Patent GB-861388-A (GB)
Title: High frequency velocity modulation apparatus
IPC: H01J23/30
Filed: 1957-03-25
Abstract: 861,388. Non reciprocal transmission devices. VARIAN ASSOCIATES. March 25, 1957 [May 30, 1956], No. 9773/57. Class 40(8) [Also in Group XL(a)] In a multistage velocity modulated amplifier such as a klystron, travelling wave tube or linear accelerator non-reciprocal field displacement attenuation means is provided. It is arranged between or in each stage (or some stages only). Each field displacement isolator has a pair of ferrite sheets 18 Fig. 2, (e.g. of ferramic J) on opposite sides of the guide and a resistive lossy strip 19 covering part of one of the ferrite sheets 18. A.D.C. biasing magnetic field B applied parallel to the planes of the ferrite sheets 18 and resistive strip 19, and which may be produced by the normal magnetic focusing structure for the tube, distorts the TE 10  wave travelling along the guide so that the forward wave passes along the side of the guide which does not have the resistive strip and is unattenuated but the reflected wave passes back along the other side of the guide and is dissipated by the resistive strip. The device may be a 4 resonator klystron, Fig. 1 in which the resonators 5, 6, 7, 8 are coupled by sections of guide 12, 13, 14 each housing two field displacement isolators 18, 19 or a 3 stage travelling wave tube in which each stage comprises one helix or other delay line, Fig. 6 (not shown), or in which a folded wave guide 26, Fig. 7, or interdigitated delay line 9, Fig. 8, is used. Successive resistive strips are arranged on different sides of the guide so that the forward wave crosses each interaction gap in the same plane. The magnetic focusing field preferably is directed from cathode to collector but if in the reverse direction the resistive strips must be on the opposite sides of the guides; it is produced by a solenoid 15, as shown in Fig. 1 or by a permanent magnet. The electron beam is velocity modulated by the input wave in resonator 5 further velocity modulation occurring in resonators 6, 7 and the final energy increment being extracted by resonator 8 and fed via waveguide 4 to the load. The waveguide coupling between the resonators in Fig. 1 may or may not be above the critical value. The guide sections 12 are coupled to the resonators 5, 6, 7, 8 by irises 9 sealed by alumina ceramic windows 11. Alternatively, the windows may be hollow cylinders surrounding the drift tube sections and sealed to the end walls of the resonators. To bring the direction of the transverse electric field component of the electromagnetic field into coincidence with the beam direction, the ferrite sheets are eliminated near the beam wave interaction spaces or successive guide sections are relatively transversely offset. Windows are provided at the input and output ends of the guide or within the waveguide sections outside the beam. In the multi-helix form, Fig. 6 (not shown) each helix both velocity modulates and extracts energy from the electron beam and in order to keep the incoming modulating signal and existing modulation carried by the beam in phase at the beginning of each stage the phase delay in the coupling guides with respect to the current modulation of the beam inside the travelling wave tube must be 2#n where n is positive, negative or zero. If less backward loss can be permitted certain of the non reciprocal lossy elements can be omitted, e.g. in Figs. 8, 9 every alternate lossy element can be omitted.