Magnetic Beam Focusing on a CRT

This short video, the first for this year, explores magnetic electron beam focusing. It showcases an early method employed in electron guns to generate the necessary sharp dot on cathode ray tube screens. Two relatively large magnets are positioned on the tube neck and mechanically moved towards or away from each other to modify the electron path, bringing it into focus on the screen. The assembly is situated over the cylindrical final anode on the electron gun. Typically, the magnets are held in position within an aluminum housing, allowing movement only along the axis of the tube neck. In the presentation, they are displayed without this housing, influencing beam deflection, which would not normally occur.

Additionally, an alternative method is demonstrated using one permanent magnet and one electromagnet. This eliminates the need for mechanical adjustment, as varying the voltage applied to the coil will alter the field strength and achieve the same focusing technique. However, this approach is relatively energy-intensive compared to later electrostatic focus methods. While it adds complexity to the electron gun design, it eliminates the requirement for bulky external magnetic assemblies on the tube neck.

The CRT featured in this video is manufactured by Cinema Television Ltd (Cintel) and was designed for early admiralty radar use. The screen diameter is just 12cm.

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