Counterpropagating Electron Beams Research Articles

Electromagnetic emission from plasma with counter-streaming electron beams in the regime of oblique instability dominance

In this study, we investigate the generation of electromagnetic emission near the second harmonic of the plasma frequency induced by pairs of counter-propagating electron beams. Such systems can naturally occur in cosmic plasmas when particle acceleration regions are closely spaced, and they can also be implemented in a laboratory device. We specifically focus on the regime where the oblique beam–plasma instability dominates. The emission mechanism relies on the coalescence of counter-propagating plasma waves with different transverse structures. It has been demonstrated that the parameters of the system necessary for efficient radiation generation can be determined using the exact linear theory of beam–plasma instability. Through particle-in-cell numerical simulations, we show that a high beam-to-radiation conversion efficiency can be achieved when the beams excite small-scale oblique plasma oscillations. Importantly, we find that the efficiency and spectral characteristics of the radiation are not dependent on the thickness of the beams. We explore two scenarios involving pairs of symmetric beams: one with relativistic beams having a directed velocity of vb=0.9c and another with sub-relativistic beams at vb=0.7c. Additionally, we consider the injection of two beams with different velocities. In all cases considered, the beam-to-radiation power conversion efficiency reaches a level of a few percent, a sufficiently high value for beam–plasma systems.

High-intensity laser pulse interaction with a counter propagating electron beam for terahertz field generation in magnetized plasmas

High-intensity laser pulse interaction with a counter propagating electron beam for terahertz field generation in magnetized plasmas

Nanosecond pulse breakdown in noble gases

The results of the investigation of the breakdown characteristics of the planar “open” discharge and open discharge with the generation of counter-propagating electron beams under excitation by pulses with nanosecond rise fronts are presented. The amplitude parameters of current and voltage and temporal characteristics of breakdown in helium, neon, and argon were measured. It is demonstrated that the breakdown in the open discharge is characterized by considerably larger electric field strengths at the same development delays as in the avalanche discharge. A similarity criterion based on the photoemission mechanism of electron generation, according to which the discharge development delay is inversely proportional to the squared working gas pressure, is obtained.

Study of character of modulation instability in cyclotron resonance interaction of an electromagnetic wave with a counterpropagating rectilinear electron beam

In this paper, the interaction of a monochromatic electromagnetic wave with a counterpropagating electron beam moving in an axial magnetic field is considered. The purpose of this study is to investigate the conditions for occurrence of modulation instability (MI) in such a system and to determine at which parameters of the incident wave the MI is absolute or convective. Methods. Theoretical analysis of the MI character is carried out by studying the asymptotic form of unstable perturbations using the saddle-point analysis. The analytical results are verified by numerical simulations. Results. Theoretically, the boundary of change in the character of MI on the plane of input signal parameters (amplitude and detuning of the frequency from the cyclotron resonance) is determined. Numerical simulations confirm that as the signal frequency increases, the regime of self-modulation, which corresponds to the absolute MI, is replaced by the stationary single-frequency transmission corresponding to the convective MI. The numerical results coincide with the analytical ones for the system, which is matched at the end. The matching is implemented by smooth increasing of the guiding magnetic field in the region of electron beam injection. Conclusion. Determining the analytical conditions for the implementation of the absolute MI is of practical interest, since the emerging self-modulation can lead to the generation of trains of pulses with the spectrum in the form of frequency combs.

Modulation instability of an electromagnetic wave interacting with a counterpropagating electron beam under condition of cyclotron resonance absorption.

In this paper, transmission of a monochromatic wave through a counterpropagating electron beam under the condition of cyclotron resonance absorption is studied by theoretical analysis and numerical simulation. Conditions of the modulation instability (MI) are analyzed. The MI strongly affects the regimes of transmission. We also derive explicit periodic stationary solutions expressed in terms of elliptic Jacobi functions, as well as bright- and dark-soliton solutions. Analysis of these solutions allows obtaining threshold values of the driving power and frequency for the different regimes of transmission, such as cyclotron absorption, multifrequency self-modulation oscillations, and stationary single-frequency propagation. The theoretical predictions are verified by numerical simulation. In this way, we obtain the conditions at which a continuous-wave driving signal disintegrates into a close-to-periodic train of microwave soliton pulses.

Entrainment, stopping, and transmission of microwave solitons of self-induced transparency in counter-propagating magnetized electron beam.

Based on numerical simulations of a boundary problem, we study various scenarios of microwave soliton formation in the process of cyclotron resonance interaction of a short electromagnetic pulse with a counter-propagating initially rectilinear electron beam taking into account the relativistic dependence of the cyclotron frequency on the electrons' energy. When a certain threshold in the pulse energy is exceeded, the incident pulse can propagate without damping in the absorbing beam, similar to the effect of self-induced transparency in optics. However, mutual motion of the wave and electrons can lead to some novel effects. For relatively small energy of the incident pulse, the microwave soliton is entrained by the electron beam opposite to the direction of the wave's group velocity. With an increase in the pulse energy, soliton stopping occurs. This regime is characterized by the close-to-zero pulse velocity and can be interpreted as a variety of the "light stopping." High-energy microwave solitons propagate in the direction of the unperturbed group velocity. Their amplitude may exceed the amplitude of the incident pulse, i.e., nonlinear self-compression takes place. A further increase in the incident energy leads to the formation of additional high-order solitons whose behavior is similar to that of the first-order ones. The characteristics of each soliton (its amplitude and duration) correspond to analytical two-parametric soliton solutions that are to be found from consideration of the unbounded problem.

Particle-in-cell simulations of THz emission from plasma by oblique collision of two-electron beams

We studied the THz radiation generated by a beam-plasma system using two-dimensional (2D) particle-in-cell (PIC) simulations. The Langmuir waves excited by two counterpropagating electron beams, via two-stream instability, collide with each other at an oblique angle, which forms a high beam-density modulation near the collision region, where both beam electrons become trapped. As a result, spatially localized Langmuir wave packets with large longitudinal-electric field amplitudes are formed, which give rise to bursts of electromagnetic radiation. Our 2D PIC simulations of the two thin, low-density, asymmetric, electron beams colliding obliquely show that a strong THz emission is obtained at the second harmonic of the plasma frequency (f = 1.0 THz), with a narrow spectral width (∼0.80%) in vacuum and significantly higher efficiency than the head-on-collision case. The efficiency of power conversion from electron beams to THz radiation measured in vacuum reaches around ∼0.0289, for a continuous injection of beams into the plasma, making it suitable for applications requiring high-power narrow-band THz radiation sources.

Operating characteristics of open discharge-based plasma switches with helium, nitrogen and oxygen

Investigations of the operating characteristics of plasma switches based on the open discharge with the counter-propagating electron beams in helium, nitrogen, oxygen, and also in their mixtures have been carried out. In spite of the differences in the mechanisms of discharge burning for various conditions, which leads to the differences in switching characteristics, devices with switching times of 0.28–1.8 ns for design without a drift space and 1–2.5 ns with it are implemented. Even at a moderate voltage U ∼ 10 kV the obtained switching efficiency is in the range of 76%–95%, which exceeds values for the other gas-discharge devices, operating at high pulse repetition rate with ∼1 ns pulse leading front.

Momentum transfer driven by fluctuations in relativistic counter-propagating electron beams

A semi-Lagrangian relativistic Vlasov–Maxwell solver is used to describe the non-linear momentum transfer between electron beams, which is driven by spontaneous fluctuations in the oblique filamentation instability (a type of Weibel mode) in two-dimensional plasmas. For counter-streaming plasmas, these filamentation instabilities may strengthen the generation of a fine structure of the distribution function in phase space. Properties of such counter-streaming beams are investigated using full-kinetic simulations and a Hamiltonian reduction technique based on the invariance of the transverse canonical momentum, the so-called multi-stream model. In the regime where , being the wavevector and d e the electron skin depth, a momentum transfer takes place when a mesoscale current filamentation mode couples with microscopic phase-space fluctuations. This regime is characterized by the appearance of filamentation modes with large wavenumbers and large amplitude fluctuations, which induce variations of the L 2-norm of the plasma in link with the self-organization of counter-streaming electron beams. We also discuss some theoretical aspects of information theory that highlight the role of the (reversible) entropy production in momentum transfer, thus by clarifying the connections between (microscopic) fluctuations of the distribution function and the fundamental aspects of the filamentation in Vlasov theory. The momentum transfer between beams is quantitatively analyzed by means of both theoretical estimations and numerical experiments.

Investigation of the characteristics and mechanism of subnanosecond switching of a new type of plasma switches. I. Devices with counter-propagating electron beams—kivotrons

Novel opportunities of waveform tailoring for controlling plasma parameters based on the development of a high-voltage gas-discharge switch with a subnanosecond breakdown time and high pulse repetition frequency are discussed. The studies of characteristics and breakdown development mechanisms of the switch based on an ‘open’ discharge—kivotron are summarized. The discharge in the switch is carried out in conditions when counter-propagating electron beams in high electric field are generated. In this case, when using helium as an operating medium, firstly, atoms are effectively excited into the resonance state by fast particles. Secondly, due to the Doppler effect, resonant photons without reabsorption reach the cathode surface, maintaining the discharge current due to photoemission. Thirdly, fast heavy particles modify the cathode surface, thereby significantly (up to an order of magnitude) increasing the photoemission coefficient. The combination of these processes leads to an increase in the switching rate with an increase in the operating voltage U and helium pressure pHe. At U > 20 kV and pHe > 10 Torr, the switching time becomes less than 100 ps both in the experiment and according to the simulation. It is preferable to use planar geometry without a drift space as a switching device, in which, on the one hand, the most complete use of EB energy is realized in creating a plasma with a high charge density, on the other hand, a small wave impedance of the switch is realized. As a result, currents of tens of kiloamperes are achieved at voltages up to 100 kV. In an interpulse period plasma in the discharge gaps recombinates fast. As a result, switches can operate up to pulse repetition frequency 100 kHz. Together these achievements open new opportunities to control plasma parameters.

Investigation of the characteristics and mechanism of subnanosecond switching of a new type of plasmas switches. II switching devices based on a combination of ‘open’ and capillary discharges—eptrons

The results of investigation of a new type of switch (eptron), which expands the opportunities of voltage waveform tailoring for low temperature plasma applications, are presented. This switch consists of the device with counter-propagating electron beams (kivotron), which acts as a plasma cathode, and capillary integrated with the kivotron into a single device. The main advantage of a discharge in a capillary is various mechanisms of charge neutralization at different plasma densities. At a low density (1010–1011) cm−3, free transport of electrons to the walls of the capillary occurs, due to which a large delay in the development of the discharge is realized. At a high plasma density, due to Debye screening, a rapid charges multiplication occurs and fast switching is realized. Under optimal conditions corresponding to the maximum value of the Townsend multiplication coefficient, a switching time less than ∼1 ns is achieved. The comparative characteristics of the kivotron and eptron due to different mechanisms of subnanosecond switching are considered. The kivotron has a much lower inductance, which allows it to receive currents of tens of kiloamperes. Eptron operates more efficiently with a small characteristic size of the capillary, preferably a few tens of millimetres square. As a result, plasma recombination in the capillary in the eptron in the interpulse interval is much faster than in the kivotron, which allows operating at pulse repetition frequency above 100 kHz and the operating voltage of tens of kilovolts. The subnanosecond switching time in the eptron is realized at currents up to ∼1 kA. Nevertheless, eptron provides new opportunities of voltage waveform tailoring, specifically for pulses generating with a subnanosecond leading edge. Particularly, it is demonstrated that lasing characteristics of BaII and HgII lasers on self-terminating transitions are significantly improved in comparison with conventional power supply.

Switching devices – EPTRON with 100 kV operating voltage and sub-nanosecond switching times

The data on the experimental study of the characteristics of a gas-discharge switching device consisting of an open discharge section with generation of counter propagating electron beams acting as an electron emitter, and a capillary discharge section providing a large time delay for the development of the discharge τd and a rapid recombination of the plasma after the passage of the current pulse are presented. The ability of this device to operate at voltages up to 100 kV, while maintaining a delay time τd of the order of several hundred ns and subnanosecond capillary breakdown τsc, is demonstrated. The τsc voltage independence in the range of helium pressures pHe = 6–30 Torr was found.

Determining the carrier-envelope phase of relativistic laser pulses via electron-momentum distribution

The impacts of the carrier-envelope phase (CEP) of a long relativistic tightly-focused laser pulse on the dynamics of a counter-propagating electron beam have been investigated in the, so-called, electron reflection regime, requiring the Lorentz factor of the electron $\gamma$ to be approximately two orders of magnitudes lower than the dimensionless laser field parameter $\xi$. The electrons are reflected at the rising edge of the laser pulse due to the ponderomotive force of the focused laser beam, and an asymmetric electron angular distribution emerges along the laser polarization direction, which sensitively depends on the CEP of the driving laser pulse for weak radiative stochastic effects. The CEP siganatures are observable at laser intensities of the order or larger than $10^{19}$ W/cm$^2$ and the pulse duration up to 10 cycles. The CEP detection resolution is proportional to the electron beam density and can achieve approximately $0.1^{\circ}$ at an electron density of about $10^{15}$ cm$^{-3}$. The method is applicable for currently available ultraintense laser facilities with the laser peak power from tens of terawatt to multi-petawatt region.

Limit characteristics of switches based on planar open discharge

The limit characteristics of switches based on open discharge (OD) with counter-propagating electron beams were studied. Such switches (kivotrons) are able to generate high-voltage pulses with a sub-nanosecond leading edge. Separately obtained limit values of the switch parameters are as follows: current density up to 1000 A cm−2, operating voltage up to 100 kV, switching time down to 100 ps, the degree of pulse compression up to 100, a pulse repetition frequency up to 100 kHz (in burst mode), and an efficiency of more than 0.9. It was shown that the fast current development arises when the discharge self-sustaining mode is caused by the photoemission from the cathodes due to the resonant radiation emitted by fast helium atoms that have a large Doppler shift with respect to the line center. As a result, the emitted radiation reaches the cathodes without absorption by the helium gas. The presence of a fast atom group with an energy determined by the applied voltage was experimentally demonstrated. The dynamics of the plasma decay was studied. It is shown that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only when the plasma density remains higher than 1012 cm−3. The main mechanism of the further plasma decay is the diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. The results of the lasing characteristics of a copper vapor laser excited by pulses generated by an OD-based switch are presented.

Generation of High-Voltage Pulses with a Picosecond Front in a Cascade Kivotron Connection

The possibility of the operation of high-current high-voltage switches based on “open” discharge with the generation of counter-propagating electron beams, that is, kivotrons, in a two-stage pulse compression scheme is demonstrated experimentally. The total degree of the initial-pulse compression, which exceeded 600 at a minimum measured front duration of a generated pulse of at most 100 ps, was achieved at a voltage of up to 18 kV and a pulse-repetition rate of 1 kHz.

Picosecond Breakdown in High-Voltage Open Pulse Discharge With Enhanced Secondary Electron Emission

A 300-ps breakdown was registered in the experiments in the high-voltage open discharge with counter-propagating electron-beams with applying voltage pulses with rise times in 10–20 ns range. Discharge operates between two cathodes and grid-anode in the midplane at 10–35 Torr in helium at voltage 5–13 kV. It is shown in Particle-in-cell Monte Carlo collision (PIC MCC) simulations that the contributions of electrons, ions and energetic atoms in ionization and excitation processes are important for current development. The influence of different emissive cathode materials (titanium, silicon carbide, and CuAlMg-alloy) on evolution of discharge current is studied in the experiment and in kinetic PIC MCC simulations. The secondary electron emission from the cathodes is found to be a dominant process at the final stage of the breakdown. The current growth rate is larger (up to 700 A/cm2ns) for the cathode with a higher secondary electron emission yield. By choosing the cathode material, applied voltage amplitude and gas pressure, the characteristic time of the gas breakdown can be reduced.

Switching of 100-kV pulses in a planar “open” discharge with generation of counterpropagating electron beams

Some results of studying a planar “open” discharge with generation of counterpropagating electron beams at high voltages and appreciable working gas pressures up to atmospheric one are presented. The possibility of its functioning in helium at a voltage above 100 kV is demonstrated, and an increase in the working voltage does not prevent fast breakdown with characteristic switching times of ~1 ns. A specific feature of this type of discharge is the existence of a gas-pressure region, where the discharge-development delay time nonmonotonically depends on the voltage due to the competition between emission processes leading to breakdown.

Boundary conditions on the plasma emitter surface in the presence of a particle counter flow: I. Ion emitter

Emission of positively charged ions from a plasma emitter irradiated by a counterpropagating electron beam is studied theoretically. A bipolar diode with a plasma emitter in which the ion temperature is lower than the electron temperature and the counter electron flow is extracted from the ion collector is calculated in the one-dimensional model. An analog of Bohm’s criterion for ion emission in the presence of a counterpropagating electron beam is derived. The limiting density of the counterpropagating beam in a bipolar diode operating in the space-charge-limited-emission regime is calculated. The full set of boundary conditions on the plasma emitter surface that are required for operation of the high-current optics module in numerical codes used to simulate charged particle sources is formulated.

Modulation of microwave radiation in the process of resonant interaction with a counter-propagating rectilinear electron beam

Modulation of microwave radiation in the process of resonant interaction with a counter-propagating rectilinear electron beam

On the mechanism of the picosecond switching phenomenon in devices based on the open discharge with counter-propagating electron beams

The subnanosecond breakdown stage in the kivotron, a switching device with counter-propagating electron beams based on the open discharge in helium, was experimentally studied. It was shown that the fast discharge stage arises when the discharge self-sustaining regime is ensured by the photoelectron emission from of the cathodes due to the resonant radiation, emitted by fast helium atoms that have large Doppler shifts with respect to the line center; as a result, the emitted radiation reaches the cathodes without imprisonment by the helium gas. Since the cross-section for the excitation of a helium atom with another (fast) helium atom increases rapidly with the energy of the fast atom, the duration of the breakdown stage strongly depends on the working voltage. The transient characteristic is modulated by microwave oscillations of ~4.4 GHz frequency generated during the discharge of kivotron self-capacitance through its self-induction. An increase in working pressure leads to suppression of oscillations. A switching time of 80 ps was achieved in a discharge circuit loaded to a resistance RL ⩾ 50 Ω. On decreasing the value of RL down to 10 Ω, the switching time increases to about 100 ps at 1.5 kA current. A minimum switching time that can be achieved via kivotron design optimization is estimated to be about 35 ps.