Experimental evidence of impact ignition: 100-fold increase of neutron yield by impactor collision

Magnetization of high-density plasma with a jet velocity of hundreds of km/s

High magnetic fields at the kilotesla scale have been experimentally generated and finding methods to fully embed such fields into high-density plasma is interesting for magnetically assisted a fast ignition scheme of inertial confinement fusion, laboratory astrophysics, and magnetically guided fast electron beam for broad applications. We investigate diffusion and embedment of an external magnetic field inwards a high-density plasma by analysis and simulation. By introducing the magnetic Péclet number, dimensional analysis indicates that the magnetizing process is sensitive to the jet velocity, temperature, and size of the plasma and gives a phenomenological scaling law of the magnetic field embedment time with an arbitrary jet velocity. The analytical results are verified by magnetic field simulation and applied in 100-g/cm^{3}, 100-μm-radius plasmas with a jet velocity of 0-400 km/s and a temperature of 50-500 eV, typically adopted in experiments. Attributed to an effective electric field from frame transformation, the magnetic field embedment time can be significantly reduced by one order of magnitude when a jetting plasma is adopted with a velocity of hundreds of kilometers per second, e.g., from 5.5 ns in a static plasma to a 0.5 ns timescale in a jetting plasma of 200 km/s. The promoted embedment process favors for various applications mentioned above.

Double-cone ignition scheme for inertial confinement fusion

While major progress has been made in the research of inertial confinement fusion, significant challenges remain in the pursuit of ignition. To tackle the challenges, we propose a double-cone ignition (DCI) scheme, in which two head-on gold cones are used to confine deuterium-tritium (DT) shells imploded by high-power laser pulses. The scheme is composed of four progressive controllable processes: quasi-isentropic compression, acceleration, head-on collision and fast heating of the compressed fuel. The quasi-isentropic compression is performed inside two head-on cones. At the later stage of the compression, the DT shells in the cones are accelerated to forward velocities of hundreds of km s^-1. The head-on collision of the compressed and accelerated fuels from the cone tips transfer the forward kinetic energy to the thermal energy of the colliding fuel with an increased density. The preheated high-density fuel can keep its status for a period of approximately 200 ps. Within this period, MeV electrons generated by ps heating laser pulses, guided by a ns laser-produced strong magnetic field further heat the fuel efficiently. Our simulations show that the implosion inside the head-on cones can greatly mitigate the energy requirement for compression; the collision can preheat the compressed fuel of approximately 300 g cm^-3 to a temperature above keV. The fuel can then reach an ignition temperature of greater than 5 keV with magnetically assisted heating of MeV electrons generated by the heating laser pulses. Experimental campaigns to demonstrate the scheme have already begun. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 1)'.

Measurement of the injecting time of picosecond laser in indirect-drive integrated fast ignition experiments using an x-ray streak camera

The injecting time of the picosecond laser in an indirect-drive integrated fast ignition experiment was measured by using an x-ray streak camera. Despite overlapping spatially and temporally in experiments, the soft x-ray signal from the nanosecond laser ablating the inner wall of an Au hohlraum and the hard x-ray signal from the bremsstrahlung radiation of hot electrons generated by a picosecond laser were separated by different image processes by filtering and collimating the two signals differently. The time sequence between the two x-ray signals was analyzed to extract the injection time of the picosecond laser relative to the hohlraum emission. By tracking the neutron yield as a function of the injection time of the picosecond laser, a clear positive correlation between the neutron yield enhancement and the derived injection times was exhibited. The heating effect of the picosecond laser was confirmed. It is concluded that this method could be used to measure the injecting time and validate the picosecond laser injection.

Solution of the Noh problem with an arbitrary equation of state

The classic self-similar solutions of the nonstationary compressible Euler equations obtained for a blast-wave propagation (Sedov, Taylor, and von Neumann), a shock-wave implosion (Guderley, Landau, and Stanyukovich), or an impulsive loading of a planar target (von Hoerner, Häfele, and Zel'dovich) have all been derived for a polytropic ideal gas. None of them can be generalized for a fluid with an arbitrary equation of state (EOS), such as the van der Waals EOS of a non-ideal-gas or a three-term EOS of a condensed material. We demonstrate here that the Noh accretion-shock problem is an exception. Its self-similar solutions exist in cylindrical and spherical geometry for fluids and materials with an arbitrary EOS. Such solutions for finite accretion-shock strength and nonuniform inflow velocity are constructed semianalytically with a model three-term equation of state that includes cold, thermal ion (lattice), and thermal electron contributions to the pressure and internal energy. Examples are presented for aluminum and copper. Other material- and EOS-specific semianalytic solutions of the Noh problem can be easily constructed using the same method for any material that in the pressure range of interest can be approximated as a dissipation-free fluid with an arbitrary equation of state.

Effect of quantum correction on the acceleration and delayed heating of plasma blocks

The interaction of laser pulses of picosecond duration and terawatt to petawatt power accelerated for the very fast undistorted plasma blocks for deuterium DD or deuterium tritium fast ignition is investigated. Based on the direct and instant conversion of laser energy into mechanical motion by nonlinear (ponderomotive) forces, any thermal pressure generation is delayed by the collision process. Following the studying of the classical collision frequency, it is found that the quantum modified collision at higher energies results in a correction by about 15% reduction of the delay.

Effect of the plasma-generated magnetic field on relativistic electron transport

In the fast-ignition scheme, relativistic electrons transport energy from the laser deposition zone to the dense part of the target where the fusion reactions can be ignited. The magnetic fields and electron collisions play an important role in the collimation or defocusing of this electron beam. Detailed description of these effects requires large-scale kinetic calculations and is limited to short time intervals. In this paper, a reduced kinetic model of fast electron transport coupled to the radiation hydrodynamic code is presented. It opens the possibility to carry on hybrid simulations in a time scale of tens of picoseconds or more. It is shown with this code that plasma-generated magnetic fields induced by noncollinear temperature and density gradients may strongly modify electron transport in a time scale of a few picoseconds. These fields tend to defocus the electron beam, reducing the coupling efficiency to the target. This effect, that was not seen before in shorter time simulations, has to be accounted for in any ignition design using electrons as a driver.