1302 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



3.3 The Helix 



The MI789 helix assembly is a rigid self-supporting structure com- 

 posed of three ceramic support rods bonded with glaze to the helix wind- 

 ing. A drawing of the helix assembly is shown in Fig. 1-4. The support 

 rods are made from Bell Laboratories F-66 steatite ceramic. This material 

 was chosen because of its low rf losses and because these losses do not 

 increase rapidly with temperature. Fig. 15 shows an enlarged photograph 

 of the glaze bonds between the winding and one of the support rods. 

 Attenuation is applied over a length of two inches starting li-^ inches 

 from the input end by spraying the helix assembly with aquadag (carbon 

 in water suspension) and then baking it. 



Supporting the winding by glazing it to ceramic support rods has the 

 following advantages : 



4.5 



V 20 MA 



n 30 MA 



O 40 MA 



A 50 MA 



Va = VARIABLE 

 Vh = 2400 volts 

 Vc= 1200 VOLTS 



0.5 



(.0 1.5 2.0 2.5 3.0 



MAGNETIC FLUX DENSITY 



3.5 



4.0 



BRILLOUIN FLUX DENSITY FOR BEAM FILLING HELIX 



Fig. 12 ^ The measurements of Fig. 11 normalized in terms of the Brillouin 

 flux density for a beam entirely filling the helix. The fact that the curves tend to 

 come together indicates that the concept of the Brillouin flux density retains some 

 meaning in the M1789. Because of the additional defocusing effects encountered 

 when the M1789 is driven to high output levels, the tube is usually used with 

 about 2.6 times the Brillouin flux density. 



