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Additive manufacturing processes
Published in Fuewen Frank Liou, Rapid Prototyping and Engineering Applications, 2019
In an EBM-melting process, as shown in Figures 6.75 and 6.76, parts are fabricated in a vacuum and at about 1,000°C to limit internal stresses and enhance material properties. The vacuum chamber is typically 10-5 torr for titanium deposition but can be set down to 10-2 for other types of metals. A layer of metal powder is spread over a platform in a vacuum chamber. To reduce residual stresses that cause distortion in a fabricated part, an electron beam gun preheats the powder layer. After the preheating is finished, the layer is selectively melted by increasing the beam power or decreasing the speed. In the melting process, electrons are emitted from a filament which is heated to over 2,500°C. The electrons are accelerated through the anode to half the speed of light. A magnetic lens brings the beam into focus, and another magnetic field controls the direction of the beam. When the electrons hit the powder, kinetic energy is transformed into heat which melts the metal powder. The power is controlled by regulating the number of electrons in the beam. The cooling process is also controlled to produce well-defined hardening. As with other processes, the parts require some final machining after fabrication. The processing in a vacuum provides a clean environment that improves metal characteristics.
Cathode Ray Tube Displays
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
Beam focusing and beam current are critical in determining the final spot size and thus the resolution of the CRT. Focusing a beam of electrons is directly analogous to focusing a beam of light; the discipline is called electron optics. Concerns familiar to optical imaging such as magnification, spherical aberration, and astigmatism also confront electron optics. As CRTs become larger, and operate at higher deflection angles, spot control becomes critical. Beam focusing is achieved using either electrostatic focusing grids or electromagnetic focusing coils. Electrostatic focus is the most extensively used technique. It can be found in use in applications from television to desktop computer monitors. Electrostatic focus is achieved by applying a succession of potentials across a complex sequence of focusing grids built into the electron gun. As designers seek to improve performance further, grid designs have become intricate [11,12]. Magnetic focus is the system of choice for all high-performance systems where resolution and brightness are design objectives. Magnetic lenses are better at producing a small spot with few aberrations. External coils in a yoke around the neck of the tube control the beam. Since it provides superior performance, electromagnetic focus is common on high-resolution commercial systems. Magnetic focus can also be achieved using permanent magnets and specialized hybrid electrostatic/magnetic focus components. Due to the tremendous impact focus has on resolution, tube suppliers continue to improve focus control [13,14]. For an excellent and comprehensive treatment of electron physics in CRTs, beam control, detailed design discussions, and other aspects of CRT devices, refer to Sol Sherr’s textbook [15].
Characterization Techniques
Published in Manjari Sharma, Biodegradable Polymers, 2021
SEM reveals the indirect information about the particle size, about the nature of cross linking between two polymers and also provides the information about the mixing pattern of the two polymers. SEM uses electrons to acquire an image. Electrons are directed towards the sample by a voltage bias. Focusing and magnification are carried out by magnetic lenses. By interactions within the sample, the electron beam is altered. These alterations are transformed to an image.
Research on performance of ionization chamber used in medical laser proton accelerator based on Garfield++
Published in Radiation Effects and Defects in Solids, 2023
Xi-Cheng Xie, X.Q. Yan, K. Zhu, Hui-Lin Ge, Ke-Dong Wang
The whole process of laser acceleration and obtaining laser pulse proton beam can be described like these procedures: firstly the laser obtains huge energy from the pump light and generates femtosecond (10−15 s), beat watt (1015 W) relativistic intensity laser (more than 1018 W/cm2). Secondly, the laser contrast is improved to better than 10−10 through the double plasma mirror of the quality improvement system, and then the laser wavefront is adjusted through the deformable mirror to optimize the beam quality. After that, the laser is focused to obtain a micro-scale spot and interacting with the nano-target. Through the photo-pressure phase-stabilizing acceleration mechanism, the high-intensity electrostatic separation field is generated, and the ultrafast proton beam of the order of 100 MeV is accelerated. In the last step, the proton beam is converged through the magnetic lens of the beam system, and the laser proton beam with high quality and low energy dispersion is obtained through deflection and energy selection, and finally transmitted to the tumor tissue of the patient for treatment through the treatment head.