Thermionic Source Research Articles

Fabrication of Lanthanum Hexaboride Single Nanowire Field Emitter and Their Field Emission Properties

Lanthanum hexaboride (LaB6) single crystals have been used as thermionic source in high resolution electron microscopes for over 50 years. LaB6 nanowires have great potential to be used as field emission electron point source, which offers larger brightness than thermionic source. We have developed a chemical vapor deposition process to synthesize LaB6 nanowires on Si substrate with a yield rate of 0.5 mg/cm2. Epitaxial growth of vertically aligned LaB6 nanowires has also been successfully realized on LaB6 films. By using an electrostatic attaching method followed by focused ion beam welding, we successfully fabricated single LaB6 nanowire field emitters. Field emission I-V curve measurement has been carried using a home-built 2010F HRTEM field emission in-situ holder. Extrapolated data shows that the nanowire field emitter has a low work function value of 2.3 e V and it generates an emission current over 20 micro A and an emission current density in the order of 10^8 A/cm2 at a low extraction voltage below 300V.

Theoretical model of the intrinsic emittance of a photocathode

A theoretical expression for the intrinsic emittance of a photocathode is developed based on a method of evaluating moments of the emission distribution function. The method is first used to reevaluate the well-known rms emittance of a thermionic source, and then, using analogous approximations but with an updated theoretical model of photoemission, an equation for the intrinsic emittance and brightness of a photocathode of comparable simplicity to the thermionic case is obtained in the limit of weak field and low temperature.

Practical considerations for high spatial and temporal resolution dynamic transmission electron microscopy

Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5×10 7 electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10 −6 s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed.

Electron plasmas: Confinement and mode structure in a small aspect ratio toroidal experiment

Toroidal electron plasmas have remained less explored due to their poor confinement properties. Their equilibrium, stability, and confinement properties are therefore not entirely understood and continue to remain a topic of intense ongoing research. Large aspect-ratio theory suggests poor confinement in toroidal devices can be overcome by the application of a radial electric field; this has been verified successfully in some of the recent experiments. In the present paper, we report the longest confinement time without these external forces. Increasing the toroidicity has helped us to generate these forces intrinsically. To this end, a trap to confine electron plasmas has been created in a small aspect-ratio (≈1.6) torus. Electrons after being injected from a thermionic source are seen to remain confined with a purely toroidal magnetic field. The confinement time is far more than known single particle drift time scales. Importantly, it is in the absence of any external electric field, additional rotational transform, and/or magnetic fields, which, although not required, in principle, may appear essential particularly due to their role in improving confinement in some of the recent large aspect-ratio traps. The successful confinement in the small aspect-ratio limit has also led to several interesting observations: the evolution of the confined plasma is marked by an interesting nonlinear (large amplitude), electrostatic wave activity. Coherent, periodic, double peak oscillations result from a low-frequency E×B motion of a toroidal vortex in a plasma that closely leans against the inner wall. As many as 16 highly phase-coherent harmonics with dominant power in m=2 suggest that the mode is not merely a center-of-charge motion. Rather, a strong coupling of modes leads to a novel nonlinear state. The predominant energy is present in the shaping of the electron cloud (m=2) and not in the displacement of the center of charge (m=1) seen in large aspect-ratio traps. The absence of any power-law tail suggests absence of any turbulence, at least on time scales longer than the wall-probe resolution (40ns). The frequency, (around 100kHz at 200G) shows an unusual shear in time: it reduces as the mode evolves, but later increases as the mode dies.

Radio frequency picosecond phototube

We propose a photon detector for recording low-level and ultra-fast optical signals, based on radio frequency (RF) analysis of low-energy photoelectrons (PEs). By using currently developed 500 MHz RF deflector, it is possible to scan circularly and detect single PEs, amplified in multi-channel plates (MCPs). The operation of the tube is investigated by means of thermionic electron source. It is demonstrated that the signals generated in the MCP can be processed event by event; by using available nanosecond electronics and that time resolution better than 20 ps can be achieved. Timing characteristics of the Cherenkov detector with RF phototube in a ‘head-on’ geometry is investigated by means of Monte Carlo simulation.

Carbon nanotubes as cold cathodes

To communicate, spacecraft and satellites rely on microwave devices, which at present are based on relatively inefficient thermionic electron sources that require heating and cannot be switched on instantaneously. Here we describe a microwave diode that uses a cold-cathode electron source consisting of carbon nanotubes and that operates at high frequency and at high current densities. Because it weighs little, responds instantaneously and has no need of heating, this miniaturized electron source should prove valuable for microwave devices used in telecommunications.

UV photon assist ionization for low temperature plasma

Abstract A new type plasma source has been developed for the generation of low temperature plasma. The plasma generation process consists of two steps, the generation of metastable neutral gas by injecting a low energy electron beam (the thermionic source) and the ionization of the metastable neutral gas by application of a UV light source. The key characteristic of this plasma source is the capability of producing extremely low temperature plasma. In the experiment, the filament heating current is 6.5 A and the electron acceleration voltage varies from 16 V to 25 V. Plasma parameters are measured by a single Langmuir probe. The plasma density increases 100%, from 4.5 × 10 9 cm −3 to 9.8 × 10 9 cm −3 in Ar 30 mTorr when the neutrals excited by the e-beam are exposed to the UV light. However, the electron temperature is still low, i.e., ∼0.5 eV. A similar result is observed in the case of Xe.

Magnetically confined high power thermionic line source emitter assembly

A directly heated thermionic high power electron beam source was constructed. The circular cross-section tungsten line cathode of length 140 mm with diameter 0.9 mm was used. Different gun design parameters were investigated and their results are discussed in detail. A uniform external magnetic field of 50 G was employed for focusing of electron beam at 180 ° deflection. The gun delivers uniform emission current density throughout the emission surface. The dimensions of the electron beam at worksite were comparable with the line cathode. The beam power of 57 kW was successfully achieved well below the saturation limit. The spring action mechanism was especially designed to avoid any cathode deformation. The gun design facilitates the reported length and emitting surface temperature of the cathode to be further increased to obtain higher emission values. The important gun features are operational reproducibility and long time stability. The evaporation rates for stainless steel have been achieved up to 1 kg/hr. An area of (280×120 mm 2) could be heat treated with the line source electron beam in few milliseconds. The gun is extremely useful for melting, evaporation and heat treatment.

An electromagnetically focused electron beam line source

A directly heated thermionic electron beam source was constructed. A tungsten wire of length 140 mm with diameter 0.9 mm was used as a cathode. An emission current of 5000 mA was achieved at an input heating power of 600 W. Cathode to anode distance of 6 mm with acceleration voltage of 10 kV was used. A uniform external magnetic field of 50 G was employed to obtain a well-focused electron beam at a deflection of 180°, with cathode to work site distance of 130 mm. Dimensions of the beam (1.25×120 mm) recorded at the work site were found to be in good agreement with the designed length of cathode. The deformation of the cathode was overcome by introducing a spring action mechanism, which gives uniform emission current density throughout the emission surface. We have achieved the saturation limit of the designed source resulting in smooth and swift operation of the gun for many hours (10–15 h continuously). The design of gun is so simple that it can accommodate longer cathodes for obtaining higher emission values. This gun has made it possible to coat large substrate surfaces at much faster evaporation rate at lower cost. It can also be useful in large-scale vacuum metallurgy plants for melting, welding and heat treatment.

Efficiency of cathodoluminescent phosphors for a field-emission light source application

The efficiency of major types of cathodoluminescent phosphors has been investigated at high-brightness (up to 30 000 cd/m2) operational conditions using thermionic electron source. The accelerating voltage was varied from 4 to 14 kV and electron beam current density was independently varied from 2 to 50 μA/cm2. Under those conditions both thermal quenching and current saturation of the phosphors were observed. Due to combined influence of those factors, it was found that the best way to characterize the phosphor is plotting the efficiency (in lm/W) versus the specific power density (in mW/cm2) graph. The resulting plots show the integrated influence of current loading and elevated temperature on the phosphor efficiency. The best efficiencies at 200 mW/cm2 and 10 kV were measured for a green color phosphor −55 lm/W, for a blue one—10 lm/W, and for a red—13 lm/W.

Thermal ionization ion mobility spectrometry of alkali salts.

Positive and negative thermal ionization ion mobility spectra (TI-IMS) of some sodium and potassium halides are reported here. The data provide the first measurement of the thermal ionization ion mobility spectrometry of inorganic compounds. A thin Nichrome filament was used as a thermionic ionization source. Sample was directly deposited on the filament, where it was heated and ionized. Each salt produced a different ion mobility pattern, but all sodium salts spectra were common in their first peak. This peak differs from the common peak observed in the spectra of potassium salts. The drift time of the second peak in all spectra was found to be linearly dependent on the size of the counteranion of the salt. Negative thermal ionization ion mobility spectra of alkali halides were also observed. An alkali halide salt (MX), in general, produced its own anion as well as some heavier ions that are thought to be hydrated X- (MX)n species. The capability of the method in quantitative analysis was demonstrated by measuring potassium impurity in sodium bromide. A detection limit of 0.01% K+ in NaBr and a linear rage of 3 orders of magnitude were obtained. The results from this study suggest that TI-IMS has potential as a field technique for the detection of some elements in samples.

Improved 20 keV injection system for the heavy-ion-beam diagnostic of the tokamak ISTTOK

The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTTOK provides a 2 mm diameter and 3.6 mrad divergence beam of Cs+ or Xe+ ions, after extraction from a plasma ion source and acceleration up to 20 keV by a gap lens and diameter definition by a set of apertures. However, the beam current is significantly reduced to 1.5 μA, therefore, complicating the HIBD measurements of the plasma parameters. Creation of the beam with a higher current and the same, or close, geometrical parameters requires a significant improvement of the ion optics of the injection system. This article describes an injection system built in the acceleration tube, and presents results of test experiments with a solid-state thermionic source (SSTS) and a monocusp plasma ion source (MPIS). The improved injection system is characterized by a good match to both ion sources. The use of the long focus ion optics of the acceleration tube allows one to obtain an output beam current up to 11 μA of Cs+ ions with SSTS and 39 μA of Xe+ ions with MPIS, and dimension preservation at the focus point. Operating on the tokamak ISTTOK, the injection system significantly increased the signal-to-noise ratio and improved the measurements of the plasma parameters.

Particle Beam Application-Present Status and Future Prospects. Particle Beam Applications for Nuclear Fusion Research. High Temperature Plasma Diagnostics Using Alkali-Metal Beams.

The application of alkali-metal ion or atomic beams to the diagnostics of magnetically confined plasmas has a long history. Thermionic ion sources are used mostly because of their ease of handling. Fine probing beams and rather large atomic collision cross sections provide good spatial and temporal resolution of the measurements. Among the variety of this kind of diagnostics, two methods are reviewed as typical examples: a heavy ion beam probe (HIBP) and beam probe spectroscopy using a neutral lithium beam (LiBP).

Getter requirements for a cathode ray tube with a diamond coated field emitter electron source

A single diamond coated field emitter (FE) tip could be used as a replacement for the thermionic cathode in small electron beam devices offering better optoelectronic performance. The most important parameter limiting the use of field emitting sources is their short lifetime, which is closely related to the pressure level within the pinched off device. Aside from the initial evacuation of the tube and bakeout, getter selection and the activation procedure would seem to play the crucial role. In the present study, the pressure of miniature cathode ray tubes (CRTs) was measured by a spinning rotor gauge (SRG) after the initial evacuation and bakeout procedures, again after activation of the getters, and finally during operation under typical conditions. After the evaporation of barium getters by the prescribed procedures, the pressure did not drop, but increased slowly. The accumulated gas was shown to be methane. In CRTs with a thermionic electron source, getter pumping action could be triggered by switching on the cathode heater, which efficiently cracked the generated methane. In CRTs with a diamond coated FE tip, there was no way to decrease the residual methane pressure to the desired (i.e., safe) level. The emitters, previously approved and characterized in ultrahigh vacuum, were thus irreversibly damaged ab initio. Ba getters consequently cannot be applied in devices applying diamond coated FE tips just because of geometrical constraints, as usually stated, but mainly because of methane generation. In CRTs with nonevaporable getters, a much lower pressure increase was registered by the SRG during several days. This means that they could maintain the residual atmosphere at the level that may provide a means for long-term operation of devices with diamond coated FE electron sources.

Response mechanisms of thermionic detectors with enhanced nitrogen selectivity.

The response mechanisms of a thermionic detector with enhanced nitrogen selectivity operating in an inert gas environment were investigated. According to accepted theory, the analyte has to contain electronegative functional groups in order for negative ions to be formed by the extraction of electrons from the thermionic source. This leads to a selective detector response for compounds containing nitro groups or multiple halogens. However, in the tests described here, polycyclic aromatic nitrogen hydrocarbons (PANHs), acridines, and carbazoles were used as reference substances. These compounds contain no electronegative functional groups. None of the investigated acridines exhibited any response from the detector, but carbazoles generated a strong structure-related detector response. By examining partial charges for all hydrogens of all individual carbazoles and acridine, it was demonstrated that the acidic hydrogen atom attached to the nitrogen heteroatom of the carbazoles has a strong influence on the detector response. Ionization of carbazoles may occur by dissociation of the nitrogen-hydrogen bond during contact with the thermionic surface. Support for this theory was provided by the linear relationship between the relative detector response and the deprotonization energy of the carbazoles (coefficients of determination of 0.90 and 0.98 for linear and quadratic models, respectively, were obtained). Further, there appeared to be no linear relationship between the detector response and electron affinity of the carbazoles, (R2 value, 0.32). Thus, the mechanism involved in ionization of the carbazoles is probably not direct electron transfer from the thermionic surface to the carbazoles. Principal component analysis (PCA) showed that the thermal conductivity of chemically inert detector gases also has an influence on the detector response. The investigated gases were helium, neon, nitrogen, carbon dioxide, and argon. It was found that thermal conductivity can be used to rank the detector response for the carbazoles, and there was no discernible response when helium, which has the highest thermal conductivity, was used as the detector gas.

Recent advances in high current vacuum arc ion sources for heavy ion fusion

For a heavy ion fusion induction linac driver, a source of heavy ions with charge states 1+–3+, ≈0.5 A current beams, ≈20 μs pulse widths and ∼10 Hz repetition rates is required. Thermionic sources have been the workhorse for the Heavy Ion Fusion (HIF) program to date, but suffer from heating problems for large areas and contamination. They are limited to low (contact) ionization potential elements and offer relatively low ion fluxes with a charge state limited to 1+. Gas injection sources suffer from partial ionization and deleterious neutral gas effects. The above shortcomings of the thermionic ion sources can be overcome by a vacuum arc ion source. The vacuum arc ion source is a good candidate for HIF applications. It is capable of providing ions of various elements and different charge states in short and long pulse bursts and high beam current density. Under a Phase-I STTR from DOE, the feasibility of the vacuum arc ion source for the HIF applications was investigated. We have modified an existing vacuum arc ion source at LBNL to produce a gadolinium ( A≈158) ion beam with >0.5 A beam current, 120 keV beam energy, ≈6 cm diameter extraction aperture and ≈20 μs pulse width. The average beam current density at the extraction grids was ≈17 mA/cm 2. We have measured that >85% Gd ions were in the 3+ charge state, the beam current fluctuation level (rms) was ≈3%, pulse-to-pulse variation of the beam (rms) was about 3%, the uniformity of the beam density over its 6 cm diameter was ⩾98% and the ion longitudinal energy spread was ⩽1%. Additional measurements were made to improve charge state purity by using other materials and employing an axial magnetic field close to the cathode. Yttrium ( A≈89), lead ( A≈207), and Ba ( A≈137) were tested at similar current parameters with Ba delivering nearly a pure charge state with >95% being in 2+ state. The results of the experiments indicate that the vacuum arc ion source is a good candidate for HIF applications.

X-Ray Microanalysis in the STEM

Abstract Commercial TEM/STEMs are ill designed for quantitative X-ray microanalysis of thin foils. There have been no fundamental advances in their design since the first instruments appeared in the mid- 1970s. These instruments had thermionic sources with useful probe sizes of ∼10 nm, small (∼ 0.1 sr) detector collection angles, illumination systems giving serious levels of stray radiation, substantial hydrocarbon and water-vapor contamination due to the poor vacuum (∼10-5Pa) and sliding o-ring seals on unstable side-entry goniometer stages. The instruments lacked a direct measure of the probe current at the specimen. Often a 60 s accumulation time resulted in enough drift or contamination that the microanalysis was not trustworthy and spatial resolution was compromised. Today's modern TEM/STEMs, apart from replacement of thermionic source with a FEG, are little better. Dedicated STEMs however, have invariably offered better X-ray performance. The first VG HB 5 DSTEM became available, also in the 1970s, and the X-ray performance improved through to the latest design (the VG HB 603) in the mid-1990s.

Photocathode source for studying two-dimensional fluid phenomena with magnetized electron columns

Magnetized electron columns are a valuable experimental tool used to study two-dimensional (2D) fluid phenomena. Traditionally, the electrons have been generated with thermionic sources, typically limiting the initial electron distribution to one filled circle and thereby restricting the range of accessible fluid phenomena. Here, we describe a new electron source based on a cesium antimonide photocathode that can generate more complicated initial electron distributions. Experiments so far have focused on the stability of 2D vortex patterns.

Exchange-correlation, dipole, and image charge potentials for electron sources: Temperature and field variation of the barrier height

Potential barrier profiles for large applied fields and/or high temperature are developed for the study of field and thermionic emission electron sources intended for radio frequency power tube applications. The numerical implementation provides a fast and flexible method to obtain the barriers which govern current density, and yet allows for complications such as nanoprotrusions, adsorbates, “internal” field emission, the sputtering of low work function emission sites, and so on. The model consists of (i) a modified form of the Wigner Lattice expansion of the electron ground state energy to evaluate the exchange and correlation potential, (ii) a simplified form of the ionic core potential to correct the “Jellium” model, (iii) a triangular representation of the barrier with a single adjustable parameter which enables both the solution of Schrödinger’s equation in terms of Airy functions and thus an exact evaluation of the electron density near the barrier, and (iv) a numerical integration of Poisson’s equation to evaluate the dipole potential and positive background boundary. An iterative calculation is performed such that the barrier used in the solution of Schrödinger’s equation becomes equivalent to the barrier predicted from the exchange-correlation and dipole potentials. As a test of the method, evaluations of the work function of various metals are made. A good correspondence is found between the potential profiles and an “analytic” image charge potential (which contains modifications to the standard image charge model). Modifications to the Richardson–Laue–Dushman and Fowler Nordheim equations, so as to obtain current density estimates, are described. The (only) adjustable parameter used to correlate theory and experimental work functions is the magnitude of the ionic core “radius,” which is often close to the actual radius of the metal ions in the test cases considered. The temperature and field dependence of the work function, which is dependent upon electron penetration of the barrier and its effect on the dipole potential, are investigated. The method is suggested to be suitable for the analysis of more complex potential barrier profiles that are encountered in actual (realistic) thermionic and field emission electron sources. The limitations of the model are discussed and methods to circumvent them are proposed.

Semianalytical model of electron source potential barriers

Potential barrier profiles are developed for the study of field and thermionic emission electron sources. The model consists of the exchange and correlation potential, an ionic core potential to correct the “jellium” model, a triangular representation of the barrier, and a numerical integration of Poisson’s equation to evaluate the dipole potential. As a test, evaluations of the work function of various metals are made which rely on “effective” ionic radii which are often close to the actual ionic radii, and a good correspondence is found between the potential profiles and an “analytic” image charge potential. Modifications to the parameters inherent in the Richardson–Laue–Dushman and Fowler–Nordheim equations, so as to obtain current density estimates, are described. The method is suggested to be suitable for the analysis of more complex potential barrier profiles which are encountered in actual (realistic) thermionic and field emission electron sources, in that it captures the behavior of the exchange–correlation potential near the surface and includes those effects in the image charge potential.