The evolutions of laser ablation plasma, expanding in strong (∼10 T) transverse external magnetic field, were investigated in experiments and simulations. The experimental results show that the magnetic field pressure causes the plasma decelerate and accumulate at the plasma-field interface, and then form a low-density plasma bubble. The saturation size of the plasma bubble has a scaling law on laser energy and magnetic field intensity. Magnetohydrodynamic simulation results support the observation and find that the scaling law (V max ∝ E p /B 2 , where V max is the maximum volume of the plasma bubble, E p is the absorbed laser energy, and B is the magnetic field intensity) is effective in a broad laser energy range from several joules to kilo-joules, since the plasma is always in the state of magnetic field frozen while expanding. About 15% absorbed laser energy converts into magnetic field energy stored in compressed and curved magnetic field lines. The duration that the plasma bubble comes to maximum size has another scaling law t max ∝E p 1/2 /B 2 . The plasma expanding dynamics in external magnetic field have a similar character with that in underdense gas, which indicates that the external magnetic field may be a feasible approach to replace the gas filled in hohlraum to suppress the wall plasma expansion and mitigate the stimulated scattering process in indirect drive ignition.
A pulsed intense magnetic field device was developed for the Shanghai Shenguang-II (SG-II) laser facility. The device using a double-turn coil with 12 mm diameter is capable of producing a peak current of 42 kA with 280 ns rising edge and 200 ns flat top width. A peak magnetic field of 8.8 T is achieved at the center of the coil. A two-section transmission line composed by a flexible section and a rigid section is designed to meet the target chamber environment of SG-II laser facility. The flexible section realizes the soft-connection between the capacitor bank and the target chamber, which facilitates the installation of the magnetic field device and the adjustment of the coil. The rigid section is as small as possible so that it can be inserted into the target chamber from any smallest flange, realizing elastic magnetic field configuration. The magnetic coil inside the chamber can be adjusted finely through a mechanical component on the rigid transmission line outside the target chamber. The adjustment range is up to 5 cm in both radial and axial directions with ∼50 µm precision. The device has been successfully operated on SG-II laser facility.
Flute instability produced by laser plasma expanding in a 10 T external magnetic field was studied in experiments. The plasma was generated by a 0.3 J ns laser ablating an aluminum target. The external magnetic field of approximately 10 T was provided by a pair of Helmholtz coils aligned parallel to the target surface. Initially, the plasma plume expands freely. The external magnetic field confines the plasma plume and, finally, forms a plasma cavity with a sharp plasma–field interface. Flute instability was observed at the plasma–field interface, which presents a salient kinetic feature rather than classical fluid instability. In the initial linear phase, the growth rate of the perturbation has good agreement with Large Larmor radius instability, which is larger than ion gyrofrequency. In the later nonlinear growth phase, the flute instability shows an obvious “fishbone” structure of kinetic instability, and the initial short wavelength perturbation shifts continually to longer wavelength mode and, finally, close to the density scale length. Our experiment reveals a new region of parameter space that reproduces the flute instability in the space experiments of an active magnetospheric particle tracer experiment and a combined release and radiation effects satellite.
Magnetized laser plasma has attracted a lot of attention in recent years especially in magnetized inertial confinement fusion, laboratory astrophysics, and industrial application. Pulsed intense magnetic field device is the core equipment of magnetized laser plasma experiment. Here in this work, an inductively coupled coil is developed to optimize the pulsed intense magnetic field device. The primary coil of a multi-turn solenoid is used instead of a single-turn coil. Then the energy of the solenoid is delivered to the secondary coil via inductively coupled transformer, which increases the current density markedly. The current generates a stronger magnetic field in the single-turn magnetic field coil. The influence of the diameter and the number of turns of the primary solenoid of the inductively coupled coil on the magnetic field are explored in experiment and simulation. It is found that for a discharge system of 2.4 μF capacitance, the optimized parameters of the primary solenoid are 35 turns and 35 mm diameter. The optimized magnetic field is 3.6 times stronger than that of the conventional directly connected single-turn coil. At a charging voltage of 20 kV, the peak magnetic field reaches 19 T in a magnetic field coil of 5 mm inner diameter. The inductively coupled coil made of CuBe solves the problem of coil expansion in intense magnetic field, and a peak magnetic field of 33 T is obtained at a charging voltage of 35 kV. The present approach creates stronger magnetic field environments. At the same time, the inductively coupled coil reduces the requirements for system inductance, so that components such as energy storage capacitors and switch can be placed far from the coil, which improves the flexibility of the experiment setup.
When an ultrashort laser pulse incidents onto a plasma mirror, there exist fast electron ejections, terahertz (THz) radiation, and harmonic generation simultaneously. We investigated the correlation of these three emission phenomena at a preplasma density gradient scale length of (0.05–1)λ and sub-relativistic laser intensity (a0 = 0.4) via particle-in-cell simulation. It is shown that THz radiation is positively correlated with fast electron ejections. As the gradient scale length increases, both enhance first, reach a maximum at 0.4λ, and then degrade at a longer scale length. Harmonic generation, on the other hand, presents the strongest radiation at a sharp surface of 0.05λ and then decays continuously at a softer gradient, indicating that it has an anti-correlation with the fast electron ejections at first (<0.4λ) but turns into a positive correlation at a softer gradient. We find that the laser energy absorption mechanism plays a vital role in the correlation among these emission phenomena. At a sharp boundary of <0.4λ gradient scale length, the Brunel mechanism is dominated, and the absorption rate increases gradually with the increasing gradient scale length. However, at the softer boundary of >0.4λ, the absorption rate decreases continuously according to stochastic heating, and the dependence on laser polarization is eventually lost. The transition of laser absorption mechanisms alters the correlation among fast electrons, THz driven by ejected fast electrons via coherent transition radiation, and harmonics excited by bounded electrons.
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