Stress wave analysis of high-voltage pulse discharge rock fragmentation based on plasma channel impedance model

High-voltage pulse discharge(HVPD)rock fragmentation controls a plasma channel forming inside the rock by adjusting the electrical parameters,electrode type,etc.In this work,an HVPD rock fragmentation test platform was built and the test waveforms were measured.Considering the effects of temperature,channel expansion and electromagnetic radiation,the impedance model of the plasma channel in the rock was established.The parameters and initial values of the model were determined by an iterative computational process.The model calculation results can reasonably characterize the development of the plasma channel in the rock and estimate the shock wave characteristics.Based on the plasma channel impedance model,the temporal and spatial distribution characteristics of the radial stress and tangential stress in the rock were calculated,and the rock fragmentation effect of the HVPD was analyzed.

The Plasma Channel Evolution Characteristics of Pulsed Flashlamps Working in an Array

This work is devoted to the study of plasma channel evolution characteristics in pulsed xenon flashlamps working in an array. Influencing factors on the plasma channel evolution process are studied, including pre-ionization pulse and neighbor fiashlamps. It has been found that neighbor flaShlamps affect the plasma channel by shaping the electric potential distribution, rather than by Lorentz force. Branching is observed in the plasma channels of the flashlamps in the middle of the array. Inconsistency also exists in the plasma channels of these flashlamps in different tests. The branching and inconsistency are both caused by the unique electric field distribution in these flashlamps. Besides, the pre-ionization pulse can help the main pulse plasma channel to develop more smoothly and faster, which will weaken the shock wave and benefit the mechanical strength of the flashlamp.

Enhanced laser-induced plasma channels in air

Plasma is a significant medium in high-energy density physics since it can hardly be damaged. For some applications such as plasma based backward Raman amplification （BRA）, uniform high-density and large-scale plasma channels are required. In the previous experiment, the plasma transverse diameter and density are 50-200 μm and 1-2 x 10^19 cm-3, here we enhance them to 0.8 mm and 8 x 10^19 cm-3, respectively. Moreover, the gradient plasma is investigated in our experiment. A proper plasma gradient can be obtained with suitable pulse energy and delay. The experimental results are useful for plasma physics and nonlinear optics.

Long Gap Discharge Guiding Experiments Using Strongly and Weakly Ionized Plasma Channels for Laser Triggered Lightning

A study to generate longer plasma channel was conucted to improve probability of triggered lightning. To generate the long laser plasma channel, a strongly and a weakly ionized plasma channels were used in series, a scheme called hybrid method. The strongly and the weakly ionized plasma channels were used for triggering an electrical leader and guiding the leader. The electrical leader propagated through the weakly ionized plasma. Then main discharge occurred between the electrodes. It is found that the streamer was accelerated with both the increase in the plasma density and the increase in an ambient electric field close to the plane electrode.

Theoretical Analysis on Propagation of Electromagnetic Wave in Preformed Narrow Plasma Channel

In the plasma sheath a narrow plasma channel generated by ultraintense laser pulses is simplified as a special cylindrical and hollow plasma waveguide with the infinite thickness of the plasma cladding. The electromagnetic wave （EM） propagation properties of the plasma channel near the cutoff and far from the cutoff are considered. Theoretical analysis shows that TE0m and TM0m and hybrid modes emerge in the plasma channel, which is influenced by the normalized frequency parameter B and numerical aperture NA. The cutoffs of the various modes are approximated. Single-mode operation is possible without a high-frequency limitation in the channel.

Low Resistance and Long Lifetime Plasma Channel Generated by Filamentation of Femtosecond Laser Pulses in Air

An experimental investigation of the length and resistance of the laser plasma channel generated by filamentation of fs laser pulses in air was presented. It was found that the length of the plasma channel was different from that of the laser filament. This phenomenon was commensurate with a special self-guide mechanism without ionization. Through increase of the laser energy the plasma channel could be prolonged and the resistance could be reduced. To get even lower resistance, more effective control would be needed over the multi-filament. The lifetime of the plasma channel was increased by a factor of 4.5 by pulse sequences, which were generated by detuning the regenerative amplifier in the chirped pulse amplification （CPA） laser system.

Laser-driven relativistic electron dynamics in a cylindrical plasma channel

The energy and trajectory of the electron, which is irradiated by a high-power laser pulse in a cylindrical plasma channel with a uniform positive charge and a uniform negative current, have been analyzed in terms of a single-electron model of direct laser acceleration. We find that the energy and trajectory of the electron strongly depend on the positive charge density, the negative current density, and the intensity of the laser pulse. The electron can be accelerated significantly only when the positive charge density, the negative current density, and the intensity of the laser pulse are in suitable ranges due to the dephasing rate between the wave and electron motion. Particularly, when their values satisfy a critical condition. the electron can stay in phase with the laser and gain the largest energy from the laser. With the enhancement of the electron energy, strong modulations of the relativistic factor cause a considerable enhancement of the electron transverse oscillations across the channel, which makes the electron trajectory become essentially three-dimensional, even if it is flat at the early stage of the acceleration.

Nonlinear propagation of an intense Laguerre-Gaussian laser pulse in a plasma channel

The nonlinear propagation of an intense Laguerre-Gaussian(LG)laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method.The evolution equation of the spot size is derived including the effects of relativistic self-focusing,preformed channel focusing,and ponderomotive self-channeling.The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size,periodically focusing and defocusing oscillation,catastrophic focusing,and solitary waves are obtained.Compared with the laser pulse with fundamental Gaussian(FG)mode,it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse,while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse.Thus,the matched condition for the intense LG laser pulse with constant spot size is released obviously,while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.

Direct electron acceleration by chirped laser pulse in a cylindrical plasma channel

We study the dynamics of single electron in an inhomogeneous cylindrical plasma channel during the direct acceleration by linearly polarized chirped laser pulse.By adjusting the parameters of the chirped laser pulse and the plasma channel,we obtain the energy gain,trajectory,dephasing rate and unstable threshold of electron oscillation in the channel.The influences of the chirped factor and inhomogeneous plasma density distribution on the electron dynamics are discussed in depth.We find that the nonlinearly chirped laser pulse and the inhomogeneous plasma channel have strong coupled influence on the electron dynamics.The electron energy gain can be enhanced,the instability threshold of the electron oscillation can be lowered,and the acceleration length can be shortened by chirped laser,while the inhomogeneity of the plasma channel can reduce the amplitude of the chirped laser.

Pulsed high-current discharge in water:adiabatic model of expanding plasma channel and acoustic wave

This paper presents the results of a theoretical and experimental study of the use of a pulsed discharge in water to obtain a strong acoustic wave in a liquid medium.A discharge with a current amplitude of 10 kA,a duration of 400 ns,and an amplitude pulsed power of 280 MW in water at atmospheric pressure created an expanding acoustic wave with an amplitude of more than 100 MPa.To describe the formation of the discharge channel,an isothermal plasma model has been developed,which made it possible to calculate both the expansion dynamics of a high-current channel and the strong acoustic wave generated by it.Our calculations show that the number density of plasma in the channel reaches 10^(20) cm^(-3),while the degree of water vapor ionization is about 10%,and the channel wall extends with a velocity of 500 m s^(−1).The calculations for the acoustic wave are in good agreement with measurements.

Generation of Preformed Plasma Channel for GeV-Scaled Electron Accelerator by Ablative Capillary Discharges

Generation of plasma channels by gated experimentally in details. A time-resolved low jitter ablative capillary discharges is investi- evolution and radial distributions of the electron density are measured, and proof-of-principle optical guiding experiment is conducted. A proper time window for optical guiding of a femtoseeond laser pulse is found. The generated low density, long plasma channel is believed to be useful in the applications as GeV-class channel-guided laser wakefield accelerators and compact X-ray femetoseeond coherent radiation sources.

Electron self-injection and acceleration in a hollow plasma channel driven by ultrashort intense laser pulses

The self-injection and acceleration of electrons in a hollow plasma channel driven by ultrashort intense laser pulses is investigated by Particle-in-Cell(PIC) simulations. It is shown that electrons from the bubble sheath will be self-injected into the hollow plasma channel and move radially towards the channel border due to the lack of focusing force in the hollow plasma channel. After several reflections near the channel wall by the strong focusing force, a self-injected electron bunch can be confined in the hollow plasma channel and quasi-phase-stably accelerated forward for the whole laser–plasma interaction process. These electrons using optical and plasma-related self-injection method can be self-organized to remain in the rear of the bubble, where the accelerating electric field is transversely uniform and nearly plateau along the propagation axis. Therefore, the self-injected electron bunch can be accelerated in a steady state without obvious oscillation and has a high quality with narrow energy spread and low divergence.

Generation of a curved plasma channel from a discharged capillary for intense laser guiding

Straight plasma channels are widely used to guide relativistic intense laser pulses over several Rayleigh lengths for laser wakefield acceleration.Recently,a curved plasma channel with gradually varied curvature was suggested to guide a fresh intense laser pulse and merge it into a straight channel for staged wakefield acceleration[Phys.Rev.Lett.120,154801(2018)].In this work,we report the generation of such a curved plasma channel from a discharged capillary.Both longitudinal and transverse density distributions of the plasma inside the channel were diagnosed by analyzing the discharging spectroscopy.Effects of the gas-filling mode,back pressure and discharging voltage on the plasma density distribution inside the specially designed capillary are studied.Experiments show that a longitudinally uniform and transversely parabolic plasma channel with a maximum channel depth of 47.5µm and length of 3 cm can be produced,which is temporally stable enough for laser guiding.Using such a plasma channel,a laser pulse with duration of 30 fs has been successfully guided along the channel with the propagation direction bent by 10.4◦.

Effects of Relativistic and Ponderomotive Nonlinearities on an Intense Laser Pulse Propagation in a Plasma Channel

The optical guiding of a moderately intense laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method.Relativistic,ponderomotive and their coupling nonlinearities are included.The conditions for periodic defocusing and focusing,as well as constant spot size propagation are given.It is found that the laser focusing is released by the coupling of relativistic and ponderomotive nonlinearities.

Characterization and Performance Evaluation of Turbulent Plasma Sheath Channel

Radio waves are highly attenuated and distorted by turbulent plasma sheath around hypersonic vehicles in near space, leading to communication blackout. The purpose of the paper is to investigate the plasma channel characteristics and the communication performances over the channel. We treat the turbulent plasma medium as a fast fading wireless channel. The coherence time and the spectrum spread of the plasma sheath channel are obtained in terms of root-meansquare(RMS). Baseband simulation scheme is proposed based on a stratified model of the plasma flow field. Results indicate that the coherence time is on the order of milliseconds and decreases rapidly with the increasing electron density turbulence. The spectrum spread due to plasma turbulence is also significant. Extensive simulations have been carried out to make communication performance evaluations. Quantitative results show that error floor takes place for PSK and QAM, while FSK with noncoherent detection is a promising method to mitigate the blackout problem.

PIC simulation of plasma properties in the discharge channel of a pulsed plasma thruster with flared electrodes

Plasma in the discharge channel of a pulsed plasma thruster(PPT) with flared electrodes is simulated by a self-developed two-dimensional code. The fully particle-in-cell method with Monte Carlo collision is employed to model the particle movement and collisions and investigate the plasma properties and acceleration process. Temporal and spatial variations of the electron density distribution and the ion velocity between electrodes are calculated and analyzed in detail.The computational results of the electron number density, which is in the order of 1023 m-3,show good agreements with experimental results of a PPT named ADD SIMP-LEX. The ion velocity distributions along the center line of the channel lead to a comprehensive understanding of ions accelerated by electromagnetic field. The electron distributions of PPT with discharge voltages varying from 1300 to 2000 V are compared. The diffusion of electrons presents strong dependency on discharge voltage and implies higher degree of ionization for higher voltage.

Plasma Characteristic of Lightning Discharge Channel

The plasma characteristics of a lightning discharge channel are reviewed.The spectrum of the natural lightning is investigated by employing the slit-less spectrograph.It is found that the spectrum characteristics are closely related to the intensity of the lightning discharge.The lines in the lightning spectrum are classified into essential lines and characteristic lines,according to the characteristics of lightning spectra with different intensities.The characteristics of the lightning channel and the radiation of the lighting plasma are analyzed in the visible and infrared regions.It is shown that,the visible spectrum of lightning is determined by the radiation generated from the early stage to the development of lightning,while the near-infrared spectrum is determined by the radiation generated after current of lightning reaches the peak.The channel temperature and the electron density are calculated using the information obtained from the lightning spectrum.Both the temperature and the density decrease with the increasing length of lightning channel.Moreover,X-rays and neutrons are produced in the process of lighting mainly due to the pinch effects.

Effects of Higher-Order Relativistic Nonlinearity and Wakefield During a Moderately Intense Laser Pulse Propagation in a Plasma Channel

Using a variational approach,the propagation of a moderately intense laser pulse in a parabolic preformed plasma channel is investigated.The effects of higher-order relativistic nonlinearity (HRN) and wakefield are included.The effect of HRN serves as an additional defocusing mechanism and has the same order of magnitude in the spot size as that of the transverse wakefield (TWF).The effect of longitudinal wakefield is much larger than those of HRN and TWF for an intense laser pulse with the pulse length equaling the plasma wavelength.The catastrophic focusing of the laser spot size would be prevented in the present of HRN and then it varies with periodic focusing oscillations.

Numerical Simulation of Dual-Channel Communication of Column Plasma Antenna Excited by a Surface Wave

This paper focuses on the application of plasma as wireless antenna. In order to reveal the radiation characteristics of column plasma antenna, we chose the finite-difference time- domain （FDTD） numerical analysis method to simulate radiation impedance and efficiencies of each channel for a few sets of plasma densities and plasma collision frequencies. Simulation results demonstrate that a plasma antenna shares similar characteristics with a metallic antenna in radiation impedance and efficiency of each channel when an appropriate setting is adopted. Unlike a metallic antenna, a plasma antenna is capable of realizing such functions as dynamic reconfiguration, digital control and dual-channel communication. Thus it is possible to carry out dual-channel communication by plasma antenna, indicating a new path for modern intelligent communication.

Dual-Channel Communication of Column Plasma Antenna Excited by a Surface Wave——Actualization and Simulation of Radiation Pattern

Along with the introduction of the concept of dual-channel communication,we utilized the finite-difference time-domain（FDTD） method to simulate and measure the radiation pattern under certain plasma densities and plasma collision frequencies.Results show that under certain settings,the radiation pattern of a plasma antenna resembles that of a metallic antenna.In contrast to a metallic antenna,a plasma antenna possesses other functionalities,such as dynamic reconfiguration and digital controllability.The data from simulation are similar to the measurement results,indicating that column plasma antenna can realize dual-channel communication.This work confirms the viability of realizing dual-channel communication by column plasma antenna,which adds a new but promising method for modern intelligent communication.