Plasma-filled Diode Research Articles

Suppression of High-Frequency Electron Beam Modulation in a Plasma-Filled Diode

The article presents the results of recent experimental studies of spontaneous electron beam modulation with a frequency of several tens of megahertz in a plasma-filled diode. The diode comprises a plasma cathode with a grid-stabilized emission boundary and a plasma anode with an open (conditionally electrodeless) movable boundary. The cathode plasma is formed by a cathodic arc and the anode plasma by toroidal plasma generators based on an anode-layer closed drift thruster. According to our previous studies, the beam current starts to modulate as the arc current goes above 50–55 A at an accelerating voltage of 12–22 kV. Here, we analyze how such electron beam modulation and its frequency are influenced by the grid geometry at a current of about 100–110 A and what methods can be used for its removal. The idea of removal consists in decreasing the effect of the accelerating voltage on the cathode plasma, for example, via a negative current feedback. It is shown that the most efficient modulation suppression is attained when the negative feedback is frequency-dependent and that the modulation frequency decreases to its complete disappearance as the grid mesh size is decreased.

Production of low-energy, high-current electron beams in a gun with a controlled explosive emission cathode

The operating characteristics of an electron gun with a controlled explosive emission cathode have been investigated. The multichannel initiation of explosive electron emission from the (copper disk) cathode occurs due to the use of simultaneously operating resistively uncoupled arc plasma sources built in the cathode. Three gun operation options were tested, namely with a vacuum, a gas-filled, and a plasma-filled diode. In the last case, a plasma anode was formed using a high-current reflective (Penning) discharge or a hybrid discharge combining a Penning discharge with vacuum arcs. The cathode unit has shown a high emissive power (a factor of 1.5–2 higher than that of a conventional gun with a plasma anode and a multiwire copper explosive emission cathode). Stable operation of the gun was observed at an accelerating voltage of amplitude 5–30 kV, in contrast to at least 17–20 kV required for stable operation of the conventional gun. The gun operation with a vacuum and a gas-filled diode provided a considerably more efficient energy transfer from the power supply to the electron beam.

OPERATIONAL CHARACTERISTICS OF THE ELECTRON ACCELERATOR BASED ON THE PLASMA-FILLED DIODE

The paper is concerned with the plasma-filled diode performance in the intensive mode regulated by means of external gas puffing. The possibility to smoothly vary the plasma parameters in the discharge gap zone, and thus, to optimize the main diode characteristics (Ucutoff, Icutoff) by the external gas puffing method has been confirmed by experiment. The introduction of additional quantity of neutral gas into the discharge causes the change in the plasma density balance due to elementary processes in physics of electronic and atomic collisions, such as ionization, dissociation, recombination. The deviation of actual voltage/current values from their maximum values can be attributed to the mismatch in the generator-load feed circuit.

Plasma Grid Cathodes Based on a Constricted Arc Discharge for Generating a Pulsed Intense Low-Energy Electron Beam in a Plasma-Filled Diode with a Longitudinal Magnetic Field

Designs are presented for plasma grid cathodes based on an arc contracted discharge. They are designed to work under conditions of an inhomogeneous magnetic field as high as 35 mT and penetrate into their gas-discharge system from the region of electron beam transport. Parameters are presented of the electronic source used to modify the surfaces of materials and products using a pulsed electron beam in order to alter their operating properties.

Resistively Decoupled Explosive-Emission Cathodes for High-Current Plasma-Filled Diodes

The design and characteristics of resistively decoupled multi-emitter explosive-emission cathodes intended for high-current plasma-filled diodes are presented. The integral glow of the emission-center plasma, the energy-density distribution over the cross section of a nonrelativistic high-current electron beam, which is formed in the plasma-filled diode, and the operation lifetime of the cathode were studied. The best results were obtained for a cathode based on TVO-1 resistors, whose wire outputs serve as emitters.

Plasma-filled diode with a rod anode for repetitive pulsed X-ray sources

AbstractThe paper presents a cylindrical diode with a rod anode for repetitive pulsed intense X-ray sources. The diode uses a massive anode of diameter 1 cm, thus increasing the lifetime of the energy converter. The diode gap is preliminary filled with ions produced by cathode flare in a vacuum discharge, allowing electron beam pinching at a voltage of <200 kV. In the mode of pinching, the diode is an efficient source of high-power X rays. At a distance of 20 cm from the X-ray source, the radiation dose per pulse is 0.4 R at a peak power of 7 × 106 R/s. The full-width at half-maximum of the radiation pulse is 40 ns. The power in the diode is 1.8 GW at a beam current of 10 kA and exceeds the maximum power of the generator operating onto a matched load with constant resistance.

The K x-ray line structures of the 3d-transition metals in warm dense plasma

The shapes and positions of the Kα1 and Kα2 x-ray lines for 3d-transition metals can vary substantially as electrons are stripped from the outer-shells. This paper shows the detailed line shapes for nickel and zinc, obtained by calculations with a multiconfiguration Dirac–Fock method that includes Breit interaction and quantum electrodynamics corrections. The line shapes can be useful in interpreting hot, dense plasmas with energetic electrons for which the K x-ray lines are optically thin, as may be produced by pulsed power machines such as the plasma-filled rod pinch diode or the plasma focus, or in short-pulsed high power laser plasmas.

Analysis of propagation of a high-current electron beam from a sectioned plasma-filled diode

We report on the results of analysis of propagation of an electron beam from a plasma-filled diode in the absence of the metal anode between the regions of beam generation and transportation. The diode parameters are 160 kA, 400 kV, and 50 GW. At a distance exceeding 10 cm behind the generation region, a beam current of 100 kA to the target and an energy density of 20 J/cm2 are attained for the beam cross-sectional area of about 200 cm2. The possibility of varying the beam current and energy density by changing the distance to the target is demonstrated.

Power Increase of the Electron Source Based on the Plasma-Filled Diode

The technique of a linear transformer driver (LTD) now allows building the generators of high-power nanosecond pulses with the current rise time of $\sim 100$ ns without intermediate power compression stages. This technique is being examined for use in high-current high-voltage electron sources based on plasma-filled diodes. The power of a plasma-filled diode is determined by the driving circuit parameters and the transition diode resistance. The problem of power rise can be solved by increasing the stored energy in the circuit inductance provided that the diode resistance rise rate is constant. In experiments on the LTD (53 nF, 480 kV), the possibility has been checked out for such increase in the stored energy. A current increase from 50 to 180 kA was obtained by increasing the current rise rate from 0.4 to 1.9 kA/ns. Stored energy has been increased, respectively, from 0.2 to 3.6 kJ. The time of energy transfer into the circuit inductance was about 120–140 ns. Maintaining the diode resistance rise rate at 0.5 $\Omega $ /ns has been shown. Diode power has been increased up to 170 GW. Further increase in power of the plasma-filled diode has been obtained using simulation circuit of the Megavolt (MV) range LTD. To simulate such a linear transformer, the driver circuit was made as Marx generator with coaxial water line (5.3 $\Omega $ , 56 ns, and 1.5 MV), providing a current input of 160 kA into the 550-nH inductance in 120 ns. Stored energy in the inductance was about 7 kJ. The following diode parameters were obtained: 150 kA, 1.9 MV, and 250 GW at a resistance rise rate more than 0.5 $\Omega $ /ns. The experiments prove that the resistance rise rate of the plasma-filled diode is constant on increasing the stored energy in the inductance up to 7 kJ. It allows scaling the power of an electron source based on the plasma-filled diode.

Explosive-emission cathode with resistive decoupling in a high-current plasma-filled diode

In the paper, the results of investigations of operation of multi-emitter explosive- emission resistively decoupled cathode in a plasma-filled diode are presented. The integral glow, current rise rate, and energy density distribution via cross section of a non-relativistic, high-current electron beam have been studied. It has been shown, that resistively decoupled cathodes look very perspective for improvement of stability and uniformity of high-current electron beams.

Foil-less plasma-filled diode for HPM generator

Plasma-filled diode regarded as perspective source of electron beam feeding HPM generator of GW power level, comparing to conventional explosive emission vacuum diode. Electron beam generation occurs in plasma double layer, where plasma boundary plays as an anode. It allows cancelling the usage of anode foils or grids in HPM generators with the virtual cathode, which could limit its life time to few shots. The presence of ions in the e-beam drift space could raise the limiting current for a drift space, but it could affect to microwave generation also. Sectioned plasma-filled diode with beam current of about 100 kA, electron beam energy of about 0.5 MV and beam current density of 1-10 kA/cm2 was realized. Cylindrical transport channel with the diameter of 200 mm and the length of about 30 cm was attached to the diode. Beam current measurements in a drift space were performed. Computer simulations of electron beam transport with the presence of ions were carried out with the 2.5D axisymmetric version of PiC-code KARAT. Obtained results would help optimizing electrodynamic system of HPM generator subjected to the presence of ions.

Capillary plasma gun for a high-current plasma-filled diode

A capillary-type plasma gun designed for generating a plasma channel with a characteristic diameter of ≈10 mm in a high-current relativistic electron diode has been tested. The dynamics of ion flow propagation and the effect of the plasma source parameters on the current amplitude of the low-resistance phase of the plasma-filled diode have been studied. A low-resistance phase duration of ≈120 ns has been attained at a current of ≈185 kA in the diode based on a unit gun with a 0.4-mm-diameter capillary. The possibility of increasing the diode current by using several capillaries in a gun is demonstrated.

Analysis of parameters of an electron beam from a plasma-filled diode

We analyze the properties of a high-current electron beam formed in an electron source based on a plasma-filled diode and a linear pulsed transformer. The beam parameters are determined by measuring bremsstrahlung X-rays and the beam current, as well as the photographs of the diode gap in the optical range, of the anode in X-rays, and beam autographs. A beam with a current of ∼100 kA and a mean electron energy exceeding 0.7 MeV for an accelerating voltage amplitude of ∼1 MV is obtained. The diameter of the generated beam is ∼1 cm. The electron beam from the plasma-filled diode makes it possible to attain a high anode power density (>1010 W/cm2) for exciting shock waves, for obtaining high pressures, and for generating powerful X-rays.

On the current density in a high-current plasma-filled diode with an explosive emission cathode

Based on the analysis of the available data from experimental studies and numerical simulations of high-current plasma-filled diodes, it is shown that the fast (∼2 cm/μs) motion of the explosive emission cathode plasma does not allow the double layer in which the high-current electron beam forms to reach a steady (or quasi-steady) state. As a result, the double layer turns out to be substantially thinner than a steady-state one and the current density in it is substantially higher. The proposed hypothesis explains well the excess of the observed current density over its calculated (stationary) value.

预充等离子体密度对杆箍缩二极管特性的影响

预充等离子体密度对杆箍缩二极管特性的影响

Plasma-filled diode based on the coaxial gun

The paper presents the results of studies of a coaxial gun for a plasma-filled electron diode. Effects of the discharge channel diameter and gun current on characteristics of the plasma and pulse generated in the diode were investigated. The electron beam with maximum energy of ≥1 MeV at the current of ≈100 kA was obtained in the experiments with a plasma-filled diode. The energy of ≈5 kJ with the peak power of ≥100 GW dissipated in the diode.

Megavolt range voltage measurement in vacuum through a short-circuited line

Method of voltage measurement at a vacuum load by means of homogeneous short-circuited vacuum-isolated line was considered. Prior to appearance of a measured high-voltage pulse, a magnetic field is formed in the line due to the bias current. Biasing provides fulfillment of magnetic electron isolation conditions and strong pressing of an electron layer down to the cathode already at the voltage wave front. As a result, a weak change of the "hot" line wave impedance is achieved during a pulse. Theoretical consideration and numerical simulation of the measuring line operation in the presence of bias current basing the applicability of the method was carried out. The method was used to determine the plasma-filled diode voltage at a megavolt voltage level. The absence of electron leakages at the voltage wave propagation of the amplitude ≈1 MV along the measuring line of the length 2.3 m with the wave impedance of 136 Ω and initial bias current of ≈6 kA was realized.

Experiments with a plasma-filled rod-pinch diode on the MIG generator

Experimental data for creating an X-ray source several millimeters in size on the basis of a plasmafilled rod-pinch diode are reported. Experiments are carried out on the MIG high-current generator. A voltage pulse applied to the diode is sharpened and shortened by injecting plasma into the interelectrode gap up to the plasma-filled diode. Radiation extraction from the vacuum chamber in the paraxial direction is provided by inverting the positions of the cathode and anode using a post-hole vacuum convolute. It is demonstrated that the energy of a low-impedance high-current generator can be deposited with a high efficiency into an electron beam focused on the 1-mm2 tip of the rod-shaped anode. With a tapered tungsten rod 1.5 mm in diameter used as an anode, a pulsed X-ray source about 1 mm in size generating 30-ns-long pulses is obtained. The absorbed dose per pulse measured with a LiF thermoluminescent dosimeter behind a 5-mm-thick aluminum screen 1 m away from the source reaches 0.042 Gy.

Transportation of high-current electron beams in plasma and plasma-filled diodes

The results of a numerical simulation of the nonstationary processes in different charge-neutralization schemes of high-current electron beams and the results of a measurement of the beam parameters are presented. Most attention is paid to beams with a current higher than the limiting Alfven current.

Plasma-filled diode in the electron accelerator on base of a pulsed linear transformer

AbstractTechnique of a linear transformer allows now to build the generators of high power nanosecond pulses with the current rise time of ~100 ns without intermediate power compression stages. This technique is being examined for use in high current high voltage pulsed accelerators. Plasma-filled diode has several advantages over standard vacuum diode that allow to improve the accelerators parameters. In this paper, plasma-filled diode experiments are described on generation of e-beam in the linear transformer accelerator. Possibility of use in the diode of isolated parallel plasma channels has been proven for the e-beam generation with cross-sectional area up to 50 cm2. The beam with current of 100 kA at voltage more than 400 kV was generated in the plasma-filled diode. Energy transmission efficiency from primary storage into a beam is about 54%.