Cathode Emission Current Research Articles

Особенности эмиссии и полевая электронная спектроскопия оксидного термокатода

The results of a study of the emission characteristics and gas release of oxide thermocathode by the method of time diagrams in the temperature range of 678÷885 °C and emission currents from 0.08 to 1.3 mA are presented, a directly proportional dependence of the partial pressures of the released gases CO and CO2 on the cathode emission current is established. The energy spectra of the emitted electrons of oxide thermocathode at a temperature of ~600 °C were measured, corresponding to the "spotted cathode" model. The dynamics of changes in the energy spectrum of the emitted electrons of the “spotted cathode” is presented, showing the existence of stages of activation, growth and deactivation of the emission of “spots” with an increase in the emission voltage.

Simulation and Optimization of CNTs Cold Cathode Emission Grid Structure

Carbon nanotubes (CNTs) show significant advantages in the development of cold cathode X-ray tubes due to their excellent field emission performance; however, there are still some problems, such as short lifetime and the low emission current of large-area CNTs. In this paper, a front-grid carbon nanotube array model was established, and the electric field intensity near the tip of the CNTs' electric field enhancement factor was analytically calculated. A simulation model of a CNT three-dimensional field emission electron gun was established by using computer simulation technology (CST). The effects of grid wire diameter, grid aperture shape, and the distribution of grid projection on the cathode surface on the cathode current, anode current, and electron transmission efficiency were analyzed. The aperture ratio was used to evaluate the grid performance, and the simulation results show that the ideal aperture ratio should be between 65% and 85%. A grid structure combining a coarse grid and a fine grid was designed, which can make the electric field intensity around the grid evenly distributed, and effectively increased the cathode emission current by 24.2% compared with the structure without the fine grid. The effect of grid aperture ratio on the electron transmission efficiency was tested. The simulation results and optimized structure can provide a reference for the grid design of cold cathode emission X-ray tubes.

Development of a High-Beam-Transparency Gridded Electron Gun Based on a Carbon Nanotube Cold Cathode

We here report on the development of an improved dual-gridded electron gun based on a carbon nanotube cold cathode that enhances electron beam transparency and reduces grid interception and loss pathways. Compared with microscale tip Spindts, the dual-gridded construction decreases challenges associated with nanomaterial growth, fabrication, and assembly. Our experimental findings show that this dual-gridded CNT electron gun can support anode output currents (cathode emission current) of up to 700 mA, with a beam-transparency of up to ~100% and a compression ratio of 1/19. A beam spot of uniform brightness is obtained and the radius of the beam spot is 1.5 mm. In addition, our studies show that the device can operate at 1/100 duty cycle continuous pulse with a stable current of around 100mA over 100 h.

Profile formation of emission current of grid plasma cathode in a longitudinal magnetic field

The paper presents a gas discharge system for a grid plasma cathode which provides the generation and transport of an intense pulsed (20–250 µs) electron beam with an energy of up to 25 keV and current of up to 500 A (against its nominal value of 300 A) in a longitudinal magnetic field of 20–50 mT. The main feature of the plasma-cathode discharge system is its stable operation with no discharge constriction in an inhomogeneous magnetic field (5–30 mT) which penetrates in the system from the beam transport region. The system shows stable initiation and operation of its main discharge without current cutoffs at an argon pressure of (2–8)×10–2 Pa, which has been attained by optimizing the geometry of its electrodes as well as the position and orientation of its permanent magnets. When transported in a magnetic field, the beam current reaches its maximum at the beam center. For controlling the cross-sectional beam homogeneity, the initial distribution of the emission current from the plasma cathode is varied by varying the magnetic field penetrating in the discharge system and the current flow at its redistributing electrode. Such corrections of the initial current from the cathode provides a more homogeneous (±10%) energy distribution over the beam cross-section after transport to a distance of 300 mm.

TH‐CD‐207B‐01: BEST IN PHYSICS (IMAGING): Development of High Brightness Multiple‐Pixel X‐Ray Source Using Oxide Coated Cathodes

Purpose:To develop and characterize a high brightness multiple‐pixel thermionic emission x‐ray (MPTEX) source.Methods:Multiple‐pixel x‐ray sources allow for designs of novel x‐ray imaging techniques, such as fixed gantry CT, digital tomosynthesis, tetrahedron beam computed tomography, etc. We are developing a high‐brightness multiple‐pixel thermionic emission x‐ray (MPTEX) source based on oxide coated cathodes. Oxide cathode is chosen as the electron source due to its high emission current density and low operating temperature. A MPTEX prototype has been developed which may contain up to 41 micro‐rectangular oxide cathodes in 4 mm pixel spacing. Electronics hardware was developed for source control and switching. The cathode emission current was evaluated and x‐ray measurements were performed to estimate the focal spot size.Results:The oxide cathodes were able to produce ∼110 mA cathode current in pulse mode which corresponds to an emission current density of 0.55 A/cm2. The maximum kVp of the MPTEX prototype currently is limited to 100 kV due to the rating of high voltage feedthrough. Preliminary x‐ray measurements estimated the focal spot size as 1.5 × 1.3 mm2.Conclusion:A MPTEX source was developed with thermionic oxide coated cathodes and preliminary source characterization was successfully performed. The MPTEX source is able to produce an array of high brightness x‐ray beams with a fast switching speed.

Investigation on emission characteristics of metal-ceramic cathode applied to industrial X-ray diode.

The industrial x-ray diode with high impedance configuration is usually adopted to generate repetitive x-ray, but its performance would be worsened due to lower electric field on the cathode of diode when a voltage of several hundreds of kV is applied. To improve its performance, a novel metal-ceramic cathode is proposed in this paper. Key factors (width, relative permittivity of ceramic, and so on) affecting electric field distribution on triple points are analyzed by electrostatic field calculation program, so as to optimize the design of this novel cathode. Experiments are done to study the characteristics including emission current of cathode, diode voltage duration, diode mean dynamic impedance, and diode impedance drop velocity within diode power duration. The results show that metal-ceramic cathode could improve diode performance by enhancing emission current and stabling impedance; the impedance drop velocity of diode with spoke-shaped metal-ceramic cathode was reduced to -5 Ω ns(-1) within diode power duration, comparing to -15 Ω ns(-1) with metal foil cathode.

Vacuum Outgassing Behavior of Carbon Nanotube Cathode with High-Intensity Pulsed Electron Emission

Experimental investigations on the vacuum outgassing of a carbon nanotube (CNT) cathode with high-intensity pulsed electron emission on a 2 MeV linear induction accelerator injector are presented. Under the 1.60 MV diode voltage, the CNT cathode could provide 1.67 kA electron beam with the amount of outgassing of about 0.51 Pa·L. It is found that the amount of outgassing, which determines the cathode emission current, depends on the diode voltage and the vacuum.

Output power variations in a plasma relativistic microwave amplifier during a 500-ns relativistic electron beam current pulse

A relativistic plasma microwave amplifier with a gain of about 30 dB and an output power of about 60–100 MW in the frequency range from 2.4 to 3.2 GHz is studied experimentally. The total duration of the output microwave pulse is equal to the duration of the current pulse of the driving relativistic electron beam (500 ns); however, the maximum output power is observed only within 200 ns. It is shown that variations in the output microwave power during the current pulse of the annular relativistic electron beam are caused by variations in the beam radius and thickness. Analysis of the experimental data and results of numerical simulations show that the thickness of the electron beam is determined by the density of the cathode emission current.

The effect of emitter geometry on lateral field emission diodes fabricated by AFM-based electrochemical nanolithography

Lateral field emission diodes were successfully fabricated using atomic force microscopy (AFM)-based electrochemical nanolithography and tetramethyl ammonium hydroxide (TMAH) wet etching method. Field emission (FE) current of the silicon emitter cathode was measured as a function of the applied anode voltage under vacuum environment. For narrowed nanogaps from 55 to 35 nm, the turn-on voltage was decreased from 21 to 16 V. The turn-on voltage of the 35 nm gap was reduced from 16 to 8 V by changing the curvature radius of the cathode tip. The sharper emitter had the lowest turn-on voltage, largest field-enhancement factor, and good stability, which were attributed to the small emitter radius at the cathode tip and very slight changes in the local field factor. These results indicate that the diodes fabricated using this technique had the lowest value of turn-on voltage ever reported for lateral silicon FE devices.

Ultra high vacuum test setup for electron gun

Ultra High Vacuum (UHV) test setup for electron gun testing has been developed. The development of next generation light sources and accelerators require development of klystron as a radio frequency power source, and in turn electron gun. This UHV electron gun test setup can be used to test the electron guns ranging from high average current, quasi-continuous wave to high peak current, single pulse etc. An electron gun has been designed, fabricated, assembled and tested for insulation up to 80 kV under the programme to develop high power klystron for future accelerators. Further testing includes the electron emission parameters characterization of the cathode, as it determines the development of a reliable and efficient electron gun with high electron emission current and high life time as well. This needs a clean ultra high vacuum to study these parameters particularly at high emission current. The cathode emission current, work function and vapour pressure of cathode surface material at high temperature studies will further help in design and development of high power electron gun The UHV electron gun test setup consists of Turbo Molecular Pump (TMP), Sputter Ion Pump (SIP), pressure gauge, high voltage and cathode power supplies, current measurement device, solenoid magnet and its power supply, residual gas analyser etc. The ultimate vacuum less than 2×10−9 mbar was achieved. This paper describes the UHV test setup for electron gun testing.

Preparation of tetrapod-shaped ZnO nanomaterial field emission cathodes by deposition method

Tetrapod-shaped ZnO nanomaterial field emission cathode was preparated by deposition method. To the high-vacuum flat capsulation with phosphor screen and cathode was applied a voltage to test the cathode emission current and the phosphor screen brightness. For the cathode with total area 13×15 cm2, the test results showed that when the volume percentage of K2SiO3 ranged from 50 × 10-3 to 83× 10-3, concentration of Ba(NO3)2 arranged from 50 × 10-4 to 77×10-4 M, and concentration of tetrapod-shaped ZnO arranged from 82×10-4 to 12×10-3M, the cathode emission current was the largest, and phosphor screen at peak brightness could reach 60 cd/cm2 The average brightness was 48 cd/cm2

Design and performance of high uniformity linear filament electron gun

We describe new features and results from the previously reported [M. Iqbal et al., Rev. Sci. Instrum. 74, 4616 (2003)] thermionic long (up to 140mm) cathode, electromagnetically focused electron beam gun. The gun which was tested up to 50kW (5000mA×10kV) achieves power density of 33kW∕cm2 at the target. The cathode temperature and emission current was uniform over a length of 100mm of the cathode. The beam density profile along the line cathode strongly relates to the temperature distribution along the line cathode. The gun has a remarkable application in heat treatment of large surface area and to coat large substrate surfaces at much faster evaporation rates with lower cost.

Monte Carlo Study on Focus Properties of Portable UltrabrightMicrofocus X-Ray Sources

The construct and electrode potential of emitting systems are veryimportant for the portable ultrahigh brightness microfocus x-raysources. The ratio of Dw/H (Dw is the diameter of Wehnelt gridaperture and H is the setting height of the cathode) and the grid biasare determinative parameters for the emission current and focusproperties of an electron beam. The Monte Carlo method is used tonumerically calculate the electron beam trajectories within a microfocusx-ray tube. The calculated results indicate that an optimum sourcesystem with the electron beam focal spot diameter smaller than10 μm at the anode, reasonable short focal length of about 76 mmand the cathode emission current of more than 30 mAcan be attained under conditions of Dw/H = 5 and grid bias Vg = −0.6 kV.

Effects of bonding materials in screen-printing paste on the field-emission properties of carbon nanotube cathodes

The effects of bonding materials in screen-printing paste on field-emission properties were investigated with respect to high-efficient carbon nanotube (CNT) cathodes. The materials used to bond the CNTs to the cathode were composed of an insulating glass frit and a conducting Ag paste. The dependence of the electrical conductivity of the CNT paste, bonding material, and CNT ink on the field-enhancement factor was investigated using the Fowler–Nordheim equation. The emission current from the CNT cathode as the result of an electron tunneling effect was increased with increasing ratio of the dielectric glass-frit fraction in the CNT paste. The larger emission current of CNT cathode containing insulating-bonding materials can be attributed to a combination of the increased effective height of the CNTs, which protrude over the film in the cathode, and an increased field-enhancement factor.

H− ion density in the plasma of a low-voltage cesium-hydrogen discharge as a function of the cathode emission current

It was shown theoretically that the increase in the cathode emission current in a low-voltage cesium-hydrogen discharge to {approx_equal}10 A/cm{sup 2} leads to an increase in the electron temperature in the anode plasma to T{sub e} {>=} 1 eV. In this regime, the rate constant for the production of H{sup -} ions via dissociative electron attachment to vibrationally excited H{sub 2} molecules is close to its maximum value and the density of H{sup -} ions is maximal (about 10{sup 13} cm{sup -3}) in the anode plasma.

Transition from sub-Poissonian to super-Poissonian shot noise in planar cold cathodes

In recently investigated cold cathodes, the energy distribution of the injected electron beam into vacuum is quite different from the hemi-Maxwellian distribution typical of thermionic cathodes. We use an ensemble Monte Carlo technique to study shot noise in planar cold cathodes in which the emission into vacuum is characterized by an average injection energy far in excess of the thermal energy typical of thermionic cathodes. For a sufficiently large voltage across the vacuum gap, the low frequency shot noise power spectrum of the anode current is found to agree with the Schottky classical result at low value of the cathode emission current. At the onset of current self-quenching due to space-charge effects, there is first a shot-noise reduction followed by a shot-noise enhancement in the anode current fluctuations for emission current densities far into the saturation regime. It is shown that this shot noise enhancement is due to large fluctuations in both energy and space of the maximum of the energy potential hump in front of the cathode.

The barrier operation mode of an electric-discharge galathea trap

Experimental plasma parameters in an electric-discharge galathea multipole trap operating in a barrier mode (“discharge with mantle”) at a magnetic field strength below 20 Oe are reported. The local plasma parameters exhibit saturation with increasing cathode emission current. The plasma energy lifetime τ E and the “ barrier” coefficient β are estimated.

Physics and technique of plasma electron sources

Reviews the properties of electron emission from a gas discharge plasma. Interest in electron emission from plasma is due to the possibility of creating reliable and effective sources of high-current electron beams. The constricted arc discharge, vacuum arc and superdense glow discharge in crossed E*H fields are used for the creation of the plasma emission surface. Grid stabilization of the plasma emission surface is used to stabilize the plasma parameters. Electron current switching is used to increase plasma cathode efficiency. The plasma cathode emission current is controlled by variation of the plasma concentration, the emission electrode potential and the crossed magnetic field. The plasma cathode electron beam source, with 200 keV energy and microsecond pulse duration, provides electron beams with a cross section up to 104 cm2, beam current from 10 to 1000 A and current density up to 1 A cm-2. The annular electron beam source provides an electron beam with 10 cm diameter, 1 mm width, 15 mu s pulse, 600 A current and current density of 75 A cm-2.

Hollow-cathode-enhanced magnetron sputtering

A hollow cathode arc electron source has been added to a conventional planar magnetron sputtering system to form a triode magnetron. The hollow cathode source was chosen in place of a filament for high emission current at low energy, lifetime, and ease of operation in high magnetic fields and reactive gases. The electrons emitted from the hollow cathode produced additional gas ionization, resulting in a denser plasma at constant voltage. Up to 60% of the hollow cathode emission current could be directly coupled into the discharge current of the magnetron. Increases in the discharge current and the deposition rate of up to 10× were recorded over the conventional magnetron at the same voltage. The hollow cathode enhanced magnetron discharges were also capable of high current operation at voltages down to 20 V, which is below the sputter threshold. The pressure of operation could also be reduced significantly, and the magnetron could be operated at full power as low as 5×10−5 Torr in argon.

PLASMA BEHAVIOR IN AN OSCILLATING-ELECTRON ION ENGINE

The behavior of the plasma within an oscillating-e lectron ion engine was investigated in detail. It was found that certain conditions of gas pressure, magnetic field strength, and cathode emission current are required to maintain the accelerating electrostatic field in the plasma within the engine. In addition, the investigation revealed two types of transient phenomena in the discharge which can occur under certain conditions. The first is a pulsing or fluctuating mode of operation which appears to be caused by depletion of gas within the engine due to the pumping action of the discharge. The second is an azimuthal rotation of plasma perturbations in the range of frequencies of 10 to 50 kc as a result of E?- X Bz/Z?2 drifts. There is a possibility that this rotation may result in an enhanced diffusion of electrons across the containing magnetic field and hence contribute to engine power losses.