Inertial Fusion Ignition Research Articles

Self-generated magnetic collimation mechanism driven by ultra-intense LG laser

Collimation control of energetic plasma beams is crucial in the laser–plasma field. In this paper, we report on a self-collimated acceleration scheme for a plasma beam using an ultra-intense Laguerre–Gaussian (LG) laser irradiating a solid target. Three-dimensional (3D) particle-in-cell simulations show that a plasma beam with a high current density is stably formed by the radiation pressure of the hollow LG laser. The initial interaction of LG laser with solid target can be approximately researched by a deformable mirror model. Under the effect of the ponderomotive force of the LG laser, the plasma converges in the center axis to form a narrow beam. An elongated strong-magnetic tunnel (B ∼ 2 kT) is self-generated around the plasma beam, capable of trapping some electrons in a region with a radius of less than 500 nm (r < 500 nm). Compared with the case driven by the conventional Gaussian laser, the beam radius size is dramatically reduced from the microscale to hundreds on the nanoscale. The beam density is increased by at least ten times. Such an interesting scheme can provide a feasible and efficient way to achieve and enhance the collimation of energetic particle beams, which may benefit the general applications of fast ignition in inertial fusion, radiotherapy, realization of high-energy density states, and so on.

Ultra-intense laser-accelerated ion beams for high-gain inertial fusion: the effect of the ion mass on the beam properties

This paper presents the results of detailed numerical studies of the properties of ion beams generated by a laser from targets with various atomic numbers under conditions relevant for ion fast ignition (IFI) of inertial fusion. The interaction of a 200 kJ 1 ps infrared (1.05 μm) laser with Li, C, Al, Ti, Cu and CH2 flat targets with the same areal mass density was numerically simulated using an advanced 2D3V particle-in-cell code. For each target, a set of ion beam characteristics important for IFI was determined. A detailed quantitative comparison of the IFI-relevant parameters of Li, C, Al, Ti and Cu ion beams and the proton beam (from the CH2 target) was made. The laser-accelerated Cu ion beam was found to achieve significantly higher values of beam intensity, fluence and ‘useful’ energy (for IFI), having a smaller angular divergence and a narrower energy spectrum than the beam of light ions or protons. Thus, it is shown for the first time that laser-accelerated heavy ion beams can achieve IFI-relevant parameters higher than light ion or proton beams and can potentially meet IFI requirements.

Role of self-generated magnetic fields in the inertial fusion ignition threshold

Magnetic fields spontaneously grow at unstable interfaces around hot-spot asymmetries during inertial confinement fusion implosions. Although difficult to measure, theoretical considerations and numerical simulations predict field strengths exceeding 5 kT in current National Ignition Facility experiments. Magnetic confinement of electrons then reduces the rate of hot-spot heat loss by >5%. We demonstrate this via magnetic post-processing of two-dimensional xRAGE hydrodynamic simulation data at bang time. We then derive a model for the self-magnetization, finding that it varies with the square of the hot-spot temperature and inversely with the areal density. The self-magnetized Lawson analysis then gives a slightly reduced ignition threshold. Time-dependent hot-spot energy balance models corroborate this finding, with the magnetic field quadrupling the fusion yield for near-threshold parameters. The inclusion of magnetized multi-dimensional fluid instabilities could further alter the ignition threshold and will be the subject of future work.

Investigation of plasma states formed under the interaction of high-power laser pulses with wire-shape Al–Cu target

Study of warm dense matter remains a very important task for understanding of many unique phenomena observing as in astrophysical research as in inertial fusion and fast ignition. In this work, we studied the parameters of plasma created by 1.7 ps laser pulses of relativistic intensity of 7 × 1018 W/cm2 in a specially designed Al–Cu wire-shape target, in comparison with a flat Cu and Al foil targets. We observed the strong emission of neutral or virtually neutral Cu Kα line from both Cu foil and Cu wire part of targets, which indicates the creation of a dense state exposed to the intense flow of hot electrons. Parameters of the plasma were evaluated by comparison of experimental spectra with the results of modeling by collisional-radiative kinetic code PrismSpec under the plasma zone approach. The using of Al foil in front of Cu wire part of target allowed avoiding the direct heating of Cu-wire and acquiring spectra of Cu K-shell emission evidently belonging to emission of warm dense matter (WDM) state. The upper estimate for the electron temperature in WDM region was found to be below 80 eV.

Energy Flow in Thin Shell Implosions and Explosions

Energy flow and balance in convergent systems beyond petapascal energy densities controls the fate of late-stage stars and the potential for controlling thermonuclear inertial fusion ignition. Time-resolved x-ray self-emission imaging combined with a Bayesian inference analysis is used to describe the energy flow and the potential information stored in the rebounding spherical shock at 0.22PPa (2.2Gbar or billions of atmospheres pressure). This analysis, together with a simple mechanical model, describes the trajectory of the shell and the time history of the pressure at the fuel-shell interface, ablation pressure, and energy partitioning including kinetic energy of the shell and internal energy of the fuel. The techniques used here provide a fully self-consistent uncertainty analysis of integrated implosion data, a thermodynamic-path independent measurement of pressure in the petapascal range, and can be used to deduce the energy flow in a wide variety of implosion systems to petapascal energy densities.

Advancing the study of hybrid-drive inertial fusion ignition and high energy density physics - Teller medal lecture at IFSA2019

The inertial confinement fusion (ICF) program toward ignition in China has made great progress since 1993. After research work on SG series with laser energy of tens kJ, SG-III laser facility with energy output of hundreds kJ is being used to study target physics prior to ignition. A new hybrid drive (HD) approach combined indirect drive (ID) and direct drive (DD) provides the HD pressure far greater than the radiation ablation pressure to drive implosion dynamics and hotspot ignition occurring in nonstagnation time when severe hotspot deformation and hydrodynamic instabilities have not fully developed. Experiments on SG-III have verified the boost effect of the HD pressure and showed lower fractions of laser backscattering and hot-electron energy. The development of the ICF program has advanced the study of high energy density physics. An energy band theory for warm dense matter has been proposed and a new code has been developed to study equation of state and transport coefficients for ICF implosion matter and the interion properties of planets.

Numerical study on the laser ablative Rayleigh–Taylor instability

The laser ablative Rayleigh–Taylor instability plays an important role in the ignition of inertial refinement fusion. An accurate simulation of this process is important to control the growth of flow instability during the implosion. In this paper, taking the simulations of the hydrodynamics, the laser energy deposition and the electronic thermal conductivity into consideration, a massively parallel laser ablative Rayleigh–Taylor instability code based on Euler method is developed. Some open source codes are used to improve the code development efficiency. The accuracy of the hydrodynamics simulation is tested by an analytical theory about the weakly nonlinear Rayleigh–Taylor instability with double interfaces. The benchmark of an one dimensional heat conductivity is used to test the accuracy of the thermal conductivity simulation. The laser ablative plane target and the laser ablative Rayleigh–Taylor instability are used to test the reliability of the code on the simulation of the whole laser ablative process. It is shown that the confidence of our numerical simulation code is high and the code framework we designed is effective. It can be a basis on studying the problems about the laser ablative instability in inertial refinement fusion.

Acceleration of carbon ion beams by an ultraviolet laser under conditions relevant for ion fast ignition of inertial fusion

The properties of carbon ion beams produced by a 100-kJ, 1-ps, KrF ultraviolet laser under conditions relevant for ion fast ignition (IFI) of DT fusion are numerically investigated using the 2D3V particle-in-cell code, and the possibility of achieving the ion beam parameters required for IFI is tested. The numerical simulations of carbon ion acceleration were carried out for flat carbon targets of various thicknesses (LT) and for various laser beam apertures (dL) on the target, while the laser pulse duration and energy were fixed. It was found that both the radiation pressure acceleration (RPA) mechanism and the sheath acceleration mechanism significantly affects the characteristics of the ion beam, with the RPA dominating in the case of thicker targets (LT ∼ 10–30 μm). The ion beam parameters depend to a significant extent on the target thickness and the distance from the target. The mean and maximum ion energy decrease with the increase of LT from 3 μm to 30 μm, while the ion beam intensity, the beam energy fluence and the total energy of the “useful part” of the ion beam (with an aperture ⩽ 50 μm) reach maximum values at LT ∼ 5–10 μm. For the optimal value of LT and a small distance x from the target (x ∼ LT), the ion beam parameters are close to or higher than what is required for IFI. However, due to the angular divergence of the ion beam, the beam intensity and fluence decrease with an increase in the distance from the target, and at x ⩾ 0.5 mm the beam parameters are below the values required for fusion ignition. To reach these values, higher laser energies and/or more sophisticated schemes of ion acceleration are required.

Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator

A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating, accelerating and guiding protons, using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism. In addition, the problems involved are studied by using two-dimensional particle-in-cell simulations. The results show that the proton beam can be collimated, accelerated and guided effectively through this type of target. Theoretically, a formula is derived for the combined electric field of accelerating protons. Compared with a proton beam without a beam collimator, the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10% respectively. Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field, and that the beam collimator plays a role in focusing energy. In addition, the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I. The more prominent point is that the proton number of type II is 2.2 times higher than that of type I. This kind of target has important applications in many fields, such as fast ion ignition in inertial fusion, high energy physics and proton therapy.

Accelerating and guiding of C6+ by an intense laser irradiating on a foil target with a tapered channel

A novel scheme with a tapered channel attached to an ultra-thin carbon foil is proposed to accelerate and guide carbon ions via breakout afterburner mechanism. Also, the problems involved are investigated by using two-dimensional particle-in-cell simulations. It is demonstrated that the tapered channel can efficiently accelerate and guide carbon ions and result in a much better quality beam with an order of magnitude higher in density and 22% larger in cut-off energy than that without the tapered channel. The enhanced reasons are analyzed in detail, which are mainly attributed to the guidance of the longitudinal electric field and the focus of the transverse electric field, as well as the convergence effect of the tapered channel. All of them are certified to guide greatly carbon ions to move along the longitudinal direction. Besides, during the simulation time, the ion beam with a tapered channel can remain eight times smaller in divergence angle than that without the tapered channel. Such a target may be beneficial to many applications such as ion fast ignition in inertial fusion, high-energy physics, and proton therapy.

Enhanced laser radiation pressure acceleration of protons with a gold cone-capillary

A scheme with a gold cone-capillary is proposed to improve the protons acceleration, and involved problems are investigated by using the two-dimensional particle-in-cell simulations. It is demonstrated that the cone-capillary can efficiently guide and collimate the protons to a longer distance and result in a better beam quality with a dense density ≥10nc, monoenergetic peak energy Ek∼1.51 GeV, spatial emittance ∼0.0088 mm mrad with divergence angle θ∼1.0° and diameter ∼0.5μm. The enhancement is mainly attributed to the focusing effect by the transverse electric field generated by the cone as well as the capillary, which can prevent greatly the protons from expanding in the transverse direction. Comparable to without the capillary, the protons energy spectra have a stable monoenergetic peak and divergence angle nearby 1.0° in longer time. Besides, the efficiency of acceleration depending on the capillary length is explored, and the optimal capillary length is also achieved. Such a target may be beneficial to many applications such as ion fast ignition in inertial fusion, proton therapy and so on.

Guiding and collimating the fast electrons by using a low-density-core target with buried high density layers

A low-density-core target with buried high density layers is proposed to improve the transport of fast electrons and involved problems are investigated by using two-dimensional particle-in-cell simulations. It is demonstrated that this target can collimate the fast electrons efficiently and lead to a better beam quality. The enhancement is attributed to the weakening of the two stream instability and the better collimation by the self-generated multilayer megagauss magnetic field as well as the baroclinic magnetic field. Comparing this to that without buried high density layers, the energy flux of fast electrons is increased by a factor of about 1.8 and has a narrower transverse distribution in space. Besides, the dependence of the efficiency on the target parameters is examined, and the optimal target parameters are also obtained. Such a target can be useful to many applications, such as fast ignition in inertial fusion.

Laser-driven relativistic electron beam for fast ignition

In the scheme of electron-driven fast ignition of inertial confinement fusion (ICF), MeV relativistic electron beams driven by ultraintense petawatt laser pulses are required and electron energy of several tens of kilojoules should be deposited in the compressed core of an imploded DT shell. The beam quality is crucial to the success of fast ignitor. In order to study the generation and transport of the relativistic electron beam, a three-dimensional high-performance parallel particle-in-cell code is developed and improved. In this review, this paper shows the researches on the generation of high-quality electron beams and the method of beam divergence controlling of our research group. Two main reasons for poor beam quality are firstly introduced: pre-plasma effect and random scattering of the magnetic field produced by the beam instability. In addition, enhancement of electron beam flux and collimation of its propagation are investigated. Four ways to obtain collimated electrons are put forward: 1) reduce the pre-plasma effect with a double cone target; 2) collimate fast electrons in specially engineered targets with the spontaneously generated magnetic field during its transportation; 3) collimate fast electrons with external magnetic field; 4) control the beam instability at the laser-plasma interface.

Collimation of Energetic Electrons from a Laser-Target Interaction by a Magnetized Target Back Plasma Preformed by a Long-Pulse Laser

It is demonstrated experimentally and by numerical simulations that the presence of a long-pulse-laser-created back plasma on the target backside can focus the relativistic electrons produced by short-pulse laser interaction with the front of a solid target. Comparing this to that without the back plasma, the number density of the fast electrons is increased by one order of magnitude, and their divergence angle is reduced fivefold. The effect is attributed to the absence of the backside sheath electric field and the collimation effect of the megagauss self-generated baroclinic magnetic field there. Such an acceleration scheme can be useful to applications requiring high-energy and charge-density electron bunches, such as fast ignition in inertial fusion.

Molecular dynamics with atomic transitions and nuclear reactions

We describe molecular dynamics particle simulations with particles that have internal structure and/or undergo reactions. These calculations give an atomic-scale description of hot plasma based on well-established microphysics and test kinetic theories used to calculate energy exchange and transport in plasma hydrodynamic simulations. The computer experiments can be given detailed diagnostics, without the usual limits of resolution of Laboratory equipment. Typical applications are non-equilibrium atomic kinetics for emission or absorption of X-ray laser radiation by solid targets and/or atomic-scale simulation of hot plasma with fusion reactions, as in inertial fusion ignition experiments.

Particle simulation code for fusion ignition

Inertial fusion ignition phenomena including effects of non-Maxwellian ion velocity distributions are studied by molecular dynamics particle simulation. 10,000 DT ions at a density 100g/cm3 and temperature ∼10keV are followed for 10 to 20ps. The simulations include ion collisions, electron–ion coupling and radiation emission and absorption. Fusion reactions produce energetic alphas and the plasma self-heats to 20–30keV. It is found that calculations starting from a variety of initial conditions evolve to approach a unique self-heating trajectory which can be called an ignition attractor. A calculation starting with 3keV DT ignites within a few ps after 2MeV alpha particles are injected and deposit ∼300MJ/g; this demonstrates that fast ions are highly effective for fast ignition of precompressed DT.

Video: Progress toward inertial fusion ignition at NIF

Video: Progress toward inertial fusion ignition at NIF

Hot electron transport and heating in dense plasma core by hollow guiding

AbstractA new scheme for cone-hollow-assisted fast ignition in inertial fusion is investigated. A hollow is attached to the tip of a conventional gold cone. The transport and heating of the high-current electrons propagating from the cone tip to the compressed fuel core along the hollow is investigated by two-dimensional hybrid simulation. Different hollow geometry sizes are examinized. It is shown that with proper hollow guiding, hot electrons can be collimated between the inner-walls of the hollow by the large interface magnetic fields appearing on the inner surface. When the beam electrons further propagate into the dense region, they are scattered into the gold hollow through collisions with the fuel electrons and ions. The resulting magnetic potential around the hollow then bends beam electrons along the gold hollow to reach the dense core.

Studies on targets for inertial fusion ignition demonstration at the HiPER facility

Recently, a European collaboration has proposed the High Power Laser Energy Research (HiPER) facility, with the primary goal of demonstrating laser driven inertial fusion fast ignition. HiPER is expected to provide 250 kJ in multiple, 3ω (wavelength λ = 0.35 µm), nanosecond beams for compression and 70 kJ in 10–20 ps, 2ω beams for ignition. The baseline approach is fast ignition by laser-accelerated fast electrons; cones are considered as a means to maximize ignition laser–fuel coupling. Earlier studies led to the identification of an all-DT shell, with a total mass of about 0.6 mg as a reference target concept. The HiPER main pulse can compress the fuel to a peak density above 500 g cm−3 and an areal density ρR of about 1.5 g cm−2. Ignition of the compressed fuel requires that relativistic electrons deposit about 20 kJ in a volume of radius of about 15 µm and a depth of less than 1.2 g cm−2. The ignited target releases about 13 MJ. In this paper, additional analyses of this target are reported. An optimal irradiation pattern has been identified. The effects on fuel compression of the low-mode irradiation non-uniformities have been studied by 2D simulations and an analytical model. The scaling of the electron beam energy required for ignition (versus electron kinetic energy) has been determined by 2D fluid simulations including a 3D Monte Carlo treatment of relativistic electrons, and agrees with a simple model. Integrated simulations show that beam-induced magnetic fields can reduce beam divergence. As an alternative scheme, shock ignition is studied. 2D simulations have addressed optimization of shock timing and absorbed power, means to increase laser absorption efficiency and the interaction of the igniting shocks with a deformed fuel shell.

Focus optimization of relativistic self-focusing for anomalous laser penetration into overdense plasmas (super-penetration)

Relativistic electron motion in a plasma due to an intense laser pulse modifies the refractive index and leads to two effects: relativistic induced transparency and relativistic self-focusing. A combination of the above two effects enables transmission of laser energy deep into plasmas which is useful for fast ignition of inertial fusion. This so-called super-penetration sensitively depends on the focal position of the laser intensity due to the inhomogeneous density profile of the plasma and convergence of the laser pulse by final focusing optics. Experiments were conducted at vacuum focused laser intensities between 3.3 and 4. 4 × 1018 W cm−2 at peak plasma densities between 23 and 75nc, where nc is the critical density of the plasma. We introduced a scenario: the laser beam diameter at nc/4 density must be smaller than the plasma wavelength to achieve whole beam self-focusing. An optimum focus was found experimentally by measuring the plasma channel, laser transmittance and electron spectra. All three data are consistent with one another and numerical calculations based on a paraxial approximation model suggest that this optimum focus corresponds to the scenario described above.