Electron emission from ferroelectric plasma cathodes

We have performed analytical studies on the radio frequency (rf) electromagnetic (EM) waves in a ferroelectric cathode with a grid electrode. During operation of the cathode, a trigger voltage induces a very strong field at the vacuum–metal–ferroelectric triple junction, which results in local saturation of the material’s dielectric property near the corresponding area. High frequency oscillations generated from the saturation then lead to the formation of standing waves. With the practical geometry and the material property used in the experiments, the oscillation frequency of the lowest mode (TM01) is found to overlap that of the domain oscillation of the ferroelectric material. This provides domains with the possibility of gaining energy from the EM wave through resonance to sustain their oscillation, which is considered to be related with electron emission. The rf pass band was studied by varying the geometry and the material properties of the cathode, and this may lead to optimization of the cathode shape.

Electron emission from ferroelectric cathodes is commonly suggested as an electron source for different application due to its special characteristics such as high current density, easy treatment and operation, etc. The technology of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) ferroelectric cathode is introduced in this paper. Stable emission current is obtained from the PMN-PT ferroelectric cathode and the characteristic of ferroelectric electron emission is deal with as well.

Recent results of laser induced electron emission from a ferroelectric cathode in the CLIC dc test setup

Electron emission from ferroelectric cathodes is observed using two kinds of Pb(Zr,Ti)O3 (PZT) material in the same cathode geometry. Beam currents are found to follow a three-halves power rule to the diode voltage, which suggests that the beam currents are “space-charge-limited”. However, the perveances of the electron guns differ by more than 100%. The difference in “space-charge-limited current” is found to be related to the difference in the frequency characteristics of the materials. This implies that domain oscillation may contribute to the strong electron emission, under our experimental conditions. The experimental results provide a key point, which may lead to a more complete understanding of electron emission from ferroelectric materials.

Emission from ferroelectric cathodes

Spontaneous electrical polarization of ferroelectric materials can be changed either by reversal or by phase transition from a ferroelectric into a nonpolar state or vice versa. If spontaneous polarization changes are induced at a submicrosecond time scale, strong uncompensated surface charge densities and related fields are generated, which may lead to the intense self-emission of electrons from the negatively charged free surface areas of the ferroelectric cathode. The nature of this self-emission differs essentially from other methods of ferroelectric electron emission and from conventional electron emission in that the latter methods are only achieved by extracting electrons with externally applied electric fields. When electron guns are constructed with ferroelectric cathodes, new design criteria have to be taken into account. The intensity, the energy, the temporal and spatial distribution and the repetition rate of the emitted electron beams can be adjusted within wide limits. The advantages of ferroelectric cathodes and the technological difficulties arising during their production, preparation, and operation are identified and discussed and solutions to the problems are proposed. Experiences with a few applications of ferroelectric electron emission are reported and suggestions for further applications are made.

Summary form only given. Ferroelectric plasma sources are known as sources of surface discharge plasma. Main advantage these sources is the ability to create layers of plasma on surfaces with large area and arbitrary shape. In vacuum, numerous discharges on the surface of ferroelectric ceramics appear in response of a driving pulse applied across a sample of ferroelectric ceramics. Electron avalanches can be initiated by field emission from triple junctions at the edges of patterned electrodes in electric fields, induced by inhomogeneities of the polarization charge. These avalanches along the surface are driven by secondary electron emission form the ferroelectric ceramics. Surface ablation and gas desorption supply neutrals, which are further ionized by avalanching electrons, resulting in a surface plasma. The depicted scenario is the same as for surface flashover; however, discharge on ferroelectrics does not turn to vacuum arc because of separation of electrodes by the ceramic bulk. This feature makes the discharge on ferroelectrics very interesting for study of the plasma formation process in the surface discharge. For instance, initial plasma density and electron temperature in the stem of the discharge can be deduced from the temporal profiles of the density in expanding plasma. For PZT ceramics, the initial plasma density was found in the range of (0.4-4)/spl times/10/sup 17/ cm/sup -3/ and the initial electron temperature of about 16-20 eV for the driving electric fields of 16-30 kV/cm. It is shown that the discharge on ferroelectrics can be realized also in a gas mode. Transition from vacuum surface discharge to the gas discharge begins at the background pressure of 1-20 Torr, and the surface discharge in this pressure range sustains the gas discharge. At the pressures between 20 and 80 Torr, formation of the discharge stems is completely suppressed, which indicates the range of neutral pressure in the stem of the surface discharge in vacuum. The gas discharge is uniform and it appears on the open surface of the ferroelectric ceramics. In Argon at a distance of 5.5 mm from the surface, the maximal plasma density of /spl sim/10/sup 13/ cm/sup -3/ was observed at pressure of /spl sim/8 Torr. Preliminary study of secondary electron emission from ferroelectrics shows that at low energies of primary electrons the total yield of electron-induced SEE from ferroelectrics is high comparing with other dielectrics and it depends on polarization of the ferroelectric material. Low value of energy which corresponds to the unity yield of SEE from ferroelectrics leads to higher desorption rate and consequently to lower threshold of the surface discharge on ferroelectrics.

Many experiments on microwave and millimeter wave tubes, which are based on ferroelectric (FE) cathodes include a magnetic field in the emission region. For such electron devices, an optimal magnetic field leads to overall performance enhancement. In this paper, the dependence of the electron emission from an FE cathode on the magnetic field is experimentally examined and presented. The electron current emitted from an annular FE electron gun, suitable for gyrotron tubes, was measured with different magnetic fields. It was found that adding magnetic field extends the emitted current until a maximal working point is reached. Magnetic field above this point reduces the emission. A ~20 % enhancement in the emitted current relative to the emission in the absence of a magnetic field is obtained. Another observation relates to the gap closure and breakdown effects. The influence of the magnetic field on these effects is also reported. A suitable working point for an electron tube is chosen according to these observations.

Summary form only given, as follows. Expellantless neutralizers can improve the propellant utilization of very low-power colloid thrusters, field emission electric propulsion (FEEP) devices, ion engines, Hall thrusters, and gridded vacuum arc thrusters. In addition, spacecraft plasma contactors and electrodynamic tether systems could reap expellant mass reduction benefits from electron emitters that require very low gas flow rates or no gas flow to perform their functions. Ferroelectric emission (FEE) cathodes operating at minimized power are currently being studied as zero expellant flow rate cathode sources. Typical cathode operation requires 0.4 to 1 kV to be rapidly pulsed across a ferroelectric ceramic, triggering electron emission through a gridded electrode on the ceramic surface. Ceramics with painted-on Ag electrodes have produced peak current densities of 10s of mA/cm/sup 2/ with pulse voltages near current emission threshold (approximately 400 to 600 V). Additionally, thin film electrode grids have been deposited on two PZT ceramics with relative dielectric constants of 1000 and 4100. The thin film grids are composed of molybdenum and are 10 and 100 nm thick, 1 mm wide, and separated into strips by 1 mm. A parametric study of these FEE cathodes examines the effects of ferroelectric dielectric constant, grid thickness, and pulse geometry for low power operation. Additionally, cathode surface modification due to operation is studied.

Strong pulsed electron emission has been observed from 12/65/35 lead lanthanum zirconate titanate ceramic composition in two different nonswitched phases at room temperature and at the temperature 100 °C. The electron emission parameters of this composition appear to be independent of phase for the two phases investigated. Fast photography and direct observation show that the strong electron emission occurs from the surface discharge plasma. The new experimental data make it possible to demonstrate the validity of the Child–Langmuir law for this electron emitter. A pulsed plasma lead lanthanum zirconate titanate ceramic cathode with burst frequency up to 100 kHz and collector current density up to 10 A/cm2 is developed.

In this paper the parameters of two types of high-current switches based on ferroelectric BaTiO3 ignition are presented. Both types of switches showed a reliable and controllable operation with a repetition rate of several Hz. The first type is a vacuum two-electrode switch ignited by the plasma which is generated by a BaTiCO3 cathode. This type of switch was tested in the voltage range of 3–25 kV and switched current amplitude of 2 –15 kA with either negative or positive polarity of the high-voltage electrode. The second type is a BaTi03 surface flashover strip-like switch ignited by a driving pulse which has an amplitude of several kV. It was shown that the application of the driving pulse (>10 kV) leads to the appearance of many non-complete surface discharges which transform further to a multi-channel discharge. This type of switch was tested in the voltage range of 1–25 kV and current amplitude of 0.5–15 kA. The design of the switches, their lifetime, the time jitter and the parameters of the switched current for different discharge conditions are presented.

The efficiency of ozonizer using silent discharge is very low as compared with the theoretical value. Therefore, many researches have been carried out in order to improve the efficiency of ozonizers. But, we cannot expect to improve the efficiency without changing the process of ozone generation.In this study, two high voltages are applied to the ozonizer which has three electrodes (the central, surface and outer electrodes) arranged coaxially, and the phase difference between applied voltages can be changed. The silent and surface discharges are superposed in the same space of the ozonizer.In this paper, the optimum experimental conditions to improve the efficiency of ozonizer using the discharge superposition method are discussed. As a result, the followings are obvious. When two kinds of discharges are superposed under the condition using the central electrode as a common-grounded electrode, the maximum ozone yield efficiency is obtained, especially at the phase difference of about 180°. However, under the condition using the surface electrode as a common-grounded electrode, the ozone efficiency is higher at the phase difference of about 90° or 270° than at the other angles.

It is widely known that electron emission is one of key initiation factors of vacuum breakdown. The electron emission can be influenced by many factors, such as the geometry of the emitter, the material properties of the emitter, the electric field distribution of the emitter, and so on. The purpose of this paper is to investigate the electron emission characteristics of tungsten, chromium and aluminum micro-protrusions under different electric fields by fractal theory modeling. Firstly, the micro- protrusions on the electrode surface are modeled based on Weirstrass-Mandelbrot (W-M) fractal function. Then, the amended thermal-field emission equations are used to calculate the emission current density. Finally, considering the Nottingham effect and resistance heating, the variation of temperature distribution and the electron emission current density under different given electric fields are calculated iteratively. The results of this paper may provide some useful information to study the electron emission characteristics and clarify the mechanism of cathode-initiated vacuum breakdown.

A pinched-beam diode consists of a thin annular cathode which emits electrons that are accelerated toward a planar anode. Once the energy deposited by the electrons on the anode is sufficiently large, a plasma forms on the anode and the electrons are strongly pinched toward the axis of the diode. An intense ion beam with a current of 100’s kA is also produced. Previous simulations of the pinched-beam diode focused on the electron dynamics. [1] In this paper, we focus on the properties of the ion beam formed in the diode. Specifically, we focus on the electric and magnetic field distributions in the diode and how these charge distributions influence the uniformity of the ion beam. There are a number of factors that can affect the overall quality of the ion beam in a pinched beam diode including spotty emission from the anode and the distribution of ion and electron charge in the diode gap. In this paper, we will present results from simulations that neglect spotty emission and focus on the role that the electron and ion charge distribution in the diode play on the overall ion beam quality. The simulations show the formation of “hot spots” in the electron flow that attract the ion flow. This causes non-uniformity in the ion flow as it passes through the electron flow. As the ion beam exits the diode region it picks up electrons either from the electron flow in the diode itself or from field emitted electrons that come from conducting surfaces located behind the cathode tips. The geometry behind the cathode tips influences how much of the neutralizing electron flow is pulled from the high-energy diode flow and how much comes from the wall emission behind the tips. Simulations results will be presented to address the effect that this geometry has on the electron hot spots and ion beam uniformity.

Two floating sheath solutions with strong electron emission in planar geometry have been proposed, a “space-charge limited” (SCL) sheath and an “inverse” sheath. SCL and inverse models contain different assumptions about conditions outside the sheath (e.g., the velocity of ions entering the sheath). So it is not yet clear whether both sheaths are possible in practice, or only one. Here we treat the global presheath-sheath problem for a plasma produced volumetrically between two planar walls. We show that all equilibrium requirements (a) floating condition, (b) plasma shielding, and (c) presheath force balance, can indeed be satisfied in two different ways when the emission coefficient γ > 1. There is one solution with SCL sheaths and one with inverse sheaths, each with sharply different presheath distributions. As we show for the first time in 1D-1V simulations, a SCL and inverse equilibrium are both possible in plasmas with the same upstream properties (e.g., same N and Te). However, maintaining a true SCL equilibrium requires no ionization or charge exchange collisions in the sheath, or else cold ion accumulation in the SCL's “dip” forces a transition to the inverse. This suggests that only a monotonic inverse type sheath potential should exist at any plasma-facing surface with strong emission, whether be a divertor plate, emissive probe, dust grain, Hall thruster channel wall, sunlit object in space, etc. Nevertheless, SCL sheaths might still be possible if the ions in the dip can escape. Our simulations demonstrate ways in which SCL and inverse regimes might be distinguished experimentally based on large-scale presheath effects, without having to probe inside the sheath.