The use of electron beam heating for cutting and welding of parts made of electro-vacuum glass ensures constant chemical composition of the material in the zone of welded joints. During melting of the glass, electron beam heating excludes the effect of environment on the composition and structure of the material and allows a more precise control of its properties. Electron beam polishing of glass creates additional opportunities for surface modification with a significant reduction of surface roughness of the part. Generation of electron beam in pressure range from 5 to 20 Pa leads to formation in the plasma treatment zone, which ensures drainage of the charge from the processed dielectric object.
Electron beam melting, welding and micromachining may find application in electronic, optical industry and instrumentation for the manufacture of parts and components of electronic and optical devices of electro-vacuum and optical glass.
The main requirements to the electron beam processing of electro-vacuum glass and to electron beam facility are shown in tables 1 and 2.
The facility includes a vacuum chamber with the pumping equipment and control system, tooling for clamping, rotation and movement of processed products, the electron source (gun) and the control unit of the electronic source. Vacuum system is equipped with dry pumps and provides the pressure of 5.0∙10-4 Pa in the chamber.
ELTA-60 electron beam gun manufactured by Tekarte is presented in Fig.1. The electron source (1) is intended for creation and formation of the electron beam. The locking mechanism (4) for separation of the volume of electron source from the rest of the gun is installed in the intermediate housing (2), which is the link between the electron source and focusing-deflecting system. Focusing-deflecting system (3) is intended for focusing and deflection of electron beam during welding. Vacuum system is designed for differential pumping of the gun and consists of a vacuum hoses with flange for connection of the turbo-molecular pump (TMP) (5). Gun's cooling system is connected in-series with the cooling system of TMP. The vacuum system keeps in the electron source the pressure not more than 10-2 Pa.
Electrons emitted by heater are accelerated by a voltage of the order of 1 kV, bombard the cathode and heating it. As the electron source the tablet of hexaboride lanthanum is used.
Adjustment and stabilization of the electron beam current is implemented by controlling the bias voltage on the control electrode. For focusing and deflection of electron beam the focusing lens and double refraction deflecting system are used. The electron source is positioned vertically on the upper plate of the chamber, the electron beam is directed downwards. The characteristics of the electron gun shown in table.3.
Electron beam facility (Fig.2) consists of the following components: vacuum chamber; box with mechanical drive; frame; tapered pipe; flush pipe; Edwards XDS35i pump; Edwards EH250 pump; Edwards STPXA4503C ISO320F pump; VAT Series 14 DN320 (14050-PE44) valve; four VAT Series 26 DN63 (26436-QE41) valves; three VAT Series 26 DN25 (26428-КE41) valves; EdwardsWRG-S wide range gauge; Edwards APG-100 active Pirani vacuum gauge; control unit. Technical characteristics of the facility are shown in table.4.
The vacuum chamber has a system of water cooling of walls, the upper and lower plates. The flange for installation of the electron beam gun and the device for its vertical movement is placed on the upper plate of the processing chamber. The door of the chamber has a water cooling system. This door has a window with a diameter of 200 mm, made of materials that provide protection from ionizing radiation.
The control module includes start-up system for vacuum pumps and control system for valves. The main part of the control module is a special industrial controller, which provides data transfer via Ethernet between the server and the control units of actuators.
The electrical equipment consists of systems for pumping control, measurement of pressures in typical points of facility, power and control of electron beam source, process gases inlet in the vacuum chamber, positioning of workpieces/items inside the vacuum chamber relative to the electron beam source, video observation, as well as for centralized control and visualization of parameters.
One of the advantages of plasma sources of electron beams is the possibility of their work under medium-vacuum conditions, however, the developed technology does not involve changing the chemical composition of electro-vacuum glass in the treatment area. Accordingly, there is a need not only pumping of the working volume to the required pressure, but also ensuring replacement of the residual atmosphere in the processing chamber by an inert gas. With this purpose the gas supply system with flow regulators is provided.
The system for process gases inlet in the vacuum chamber consists of two identical channels. The gas puffing channel includes a shut-off valve and РРГ-10 (MFC, Brooks) gas flow controller. The following operating modes are provided:
• two independent channels, each of which supports a preset gas flow;
• one channel operates in the fixed gas flow mode, the other stabilizes the pressure in the chamber;
• two channels operate in the mode of pressure stabilization in the chamber, maintaining a predetermined ratio between the mass consumptions.
The workpiece is placed on a water-cooled table (Fig.3), which is mounted inside the chamber on its lower plate. The table is equipped with stepper drives (1), (2) and provides the movement of treated sample of 200 mm in the horizontal plane in two mutually perpendicular directions. If the sample is fixed in the rotating faceplates (3), then his rotation around the axis parallel to one of axes of horizontal movement is provided. For focusing of an electron beam on the treated surface the vertical movement of the electron beam gun of 100 mm is provided. The solution described allows electron beam processing both flat and cylindrical surfaces of products.
The positioning system is used to change the position of workpieces/items in the vacuum chamber relative to the electron beam source and includes an XY table, a spindle to rotate the part, and the drive to move the electron beam source relative to parts in the vertical axis for focusing the electron beam.
The positioning system consists of FL57STH four stepper motors, four SMD-4.2 stepper drivers, power supplies for drivers and of four EM253 extension modules in master controller. The controller sets the required speed of movement, times of acceleration and deceleration, searches the reference marks, monitors the status of position sensors. The connection between the controller and drivers of stepper motors is carried out by Step/Direction interface.
The project was performed in the framework of the State Contract No. 13411.1006899.11.063 with the financial support of the Ministry of industry and trade of the Russian Federation.