Experimental investigation of three‐dimensional (3‐D) effects of magnetic reconnection dynamics has been extended by use of axially colliding spheromaks [M. Yamada et al., Phys. Fluids B 3, 2379 (1991)]. The two toroidal shape spheromak plasmas with major radii of 15–20 cm and with parallel toroidal currents of up to 30 kA collide to merge in an external equilibrium field. It is important to note that the present experimental setup allows one to investigate magnetic reconnection comprehensively from both local and global points of view. Reconnection angle θ between the merging field lines is varied by changing the polarity of the internal toroidal field and the magnitude of an external toroidal field. It is observed that the speed of counterhelicity merging with θ∼180° is about three times faster than that of cohelicity merging with θ∼90°. This suggests the significance of a 3‐D effect on the reconnection process. This difference is attributed to the property of the neutral current sheets with and without the magnetic field component parallel to the reconnection (X) line. In the counterhelicity merging, the neutral current sheet is compressed in much shorter time than in the cohelicity merging, resulting in much higher current density and subsequent faster decay of the current sheet. This induces a faster magnetic reconnection. The reconnection speed increases proportionally with the initial approaching speed of the spheromaks, suggesting that a compressible driven reconnection model is consistent with the present reconnection experimental results.

Experimental Investigation of Three Dimensional Magnetic Reconnection by Use of Two Colliding Spheromaks

An analytical model for the linear Richtmyer-Meshkov instability in solids under conditions of high-energy density is presented, in order to describe the evolution of small perturbations at the solid-vacuum interface. The model shows that plasticity determines the maximum perturbation amplitude and provides simple scaling laws for it as well as for the time when it is reached. After the maximum amplitude is reached, the interface remains oscillating with a period that is determined by the elastic shear modulus. Extensive two-dimensional simulations are presented that show excellent agreement with the analytical model. The results suggest the possibility to experimentally evaluate the yield strength of solids under dynamic conditions by using a Richtmyer-Meshkov-instability-based technique.

Richtmyer-Meshkov instability in elastic-plastic media

The Lawson criterion is a key concept in the pursuit of fusion energy, relating the fuel density $n$, pulse duration $\tau$ or energy confinement time $\tau_E$, and fuel temperature $T$ to the energy gain $Q$ of a fusion plasma. The purpose of this paper is to explain and review the Lawson criterion and to provide a compilation of achieved parameters for a broad range of historical and contemporary fusion experiments. Although this paper focuses on the Lawson criterion, it is only one of many equally important factors in assessing the progress and ultimate likelihood of any fusion concept becoming a commercially viable fusion-energy system. Only experimentally measured or inferred values of $n$, $\tau$ or $\tau_E$, and $T$ that have been published in the peer-reviewed literature are included in this paper, unless noted otherwise. For extracting these parameters, we discuss methodologies that are necessarily specific to different fusion approaches (including magnetic, inertial, and magneto-inertial fusion). This paper is intended to serve as a reference for fusion researchers and a tutorial for all others interested in fusion energy. 

Progress toward fusion energy breakeven and gain as measured against the Lawson criterion

A double-turn capacitor-coil is used to produce a magnetic field (38.5 T) and construct a topology of magnetic reconnection in a low-β (β < 1) plasma environment. The device is constructed with two metallic U-turn coils connecting two parallel metallic disks. High energy lasers are employed to ablate one disk spontaneously driving two currents in the two coils, which produces an interactive magnetic field topology. We demonstrated through experiments and numerical simulations that the reconnection process takes place between two non-uniform magnetic fields created by the coils, and that the plasma state and the associated magnetic topology in the process can be seen via the technology of the optical probe beam and the proton backlight.

https://iopscience.iop.org/article/10.1088/1361-6587/aabaa9/pdf

Low-β magnetic reconnection driven by the intense lasers with a double-turn capacitor-coil

A unified analytical approach to study the effects of elasticity, viscosity, and magnetic fields on the Richtmyer–Meshkov (RM) instability by using the impulsively accelerated model is presented. This model clarifies the discontinuity in the oscillation periods and yields the asymptotic decaying rate in elastic solids. It reveals that the complex eigenvalues produce better results compared with the numerical simulations for RM instability in viscous fluids and resolves the standing controversy between the analytical theory and numerical simulations at a vacuum/fluid interface. At last, it easily retrieves the results when the normal or tangential magnetic field is present. Those good agreements, between numerical simulations and theoretical analysis, would enable the model to be valuable in more complex situations such as in the elastic–plastic slabs with or without the presence of magnetic fields, as well as in the nonlinear regime.

A unified model to study the effects of elasticity, viscosity, and magnetic fields on linear Richtmyer-Meshkov instability

https://aip.scitation.org/doi/pdf/10.1016/j.mre.2017.07.003

Formation of Field Reversed Configuration (FRC) on the Yingguang-I device

Investigation of convergent Richtmyer-Meshkov instability at tin/xenon interface with pulsed magnetic driven imploding

The invention discloses an electron density measuring system and method adopting fiber laser interference and based on a mathematical contrast method. The method comprises the following steps: 1, an all-fiber laser interference system based on a mathematical contrast phase shift demodulation method is adopted to demodulate phase shift caused by electron density with the mathematical contrast method; 2, the all-fiber laser interference system based on the mathematical contrast phase shift demodulation method is adopted to acquire space-time distribution information of plasma electron density with an Abel inversion data processing method. The complexity of the laser interference system can be reduced greatly, limits of performance of functional components and parts of a circuit and demodulation precision are avoided, and phase shift caused by electron density can be solved more precisely. Compared with the traditional system established on the basis of optical components, the all-fiber laser interference system has the advantages that experiment efficiency (time saving) and system reliability can be improved greatly, and a complex focusing adjustment process of the traditional optical-light path interference system before each experiment is omitted.

Electron density measuring system and method adopting fiber laser interference and based on mathematical contrast method

To produce broadband electromagnetic pulse (EMP) radiation, we design and test a sub-ns pulse system which is utilized to drive an axial mode helix antenna. This sub-ns pulse system consists of a 10-stage Marx generator, an intermediate storage capacitor, an impedance transforming line, and can generate a high voltage of 570 kV with a rise time of 1–1.5 ns on a 300-Ω-resistance load. The diameter, pitch and turns of the helix antenna are 29 cm, 19.5 cm and 5, respectively, with a design frequency of 300 MHz. When the charging voltage of the Marx generator is 30 kV, the electric field of radiated EMP measured 3m from the source is 35 kV/m with a central frequency of 250 MHz. The relatively spectral bandwidth is 25%. The radiated EMP results are reproducible and the system will work in repetition mode if the available dc power is utilized.

http://iopscience.iop.org/article/10.1088/0256-307X/28/9/090702/pdf

Experiments on Broadband EMP Radiation with an Axial Mode Helix Antenna

A compact ten stage Marx with six parallel 40kV/5600pF capacitors per stage, developed for various pulsed power applications, is described. Every two stages have a common unattached gas switch so there are five switches that of two are trigger switch and three are self-breakdown switch. The spark gaps and capacitors are arranged radially at sixty degree intervals. Unique features of the Compact Marx include a peaking switch and a peaking capacitor that formed by the structure. It is 43cm in diameter and 67cm in height. It can generate as high as 250kV less than 5ns rise time pulse into a 100-150 ohm load.

Qizhi Sun

Papers

Formation of Field Reversed Configuration (FRC) on the Yingguang-I device

Investigation of convergent Richtmyer-Meshkov instability at tin/xenon interface with pulsed magnetic driven imploding

Electron density measuring system and method adopting fiber laser interference and based on mathematical contrast method

Experiments on Broadband EMP Radiation with an Axial Mode Helix Antenna

Compact repetition fast rise time Marx generator