Research on plasma electron density distribution based on microwave diffraction

In this paper, a non-contact plasma microwave diffraction measurement method is proposed, which can obtain the electron density at different diameters of the cylindrical plasma. There is a lot of diffraction when a non-focused antenna is used to transmit plasma. As we all know, when the frequency of the incident microwave is lower than the characteristic frequency of the plasma, the microwave cannot be transmitted through the plasma, so this interface can be regarded as a metal. According to the microwave diffraction of the plasma, the size of the plasma correspond-ing to the characteristic frequency can be obtained. Furthermore, by sweeping the incident elec-tromagnetic wave, the size of plasma with different characteristic frequencies can be obtained, and the distribution of electron density can be obtained. To verify the method, a cylindrical plasma was measured by microwave diffraction, in which the electron density of the plasma column gradually decreased along the increase in radius. According to the diffraction of the plasma column at different frequencies, the distribution of the electron density along the diame-ter is obtained. And compared with the transmission diagnosis method, the validity and accuracy of this method are verified. In non-uniform high-temperature plasma, the diffraction method greatly improves the accuracy of spatial diagnosis compared with traditional transmission diag-nosis.

Studies on Underwater Electric Discharge and Associated Pressure Waves

<p>Pulsed electric discharges in a liquid with the sufficiently wide range of energy contributions to them can generate diverging shock waves. А significant part of this energy is carried away by these waves from the center of the system to its periphery. At the same time, pulsed plasma-liquid systems limited by reflecting walls of both cylindrical and spherical geometry are insufficiently studied. A fundamental feature of such systems is the generation of a sequence of both diverging and converging (reflected) shock waves by a single pulse discharge. It was shown earlier that in a cylindrical plasma-liquid system with a height of the cylinder (h) comparable with the interelectrode distance (d), radius of the cylinder base R (at R >> h), when discharge current is increased, the ratio of the second diverging shock wave amplitude to the amplitude of the first diverging shock wave can be → 1. This leads to effective return of the energy carried away to the periphery back to the center of the system by converging shock waves. The collapse of the converging shock waves and initiated processes in the center of such plasma-fluid systems can be very interesting. The paper presents the results of experimental studies of pulsed cylindrical plasma-liquid system using both H₂O and a mixture of H₂O / D₂O and pure D₂O as a liquid. The energy-storage capacitor is charged by using a high voltage DC power supply (up to 70 kV).</p>

Studies on Underwater Electric Discharge and Associated Pressure Waves

<p>Pulsed electric discharges in a liquid with the sufficiently wide range of energy contributions to them can generate diverging shock waves. А significant part of this energy is carried away by these waves from the center of the system to its periphery. At the same time, pulsed plasma-liquid systems limited by reflecting walls of both cylindrical and spherical geometry are insufficiently studied. A fundamental feature of such systems is the generation of a sequence of both diverging and converging (reflected) shock waves by a single pulse discharge. It was shown earlier that in a cylindrical plasma-liquid system with a height of the cylinder (h) comparable with the interelectrode distance (d), radius of the cylinder base R (at R >> h), when discharge current is increased, the ratio of the second diverging shock wave amplitude to the amplitude of the first diverging shock wave can be → 1. This leads to effective return of the energy carried away to the periphery back to the center of the system by converging shock waves. The collapse of the converging shock waves and initiated processes in the center of such plasma-fluid systems can be very interesting. The paper presents the results of experimental studies of pulsed cylindrical plasma-liquid system using both H₂O and a mixture of H₂O / D₂O and pure D₂O as a liquid. The energy-storage capacitor is charged by using a high voltage DC power supply (up to 70 kV).</p>

WAKEFIELD EXCITATION BY A RAMPED ELECTRON BUNCH TRAIN IN A PLASMA-DIELECTRIC WAVEGUIDE

A linear theory of wakefield excitation by a ramped electron bunch train in a cylindrical plasma-dielectric waveguide is presented. It is shown that during an excitation process the drive bunches are in the focusing field due to the radial electric field excitation of the plasma wave. The possibility of both obtaining a high transformer ratio and focusing the drive and witness bunches is demonstrated.

Radial distribution of the plasma potential in a cylindrical plasma column with a longitudinal magnetic field

The possibility of controlling the electrostatic field distribution in plasma has yielded wide prospects for modern technologies. As a magnetic field primarily allows for creating electric fields in plasma, it serves as an additional obstacle for the current flow through a medium. In the present paper, an axially symmetric system is considered in which the magnetic field is directed along the axis and concentric electrodes are located at the ends. The electrodes are negatively biased. A model which solves the problem of the radial distribution of the plasma potential inside the cylindrical plasma column supported by the end electrodes is proposed. The most commonly encountered configurations of the electrical connection for the end electrodes are considered, and the particular solutions to the problem of the radial distribution are presented. The contribution of ions and electrons to the transverse conductivity is evaluated in detail. The influence of a thermionic element on the radial profile of the plasma potential is considered. To verify the proposed model, an experimental study of the reflex discharge is carried out with both cold electrodes and a thermionic element on the axis. A comparison of the computational model results with experimental data is given. The presented model makes it possible to solve the problem concerning the plasma potential distribution in the case of an arbitrary number of end electrodes, and also to take into account the inhomogeneity of the distribution of plasma density, neutral gas pressure and electron temperature along the radius.

Fast electron transport dynamics and energy deposition in magnetized, imploded cylindrical plasma

Inertial confinement fusion approaches involve the creation of high-energy-density states through compression. High gain scenarios may be enabled by the beneficial heating from fast electrons produced with an intense laser and by energy containment with a high-strength magnetic field. Here, we report experimental measurements from a configuration integrating a magnetized, imploded cylindrical plasma and intense laser-driven electrons as well as multi-stage simulations that show fast electrons transport pathways at different times during the implosion and quantify their energy deposition contribution. The experiment consisted of a CH foam cylinder, inside an external coaxial magnetic field of 5 T, that was imploded using 36 OMEGA laser beams. Two-dimensional (2D) hydrodynamic modelling predicts the CH density reaches 9.0 g cm − 3 , the temperature reaches 920 eV and the external B-field is amplified at maximum compression to 580 T. At pre-determined times during the compression, the intense OMEGA EP laser irradiated one end of the cylinder to accelerate relativistic electrons into the dense imploded plasma providing additional heating. The relativistic electron beam generation was simulated using a 2D particle-in-cell (PIC) code. Finally, three-dimensional hybrid-PIC simulations calculated the electron propagation and energy deposition inside the target and revealed the roles the compressed and self-generated B-fields play in transport. During a time window before the maximum compression time, the self-generated B-field on the compression front confines the injected electrons inside the target, increasing the temperature through Joule heating. For a stronger B-field seed of 20 T, the electrons are predicted to be guided into the compressed target and provide additional collisional heating. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.