The limits of beam-to-radio frequency (RF) efficiency, which are achievable using klystrons, are discussed in this article. Interaction between the electron beam and RF field in the output cavity is also investigated. A 100% efficiency could be achieved for certain RF fields and point-like beams. However, efficiency is reduced when fringe field, space–charge effect, and transverse beam distribution are considered. Numerical calculations on these factors are presented to investigate the power extraction mechanism in the output cavity, which will help in the optimization of future klystron designs. Large-signal klystron simulation code KlyC is used for 1-D and 1.5-D calculations.

Study on the Efficiency of Klystrons

A compact X-band (9.3 GHz) RF rotary joint has been developed in the accelerator laboratory of Tsinghua University. RF measurements on the rotary joint using VectorNetwork Analyser showed good results. In recent highpower tests, the RF rotary joint was operated under a 1.6 MW X-band magnetron. The incident power, the transmitted power and the pulse width of this rotary joint have been measured. The transmitted power kept stable in different rotation angles. In this paper, the setup and results of the high-power tests of this RF rotary joint are presented.

High-Power Test of a Compact X-Band RF Rotary Joint

A multi-beam klystron at 5.712GHz has been designed with efficiency of more than 80%. It can generate a pulse with output power of about 3MW and a pulse length of 5 μs. Space charge effect and large signal theory, which both increase the accuracy theoretically, are considered in the simulation. A series of parameters of cavities are given after optimizing, including the frequency, R/Q, Q0 and Qe. This paper describes the beam dynamics design of the klystron as well as a preliminary mechanical design.

Design of a C-Band High-Efficiency Multi-Beam Klystron

In this paper, the design and simulation of an ultra-high power rotary joint for X-band frequency are discussed. The proposed rotary joint consists of a mode converter at the beginning and the end, a contactless circular waveguide, and an external choke including ball bearings and impedance-matching stubs in the middle part. At the beginning and the end parts, by converting the TE10 mode of a rectangular waveguide to the TE01 mode of a circular waveguide, a symmetrical mode is created, which enables successive rotations of the structure without disturbing the electrical current. The use of circular TE01 mode in the proposed rotary joint leads to the power handling of 52 megawatts in the structure. The middle circular waveguide consists of choke and stubs for matching the contactless circular waveguide. The simulation results show that the proposed structure has a transmission loss of less than 0.4 dB and a return loss of better than 15 dB in the 10 GHz frequency.

Design and Simulation of Ultra High Power X-band Rotary Joint with a Matching Choke

A novel X-band (11.424 GHz) radio frequency (RF) power splitter has been designed, fabricated and tested at Tsinghua University. With E-bend waveguide structures and a modified concave bottom polarizer introduced in this work, a compact geometry is achieved. The RF power division ratio of two output ports can be adjusted arbitrarily by changing the position of a short circuit piston, controlled by a servo motor precisely. The input port maintains a good match at any output ratio. The parameters are optimized to acquire high RF power transmission capacity and large bandwidth . The cold test indicated that the max input reflection is −33.1 dB and the max output transmissions are −0.07 dB and −0.12 dB respectively, which agrees with the simulation results well. The high-power test was operated on Tsinghua High Power Test Stand. After days of conditioning, the RF power splitter can be operated steadily at 53 MW 250 ns input power with 20 Hz repetition frequency.

Development of a compact X-band variable-ratio RF power splitter

RF rotary joints allow the independent movement between the RF power source and the accelerating tube of a linear accelerator (linac). In this paper, the design of a compact X-band (9.3 GHz) high-power RF rotary joint is presented. Simulation results illustrate that RF parameters (the scattering matrix) of this rotary joint keep stable in the arbitrary rotation angle. The maximum return loss is about -30 dB, the insert loss is less than 0.11 dB, and the variance of output phase shifts is below 1 degree while rotating the joint. RF measurement on the rotary joint using Vector-Network analyser is also conducted and presented in this paper.

Development of a High-Power X-Band RF Rotary Joint

A 32-beam klystron working at 5.712 GHz has been designed with efficiency of 70% and output power of 3.4 MW. Core oscillations method (COM) is chosen to bunch electrons. The code KlyC is used for 1-D and 1.5-D calculation and a series of parameters are given after optimizing, including the position, frequency, R/Q, Q₀ and Qe of cavities. CST/PIC is used to make the final design and coaxial cavities are used. This paper describes 1-D, 1.5-D and 3-D beam dynamics of the klystron, compares their differences, and analyses effect of these differences on efficiency.

/pdf/there-dimensional-simulation-of-a-c-band-32-beam-klystron-1m0h6f9gvu.pdf

There-Dimensional Simulation of a C-Band 32-Beam Klystron

This paper presents a design of a compact variable X-band RF power splitter. The RF power splitter includes one input port and two output ports, and the power divi-sion ratio can be adjusted by changing the position of a short circuit piston. This system keeps a good match (less than -40 dB) at any power division ratio. An E-bend waveguide structure is selected to make the geometry more compact (11cm in length, 3.5cm in width and 5 cm in height). Special studies was conducted to sustain a low surface electrical field (maximum 65 MV/m at 100 MW input), and large bandwidth (250MHz). This power split-ter is designed for high-power test stand at Tsinghua Uni-versity.

/pdf/design-of-a-compact-variable-x-band-rf-power-splitter-4yu7mbx73d.pdf

Design of a compact variable x-band rf power splitter

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Development of a High-Power X-Band RF Rotary Joint

There-Dimensional Simulation of a C-Band 32-Beam Klystron

Design of a compact variable x-band rf power splitter