Concave Cathode Research Articles

Developing a desk-top electron beam micro-machining system in the low- pressure argon atmosphere

Electron beam machining (EBM) is a well-established technology for various industries. Conventional EBM systems use high-voltage for electron beam generation. The EBM could be used for various processing such as drilling, welding and surface modification by converting kinetic energy of electrons to thermal energy of the substrate. The core technology of EBM is electron beam generation. Traditionally, magnetic lens systems are used to focus the electron beam and magnetic coils are used for beam deflection. However, most EBM systems available on the market are designed for macro size work-piece machining purposes. These machines have large chambers and equipped with high power supplies. Thus causes difficulty for micro-machining on small work-pieces. Therefore, in order to perform micro-machining for tiny work-piece, a desk-top EBM system with a vacuum chamber smaller than 5 L was developed dedicated for micro machining processes. A transformer of microwave oven is used as power supply. As an electron beam source by using concave cathode, which focuses the resulting electron beam in argon ambient. Argon ambient also allows to avoid contamination during the process of machining. In order to obtain excellent results of micro machining operation on smaller work-pieces, the electron beam behavior of the desk-top EBM was investigated in this study. The experimental results show that dissimilar materials welding, small hole drilling and surface modification could be carried out by this desk-top EBM. It is expected that the desk-top EBM can be used for commercial applications especially for micro machining in near future.

Nonadiabatic Effects on Beam-Quality Parameters for Frequency-Tunable Gyrotrons

We propose an unconventional electron gun structure in which the emitter is located on a concave cathode surface with a non-uniform electric field. Such a design violates the intuition that an emitter should place close to a uniform electric field to reduce the velocity spread. The commonly employed design guide based on the adiabatic condition predicts a huge velocity spread of 24%, but the simulation using EGUN code and verified with CST particle studio shows a very low spread of 2.8%. Examining the magnetic moment and the kinetic energy of the beam reveals that the electrons experience a relatively long acceleration process due to the much weak electric field. That's why the non-adiabatic effect matters. In addition to the cyclotron compression and the E$\times$B drift, the "resonant" polarization drift plays a crucial role in reducing the overall velocity spread.Simulations show a decent beam quality with the pitch factor of 1.5 and the transverse velocity spread of 2.8% over a wide range of the magnetic field (7.4-8.0 T) and the beam voltage (12-22 kV) with a high structural tolerance. The promising results with the wide working range enable the development of continuous frequency-tunable gyrotrons.

Design study of a low-emittance high-repetition rate thermionic rf gun

We propose a novel gridless continuous-wave radiofrequency (rf) thermionic gun capable of generating nC ns electron bunches with a rms normalized slice emittance close to the thermal level of 0.3 mm mrad. In order to gate the electron emission, an externally heated thermionic cathode is installed into a stripline-loop conductor. Two high-voltage pulses propagating towards each other in the stripline-loop overlap in the cathode region and create a quasielectrostatic field gating the electron emission. The repetition rate of pulses is variable and can reach up to one MHz with modern solid-state pulsers. The stripline attached to a rf gun cavity wall has with the wall a common aperture that allows the electrons to be injected into the rf cavity for further acceleration. Thanks to this innovative gridless design, simulations suggest that the bunch emittance is approximately at the thermal level after the bunch injection into the cavity provided that the geometry of the cathode and aperture are properly designed. Specifically, a concave cathode is adopted to imprint an Ƨ-shaped distribution onto the beam transverse phase-space to compensate for an S-shaped beam distribution created by the spherical aberration of the aperture-cavity region. In order to compensate for the energy spread caused by rf fields of the rf gun cavity, a 3rd harmonic cavity is used. A detailed study of the electrodynamics of the stripline and rf gun cavity as well as the beam optics and bunch dynamics are presented.

Low energy electron generator design and depth dose prediction for micro-superficies tumors treatment purposes

We investigate deposited energy and linear energy transfer (LET) of low energy ejection electrons in air and water layers of a generator design via a plasma source. A structured model of a concave cold cathode electron generator was designed and simulated by using Monte Carlo n-particle version X 2.7.0 (MCNPX) code. A negative dc high voltage was applied to a concave cathode up to −12 kV to determine electron energy activity. Results determined that the geometric dimensions of field size toward the anode increased in relation to the angle of the conic beam, widening the accumulated bulks. The increased field size increased the anode current, which also resulted in an increase of electron energy, a reduction in LET, a stretched build-up area and a dose curve that shifted to a higher depth. The biological effect of low energy electron radiation can be increased with an increase of LET; as the depth dose decreased, the electron energy increased at the same time. The study of electron irradiation as a conic beam from an electron generator may provide an accurate investigation of the indirect effect of low energy electrons on bystander cells.

Research of Bearing Crown Roller-Raceway by Non-Uniform Interelectrode Gap Electrochemical Mechanical Machining

Electrochemical mechanical machining (ECMM) is introduced to the manufacture of bearing crown roller-raceway in consideration of the very problems appearing in traditional machining methods. Firstly, numerical model on the non-uniform interelectrode gap ECMM is established and anode crown shapes under different cathode shapes are analyzed. Afterwards, according to the requirement of anode crown related cathode shape is formed through numerical simulation. In the end, the crown of bearing roller-raceway is machined by practical concave cathode with improved surface quality and certain crown face.

Focusing Effect in Concave Cathode Model Using Computer Simulation

It is well known that carbon nanotube (CNT) is an excellent electron field emitter. Because of its merits, CNTs have been applied recently for X-ray source for imaging. In X-ray imaging, both the emission current and the spot size of the electron beam are crucial. To fulfill these, a concave cathode structure is proposed as an alternative, and the simulation results based on the trajectories of time of flight of the electron beam are presented.

Simulation of Field-Emission Triode Using Carbon Nanotube Emitters

An X-ray source based on a field-emission electron source is currently being developed using carbon nanotubes (CNTs) for applications to biological, medical and material technologies. In the design of triode-type X-ray sources, it is important to ensure that fine beam focus and efficient electron emission can be simultaneously obtained. Therefore, the geometrical parameters, such as electrode shape and grid mesh design, including mesh wire thickness and mesh opening area, and the gaps between parts (for example, CNT cathode and grid mesh), must be optimized. In this study, for four different types of triode structures, the beam focusing performances and emission currents, with respect to grid mesh design, were simulated. The results show that the concave-shape cathode structure allows the optimum spot size and structural simplicity among the structures tested in this study. From the simulation results, it was estimated that a beam spot size of 1.5 mm can be reduced to less than 150 µm on the anode plate with the aid of the concave cathode.

Enhanced energy deposition efficiency of glow discharge electron beams for metal surface treatment

The energy deposition efficiency and focal spot dynamics of electron beams produced by pulsed cold-cathode high-voltage glow discharges for metal surface treatment are investigated for two different cathode geometries. A concave cathode geometry in which the focusing is dominated by the convergence of the electric field lines in the cathode fall region is compared with flat cathode in which the focusing is exclusively caused by the self-generated magnetic field. Results of the treatment of AISI 4140 carbon steel samples show that the concave cathode geometry significantly increases the efficiency, reduces the threshold power necessary for melting, and is less sensitive to variations in the discharge parameters and sample position. The results of numerical modeling indicate that the observed increase in efficiency is caused by the longer persistence of the focal spot on the sample. The model can be used to predict the discharge parameters required for a desired treatment.

Proposal for a new self-focusing configuration involving porous silicon for field emission flat panel displays

Field emission displays (FEDs) work in principle in a similar way with conventional cathode ray tubes (CRTs), namely emitted electrons excite phosphors which in turn emit visible light. However, FEDs obtain the electrons through field emission from a distributed network of sharp emitters, and there is no special deflection system for them. The size of the light spot associated with an island of emitters (a pixel) depends on many factors, such as display geometrical dimensions and operating conditions. In this article, a special self-focusing configuration for FEDs is analyzed using a combined analytical and numerical approach. The proposed configuration involves a distributed network of electron emitting areas which are concave in shape and not plane as usual, one for each phosphor pixel. The cathode concave areas are covered with a porous silicon layer, having sharp fibrils with several nanometers radius of curvature. The concave shape has a built-in electron self-focusing feature but, as a by-product, decreases the electric field compared to the plane cathode situation. However, the field multiplication approximation applies in this case, as concerns the electric field enhancement due to both the concave cathode and fibrils. It is shown that high enough electric fields are obtained, allowing electron field emission to occur. Analytical equations are provided for the electric field on the cathode concave surface. These equations are then used in a numerical model, taking into account the electric field enhancement associated with the fibrils, their mutual influence included. This approach allows the computation of the field emitted current as function of model parameters.

Spot deposition by focusing in Penning discharge sputtering

In order to obtain an efficient spot-sized deposition in Penning discharge sputtering, an electrode configuration for focusing of sputtered atoms has been designed and evaluated. In the apparatus designed, a conical beam of sputtered atoms is produced by a combination of a concave cathode, a planar cathode with a ring-shaped slit, and a hollow conical frustum anode between the two cathodes. By adjusting the distance of the electrodes a sharp focus is obtained on a substrate which lies behind the planar cathode. The focusing position can be controlled easily. When a circular disk gold film is deposited on the glass substrate, its diameter became smaller in inverse proportion to the distance between the substrate and planar cathode. With an input power of 4.5 W the deposition rate of the gold film is found to be 1 nm/s. It has been shown that the technique is very suitable for spot-sized film deposition on various substrates and for other applications such as dental-caries-preventing coating on extracted teeth and the coating for fine adjustment of resonant frequency of quartz-crystal wafers.

Optical characteristics of the hollow anode discharge

A new type of discharge realized in a diode consisting of a concave cathode and a special type of a hollow anode is presented. The concave cathode focuses electrons toward the hollow anode increasing the density of high-energy electrons and the efficiency of excitation and ionization in the hollow anode. The distinctive arrangement of dark and luminous regions between the cathode and anode has been observed in the discharge.

Hollow anode ion–electron source

A new type of ion–electron source, based on a gas discharge in a diode with concave cathode and a special type of hollow anode, is presented. In this type of discharge, the maximum plasma density is localized at the exit aperture. Constant hydrogen ion currents in excess of 1 mA have been obtained with a 1-mm-diam exit aperture and an extraction voltage of 3 kV. The power supplied to the source, whose volume is less than 0.5 cm3, is only a few watts.

Spectroscopy of hollow anode discharge

Hollow anode discharge represents a new type of discharge which is realized in a diode consisting of a concave cathode and a special type of hollow anode whose inner surface only is conductive. The concave cathode focuses electrons toward the hollow anode, increasing the density of high energy electrons and the efficiency of excitation and ionization in the hollow anode. The optical spectrum, which consists of ion lines only, indicates that in the hollow anode a highly ionized plasma is obtained. The spectrum does not consist of cathode (or anode) material lines.

Pierce-Type Electron Gun with a Large Concave LaB6 Cathode

A Pierce-type electron gun with a concave cathode of lanthanum hexaboride (LaB6) 30 mm in diameter is described. At an accelerating voltage of 3 kV and a cathode temperature of 1340°C, a focussed electron beam of DC 300 mA was obtained.