Laser Quad Research Articles

Ultrafast random polarization beam smoothing driven by counter-rotating wavefronts via the optical Kerr effect

We propose an ultrafast random polarization smoothing scheme to achieve ultrafast speckle movement and polarization randomization of the focal spot. In the scheme, each beamlet in a laser quad is driven by a rotating petal-like wavefront, giving rise to the ultrafast redistribution of the speckles within the focal spot. The rotating wavefront is induced by an optical Kerr medium pumped by a laser beam with rotating intensity distribution that is generated by the superposition of two Laguerre–Gaussian beams carrying conjugate vortex phase and frequency shift. In addition, by properly selecting the topological charges of the pump beams and polarization states of the beamlets, the beamlets in a quad can be divided into two sets with counter-rotating wavefronts and orthogonal polarizations, which further randomizes the polarization of the focal spot. Results indicate that the proposed scheme can reduce the contrast by more than 70% and the degree of polarization by more than 75% within several picoseconds, by utilizing the rotating phase pumped by two Laguerre-Gaussian beams with a peak intensity of 0.18GW/cm2 and a frequency shift of 1 THz. The ultrafast random polarization smoothing scheme is expected to suppress the growth of Laser Plasma Instabilities (LPIs) including the filamentation and backscatter in indirect-drive inertial confinement fusion (ICF) facilities.

Analysis and Optimization of Irradiation Characteristics of Laser Quads on Hohlraum Wall Based on Broadband Laser Beams Smoothed Using Induced Spatial Incoherence and De-Diffraction Lens Array

Analysis and Optimization of Irradiation Characteristics of Laser Quads on Hohlraum Wall Based on Broadband Laser Beams Smoothed Using Induced Spatial Incoherence and De-Diffraction Lens Array

Optimized dynamic interference smoothing via asynchronous phasemodulation of SSD

In dynamic interference smoothing (DIS) scheme, the speckles of focal spots are swept in multi-directions within several picoseconds, leading to the improvement of irradiation uniformity. In this work, the theoretical limitation of the smoothing performance of DIS scheme is revealed and then an optimized dynamic interference smoothing (ODIS) scheme is proposed. The ODIS scheme is implemented by applying asynchronous sinusoidal phase modulations induced by electro-optic modulators with time-delayed voltage signals to the beamlets in a laser quad. Results indicate that the irradiation uniformity of the ODIS scheme is further improved than that of DIS scheme owing to the asynchronous spatiotemporal phases. In addition, the polarization of the ODIS scheme also rotates on picosecond timescale, which may be potential to mitigate the parametric backscattering. Moreover, though the ODIS scheme has a relatively high requirement for temporal synchronization of beamlets, it shows a wide tolerance for the spatial wavefront distortion.

Analysis of the smoothing performance of a rapid polarization rotation smoothing scheme.

In order to improve the irradiation uniformity on the target plane and reduce the parametric backscattering in indirect-drive inertial confinement fusion facilities, a rapid polarization rotation smoothing scheme is proposed. In this scheme, there is a central wavelength shift between adjacent beams in a laser quad. Two of these beams are transformed into left-rotating circularly polarized beams by using polarization control plates, and the other two are transformed into right-rotating circularly polarized beams. Moreover, conjugate spiral phase plates are adopted to transform the beams into conjugate Laguerre-Gaussian beams. As a result of the frequency beat among the beams with counter-rotating polarization and conjugate spiral phases, both the intensity distribution and polarization of the focal spot would rotate rapidly on the picosecond timescale. The physical model of the rapid polarization rotation smoothing scheme is built up, and the smoothing performance affected by the parameters of spiral phase plates, polarization control errors, wavelength shift, and wavefront distortions is analyzed in detail. The results indicate that the rapid polarization rotation smoothing scheme has a relatively wide tolerance to the polarization errors and the spiral phase plates, and can improve the smoothing performance effectively with the combination of the smoothing by spectral dispersion scheme.

Untrafast smoothing scheme based on dynamic interference structure between beamlets of laser quad

Aiming at the high requirements for illumination uniformity on the target in laser-driven inertial confinement fusion (ICF) facilities, an ultrafast smoothing method based on dynamic interference structure between beamlets of a laser quad is proposed. The basic principle of this scheme is to use a conjugate phase plate array to add the conjugate phase modulation to the multiple beamlets of a laser quad with a certain wavelength difference. Consequently, every two beamlets are coherently superposed in the far field to generate a dynamic interference pattern, resulting in the fast redistribution of the speckles introduced by continuous phase plate inside the focal spot and further improving the illumination uniformity on the target on a picosecond timescale. The coherent beamlets with a certain wavelength difference can be generated by using a broadband seed laser. Taking the laser quad of the typical ICF facilities for example, the physical model of the ultrafast smoothing method based on dynamic interference structure of beamlets is built up. The influences of the phase-plate type, the peak-to-valley value of the phase modulation and the wavelength difference between the beamlets are analyzed quantitatively, and the smoothing characteristics of the focal spot are discussed in detail and compared with those from the traditional temporal smoothing scheme such as smoothing by spectral dispersion. The results indicate that the directions of the moving speckles in the focal spot are determined by the phase-plate type. However, the required time to achieve stable illumination uniformity, i.e, the decay time, is determined by the wavelength difference between the beamlets. Moreover, the illumination uniformity on the target becomes better with the increase of peak-to-valley value of the phase modulation at first and then remains almost the same. Thus, the ultrafast smoothing method based on dynamic interference structures with well-designed phase arrays and wavelength combinations of the beamlets can realize the multi-directional and multi-dimensional speckle sweeping inside the focal spot, and further improving the irradiation uniformity on the target within several picoseconds or sub-picoseconds. Combining with the traditional beam smoothing scheme, better illumination uniformity can be achieved on an ultrashort timescale. This novel scheme can be used as an effective supplement to the existing temporal beam smoothing techniques.

Smoothing scheme for intensity sweep and polarization rotation at a subpicosecond timescale

A novel scheme for decreasing the scattering levels of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), as well as for improving illumination uniformity of focal spots, is proposed. In this scheme, four beamlets in a laser quad are frequency shifted, spatial phase shaped by conjugate phase plates, and counter-rotating circularly polarized by polarization control plates before being focused by wedged lenses. Consequently, the intensity sweep and the polarization rotation of the focal spot in the timescale of a subpicosecond are achieved, resulting in the reduction of the scattering level of SRS and SBS, and also the improvement of the illumination uniformity when the frequency shift among beamlets is comparable to the parametric backscattering growth rate and the filamentation instability growth rate. The theoretical model of the novel smoothing scheme has been established, and the influences of some key parameters including spatial modulation type, peak-to-mean value of phase distortion, and frequency shift have been discussed. Results show that the uniformity of the focal spot of this novel scheme is improved much faster than that of the conventional temporal smoothing scheme, i.e., smoothing by spectral dispersion. The parametric instabilities are significantly reduced and both phase distortion and modulation depth of the laser quad have a large tolerance, opening a new dimension for controlling the incident light in laser plasma interaction.

Combined implementation of smoothing technologies for improving illumination uniformity of laser quad in multi-directions

ABSTRACTAzimuthal smoothing (AS) that enables the focal spot of a laser quad to rotate in a period of a few picoseconds can improve the illumination uniformity. However, the smoothing performance of azimuthal smoothing is worse than that of smoothing by spectral dispersion (SSD) due to the low rotation speed in the center area of the focal spot. A combined implementation of the schemes including smoothing by spectral dispersion, radial smoothing (RS) and azimuthal smoothing is discussed to achieve better and faster illumination uniformity. The theoretical models of the combined smoothing scheme have been built up to analyse the smoothing performance. The key parameters affecting the smoothing performance of the combined smoothing scheme are analysed and optimized, including the topological charge and the wavelength difference. The simulation results show that the illumination uniformity of the laser quad can be significantly improved in multi-directions by the combined smoothing scheme.

Optimization of radial smoothing scheme for improving illumination uniformity of laser quad

Radial smoothing scheme based on the optical Kerr effect can add periodical spherical wavefront modulation to the laser beam and make the speckles within the focal spot in far field sweep periodically in radial direction, which improves the illumination uniformity of the laser beam on target in inertial confinement fusion facilities. In order to analyze the smoothing performance of radial smoothing scheme in a laser quad, the corresponding theoretical model was established and the temporal characteristic parameters of the pump laser were further analyzed and optimized. The results show that the fluctuation of the smoothing performance with time can slow down by appropriately adjusting the delay time between the laser beam and pump laser pulses in a laser quad. Furthermore, the different sweep period of the incoherent beams can be obtained by altering the pulse duration of pump laser pulses in a laser quad. Consequently, the group velocity of intensity envelope becomes faster with the increase of the frequency difference between incoherent beams. What’ more, a better smoothing performance can be achieved by applying the appropriate spatial deflection between the laser beam and pump laser pulses in a laser quad, and the deflection angle of the pump laser in a laser quad was further optimized to improve the illumination uniformity on target significantly. Radial smoothing with obliquely incident pump laser can add a spherical wavefront to the laser beam, and further makes the speckles redistribute rapidly both in radial and transverse directions. With appropriate incident direction and angle, RS can achieve a much better final smoothing performance than 2D-SSD.

Ultrafast azimuthal beam smoothing scheme based on vortex beam

The illumination uniformity of laser beams in inertial confinement fusion (ICF) facility is a key factor, which plays a crucial role in suppressing the laser plasma instabilities. However, the prevailing beam smoothing techniques cannot meet all the requirements for improving the irradiance uniformity of laser beams and mitigating the laser plasma instabilities, which are determined by the high-frequency spatial modulations and the fine-scale speckles of the focal spots. An ultrafast azimuthal beam smoothing scheme based on vortex beams is proposed in this paper. In this scheme, two of the four beams in a laser quad are transformed from super-Gaussian (SG) beams into vortex beams by inserting two spiral phase plates with opposite topological charges into the beam path, whereas the other two SG beams remain unchanged. By controlling the polarization and the center wavelength of each beam, the SG beam and the transformed vortex beam in the quad are coherently superposed on the target plane, so are the remaining two beams. Owing to the difference in central wavelength and the existence of the topological charges, two focal spots rotating in a period of a few picoseconds are generated in the target plane, which can redistribute the speckles quickly in temporal domain and thus improve the irradiance uniformity of the laser quad. By establishing the physical model of the azimuthal smoothing scheme, the smoothing characteristics including the rotation period, the illumination uniformity and the fractional-power-above-intensity of the focal spots are analyzed in detail. In order to improve the smoothing characteristics significantly, the novel smoothing scheme is further combined with another ultrafast smoothing scheme, i.e. radial smoothing scheme. The influence of the key parameters of the combined smoothing scheme on the illumination uniformity and on the smoothing velocity are discussed. Results indicate that the azimuthal smoothing scheme can achieve the ultrafast smooth of the laser quad in the azimuthal direction and the best illumination uniformity within a few picoseconds as well. Though the degree of improvement in the irradiance uniformity of the azimuthal smoothing scheme is lower than that of the radial smoothing, the combination of these two schemes can improve the uniformity effectively and rapidly. The novel smoothing scheme provides a potential smoothing approach for the high-power laser facilities.

Ultrafast smoothing scheme for improving illumination uniformities of laser quads.

The beam smoothing technology, smoothing by spectral dispersion, plays an important role in improving the illumination uniformities of the lasers in inertial confinement fusion facilities. However, due to the limitations of the modulation frequency of the electro-optic modulator, the uniformity of the lasers approaches an asymptotic value after tens of picoseconds that are much longer than the response time of laser plasma instabilities. To the best of our knowledge, this is the first time that an ultrafast smoothing approach for improving the uniformity of a laser quad in both radial and azimuthal directions in the picosecond scale was proposed. Among the four individual beams in a quad, two of them were smoothed by independent ultrafast focal zooming, and the rest were transformed into Laguerre-Gaussian (LG) beams that carry same topological charges with opposite signs. The focal spots of these two LG beams were coherent superposed, and their intensity distributions rotated rapidly in a period of several picoseconds. As a result of the focal zooming and rotation, an ultrafast and significant improvement of the uniformity of a laser quad was achieved.

基于复合型光栅组合的多色集束匀滑方案

基于复合型光栅组合的多色集束匀滑方案

Analysis and optimization of radial smoothing based on optical Kerr effect for irradiation improvement

In radial smoothing scheme, taking a super-Gaussian pulse train obtained by the pulse stacking scheme based on fibers and spatial shaping technology based on serrated-aperture apodizers as the pump laser, due to the hemispherical shape of the optical Kerr medium, the induced refraction index by the interaction of the optical Kerr medium and the pump laser is spherically distributed with periodical variation. Consequently, the transmission wavefront of the laser quads in the beamline is periodically modulated, resulting in the rapidly and periodic focal zooming in far field. This focal zooming smoothes the speckles on target plane in the radial direction in the sense of averaged over a finite time interval. The performance of the pump laser and the optical Kerr medium strongly affect the radial smoothing effect. In order to obtain better smoothing effect as that of smoothing by spectral dispersion, the propagation model of laser quads in the beamline with the radial smoothing scheme has been built up and further used to optimize the parameters of the pump laser and the optical Kerr medium. The beam smoothing effects of the joint use of continuous phase plate and polarization control plate with smoothing by spectral dispersion, as well as radial smoothing have been analyzed and compared in detail. Results indicate that, the delay time between each super-Gaussian pulse in the pump laser should be matched with the pulse width of each super-Gaussian pulse to achieve the best and stable radial smoothing effect, while the fluctuation of the peak intensity of each super-Gaussian pulse in the pump laser would degrade the radial smoothing effect. The selection of the optical Kerr medium directly determines its thickness and peak intensity of the pump laser to obtain the required wavefront modulation, which affects the feasibility of the radial smoothing scheme.

A novel arrangement scheme of laser quads for spherical hohlraum in laser indirect-driven facility

In traditional laser quads arrangement schemes for spherical hohlraum in indirect-driven laser facilities, the laser quads to bring about the laser entrance hole (LEH) to close when they are incident at a large angle (55), while the complicated cross and overlap of laser quads inside the spherical hohlraum may be generated when they are incident at a small angle (35). To overcome these problems, a novel laser quads arrangement scheme for spherical hohlraum is proposed. The laser quads injected into the single LEH are divided into two cones (the incident angle of the inner cone is 35, and that of the outer cone is 55). Furthermore, the contrast and the fractional power above the intensity have been proposed to evaluate the irradiation uniformity of single laser quad, while the dispersion degree and the duty ratio are proposed to evaluate the distribution uniformity of all laser quads on the spherical hohlraum wall. Based on the beam smoothing scheme implemented by the combination of one-dimensional smoothing by spectral dispersion, the continuous phase plate and polarization control plate, the propagation model of laser quads in the spherical hohlraum has been built up, and further used to analyze the irradiation uniformity of single laser quad and all the laser quads on the spherical hohlraum wall. On this basis, the irradiation characteristics on the LEHs and the spherical hohlraum wall, and the propagation characteristic of laser quads in the novel and traditional laser quads arrangement schemes have been analyzed and compared. Results indicate that, compared with the traditional arrangement scheme of laser quads, the novel laser quads arrangement scheme has following advantages: The irradiation uniformity on the spherical hohlraum wall of single laser quad and all laser quads remains unchanged. Not only the LEH closure problem can be alleviated, but also the complicated cross and overlap of laser quads inside the spherical hohlraum in the traditional scheme could be avoided. The novel scheme may provide useful reference for the design of spherical hohlraum structure in laser indirect-driven facilities due to its obvious advantages over the traditional scheme.

激光间接驱动柱形腔壁辐照特性

激光间接驱动柱形腔壁辐照特性

激光间接驱动柱形腔壁辐照特性

激光间接驱动柱形腔壁辐照特性

间接驱动装置中柱形腔内激光束传输叠加特性研究

间接驱动装置中柱形腔内激光束传输叠加特性研究

间接驱动装置中球形腔内激光束传输特性

间接驱动装置中球形腔内激光束传输特性

Insensitivity of the octahedral spherical hohlraum to power imbalance, pointing accuracy, and assemblage accuracy

The random radiation asymmetry in the octahedral spherical hohlraum [K. Lan et al., Phys. Plasmas 21, 0 10704 (2014)] arising from the power imbalance, pointing accuracy of laser quads, and the assemblage accuracy of capsule is investigated by using the 3-dimensional view factor model. From our study, for the spherical hohlraum, the random radiation asymmetry arising from the power imbalance of the laser quads is about half of that in the cylindrical hohlraum; the random asymmetry arising from the pointing error is about one order lower than that in the cylindrical hohlraum; and the random asymmetry arising from the assemblage error of capsule is about one third of that in the cylindrical hohlraum. Moreover, the random radiation asymmetry in the spherical hohlraum is also less than the amount in the elliptical hohlraum. The results indicate that the spherical hohlraum is more insensitive to the random variations than the cylindrical hohlraum and the elliptical hohlraum. Hence, the spherical hohlraum can relax the requirements to the power imbalance and pointing accuracy of laser facility and the assemblage accuracy of capsule.

Octahedral spherical hohlraum and its laser arrangement for inertial fusion

A recent publication [K. Lan et al., Phys. Plasmas 21, 010704 (2014)] proposed a spherical hohlraum with six laser entrance holes of octahedral symmetry at a specific hohlraum-to-capsule radius ratio of 5.14 for inertial fusion study, which has robust high symmetry during the capsule implosion and superiority on low backscatter without supplementary technology. This paper extends the previous one by studying the laser arrangement and constraints of octahedral hohlraum in detail. As a result, it has serious beam crossing at θL≤45°, and θL=50° to 60° is proposed as the optimum candidate range for the golden octahedral hohlraum, here θL is the opening angle that the laser quad beam makes with the Laser Entrance Hole (LEH) normal direction. In addition, the design of the LEH azimuthal angle should avoid laser spot overlapping on hohlraum wall and laser beam transferring outside hohlraum from a neighbor LEH. The octahedral hohlraums are flexible and can be applicable to diverse inertial fusion drive approaches. This paper also applies the octahedral hohlraum to the recent proposed hybrid indirect-direct drive approach.

New designs of LMJ targets for early ignition experiments

The LMJ experimental plans include the attempt of ignition and burn of an ICF capsule with 40 laser quads, delivering up to 1.4MJ and 380TW. New targets needing reduced laser energy with only a small decrease in robustness are then designed for this purpose. A first strategy is to use scaled-down cylindrical hohlraums and capsules, taking advantage of our better understanding of the problem, set on theoretical modelling, simulations and experiments. Another strategy is to work specifically on the coupling efficiency parameter, i.e. the ratio of the energy absorbed by the capsule to the laser energy, which is with parametric instabilities a crucial drawback of indirect drive. An alternative design is proposed, made up of the nominal 60 quads capsule, named A1040, in a rugby-shaped hohlraum. Robustness evaluations of these different targets are in progress.