Abstract
Keywords
1 Introduction
In inertial confinement fusion (ICF) experiments, laser plasma instabilities (LPIs) such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and two-plasmon decay (TPD) instability are never avoided, even if several approaches, including direct-drive[1–4], indirect-drive[5–7] and hybrid-drive[8], have been proposed to achieve ICF. Generally, LPIs are expected to be suppressed in ICF experiments because they do harm to ICF[1,5]. Backward SRS (BSRS) and SBS take away part of the laser energy and weaken the drive intensity, side SRS (SSRS) and side SBS change the laser propagation direction, which results in uncontrollable drive symmetry, and SRS and TPD generate super-hot electrons that may decrease the implosion efficiency.
Significant progresses in LPI studies have been achieved during recent decades[9–11], but there are still many LPI problems remaining unclarified. Among them, nonlinearities[12] are the most important and highlighted, since nonlinear processes govern the saturation of LPIs (which decide the final energy loss from LPIs) in ICF experiments. Nonlinearities are usually involved in the behaviors of daughter waves, including the scattered lights or the electrostatic waves. The most common nonlinearities involved in electrostatic waves are high-order harmonics generation (HHG)[13,14], particle trapping[15,16], daughter wave decay[17–20], wave collapse[12], etc. The parameter
In this paper, with the help of 2D particle-in-cell (PIC) simulations[24], we discuss the competition among three instabilities, namely two secondary instabilities, which are the TPD of the backscattered light and filamentation of the EPWs, and a primary instability, which is the SSRS of the laser. As discussed in our previous work[22], the TPD of the backscattered light will be excited in a plasma with an electron density near
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2 Particle-in-cell simulation setups
Two simulations, named cases 1 and 2, are performed in this paper. Except for the laser intensity, all the simulation parameters are the same in both cases. The laser intensities in cases 1 and 2 are 4.45
Since
Figure 1.Calculated wave numbers of the scattered lights and EPWs of SRS for and keV according to
3 Analysis of simulation results
Since the p-polarized laser is propagating in the
Figure 2.Snapshots of in
Figure 3.Snapshots of in
Based on the above analysis, Figure 2 gives four snapshots of the
Figure 4.Snapshots of in real space,
Several reasons, such as Langmuir wave collapse and TPMI, could be responsible for filamentation of the EPWs. Langmuir wave collapse is excluded for BSRS because it occurs when
Figure 5.Snapshots of the electron distribution functions in
To analyze why TPD is suppressed, we give a more detailed analysis for case 2 in Figure 4. In homogeneous plasmas, SRS is subject to both absolute and convective instability. For BSRS, the scattered light is counter-propagating with the laser, so the EPW near the laser incident side is more amplified and has a higher intensity. For FSRS, the intensity distribution is opposite to that of BSRS because the scattered light has the opposite propagation direction to BSRS. As a result, nonlinearities should be initially excited near the left-hand boundary (defined as region I in this paper, i.e.,
Distributions of electrons in phase space are employed to reflect the trapping process. In phase space, the trapping structure is a series of ‘phase islands’ with their center near the phase velocity of the EPWs. For the simulation parameters used in this paper, the phase velocity of the EPW is
4 Discussion and conclusion
It should be mentioned that 3D PIC simulations could be more powerful to research LPIs because more physics are included. For example, in inhomogeneous plasmas, tangential SSRS in which the scattered light is propagating in the
In conclusion, with the help of 2D PIC simulations, we investigate the competition among several instabilities, including TPD, of the backscattered light, filamentation of the EPWs generated by both BSRS and FSRS, and SSRS of the laser. The simulation results show that when the laser intensity is near
References
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