Impacts of Direct Drive Laser Focal Spot Size on Ignition Performance of Hybrid Drive Inertial Confinement Fusion

doi:10.19596/j.cnki.1001-246x.8619

Abstract

Abstract:

The direct drive laser is the key factor in hybrid drive inertial confinement fusion. Its drive asymmetry has a great influence on the ignition performance of nuclear fusion. Using the same laser power, the impacts of the direct drive laser focal spot size on the ignition performance of a hybrid drive model are studied. It is shown that the size of the laser focal spot is a key parameter to influence the ignition performance of the hybrid drive model. When the size of the laser focal spot is 1 500 μm, the neutron yield of the target is close to the one got by the one dimensional implosion. When the 1 400 μm laser focal spot is used, the neutron yield is 40% of the one got by the one dimensional implosion. However, it is failed to ignite for the 1 200 μm laser focal spot. The high drive asymmetry caused by the small laser focal spot can increase the adiabat in the fuel. The fuel compressibility will decrease when the adiabat is increased, which goes against creating the ignition condition. In this way, the ignition wave is weak. Meanwhile, the high drive asymmetry can lead a high perturbation of the fuel areal density. The perturbation growth of the fuel areal density can make the shell asymmetry grow greatly when the ignition wave is formed. Under the conditions of the weak ignition wave and the high fuel areal density perturbation, the fuel spike with a high density is hard to be ignited. Therefore, the positive feedback between the increase of hotspot temperature and the ignition is restrained. Meanwhile, the fuel spike can decrease the hotspot temperature. The fuel bubble can lead the hotspot expand rapidly. All these factors will make the ignition performance decrease when a small laser focal spot is used.

Key words: hybrid drive, size of laser focal spot, laser overlap, drive asymmetry, inertial confinement fusion

Zhiyuan LI, Jiwei LI, Lifeng WANG, Zhensheng DAI, Jianfa GU, Minqing HE, junfeng WU, Wenhua YE, Xiantu HE. Impacts of Direct Drive Laser Focal Spot Size on Ignition Performance of Hybrid Drive Inertial Confinement Fusion[J]. Chinese Journal of Computational Physics, 2023, 40(2): 189-198.

Figures/Tables 15

Fig.1 One-dimensional target design (a) capsule material; (b) radiation source and laser source

Table 1 One dimensional implosion parameters

t_imp/ns	V_imp/(km · s^-1)	Adiabat	Bangtime/ns	Stagnation time/ns	at the stagnation time			Neutron yield	Yield/MJ
t_imp/ns	V_imp/(km · s^-1)	Adiabat	Bangtime/ns	Stagnation time/ns	P_hs/Gbar	Ti_hs/keV	ρR_fuel/(g · cm^-2)	Neutron yield	Yield/MJ
7.72	445	3.7	7.98	7.95	1 069	8.7	1.04	7.08 × 10¹⁸	20

Fig.2 The distribution of the laser intensity

Table 2 Two-dimensional implosion parameters

焦斑尺寸/μm	t_imp/ns	V_imp/(km·s^-1)	Adiabat	Bangtime/ns	Stagnation time/ns	at the stagnation time			Neutron yield	Yield/MJ
焦斑尺寸/μm	t_imp/ns	V_imp/(km·s^-1)	Adiabat	Bangtime/ns	Stagnation time/ns	P_hs/Gbar	Ti_hs/keV	ρR_fuel/(g·cm^-2)	Neutron yield	Yield/MJ
1 200	7.68	423	5.37	7.98	7.97	312	4.57	0.9	6.80 × 10¹⁶	0.192
1 400	7.7	430	4.05	8.03	7.97	516	5.94	0.96	2.89 × 10¹⁸	8.14
1 500	7.75	439	3.74	8.02	7.99	934	7.34	1.14	7.00 × 10¹⁸	19.90
1D	7.72	445	3.7	7.98	7.95	1 069	8.7	1.04	7.08 × 10¹⁸	20.00

Fig.3 Evolution of RMSE about (a) laser intensity on critical surface and (b) hybrid drive pressure

Fig.4 Evolution of RMSE about (a) fuel areal density and (b) hotspot interface

Fig.5 The contributions of the key implosion parameters on ITF

Fig.6 Evolution of the adiabat in the fuel layer under the conditions of the three cases

Fig.7 The shock waves' contour when the indirect drive shock comes into the DT gas region under the condition of 1 200 μm focal spot radius

Fig.8 The relationship between the ion temperature and the hotspot areal density and the relationship between the hotspot mass and the hotspot areal density under the different focal spot radius

Fig.9 The ion temperature contour and the density contour of the fuel and hotspot under the condition of 1 200 μm focal spot radius

Fig.10 The ion temperature contour and the density contour of the fuel and hotspot (a) 1 400 μm; (b) 1 500 μm

Table 3 Implosion parameters of the optimized design

t_imp/ns	V_imp/(km · s^-1)	Adiabat	Bangtime/ns	Stagnation time/ns	at the stagnation time			Neutron yield	Yield/MJ
t_imp/ns	V_imp/(km · s^-1)	Adiabat	Bangtime/ns	Stagnation time/ns	P_hs/Gbar	Ti_hs/keV	ρR_fuel/(g · cm^-2)	Neutron yield	Yield/MJ
7.88	420	3.35	8.17	8.12	609	5.9	1.18	4.9 × 10¹⁸	13.8

Fig.11 Evolution of RMSE about the fuel areal density and the hotspot interface for the optimized design

Fig.12 The ion temperature contour and the density contour of the fuel and hotspot for the optimized design

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