Limiting Parameters of the Plasma Opening Switch

Implementing programs for nuclear fusion research and X-ray generation requires the creation of superpower generators based on plasma opening switches (POSs) capable of commutating currents as high as several tens of megaamperes at output voltages of up to 5 MV and higher. The physical mechanisms limiting the POS voltage are investigated. It is shown that, as the generator voltage U _g increases, the voltage multiplication factor k = U_POS/U_g (where U_POS is the POS voltage) decreases. An explanation for such a dependence is proposed, and the maximum value of the POS voltage is estimated. A POS design that enables operating in the above current and voltage ranges is considered. The design is based on applying an external magnetic field to the POS interelectrode gap, increasing the initial generator voltage, and decreasing the linear (along the perimeter of the outer electrode) density of the charge passing through the POS during the conduction phase.     

Experimental verification of the technique for calculating a plasma opening switch and its matching to the load

A technique for calculating a plasma opening switch in an external magnetic field and its matching to a load the impedance of which increases with time was verified experimentally. The experiments were performed in the RS-20 facility both in the absence of a load and with various inductive loads. The amplitude of the voltage pulse at the input of the plasma opening switch was 0.36–0.84 MV, the current amplitude was 280–320 kA, and the pulse duration was ～2 μs, whereas the corresponding parameters of the output pulse were 0.8–3.2 MV, 0–240 kA, and ～100 ns.     

Outlook for the use of microsecond plasma opening switches to generate high-power nanosecond current pulses

An analysis is made of the current break process in microsecond plasma opening switches and their possible application in high-current generators. Necessary conditions are determined for generating megavolt pulses in the erosion mode of a plasma opening switch with the gap insulated by an external magnetic field. Under these conditions, efficient sharpening of high-power submegampere current pulses can be achieved. The possibility of using plasma opening switches operating at voltages of 5–6 MV to generate X-ray and gamma emission is discussed. The main operating and design parameters of a six-module plasma opening switch with a current pulse amplitude of 3.7 MA and voltage of 4–6 MV for use in the MOL generator, which is the prototype of one of the 24 modules of the projected Baikal multimegajoule generator, are estimated by using the available scalings.     

Simulation of the Plasma Afterglow in the Discharge Gap of a Subnanosecond Switch Based on an Open Discharge in Helium

The phenomenon of subnanosecond electrical breakdown in a strong electric field observed in an open discharge in helium at pressures of 6–20 Torr can be used to create ultrafast plasma switches triggering into a conducting state for a time shorter than 1 ns. To evaluate the possible repetition rate of such a subnanosecond switch, it is interesting to study the decay dynamics of the plasma remaining in the discharge gap after ultrafast breakdown. In this paper, a kinetic model based on the particle-in-cell Monte Carlo collision method is used to study the dynamics of the plasma afterglow in the discharge gap of a subnanosecond switch operating with helium at a pressure of 6 Torr. The simulation results show that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only while the plasma density remains higher than 10¹² cm^?3; the main mechanism of the further plasma decay is 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. Over nearly the same time, plasma electrons become thermalized. The afterglow time can be substantially reduced by applying a positive voltage U_c to the cathode. Since diffusive losses are limited by the ion mobility, the additional ion drift toward the wall significantly accelerates plasma decay. As U_c increases from 0 to +500 V, the characteristic time of plasma decay is reduced from 35 to 10 μs.     

Effect of an external RF field on the beam-plasma discharge in a magnetic field

The effect of an RF field on a steady-state beam-plasma discharge with a plane electrode placed parallel to a sheetlike electron beam is studied experimentally. The plasma parameters were measured by a single probe, and the electron distribution function was determined with the use of an electrostatic analyzer. The energy and current of the electron beam were E _B=2.5 keV and J _B=0.05–1.5 A, respectively. The working pressure was p=2×10^?5–10^?3 torr. The frequency of the external RF field was 13.56 MHz. Both the steady-state regimes in which the RF field had no effect on the plasma parameters and regimes with a pronounced effect of the RF field were observed. The experiments show that the regime of the discharge depends strongly on the plasma density and the magnetic field. The parametric instability is studied theoretically in the weak-turbulence approximation. It is shown that, due to the decay nature of the spectrum of plasma oscillations, the onset of instability is accompanied by the transfer of the energy of fluctuations over the spectrum, from the pump frequency toward its harmonics.     

Additional ECR heating of a radially inhomogeneous plasma via the absorption of satellite harmonics of the surface flute modes in a rippled magnetic field

A theoretical study is made of the possibility of additional heating of a radially inhomogeneous plasma in confinement systems with a rippled magnetic field via the absorption of satellite harmonics of the surface flute modes with frequencies below the electron gyrofrequency in the local resonance region, ε₁ (r ₁) = [2πc/(ωL)]², where ε₁ is the diagonal element of the plasma dielectric tensor in the hydrodynamic approximation, L is the period of a constant external rippled magnetic field, and the radical coordinate r ₁ determines the position of the local resonance. It is found that the high-frequency power absorbed near the local resonance is proportional to the square of the ripple amplitude of the external magnetic field. The mechanism proposed is shown to ensure the absorption of the energy of surface flute modes and, thereby, the heating of a radially inhomogeneous plasma.     

Influence of a Nitrogen Admixture on the Anomalous Memory Effect in the Breakdown of Low-Pressure Argon in a Long Discharge Tube

The memory effect (the dependence of the dynamic breakdown voltage U _b on the time interval τ between voltage pulses) in pulse-periodic discharges in pure argon and the Ar + 1%N₂ mixture was studied experimentally. The discharge was ignited in a 2.8-cm-diameter tube with an interelectrode distance of 75 cm. The measurements were performed at gas pressures of P = 1, 2, and 5 Torr and discharge currents in a steady stage of the discharge of I = 20 and 56 mA. Breakdown was produced by applying positive-polarity voltage pulses, the time interval between pulses being in the range of τ = 0.5–40 ms. In this range of τ values, a local maximum (the anomalous memory effect) was observed in the dependence U _b (τ). It is shown that addition of nitrogen to argon substantially narrows the range of τ values at which this effect takes place. To analyze the measurement results, the plasma parameters in a steady-state discharge (in both pure argon and the Ar + 1%N₂ mixture) and its afterglow were calculated for the given experimental conditions. Analysis of the experimental data shows that the influence of the nitrogen admixture on the shape of the dependence U _b (τ) is, to a large extent, caused by the change in the decay rate of the argon afterglow plasma in the presence of a nitrogen admixture.     

Nonlinear dynamics of beam–plasma instability in a finite magnetic field

The nonlinear dynamics of beam–plasma instability in a finite magnetic field is investigated numerically. In particular, it is shown that decay instability can develop. Special attention is paid to the influence of the beam?plasma coupling factor on the spectral characteristics of a plasma relativistic microwave accelerator (PRMA) at different values of the magnetic field. It is shown that two qualitatively different physical regimes take place at two values of the external magnetic field: B ₀ = 4.5 kG (Ω ~ ωB _p ) and 20 kG (Ω _B ? ω_p). For B ₀ = 4.5 kG, close to the actual experimental value, there exists an optimal value of the gap length between the relativistic electron beam and the plasma (and, accordingly, an optimal value of the coupling factor) at which the PRMA output power increases appreciably, while the noise level decreases.     

Nonlinear dynamics of Kelvin-Helmholtz instability in a finite-width plasma flow

The nonlinear stage of Kelvin-Helmholtz (KH) instability in a finite-width plane-parallel plasma flow is analyzed. The analysis is performed by means of two-dimensional numerical simulations with the use of ideal magnetohydrodynamic equations describing isothermal plasma flows propagating along the magnetic field. The influence of the magnetic field strength, the plasma temperature, and the ratio of the flow width to the width of the transition layer on the formation of vortex layers and large-scale flow perturbations is investigated. It is shown that, if the wavelength of periodic perturbations is shorter than the flow width, the symmetric and antisymmetric modes develop in a qualitatively similar manner. For waves with wavelengths longer than the flow width, the development of such modes is very different due to the mutual influence of the flow boundaries. Analysis of the development of instability at different values of the Alfvén Mach number M _A shows that long-lived vortices with a characteristic scale length on the order of the flow width appear in a weak magnetic field for both symmetric and antisymmetric modes; however, the vortex geometries for these modes are different. In a strong magnetic field, M _A ～ 5, the phase of vortex decay for both types of modes occurs faster than in a weak field; however, in the case of an antisymmetric mode, large-scale perturbations of the flow boundary are retained for a longer time. Analysis of the evolution of the initial disturbance produced by an ensemble of random small perturbations (noise) at different plasma temperatures shows that, for a flow width comparable with the width of the transition region, the development of KH instability is always antisymmetric in character and leads to well-developed large-scale perturbations of the flow as a whole. For a cold plasma with C _S < 0.5U (where C _S is the speed of sound and U is the flow velocity), in contrast to hot plasma with C _S > 0.5U, the development of KH instability leads to the growth of the antisymmetric mode even if the flow width is much larger than the width of the transition region.     

Suppression of the rayleigh-taylor instability of a low-density imploding liner by a longitudinal magnetic field

The possibility of suppressing the Rayleigh-Taylor instability in a low-density plasma, Π=ω _pi ² Δ²/c²?1 (where Δ is the thickness of the current-carrying slab), is investigated for the case in which the electron currents are much higher than the ion currents. The suppression of this instability in an imploding cylindrical liner by an axial external magnetic field \(B_{0z} \) is considered. It is shown that, for the instability to be suppressed, the external magnetic field \(B_{0z} \) should be stronger than the magnetic field B_0θ of the current flowing through the liner.     

Effect of the transverse magnetic field on turbulence and parameters of a plasma column in the L-2M stellarator

The influence of magnetic configurations with magnetic hills or wells on the parameters of a plasma column and turbulence characteristics were studied in experiments in which the plasma was created and heated by a microwave beam at the second harmonic of the electron cyclotron frequency. Calculations show that, for 〈β〉=(1.5?2)×10^?, a configuration with a magnetic well takes place and the Mercier criterion for stability of the ideal MHD modes is satisfied. It is shown that the compensation of the Shafranov shift of the plasma column by a transverse (vertical) field (B _v /B ₀ =5×10^?3) leads to a configuration with a magnetic hill in which the Mercier stability criterion is violated in the central region of the plasma column. It is experimentally shown that the stored plasma energy in the magnetic-hill configuration is reduced by one-half in comparison with the magnetic-well configuration. In the case of a magnetic hill, the energy of fluctuations increases both in the plasma core and near the separatrix, and the quasi-regular components of the wavelet spectra grow. When the Shafranov shift is compensated only partially (B _v/B ₀～3×10^?3) and the system is near the instability threshold, the stored plasma energy and the central electron temperature are somewhat higher, and the radiation power of fast electrons from non-Maxwellian tails at the second harmonic of the electron gyrofrequency decreases. It is found that the wavelet spectra of fluctuations change, the coherence coefficient for spectral components increases, and the radial electric field near the separatrix decreases.     

Magnetic field excitation during electron beam injection from the Intercosmos-25 satellite (APEX)

Results of active experiments on electron beam injection from the Intercosmos-25 satellite into the ionospheric plasma are presented. A quasistatic magnetic field and the VLF-wave magnetic component are excited when an unmodulated electron beam with a current of I _be ?0.1 A and energy of ? _be =mv ²/2?10 keV is injected into the ambient plasma. The magnetic field excitation is attributed to the onset of plasma gradient instabilities.     

Stability of localized MHD perturbations in confinement systems with closed magnetic field lines

Conditions are determined for the stability of a finite-pressure plasma against perturbations localized near a magnetic field line in a magnetic confinement system without average minimum-B. The marginal stability (ω²=0) is achieved at the pressure profile p∝U ^?5/3 (where $U = \oint {\frac{{dl}}{B}}$ ), provided that the pressure is lower than a certain critical value above which an unstable incompressible mode in which the displacement as a function of the coordinate along the field line has zeros appears at some magnetic field line.     

Dynamics of the plasma injected into the gap of a plasma opening switch across a strong magnetic field

A method is proposed to increase the linear charge density transferred through a plasma opening switch (POS) and, accordingly, reduce the POS diameter by enhancing the external magnetic field in the POS gap. Results are presented from experimental studies of the dynamics of the plasma injected into the POS gap across a strong magnetic field. The possibility of closing the POS gap by the plasma injected across an external magnetic field of up to 60 kG is demonstrated.     

Polarization Orientation,Piezoelectricity, and Energy Harvesting Performance of Ferroelectric PVDF‐TrFE Nanotubes Synthesized by Nanoconfinement

1D nanostructures of soft ferroelectric materials exert promising potential in the fields of energy harvesting and flexible and printed nanoelectronics. Here, improved piezoelectric properties, energy‐harvesting performance, lower coercive fields, and the polarization orientation of poly(vinylidene fluoride–trifluoroethylene) (PVDF‐TrFE) nanotubes synthesized with nanoconfinement effect are reported. X‐ray diffraction (XRD) patterns of the nanotubes show the peak corresponding to the planes of (110)/(200), which is a signature of ferroelectric beta phase formation. Piezoforce spectroscopy measurements on the free‐standing horizontal nanotubes bundles reveal that the effective polarization direction is oriented at an inclination to the long axis of the nanotubes. The nanotubes exhibit a coercive field of 18.6 MV m^?1 along the long axis and 40 MV m^?1 (13.2 MV m^?1 considering the air gap) in a direction perpendicular to the long axis, which is lower than the film counterpart of 50 MV m^?1. The poled 200 nm nanotubes, with 40% reduction in poling field, give larger piezoelectric d₃₃ coefficient values of 44 pm V^?1, compared to poled films (≈20 pm V^?1). The ferroelectric nanotubes deliver superior energy harvesting performance with an output voltage of ≈4.8 V and power of 2.2 μW cm^?2, under a dynamic compression pressure of 0.075 MPa at 1 Hz.     

Formation of a strong electric field resulting in the excitation of microplasma discharges at the edge of a dielectric film on a metal in a plasma flow

Results are presented from experimental and analytical studies of the processes resulting in the excitation of microplasma discharges (MPDs) on a metal surface partially covered with a thin dielectric film under the action of an external plasma flow in vacuum. It is shown experimentally that MPDs are excited at the interface between the open metal surface and the region covered by the dielectric film. The probability of MPD excitation is investigated as a function of the thickness of the dielectric film deposited on the metal. It is found that, for a film thickness of 1 μm, the probability of MPD excitation is close to unity. As the film thickness decreases below ~10 nm or increases above ~10 μm, the probability of MPD excitation is reduced by more than two orders of magnitude. A two-dimensional kinetic numerical code is developed that allows one to model the processes of Debye sheath formation and generation of a strong electric field near the edge of a finite-thickness dielectric film on a metal surface in a plasma flow for different configurations of the film edge. It is shown that the maximum value of the tangential component of the electric field is reached at the film edge and amounts to E _max ≈ |φ₀|/2d (where φ₀ < 0 is the electric potential applied to the metal and d is the film thickness), which for typical conditions of experiments on the excitation of MPDs on metal surfaces (φ₀ ≈–400 V, d ≈ 1 μm) yields E _max ≈ 2 MV/cm. The results of kinetic simulations confirm the qualitative idea about the mechanism of the formation of a strong electric field resulting in the excitation of MPDs at the edge of a dielectric film on a metal surface in a plasma flow and agree with experimental data.     

Experimental Study of Heating of a Liquid Cathode and Transfer of Its Components into the Gas Phase under the Action of a DC Discharge

An atmospheric-pressure dc discharge in air (i = 10–50 mA) with metal and liquid electrolyte electrodes was studied experimentally. An aqueous solution of sodium chloride (0.5 mol/L) was used as the cathode or anode. The electric field strength in the plasma and the cathode (anode) voltage drops were obtained from the measured dependences of the discharge voltage on the electrode gap length. The gas temperature was deduced from the spectral distribution of nitrogen emission in the band N₂(C³Π _u → B³Π _g , 0–2). The time dependences of the temperatures of the liquid electrolyte electrodes during the discharge and in its afterglow, as well as the evaporation rate of the solution, were determined experimentally. The contributions of ion bombardment and heat flux from the plasma to the heating of the liquid electrode and transfer of solvent (water) into the gas phase are discussed using the experimental data obtained.     

Electrostatic electron vortex structure in a plasma in an external magnetic field

A previously developed method for describing vortex structures is used to construct electrostatic vortices in a plasma in an external magnetic field. An equation for the radial electric field that gives rise to azimuthal electron drift in crossed electric (E _r ) and magnetic (B _z ) fields is derived without allowance for the magnetic field of the electron currents. Two types of the resulting electrostatic vortex structures with a positive and a negative electric potential at the axis are analyzed. The results obtained are compared with experimental data on vortex structures.     

Kelvin-Helmholtz instability of a cylindrical plasma flow with an arbitrary temperature

The dispersion relation for Kelvin-Helmholtz magnetohydrodynamic instability of a cylindrical plasma flow in a longitudinal magnetic field is studied with allowance for plasma compressibility. Stability of the system in a wide range of plasma parameters is thoroughly analyzed in the incompressible plasma approximation. Using the results obtained, a diagram of the system stability is constructed in terms of the magnetic field and the ratio between the plasma densities in the flow and the ambient space. It is shown by numerically solving the dispersion relation for the case of a compressible plasma that perturbations with scale lengths on the order of the flow diameter and larger can develop even at a zero temperature. For low ion-sound velocities, c _S ²/U ₀² < 0.25, the growth rate of the axisymmetric mode with m = 0 is much smaller than that of non-axisymmetric modes. It is shown that, in an incompressible plasma, the eigenmodes are damped monotonically with distance from the flow. In plasma with a finite temperature, the character of damping is oscillatory; in this case, the lower the plasma temperature, the larger the distance at which the ambient plasma is perturbed.     

Relaxation of plasma rotation in toroidal magnetic confinement systems

A study is made of the relaxation of plasma rotation in nonaxisymmetric toroidal magnetic confinement systems, such as stellarators and rippled tokamaks. In this way, a solution to the drift kinetic equation is obtained that explicitly takes into account the time dependence of the distribution function, and expressions for the diffusive particle fluxes and longitudinal viscosity are derived that make it possible to write a closed set of equations describing the time evolution of the ambipolar electric field E and the longitudinal (with respect to the magnetic field) plasma velocity U₀. Solutions found to the set of evolutionary equations imply that the relaxation of these two parameters to their steady-state values occurs in the form of damped oscillations whose frequency is about 2v_T/R (where v_T is the ion thermal velocity and R is the major plasma radius) and whose damping rate depends on the ion-ion collision frequency and on the magnetic field parameters. In particular, it is shown that, for tokamaks with a slightly rippled longitudinal magnetic field, the frequency of oscillations in the range q>2 (where q is the safety factor) is, as a rule, much higher than the damping rate. For stellarators, this turns out to be true only of the central plasma region, where the helical ripple amplitude ? of the magnetic field is much smaller than the toroidal ripple amplitude δ=r/R.