initiate the LDI process.

Transition to the two-plasmon decay for increasing plasma density scalelengths

Two plasmon decay (TPD) instability is an important mechanism for laser energy deposition in underdense plasma even for ultrashort laser pulses provided that the laser intensities are high enough. The excitation of this instability depends also heavily on the plasma density scalelength. Important features associated with the TPD are the 3/2 harmonics emission and hot electron generation. Their angular distributions from numerical simulations are presented, showing the transition of the stimulated Raman scattering to the TPD when increasing the plasma density scalelengths.

Convective Two - Plasmon Decay Instability in Inhomogeneous Collisional Plasma

Quantitative description of the two plasmon - decay (TPD) in inhomogeneous collisional plasma is given through the solution of the slow coupling equations of the decay modes. The initial TPD growth rate and the threshold pump intensity for the instability to occur are calculated in a homogeneous unmagnetized collisional plasma. In inhomogeneous plasma with a linear density ramp, the spatial amplification factor and the pump threshold intensity for the convective TPD instability has been calculated.

A fluid-kinetic model for the two plasmon decay instability

A solution for the gain rates and plasmon frequencies of the two plasmon decay instability is obtained using a fluid-kinetic model. This work is of relevance to high temperature, short-pulse laser plasma interactions where appreciable instability is expected in regions of plasma where the phase velocity of the electrostatic waves may not be assumed to be much larger than the thermal velocity of the electrons. The model is evaluated by comparing its results for the stimulated Raman scattering process with a fully kinetic treatment.

A versatile matrix-based solution for the two plasmon decay instability

A Floquet-based solution of the equations governing the two plasmon decay (TPD) instability which is valid over a range of plasma densities is presented. The capability to include a large number of modes allows the validity of conventional fluid-based analytic solutions at densities differing from quarter-critical to be assessed. The conventional assumptions are upheld in the absence of a magnetic field, however, it is shown that the extra modes have a significant effect once a static magnetic field is present in the plasma.

A variational approach to parametric instabilities in inhomogeneous plasmas IV: The mixed polarization high-frequency instability

The theories of the two plasmon decay (TPD) instability and stimulated Raman scattering (SRS) are unified and presented using a variational formulation. The mixed polarization high-frequency instability is the resulting generalization which has SRS and TPD as special cases. The most unstable mode’s properties are derived and shown as a function of perpendicular wave number. For vanishing wave number we recover the Raman backscattering result and for wave numbers a few times larger than the square root of the homogeneous plasma growth rate normalized to the pump frequency, the two plasmon decay instability modes are recovered. This transition is accompanied by a change of polarization from a purely electromagnetic transversely polarized wave in the case of Raman to a strictly electrostatic or longitudinally polarized wave in the case of TPD, and admixtures in between. Experimental signatures of these modes are given and a method is proposed by which the density scale length and electron temperature would be simultaneously inferred from the width and height of the spectral signature corresponding to these hybrid modes in an ideal experiment.

Saturation of radiation-induced parametric instabilities by excitation of Langmuir turbulence

Progress made in the last few years in the calculation of the saturation spectra of parametric instabilities which involve Langmuir daughter waves will be reviewed. These instabilities include the ion acoustic decay instability, the two plasmon decay instability (TPDI), and stimulated Raman scattering (SRS). In particular we will emphasize spectral signatures which can be directly compared with experiment. The calculations are based on reduced models of driven Langmuir turbulence. Thomson scattering from hf-induced Langmuir turbulence in the unpreconditioned ionosphere has resulted in detailed agreement between theory and experiment at early times. Strong turbulence signatures dominate in this regime where the weak turbulence approximation fails completely. Recent experimental studies of the TPDI have measured the Fourier spectra of Langmuir waves as well as the angular and frequency spectra of light emitted near 3/2 of the pump frequency again permitting some detailed comparisons with theory. Thomson scattering measurements of the Langmuir wave spectra from SRS are consistent with the saturation by secondary and tertiary decay of the primary SRS Langmuir waves. Scaling laws derived from a local model of SRS saturation are compared with full simulations and recent Nova experiments.