The Magnetically Focused Radial Beam Vacuum Tube 



By A. M. SKELLETT 



A new type of vacuum tube is described in which a flat radial beam of elec- 

 trons in a cylindrical structure may be made to rotate about the axis. Features 

 of the tube are its absence of an internal focusing structure and resultant sim- 

 plicity of design, its small size, its low voltages, and its high beam currents. 

 The focusing of the beams and their directional control are accomplished by 

 the magnetic fields in small polyphase motor stators. A time division multiplex 

 signaling system for 30 channels using these tubes is brieflj' described. 



IT HAS long been recognized that the substitution of electron beams for 

 mechanical moving parts would offer decided advantages in many applica- 

 tions in the field of communications. The high voltages .equired for the 

 usual cathode-ray type of tube and the very low currents obtainable there- 

 from prevent their use in most such proposals; their complicated guns and 

 their large sizes are also undesirable features. The kind of tube described 

 herein has no focusing structure, is small in size, requires only low voltages, 

 utilizes the cathode power efficiently, and produces beam currents of the 

 same order of magnitude as the space currents of ordinary vacuum tubes. 



Figure 1 shows the elementary tube structure. It consists, in the simplest 

 case, of a cyhndrical cathode of the sort in common use in vacuum tubes, sur- 

 rounded by a cylindrical anode structure. When this structure is made 

 positive with respect to the cathode and there is no magnetic field in the 

 tube, the electrons flow to the anode structure in all directions around the 

 axis. When a uniform magnetic field is applied with its direction at right 

 angles to the axis, the electrons are focused into two diametrically opposite 

 beams as shown. The beams are parallel to the fines of force of the magnetic 

 field so that if the field is rotated the beams move around with it. Thus the 

 magnetic field serves both to focus the electrons and to direct the resulting 

 beams to different elements of the anode structure. 



If ordinary commercial cathodes are used with anode structures an inch 

 or two in diameter, 100 volts or less on the anode will draw the full space 

 current for which the cathode was designed. The application of the mag- 

 netic field will then focus from 85 to 90 per cent of this electron current into 

 the two beams, the remaining 10 or 15 per cent being lost at the cathode due 

 to an increase in the space charge which the magnetic field produces. Some 

 of the smaller tubes produce beam currents of more than 5 milliamperes with 

 only 50 volts on the anode structure, and in some of the tubes with larger 

 cathodes beam currents of 50 milliamperes or more are easily obtainable. 

 The magnetic field strengths range from 50 to 300 gauss. 



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