METHOD AND FACILITY FOR NANOSECOND LASER SPATIAL-TEMPORAL CONTROL IN A DIRECT-DRIVE LASER FACILITY WITH DOUBLE-CONE IGNITION SCHEME

A nanosecond laser spatial-temporal control method for a direct-drive laser facility based on Double-Cone Ignition, while satisfying the near-isentropic compression waveform required for driving implosion, redistributes the temporal waveforms of all sub-beams in the facility. A small portion of the beams are used to generate a low-energy foot pulse, which is focused on a larger target surface. Under the action of the low-energy foot pulse, the target surface contracts slowly inward. By the end of the duration of the low-energy foot pulse, the target surface has contracted to a smaller size. At this moment, the remaining majority of the beams are used to generate the main drive pulse, which is focused on this smaller target surface, thereby achieving zooming.

TECHNICAL FIELD

The present invention relates to cross-beam energy transfer (CBET) and target irradiation uniformity in laser-direct-drive inertial confinement fusion (ICF) facilities, and specifically a nanosecond laser spatial-temporal control method and facility based on Double-Cone Ignition direct-drive scheme.

BACKGROUND OF INVENTION

Inertial Confinement Fusion (ICF) driven by lasers commonly employs two types of driving methods: direct drive and indirect drive. Indirect drive refers to a process where the laser does not directly irradiate the target capsule. Instead, it irradiates the walls of a hohlraum, generating X-rays which are then used to heat and compress the target capsule. The advantage of indirect drive is that the radiation source for driving the ablation is more uniform. However, the process of generating X-rays inevitably leads to energy losses, which in turn increases the demand for higher laser energy. Moreover, the Laser-Plasma Interactions (LPI) within the hohlraum are relatively complex. Direct drive, on the other hand, requires multiple beams to uniformly irradiate the target capsule which contains cryogenic nuclear fuel. The energy coupling efficiency of direct drive, where the driving laser energy is transferred to the deuterium-tritium (DT) fuel, is approximately 5 to 6 times higher than that of indirect drive. However, direct drive demands much higher degree of uniformity in laser beam irradiation. Research indicates that direct drive requires the irradiation uniformity on the surface of the target capsule to be within 1%. Direct drive can effectively convert laser energy into the kinetic energy of the target shell, compressing and gathering the fusion fuel to high densities, thereby meeting the conditions for thermonuclear ignition. (ρR≥0.3 g/cm2, in this context, ρ represents the mass density of the fuel, and R represents the radius of the target capsule.)

In direct-drive ignition designs, the laser beam spot size is close to the target focal spot diameter. Cross-beam energy transfer (CBET) between multiple beams can lead to energy losses. CBET is a type of laser-plasma instability that essentially results from the overlap of multiple beams, causing in a significant energy transfer via diffraction on the ion acoustic wave gratings in the overlapping regions. It directly affects the uniform compression of the target in ICF, impacts the laser-target coupling efficiency, and thereby reduces the fusion yield. Research has shown that reducing the spot size can compensate for the kinetic energy lost due to CBET. However, the reduction simultaneously increases low-order mode perturbations and reduces irradiation uniformity, affecting implosion performance. Dual-state focal zooming can mitigate both CBET and low-order mode perturbations, leading to a more stable implosion process.

The traditional direct-drive configuration on OMEGA requires 60 beams to uniformly irradiate the entire spherical surface with a diameter of approximately 850 μm. The power balance requires that the root-mean-square (RMS) power imbalance be less than 1% in any 100-ps pulse interval. Therefore, OMEGA has extremely high requirements for pulse temporal synchronization, control precision of the temporal-power profile of each beam, and consistency of the temporal-power profiles among multiple beams. To reduce the impact of CBET, the OMEGA facility can only employ a two-step focal zooming scheme based on spatial-division multiplexing, which involves using time-varying wave-front and zooming phase plates (ZPP) to superimpose different phase structures on the beams at different moments, ultimately achieving a far-field focal spot size that varies with time. The zooming scheme based on spatial-division multiplexed can effectively overcome energy losses and CBET effects, but it has several major issues:

First, OMEGA uses ring-shaped beams to generate the main drive pulse, which requires a high requirement for the incident light field distribution. Second, the energy utilization rate of the incident beams is relatively low, with a maximum of only 75%. Third, since the foot pulse is non-saturated linear amplification in the whole amplification chain so its stability is more challenging to control than the main pulse. Excessive power fluctuations in the foot-pulse can degrade the irradiation uniformity during the isentropic compression process of the entire target sphere and affect the mismatch of timing and strength of the shockwave, thereby reducing the areal density. (See D. H. Froula et al., “Mitigation of cross-beam energy transfer: Implication of two-state focal zooming on OMEGA,” Physics of Plasmas 20, 102704 (2013).) The laser pulse shape in the Double-Cone Ignition (DCI) scheme employs a near-isentropic nanosecond compression waveform, which is used to compress fusion fuel isentropically within two gold cones placed in opposition. Under the constraining effect of the gold cones, the fusion fuel reaches extremely high velocities and densities, and is confined to be ejected from the cone apertures. Subsequently, the plasma ejected from one cone collides with that from the opposing cone, thereby increasing the temperature and density of the fusion fuel. Since all beams in the Double-Cone Ignition scheme are constrained by the gold cones to only irradiate the same conical surface, the requirement for power balance between beams can be reduced. The focus is only on the intensity balance of all beams superimposed on the two conical surfaces.

SUMMARY OF THE INVENTION

To address the shortcomings of the existing technologies, the present invention provides a nanosecond laser spatial-temporal control method and facility for Double-Cone Ignition direct-drive scheme. Assuming that the total number of nanosecond laser pulses on both cones in the Double-Cone Ignition direct-drive scheme is n, for the n/2 laser pulses on a single cone, using a portion of them, i.e., m beams, to generate a low-energy foot pulse focused on a target surface with an initial radius of Rt to provide better illumination uniformity, and using the remaining (n/2−m) beams to generate a high-energy main drive pulse focused on a relatively smaller Rb(Rb=0.7 Rt) to mitigate cross-beam energy transfer (CBET) and enhance laser-target coupling efficiency.

The present invention provides a nanosecond laser spatial-temporal control method for Double-Cone Ignition direct-drive laser facility, characterized by redistributing the temporal waveforms of all nanosecond laser pulses in the facility while satisfying the near-isentropic compression waveform required for driving implosion. The method of the present invention comprises the following steps:

In the present invention, the requirements for the m nanosecond laser pulses generating the low-energy foot pulse are as follows:

The apparatus for implementing the above-mentioned nanosecond laser spatial-temporal control method for the Double-Cone Ignition direct-drive laser facility of the present invention comprises, in sequence along the direction of the front-end seed source transmission, a spectral dispersion grating, a transmission amplification component, a frequency-doubling and tripling crystal, a continuous phase plate, a polarization control plate, a focusing lens, and a target surface. The seed light output from the front-end seed source enters the transmission amplification component after passing through the spectral dispersion grating for beam aperture and energy amplification. It is then converted from the fundamental frequency to the third harmonic by the frequency-doubling and tripling crystal. The continuous phase plate shapes the far-field focal spot into a spot with a certain shape and size. After passing through the polarization control plate, the focusing lens focuses the light on the far-field target surface to form a shaped focal spot. After passing through the spectral dispersion grating, the continuous phase plate, the polarization control plate, and the focusing lens, the far-field light field distribution of each nanosecond laser pulse on the target surface can be represented as:

wherein

is the near-field distribution of each nanosecond laser pulse, λ3ωj, k3ωj; and ω3ωj; are the center wavelength, wave vector, and frequency corresponding to each nanosecond laser pulse, respectively, φCPP and φSSD are the phase modulations of the beam by the continuous phase plate and the spectral dispersion grating, respectively, f is the focal length of the focusing lens, “FT” denotes the Fourier transform, φrandom represents the random phase distortion introduced during the beam propagation, which can be expressed as:

over the integration time Δt, the far-field light intensity distribution is given by:

wherein, Δt is the integration time.

In the present invention, each nanosecond laser pulse is generated by a different front-end seed source, thereby enabling independent control of the temporal and spectral characteristics of each beam.

Further, the seed laser can be tuned in terms of center wavelength.

Further, the temporal pulse shaping unit can achieve precise control of the temporal-power profile, thereby enabling high-precision control of the timing of each nanosecond laser pulse and high-precision adjustment of the time delay, with a control accuracy at the picosecond level.

Further, the phase modulation unit can control the spectrum.

Further, the combination of the spectral dispersion grating and the phase modulation unit in the front-end seed source can produce a time-varying speckle structure, thereby smoothing the focal spot over a specific period of time.

Further, the continuous phase plates used for each nanosecond laser pulse during propagation have different target focal spot sizes. For the low-energy foot pulse, the target focal spot size is slightly smaller than the initial target surface with radius Rt to provide better illumination uniformity. For the high-energy main drive pulse, the target focal spot size is slightly smaller than the target surface with radius Rb to reduce cross-beam energy transfer (CBET) and improve laser-target coupling efficiency.

Further, the polarization state of the polarization control plate can be linearly polarized light along the tangent or normal direction of the position on the ring where the beam is located for different nanosecond laser pulses, or it can be left-hand or right-hand circularly polarized light, or elliptically polarized light, with the aim of reducing the correlation between the sub-beams.

Further, the target positions for the m low-energy foot pulses and the (n−m) high-energy main drive pulses are different. The target positions for the (n−m) high-energy main drive pulses are slightly offset to the back.

The advantages of the present invention are as follows:

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is provided in conjunction with the accompanying drawings. It is necessary to point out that the described embodiments are used to further describe the invention and do not limit the scope of protection of the invention.

This example primarily considers the physical requirements for compressing the target capsule in the Double-Cone Ignition direct-drive laser facility described in the present invention. It provides the physical basis for the “4 foot pulses+12 main pulses” zooming scheme design based on the Double-Cone Ignition direct-drive laser facility. The beam aperture is 375 mm×375 mm. The baseline time-power curve for the Double-Cone Ignition direct-drive laser facility requiring irradiation on a single cone is shown in FIG. 1A. This curve is evenly distributed among 16 nanosecond laser pulses. The contrast of the baseline time-power curve on a single path is 55:1, with a peak power density of 711 MW/cm2 and an energy density of 2.97 J/cm2.

To improve the control precision of the foot pulse and maximize the output capability of the facility, 4 beams are used to generate only the foot pulse, with a duration of 0-10 ns and a total energy of 6.20 kJ. The remaining 12 beams are used to generate only the main drive pulse, with a duration of 10-15 ns and a total energy of 60.54 kJ. The sum of these 16 nanosecond laser pulses meets the baseline time-power curve for irradiation on a single cone.

Thus, the contrast of a single foot pulse is 13:1, with a peak power density of 666 MW/cm2 and an energy density of 1.16 J/cm2. The contrast of a single main drive pulse is 3.7:1, with a peak power density of 948 MW/cm2 and an energy density of 3.57 J/cm2. The time-power curves of the single foot pulse and single main drive pulse are shown in FIG. 1B. Compared with the baseline time-power curve, the waveform contrast is significantly reduced, thereby effectively improving pulse stability.

Since the 12 main drive pulses begin 10 ns after the 4 prepulses, these 12 main drive pulses use a small focal spot CPP (Continuous Phase Plate), while the other 4 foot pulses use a large focal spot CPP to achieve the zooming scheme.

Referring to FIG. 2, which shows the beam arrangement on a single cone of the Double-Cone Ignition direct-drive laser facility.

Referring to FIG. 3, which first considers the irradiation uniformity of the 4 foot pulses on the large focal spot in the far field. The angle between these 4 foot pulses and the polar axis is 26°. Each foot pulse sequentially passes through the front-end seed source (1), spectral dispersion grating (2), transmission amplification component (3), frequency-doubling and tripling crystal (4), continuous phase plate (5), polarization control plate (6), focusing lens (7), and target surface (8) during generation, transmission, and focusing.

The relevant parameters of the spectral dispersion grating (2) are as follows: temporal phase modulation frequency. fm=17 GHz, modulation depth m=2.4, grating dispersion coefficient δθ|δλ=215.68 urad/nm.

The target focal spot of the continuous phase plate (5) is an elliptical focal spot, which is projected onto the target surface as a circular focal spot with a radius slightly less than 1000 μm.

The focal length of the focusing lens (7) is 5 m. The initial radius of the target surface (8) is Rt=1000 um. The fundamental frequency center wavelength of each foot pulse is 1053 nm, and the third harmonic center wavelength is 351 nm. The pointing accuracy is 30 μm rms.

To quantitatively analyze the variation of focal spot uniformity with integration time, the flux contrast C of the focal spot is used for evaluation. A smaller flux contrast C indicates better focal spot uniformity. The calculation method for C is as follows:

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Wherein, Ii,j(xf, yf) at the focal plane (xf, yf) intensity at the location, Imean(xf, yf) the average intensity at the focal plane.

When quantitatively analyzing the focal spot uniformity, the flux contrast of the focal spot is calculated for the region with an 80% encircled energy ratio, as shown in FIGS. 4A and 4B. FIG. 4A shows the variation of irradiation uniformity on the target surface 8 with integration time for the 4 foot pulses. At an integration time of 100 ps, the irradiation uniformity is 0.1740, and the irradiation situation on the target surface is shown in FIG. 5A.

Similarly, the irradiation situation of the 12 main drive pulses on the smaller focal spot is considered. These 12 main drive pulses comprise 4 beams with an angle of 32° to the polar axis and 8 beams with an angle of 45° to the polar axis.

For these 12 main drive pulses, the radius of the target surface 8 is 0.7 Rt. The variation of the flux contrast of the focal spot within the region of the 80% encircled energy ratio with integration time is shown in FIG. 4B. At an integration time of 100 ps, the irradiation uniformity is 0.2072, and the irradiation situation on the target surface is shown in FIG. 5B.

This example primarily considers the physical basis for the “2 foot pulses+14 main pulses” zooming scheme design based on the Double-Cone Ignition direct-drive laser facility. The beam aperture is 375 mm×375 mm. The time-power curves of the single foot pulse and single main drive pulse obtained through this division method are shown in FIG. 6.

In this way, the peak power density allocated to a single foot pulse is 1332 MW/cm2, and the energy density of the single foot pulse is 2.32 J/cm2; the peak power density allocated to a single main drive pulse is 812 MW/cm2, and the energy density of the single main drive pulse is 3.06 J/cm2.

In this zooming scheme, two adjacent prepulses from the 4 foot pulses described in Example 1 are selected as the two foot pulses for Example 2, acting on the target surface with radius Rt.

The uniformity of the focal spot is then quantitatively analyzed. At an integration time of 100 ps, the flux contrast is calculated for the region within the 80% encircled energy ratio of the focal spot, resulting in a value of 0.1157. The irradiation situation on the target surface is shown in FIG. 7A.

Similarly, the irradiation situation of the remaining 14 main drive pulses on the smaller focal spot is considered. The radius of the target surface 8 is 0.7 Rt. At an integration time of 100 ps, the flux contrast of the focal spot within the 80% encircled energy ratio region is 0.2117. The irradiation situation on the target surface is shown in FIG. 7B.

Compared with Example 1, although the irradiation uniformity on the larger focal spot using 2 foot pulses in Example 2 is better (this is because the two beams can have their polarization directions controlled to be mutually perpendicular, resulting in incoherent superposition on the target surface), the peak power density on a single foot pulse in Example 2 is too high, which can easily cause damage to optical components.

The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the invention. For those skilled in the art, the invention can also be subject to various modifications and variations.

The method of the present invention, through the redistribution of the time-power curves of all sub-beams, reduces the waveform contrast of the sub-beams, allowing for higher precision control of the time-power curve on the target surface. It also effectively mitigates the energy loss caused by cross-beam energy transfer (CBET) and improves the laser-target coupling efficiency. Additionally, it enhances the irradiation uniformity of the target.