Anode-cathode Gap Research Articles

Spoke-Type Structures in an Ion Diode with Magnetic Insulation of Electrons

The article presents the results concerning the cross-sectional energy density distribution of a pulsed ion beam for two types of diodes with electron open drift: with external magnetic insulation (250 kV, 80 ns, 0.6 T) and with self-magnetic insulation of electrons (250–300 kV, 120 ns, 0.8 T). Anode plasma is formed using a breakdown along the surface of the anode dielectric coating (single-pulse mode) or explosive electron emission (with double opposite-polarity pulses). It was found that, when the energy density of the ion beam exceeds ≈0.4 J/cm2, periodic spoke-type structures with a step of 3–6 cm in the beam cross section are formed. The processes of formation of such a structure—nonuniform density of anode plasma and self-organization of anode and/or cathode plasma in crossed electric and magnetic fields—are analyzed. It is shown that the formation of local plasma regions in the anode–cathode gap of an ion diode can cause the formation of a periodic structure of the cross-sectional energy density distribution.

Hysteresis between gas breakdown and plasma discharge

In direct-current (DC) discharge, it is well known that hysteresis is observed between the Townsend (gas breakdown) and glow regimes. Forward and backward voltage sweep is performed using a one-dimensional particle-in-cell Monte Carlo collision (PIC-MCC) model considering a ballast resistor. When increasing the applied voltage after reaching the breakdown voltage (Vb), transition from Townsend to glow discharges is observed. When decreasing the applied voltage from the glow regime, the discharge voltage (Vd) between the anode–cathode gap can be smaller than the breakdown voltage, resulting in a hysteresis, which is consistent with experimental observations. Next, the PIC-MCC model is used to investigate the self-sustaining voltage (Vs) in the presence of finite initial plasma densities between the anode and cathode gap. It is observed that the self-sustaining voltage coincides with the discharge voltage obtained from the backward voltage sweep. In addition, the self-sustaining voltage decreases with increased initial plasma density and saturates above a certain initial plasma density, which indicates a change in plasma resistivity. The decrease in self-sustaining voltage is associated with the electron heat loss at the anode for the low pd (rarefied) regime. In the high pd (collisional) regime, the ion energy loss toward the cathode due to the cathode fall and the inelastic collision loss of electrons in the bulk discharge balance out. Finally, it is demonstrated that the self-sustaining voltage collapses to a singular value, despite the presence of a initial plasma, for microgaps when field emission is dominant, which is also consistent with experimental observations.

Time-of-flight technique for estimation of the energy of runaway electron bunches formed in magnetized gas diodes

Time-of-flight estimates of the kinetic energy of magnetically insulated bunches of moderately relativistic runaway electrons (RAEs) were obtained with the use of an air diode equipped with a drift section. The measurements have demonstrated that the bunch energy can be controlled by changing the conditions for RAE emission in an air diode with a sharply inhomogeneous electric field, which can be done by varying the cathode geometry and material and the length of the cathode-anode gap. It has been demonstrated that if the measured electron current peaks are resolved to within 10 ps, and the bunch duration and spread in RAE emission moments are close to this resolution, the characteristic energies of the bunches can be estimated with an accuracy of ≈10%. Examples of energy measurements are given for paraxial and tubular bunches accelerated at cathode potentials of up to −250 kV. The estimated characteristic energies of the bunches, even not corrected for drift losses in the fill gas, turned out to be greater than the energies that could be achieved for bunches accelerated in a vacuum diode with due account for the variations in diode voltage.

On the two-dimensional Brillouin flow

The Brillouin flow is a rectilinear, sheared electron fluid flow in a crossed electric field (E) and magnetic field (B), in the E × B direction with zero flow velocity and zero electric field at the surface with which the flow is in contact. It is broadly considered as the equilibrium electron flow in high power crossed-field devices including the magnetron and magnetically insulated transmission line oscillator. This paper provides an examination of Brillouin flow in two dimensions, in a cylindrical geometry where the anode radius changes abruptly at a single axial location, while the cathode surface has a constant radius. Our simulation confirms the proof that there is no equilibrium Brillouin flow solution for such a geometry. It further reveals that this change in the anode radius introduces novel bunching of the electrons within the Brillouin hub. This bunching occurs at low frequencies and is very pronounced if the Brillouin flow is from the small gap region to the large gap region, but is minimal if the Brillouin flow is from the large gap region to the small gap region. New insights are provided into the physical processes that initiate and sustain the bunching processes that are unique for a crossed-field diode, as compared with a non-magnetized diode. We argue that this enhanced bunching, and its concomitant formation of strong vortices, is not restricted to an abrupt change in the anode–cathode gap spacing.

Performance improvement studies of axially partitioned dual band magnetically insulated line oscillator

In magnetically insulated line oscillator (MILO)s, a proportion of device current is responsible for the self-generated magnetic field that aids electron flow confinement in the anode cathode gap, thereby suitable RF generation. The device currents should not be high due to its direct impact on device efficiency, and electron energy depositions at load can lead to anode plasma formation. Also, the lower currents can facilitate the device to operate at larger voltages and/or for longer operational times. In this article, couple of axially partitioned dual band MILOs (DBMILO) designs that operate at lower device currents in comparison with the existing axially partitioned DBMILOs in the literature is presented. For one design with the radius of the slow wave structure (SWS), i.e., RSWS = 40 mm, the injection of the DC electron beam parameters 505 kV and 49 kA, generated an output RF power of ∼5.8 GW oscillating at 3.71 and 10.16 GHz (S-and X- bands) concurrently with ∼23.8% conversion efficiency. However, the other design (RSWS = 40.6 mm) at beam voltage 505 kV and device current of 47 kA generated an output RF power of >3.85 GW oscillating at 3.5 and 10.5 GHz, concurrently, where the frequencies are harmonically related. Thus, the proposed designs yielded significant device current reduction to ∼47 and ∼49 kA from ∼56 kA (existed), respectively, thereby drop of 16% and 12.5% to its counter designs, respectively. Also, there is substantial reduction in the electron energy depositions, thus the occurrence of anode plasma formations. Moreover, the peak conversion efficiency is enhanced to 23.4% and 16.4%, from 12.8%.

Modeling and optimization of a relativistic magnetron with transparent cathode and output mode

Abstract This paper outlines the results of particle-in-cell simulations of a relativistic magnetron with six cavities and a transparent cathode configuration. Excitation of the π mode in the interaction region was attained, which in turn led to $\textrm{TE}_{11}$ mode emission of microwaves to the waveguide. This mode transformation was achieved with a non-symmetric diffraction output, consisting of four large and two small tapered cavities. Simulations were performed with a voltage across the anode-cathode gap varying from 164 to 356 kV, and axial magnetic field strengths between 0.24 and 0.34 T. Maximum efficiency of 37% was obtained with a peak output power of 590 MW, having a voltage of 261 kV and a magnetic field of 0.30 T. Furthermore, a frequency of 2.57 GHz and a rise time of microwaves at the waveguide of 15 ns were demonstrated. The electron leakage current was shown to decrease from ∼10 $\%$ to less than $1\%$ when employing a longer interaction region, while still exhibiting good performance. Additionally, we show that there is an optimal range of voltages given a magnetic field, for which π mode excitation with high efficiency is attained.

Backward fast electrons supported by ionization wave passing through the grid cathode

In this paper, the effect of appearance of fast electrons behind the grid cathode in the direction reverse to the anode is studied computationally. Fast electrons are observed within 0.5 ns after application of a short voltage pulse. The results obtained confirm the possibility of generation of backward fast electrons (some of them are in a runaway mode) and explain the main trends of this process. It is shown that backward fast electrons are supported by the ionization wave (IW). The IW evolution proceeds via two phases. During the first phase, first fast electrons are observed moving toward the anode. Then, multiple individual IWs starting from each individual cathode wire are formed in the anode–cathode gap. The duration of this stage is 0.3 ns and corresponds to the pulse rise time. At the second phase, the separate individual IWs merge in a single flat ionization wave. The IW penetrates through the cathode wires and propagates in the direction reverse to the anode. The preferred direction of fast electrons propagation also reverses. Now, the trajectories of fast electrons are mainly directed away from the anode. The duration of the recorded flux of fast electrons is of the order of a few picoseconds. This time interval correlates with the available experimental data.

Power flow in magnetically insulated transmission lines with ion backscatter effects

Ion backscattering off of surfaces in magnetically insulated transmission lines (MITLs) is often ignored in kinetic simulations of MITL power flow. Backscattering reduces ion current losses and the surface impact heating, which dictates the rate at which surface-adsorbed contaminants are liberated into the anode–cathode gap. Backscatter probabilities are difficult to implement in a kinetic code because there are limited data for incident ion energies less than a few keV. This paper presents an analytic model based on the Rutherford scattering formula that reproduces the measured backscatter probabilities at high incident energies and transitions to the highly reflective behavior expected at low incident energies. The backscatter model is implemented in power flow simulations, which are validated with current loss experiments conducted on the 0.4 TW Mykonos accelerator at Sandia National Laboratories. This simulation setup is then used in a high-current Z-machine shot. Backscatter effects are found to be unimportant in the low-current Mykonos regime but significantly reduce current losses at the Z-machine scale.

Acceleration of electrons in a combined magnetic trap in synchronous mode

The electron energy distribution function (EEDF) and the spatial profile of the electron density in the cathode–anode gap in a helium discharge are calculated within a one-dimensional model by the Monte Carlo method. Numerical studies are performed for experimental conditions known from the literature in a discharge with a hollow cathode: the cathode–anode distance of 3 cm, the helium pressure of 0.75 Torr, and the electric field strength in the discharge gap of 1.3 V/cm. The calculations are performed without and with allowance for the anode potential drop and the effect of electron reflection from the anode. The dependence of the form of EEDF on the energy spectrum of the electron source used in the calculations is also studied. In all variants of calculations, the main feature of the EEDF is retained, that is, a significant depletion of the low-energy part of the distribution function due to the effect of electron absorption by the anode. The calculated EEDF and the spatial profile of the electron density are compared with the available experimental data.

Study of the Effect of the Anode on EEDF and the Spatial Profile of the Electron Density in a Discharge with a Hollow Cathode in Helium

The electron energy distribution function (EEDF) and the spatial profile of the electron density in the cathode–anode gap in a helium discharge are calculated within a one-dimensional model by the Monte Carlo method. Numerical studies are performed for experimental conditions known from the literature in a discharge with a hollow cathode: the cathode–anode distance of 3 cm, the helium pressure of 0.75 Torr, and the electric field strength in the discharge gap of 1.3 V/cm. The calculations are performed without and with allowance for the anode potential drop and the effect of electron reflection from the anode. The dependence of the form of EEDF on the energy spectrum of the electron source used in the calculations is also studied. In all variants of calculations, the main feature of the EEDF is retained, that is, a significant depletion of the low-energy part of the distribution function due to the effect of electron absorption by the anode. The calculated EEDF and the spatial profile of the electron density are compared with the available experimental data.

Revealing the plasma confinement behavior of an axial ring cusp hybrid discharge in a miniature ion thruster using PIC/MCC simulation

Micro direct current (DC) ion thrusters have broad application prospects as propulsion systems for micro spacecrafts due to their advantages of high discharge reliability and efficiency. Experiments in the literature show that the plasma discharge under the axial ring cusp hybrid (ARCH) magnetic field has higher discharge efficiency in the discharge chamber of micro DC ion thruster. In this paper, a 2d-3v axisymmetric particle-in-cell/Monte Carlo collision numerical model is developed for the ARCH discharge. This model takes the thermal electron emission including the Schottky effect, various collision processes including the Bohm-type anomalous conductivity and the uneven background gas density distribution in the cathode-anode gap into account. The spatial distributions of plasma characteristics are presented and the advantages of ARCH discharge compared with traditional 3-ring discharge in the discharge chamber of the micro DC ion thruster are analyzed. The longer electron path length and the change of ionization region improve the discharge efficiency in small-scale discharges. Two primary methods for the discharge confinement in the miniature ion thrusters, that is, the magneto-static ‘cusp confinement’ through magnetic cusps and the electrostatic ‘sheath refection confinement’ through the backplate with the lower potential are investigated. The sensitivity of macroscopic current characteristics and microscopic plasma characteristics in the ARCH discharge to the magnetic field strength and backplate biased potential are explored. It is found that there is an optimal magnetic field to maximize the utilization of propellant and minimize the discharge loss. The electrostatic ‘sheath refection confinement’ is conducive to the reduction of discharge loss, however, it is also accompanied by the decline of propellant utilization. The above results provide further support for the design optimization of the micro DC ion thruster discharge chamber.

Physical Factors Governing the Shape of the Miram Curve Knee in Thermionic Emission

In a current density versus temperature (J-T) (Miram) curve in thermionic electron emission, experimental measurements demonstrate there is a smooth transition between the exponential region and the saturated emission regions, which is sometimes referred to as the "roll-off" or "Miram curve knee". The shape of the Miram curve knee is an important figure of merit for thermionic vacuum cathodes. Specifically, cathodes with a sharp Miram curve knee at low temperature with a flat saturated emission current are typically preferred. Our previous work on modeling nonuniform thermionic emission revealed that the space charge effect and patch field effect are key pieces of physics which impact the shape of the Miram curve knee. This work provides a more complete understanding of the physical factors connecting these physical effects and their relative impact on the shape of the knee, including the smoothness, the temperature, and the flatness of the saturated emission current density. For our analyses, we use a periodic, equal-width striped ("zebra crossing") work function distribution as a model system and illustrate how the space charge and patch field effects restrict the emission current density near the Miram curve knee. The results indicate there are three main physical parameters which significantly impact the shape of the Miram curve. Such physical knowledge directly connects the patch size, work function values, anode-cathode voltage, and anode-cathode gap distance to the shape of the Miram curve, providing new understanding and a guide to the design of thermionic cathodes used as electron sources in vacuum electronic devices (VEDs).

Impact of power flow on Z-pinch loads

Magnetically insulated transmission lines (MITLs) are used to deliver tens of MA to a Z-pinch load. The MITLs suffer current losses due to contaminant plasma located in the anode–cathode gap which is swept toward the load along the power flow. The swept up contaminant plasma can deposit mass and energy onto the load resulting in deformations or the seeding of macroscopic instabilities. This paper discusses 2D fully kinetic simulations of the contaminant plasma evolution which predict the current losses and the flux of mass and energy onto the load. The effects of a dynamic, i.e., imploding, load are shown to increase both the current loss and the mass and energy flux. The MITL used is a conical, radially converging design which is a feature common to MA-scale Z-pinch accelerators.

Improving the electrostatic design of the Jefferson Lab 300 kV DC photogun.

The 300kV DC high voltage photogun at Jefferson Lab was redesigned to deliver electron beams with a much higher bunch charge and improved beam properties. The original design provided only a modest longitudinal electric field (Ez) at the photocathode, which limited the achievable extracted bunch charge. To reach the bunch charge goal of approximately few nC with 75ps full-width at half-maximum Gaussian laser pulse width, the existing DC high voltage photogun electrodes and anode-cathode gap were modified to increase Ez at the photocathode. In addition, the anode aperture was spatially shifted with respect to the beamline longitudinal axis to minimize the beam deflection introduced by the non-symmetric nature of the inverted insulator photogun design. We present the electrostatic design of the original photogun and the modified photogun and beam dynamics simulations that predict vastly improved performance. We also quantify the impact of the photocathode recess on beam quality, where recess describes the actual location of the photocathode inside the photogun cathode electrode relative to the intended location. A photocathode unintentionally recessed/misplaced by sub-millimeter distance can significantly impact the downstream beam size.

Investigation of the switching characteristics of high-pressure subnanosecond gas dischargers with the purpose of a sharp increasing of the breakdown voltages and the switching speed

The paper analyzes the data obtained in the subnanosecond time range on the times (t br) and speeds (V br) of switching of hydrogen diode dischargers. These data were obtained in a wide range of hydrogen pressures (p) and the degree of the discharge gap overvoltage (the length of the cathode–anode gap d) in a uniform electric field. It is shown that the reduced strength of the average electric field E br/p in the discharge gap at the moment of the beginning of the breakdown significantly decreases when the gas pressure increases from 5 atm to 50 atm. An increase in pressure from 50 atm to 60 atm leads to a sharp (by 40% ÷ 135%, depending on the d) increase in the pulse breakdown voltage (U br) and an increase in E br/p. In proportion to the growth of E br/p the switching speed V br of the discharge gas gap increases. The observed effect is explained by the change in the discharge initiation mechanisms. The limitation of U br and V br in the hydrogen pressure range from 5 atm to 50 atm occurs as a result of gas ionization by runaway electrons and the subsequent development of a multi-avalanche discharge in the volume of the discharge gap. With a further increase in pressure, the discharge develops according to the streamer type. To design ultrafast gas dischargers of the subnanosecond range intended for switching high voltages, it is necessary to select an appropriate range of working gas pressures in order to ensure the development of a streamer-type discharge.

Radial oscillation of intense relativistic electron beam in low-magnetic-field foil-less diode

The radial oscillation of an intense relativistic electron beam possesses two main features of the spatial period and the radial oscillation amplitude in a low-magnetic-field foil-less diode, and the large radial oscillation extremely limits the beam–wave conversion efficiency and stability of a high-power microwave device. Thus, the formation mechanism of the radial oscillation is analyzed in detail. The results show that the radial oscillation of an electron beam consists of a great number of electrons with different Larmor radii and guiding centers, and the large radial oscillation is mainly caused by the strong radial electric field and the directional difference between the electric field and the magnetic field in the anode–cathode gap. A low diode voltage or a proper large anode radius is beneficial to improve the beam quality. Considering that cathode plasmas have a dominant effect on the spatial distribution of electrons, the explosive emission model was improved with cathode plasmas, and the consistency between simulation and experimental results becomes better.

Effect of non-uniform magnetic field on radial oscillation of electron beam in a low-magnetic-field foilless diode

A theory regarding a non-uniform magnetic field with a parallel gradient is presented. The research results show that a proper non-uniform magnetic field can greatly reduce the transverse momentum of an electron beam and even eliminate its gyration motion, and it depends on the gradient of the magnetic field and the phases of electrons entering and leaving the local magnetic field region. Thus, a magnetic field that decreases along the axial direction is proposed to suppress the radial oscillation of the electron beam. However, in the drift tube, the suppression of the radial oscillation is not obvious, because the large phase differences among electrons lead to a mismatch between the electron beam and the non-uniform magnetic field. Further studies found that the non-uniform magnetic field applied in the anode-cathode gap can not only reduce the phase differences among electrons, but also effectively transform the transverse momentum of the electron beam into its axial momentum. The results obtained by PIC simulations and experiments consistently confirm that the non-uniform magnetic field can significantly suppress the radial oscillation of the electron beam in a low-magnetic-field foilless diode.

Characterization of self-magnetic pinch (SMP) radiographic diode performance on RITS-6 at Sandia National Laboratories. I. Diode dynamics, DC heating to extend radiation pulse

Radiographic diodes focus on an intense electron beam to a small spot size to minimize the source area of energetic photons for radiographic interrogation. The self-magnetic pinch (SMP) diode has been developed as such a source and operated as a load for the six-cavity radiographic integrated test stand (RITS-6) inductive voltage adder driver. While experiments support the generally accepted conclusion that a 1:1 aspect diode (cathode diameter equals anode–cathode gap) delivers optimum SMP performance, such experiments also show that reducing the cathode diameter, while reducing spot size, also results in reduced radiation dose, by as much as 50%, and degraded shot reproducibility. Analysis of the effective electron impingement angle on the anode converter with time made possible by a newly developed dose-rate array diagnostic indicates that fast-developing oscillations of the angle are correlated with early termination of the radiation pulse on many of the smaller-diameter SMP shots. This behavior as a function of relative cathode size persists through experiments with output voltages and currents up to 11.5 MV and 225 kA, respectively, and with spot sizes below approximately few millimeters. Since simulations to date have not predicted such oscillatory behavior, considerable discussion of the angle behavior of SMP shots is made to lend credence to the inference. There is clear anecdotal evidence that DC heating of the SMP diode region leads to stabilization of this oscillatory behavior. This is the first of two papers on the performance of the SMP diode on the RITS-6 accelerator.

Влияние анодной и катодной плазмы на работу электронного диода со взрывоэмиссионным катодом

The results of modeling and experimental investigation of the formation of anode and cathode plasmas in a vacuum diode with an explosive-emission cathode during the generation of a pulsed electron beam with a current density of 0.3-0.4 kA/cm^2 and an accelerating voltage of 300-500 kV are presented. It is shown that the concentration of the anode plasma does not exceed 10^10 cm^-3 and it does not significantly contribute to the operation of the diode. However, the complete desorption of molecules from the working surface of the explosive-emission cathode and the high efficiency of shock ionization of atoms ensure the formation of a cathode gas plasma with a concentration of 10^16 cm^-3. It is found that the charge of the explosive-emission plasma layer is significantly less than the charge of the electron beam and the main source of electrons is not an explosive-emission plasma, but a cathode gas plasma. In this case, the electron current is limited by the concentration of the cathode plasma. The use of a cathode with a developed surface (a cathode with a carbon fabric coating) allows increasing the total charge of the electron beam by more than 1.5 times without changing the cathode diameter and the anode-cathode gap.

Influence of anode and cathode plasmas on the operation of an electronic diode with an explosive-emission cathode

The results of modeling and experimental investigation of the formation of anode and cathode plasmas in a vacuum diode with an explosive-emission cathode during the generation of a pulsed electron beam with a current density of 0.3-0.4 kA/cm2 and an accelerating voltage of 300-500 kV are presented. It is shown that the concentration of the anode plasma does not exceed 1010 cm-3 and it does not significantly contribute to the operation of the diode. However, the complete desorption of molecules from the working surface of the explosive-emission cathode and the high efficiency of shock ionization of atoms ensure the formation of a cathode gas plasma with a concentration of ~1016 cm-3. It is found that the charge of the explosive-emission plasma layer is significantly less than the charge of the electron beam and the main source of electrons is not an explosive-emission plasma, but a cathode gas plasma. In this case, the electron current is limited by the concentration of the cathode plasma. The use of a cathode with a developed surface (a cathode with a carbon fabric coating) allows increasing the total charge of the electron beam by more than 1.5 times without changing the cathode diameter and the anode-cathode gap. Keywords: high-current electron beam, explosive emission, plasma concentration, electronically stimulated desorption.