A magnetic circuit for 2450 MHz, as shown in Figure 64, was also designed. The required 3.38 kG can be obtained with this magnetic circuit. The Sm-Co magnet weighs approximately 6.25 oz with a magnetic height of 1.47 cm. The cathode back-bombardment power of 150 watts dissipated in the cathode requires an external radiator for its radiation into space. The radiator is conductively coupled to the cathode resulting in a cathode heat flow path which is approximately 0.3 cm longer than the magnet. The pyrolytic graphite radiator will be approximately 0.8 of the cathode diameter, but could be tapered larger at the top if necessary. With a thermal path of 2.4 cm, the cathode AT will be 132°C for the 2450-MHz Amplitron. By tapering the heat flow channel, the area could be increased more than half way, so that the AT could be reduced by approximately one third to 88°C. The magnetic circuit of the SSPS Amplitron operating at 3.3 GHz is shown in Figure 65. This radially-gaussed samarium-cobalt magnetic circuit supplies the required 4.55 kG and a desirable field shape in the interaction space. The magnet is 1.7 cm high and weighs approximately 6.26 oz.The cathode AT is 243°C and can be reduced to approximately 161 °C by tapering the heat flow channel. The AT in the cathode will not be a serious problem at either 2.45 or 3.3 GHz. Although the magnetic flux requirement is greater at the higher frequency, the volume and magnetic gap at 3.3 GHz results in the weight of the Sm-Co magnet being the same as at 2.45 GHz. The magnetic circuit for the SSPS Amplitron may consist of radially-gaussed Sm-Co magnets (which weigh approximately 6.26 oz), pole pieces for field shaping, and a steel flux return path (which would add approximately 3 oz to the weight of the magnetic circuit). Additional work on the magnetic circuit could result in a reduction of the magnetic weight by approximately 25%. Experience has shown that it is desirable to have the pole pieces for field shaping on the cathode structure to increase the operating efficiency of CW tubes. This results in a sharp knee on the V-I curve, which is necessary for CW tubes that operate at a low plate current. A computer plot of a Sm-Co magnetic circuit where the pole piece is attached to the cathode is shown in Figure 66. This is represented by the gap between the field shaping pole piece and the Sm-Co magnet. This magnetic circuit develops the required 4550 gauss in the interaction region for an SSPS Amplitron operating at 3.3 GHz. The weight of this radially-gaussed Sm-Co magnet is 5 oz. The study showed that the SSPS Amplitron could operate at either 2.45 or 3.30 GHz. Pertinent parameters are presented below: 2.45 GHz 3.3 GHz Max Temp. Vane temperature 380°C 405°C 460°C Cathode temperature 413°C 486°C 600°C Sm-Co magnet weight 6.26 oz 6.26 oz —
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