Ba Dispenser Cathodes Research Articles

Studies of the Influence of ZrH2 Addition to the Impregnant on the Performance of Yttrium Oxide–Tungsten Matrix Ba Dispenser Cathodes

The cathode in the magnetron should have good secondary emission property and a certain thermionic property. In this article, ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was added as an activator to obtain rare earth-doped tungsten matrix impregnated cathode to improve the emission property. Cathodes with ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Y15-411-Zr cathode) or without ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Y15-411 cathode) were prepared by a spray drying method combined with high-temperature sintering in dry hydrogen atmosphere. The emission properties and emission mechanism have been studied. The results showed that thermionic emission performance and secondary emission performance of Y15-411-Zr cathode are 3.4 times and 1.2 times higher than those of Y15-411 cathode, respectively. With the addition of ZrH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , yttrium was produced during activating process, which contributed to the better emission properties of Y15-411-Zr cathode. Furthermore, the work function of barium and oxygen adsorbed on yttrium oxide and yttrium surface was calculated using the first principle. The results showed that the lowest work function of Ba-O absorbed on Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Y is 1.9 and 1.6 eV, respectively.

Design Consideration of 650-MHz Circular Electron–Positron Collider Klystron and Simulation of Its Beam Tester**Supported by the National Natural Science Foundation of China under Grant No 11475201, and the Innovation and Technology Fund of Institute of High Energy Physics.

We present the first phase R&D for the 800 kW cw, 650 MHz klystron for the future circular electron–positron collider (CEPC) project in China. The CEPC requires 192 klystrons and it is desired to be designed in the Institute of High Energy Physics, CAS, and manufactured domestically. Therefore, we present the manufacturing schedule of this project; the three-stage development from the beam test tube to the klystron having a high efficiency structure. Design of the beam test tube that comprises electron gun and collector is presented. First, gun simulation having a modulating anode is performed using DGUN software. The uniform beam trajectories with a beam perveance of 0.64 μA/V3/2 are simulated. We employ a Ba-dispenser cathode of radius 35 mm with ϕ10 hole at the center and obtain a current density on cathode less than 0.45 A/cm2. The beam trajectories are also simulated over beam test tube of length about 2 m with a magnetic field of 213 Gauss. Thermal analysis of the collector is performed using the ANSIS-CFX code and hence the cooling structure is determined. Mechanical design is almost carried out and it is in manufacturing stage.

A high-current electron gun for the electron beam ion trap at the National Superconducting Cyclotron Laboratory

The Electron Beam Ion Trap (EBIT) in NSCL's reaccelerator ReA uses continuous ion injection and accumulation. In order to maximize capture efficiency and minimize breeding time into high charge states, the EBIT requires a high-current/high current-density electron beam. A new electron gun insert based on a concave Ba-dispenser cathode has been designed and built to increase the current transmitted through the EBIT's superconducting magnet. With the new insert, stable EBIT operating conditions with 0.8 A of electron beam have been established. The design of the electron gun is presented together with calculated and measured perveance data. In order to assess the experimental compression of the electron beam, a pinhole CCD camera has been set up to measure the electron beam radius. The camera observes X-rays emitted from highly charged ions, excited by the electron beam. Initial tests with this camera setup will be presented. They indicate that a current density of 640 A/cm(2) has been reached when the EBIT magnet was operated at 4 T.

Study on the Emission Properties of the Impregnated Cathode With Nanoparticle Films

The effect of thin-film characteristics on the thermionic emission of dispenser cathodes has been investigated. Nanoparticle Ir thin films were grown by magnetron sputtering at room temperature. Microstructures of these thin films grown at different sputtering pressures were observed using a scanning electron microscope. The results showed that the particle size of the films depended on the deposition rate in the nucleation stage of the Ir films, which could be well controlled by the sputtering pressure. N-type cathodes were fabricated by impregnating tungsten matrices of 25% porosity with 6:1:2-type barium calcium aluminate and then coating them with nanoparticle Ir thin films, 200–500 nm in thickness, in hydrogen at 1200 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ} \hbox{C}$</tex></formula> . A comparison was made in the emission uniformity between an N-type cathode (defined here as a Ba dispenser cathode coated with a layer of metal nanoparticles) and a traditional M-type cathode using a thermionic electron microscope. The chemical components of the vacuum background, the evaporants from an N-type cathode, and an M-type cathode were respectively analyzed using a time-of-flight mass spectrometer. The evaporating rates of the two types of cathodes and their dependence on the cathode temperatures were also investigated. Finally, the electron emission performance of the N-type cathodes was studied.

Preparation and performance analysis of barium dispenser cathodes

The intercorrelation of aluminate properties, such as structure, component and metal Os film coating with electron emission capability and longevity of Ba dispenser cathodes, has been investigated. It is shown that reactants react completely and the percentage of tribarium aluminates is larger in the normal environment, and that at 1723 K the pulse of cathode impregnated in the above aluminates is larger. Aluminates sintered under the normal environment constitute the multiphase formation, but aluminates sintered under the special environment with Ar gas injection compose the single-phase formation by means of X-ray diffraction (XRD). Both the surface and the fracture surface of cathode are full of aluminates evenly distributed by using Sirion 200 superhigh resolving power field emission scanning electron microscope (SEM). The emission current density of cathode-impregnated aluminates sintered in the special environment with Ar gas injection is larger than that in the common environment by the analysis of diode experiments. The emission capability of Os-coated cathode prepared by laser ablation deposition (LAD), which has good characteristics of low temperature is higher, than that without metal film coating cathodes.

The tungsten powder study of the dispenser cathode

The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes has been investigated for the different grain morphologies. It is shown that a fully cleaning step of the tungsten powder is so necessary that the tungsten powder will be reduction of oxide in hydrogen atmosphere above 700 °C. The porosity of the tungsten matrix distributes more even and the closed pore is fewer, the average granule size of the tungsten powder distributes more convergent. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger by the analysis of the electronic microscope and diode experiment.

Life-limiting mechanisms in Ba-oxide, Ba-dispenser and Ba-Scandate cathodes

Ba-oxide, Ba-dispenser and Ba-Scandate cathodes have been continuously improved in their emission performance in the past decades. Ba-oxide and Ba-dispenser cathodes are also the dominant types of thermionic cathodes used in most vacuum tube applications. When improvements in emissive properties are introduced, their impact on cathode life – where several years in a vacuum tube environment are typically required – also needs to be known. Hence, the investigation of cathode life-limiting effects is the basis of accelerated life predictions and of further cathode improvement. In this contribution, the main effects limiting the operating life of Ba/BaO-based thermionic cathodes are discussed, especially related to intrinsic dispensation and resupply to the emissive surface. Emission poisoning induced by adsorption of poisonous gases will not be addressed here. We will stress common points and point out the differences between the three types.

Correlation of emission capability and longevity of dispenser cathodes with characteristics of tungsten powders

The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes are investigated for three different grain morphologies. Best results of tungsten cathode life were found for isoaxis polyhedron morphology in combination with certain powder and matrix parameters.

Ion bombardment investigations of impregnated cathodes

Ion bombardment is one of the important factors limiting the performance of impregnated cathodes (=Ba dispenser cathodes) in high end television tubes or in colour monitor tubes. Hence, when designing a new gun with, e.g. higher electron beam current density, it is important also to model the influence of ion bombardment. Therefore, relations between basic parameters as a function of temperature need to be known quantitatively. In this paper, the emission slump of impregnated cathodes has been analyzed in a diode configuration in UHV with a differentially pumped Ar ion gun. The emission degeneration during and regeneration periods after ion bombardment have been investigated as function of cathode temperature, ion current and ion energy. One of the important results is, that the degeneration time coefficient is only weakly dependent on ion energy. The data matrix obtained can be used to improve the ion bombardment model applied in new electron gun design.

Low temperature and cold emission of scandate cathodes

The unprecedented emission capability of top-layer scandate cathodes prepared by laser ablation deposition (LAD) was already reported [1,2] yielding 460 A cm −2 at 1030 °C true temperature, which is a factor of about 20 higher than for the best impregnated (I) cathodes. Therefore their emission behaviour has been investigated down to room temperature, yielding 0.5 A cm −2 at 500 °C and 0.001 A cm −2 at 300 °C. The measurements underline, that the work function (WF) remains about constant over a wide temperature range. The effective work function with the theoretical thermionic constant of 120.4 A (cm K) −2 amounted to 1.42 eV, a Richardson plot determination resulted in a Richardson WF of 1.16 eV and a Richardson constant of 8.5 A (cmK) −2. Field emission was observed at room temperature and at about 300 °C for LAD-top-layer scandate cathodes (‘Sc’/Re–I) and was compared with other Ba dispenser cathodes. The threshold field strengths were 3.2 V μm −1 for ‘Sc’/Re–I, 12.4 V μm −1 for Re–I and 15.7 V μm −1 for W–‘I. The emission stability at low operating temperatures is dependent on residual gas pressure and composition.

Surface analysis of thermionic dispenser cathodes

Surface analysis of thermionic Ba dispenser cathodes with W, Re, Ir and Os/Ru substrates was performed by means of low-energy ion scattering and Auger electron spectroscopy. It was found that the Ba–O complex responsible for the low work functions of the cathodes has similar properties on the different substrates: the atomic Ba/O ratio is close to unity on all cathodes, with the O atoms positioned in a plane below the Ba atoms. The bonding between the Ba and O atoms as observed in the low-energy Auger spectra is also almost identical on all cathodes. However, a significant difference in the absolute Ba–O coverage was observed for the various cathodes.It was found that the work functions of the cathodes are directly determined by the Ba–O coverage of the surface, with a larger coverage resulting in a lower work function of 2.02, 1.93, 1.89 and 1.85eV for W, Re, Ir and Os/Ru substrates, respectively. The relationship between Ba–O coverage and work function can be described by a simple relation using the density of Ba–O dipoles and a single value for the dipole moment for all cathodes. For all substrates, the density of Ba–O dipoles is below the value that would result in the lowest work function. The work function of the cathode is thus limited by the density of the Ba–O dipoles, which is determined by the strength of the bonding of the O atoms in the Ba–O complex with the substrate atoms. The results are discussed with respect to previous experimental and theoretical investigations of cathode surfaces.

Ba losses due to oxygen adsorption on Ba-dispenser carthodes

The emission stability against ion bombardment and oxygen adsorption was investigated on thermionic Ba-dispenser cathodes with different top layers (W, Os/Ru, Re, Ir, and Re+Sc2O3) as a function of temperature and oxygen partial pressure. From these data, Ba and BaO reactivation energies and oxygen desorption energies were derived. During ion bombardment, two degradation/regeneration channels were observed due to the sequential carrying off of the emitting dipole and the corresponding resupply of Ba and BaO. Also two degradation/regeneration channels were detected during oxygen poisoning due to adsorption of oxygen and oxygen induced desorption of the emitting dipole or parts thereof and corresponding oxygen desorption and regeneration of the BaO dipole. The different top layers showed Ba and BaO reactivation energies of 1.5–2.6 and 2.0–2.93 eV, respectively. The values determined from the poisoning measurements were 0.3–0.7 eV larger that those from ion bombardment measurements. The oxygen desorption energies were between 1.5 (Sc/Re) and 2.91 eV (W).

Low temperature properties of Ba-dispenser cathodes

The low temperature emission properties of Ba-dispenser cathodes with different metal and metal+oxide top layers were determined as a function of anode voltage and operating time. A “Sc”/Re–I cathode showed room temperature emission, which was nearly four magnitudes larger than for a W–I cathode. At 300 °C, thermionic and field emission were both found superimposed. A Fowler–Nordheim fit derived a remarkable low field emission threshold of 3–4 V/μm. Nevertheless, emission degradation with time was observed due to oxidation of the Ba–O dipoles. At temperatures below 600 °C, oxygen adsorption time constants could be lesser than time constants for desorption of oxygen and resupply the Ba due to oxygen induced desorption, which led to a continuous degradation of emission current. Using oxygen desorption energies and energies required for the Ba resupply (determined from high temperature measurements), we could estimate boundary condition for stable long-term emission. Above 600 °C, required low background pressures in the order of 10−8 mbar are easily obtained. However, at temperatures below 200 °C, stable long-term emission will only be possible at very low total background pressures of at least 10−13 mbar.

Supply and loss mechanisms of Ba dispenser cathodes

Ba dispenser cathodes, especially W–I, Re–I, Os/Ru–I and Sc 2O 3/Re top-layer Scandate cathodes have been investigated with respect to Ba resupply, Ba evaporation and ion bombardment behaviour. Since a 411 impregnated base was used for all these types, the same Ba evaporation activation energy of 3.15 eV was found despite different surface coatings. These and further results are compared with literature data. From ion bombardment (=IB) investigations different sputter yields are derived. With respect to IB stress tests, dynamic shielding of the surface complex rest by Ba sitting on top is introduced, which allows the extrapolation of resistivity data to real tube conditions. Information about the Scandate surface complex is evaluated.

Dynamic shielding during ion bombardment of Ba dispenser cathodes

Ion bombardment (IB) resistivity is a key parameter for cathodes to be operated in TV tubes and CMT's over lifetimes of several years. Thus, accelerated stress tests with much higher ion dose rates than those present in real tube environment are normally used. Here, IB resistivity of Os/Ru-I and top layer Scandate cathodes was investigated. It was observed by variation of the dose rates, that Ba resupply modified the survived ion dose and led to a dynamic shielding of the rest of the surface complex/dipole below Ba. Enhanced IB resistivity w.r.t. total ion dose was observed for reduced ion dose rates. Based on a model of shielding an extrapolation from stress tests to real tube conditions has been established, where the correction factor is in the range of 1–8, depending on the test conditions.