# Bandwidth Enhancement of a V-Band Klystron with Stagger-Tuned Multiple Radial Re-Entrant Cavities  
[PERSON]  
1 National Institute of Technology Mizoram, Aizawl 796012, India; [EMAIL_ADDRESS] (S.M.) [PERSON]  
1 National Institute of Technology Mizoram, Aizawl 796012, India; [EMAIL_ADDRESS] (S.M.) [PERSON]  
1 National Institute of Technology Mizoram, Aizawl 796012, India; [EMAIL_ADDRESS] (S.M.) [PERSON]  
2 CSIR-Central Electronics Engineering Research Institute, Pilani 333031, India  
23 Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India  
3  
[PERSON]  
1 National Institute of Technology Mizoram, Aizawl 796012, India; santigopal. current modulation in the beam by forming the denser electron bunch as the beam traverses through the successive intermediate cavities. The signal is amplified by the transfer of the kinetic energy of the electron beam when the bunches pass the output cavity, and the cavity fields decelerate their motion. Under this condition, maximum power is extracted from the beam at the gap of the output cavity, and the RF field grows. The gain of this amplifier can be up to 60 dB, with output power up to tens of MW, depending upon the operating frequency and configuration of the device. Usually, the bandwidth of this amplifier can reach up to 10% in some devices [5; 6]. The Klystron bandwidth is determined by two main factors: the interaction phenomena between an electron beam and RF waves and the quality factors of the output resonator circuit. Optimization of the interaction phenomena is achieved by the detuning of intermediate resonators and reduction in their Q-factors [7]. The use of multi-cavity interaction structures provides high interaction impedance, resulting in the possibility of efficient beam-wave interaction. Over an extended period, several design categories for enhancing bandwidth in Klystron have been researched [8; 9; 10; 11; 12; 13; 14; 15; 16]. Sheet beam type Klystrons, extended interaction type Klystrons, and triaxial Klystrons are popular candidates among them [17; 18; 19; 20; 21]. However, the fabrication difficulties limit their use. Using radial re-entrant cavities simplifies Klystron design and fabrication technology and provides higher shunt impedance and R/Q. The detailed design approach of Klystron employing radial re-entrant cavities (in synchronous tuning mode) stems from the article reported in [22]. The radial re-entrant type Klystrons, as reported in [22], are one of the potential candidates for efficient beam-wave interaction. In contrast to narrowband Klystron, to enhance the bandwidth, the use of more cavities and various tuning methods have been employed [23; 24; 25; 26]. The asynchronous tuning technique is a popular choice to enhance the bandwidth of Klystron tubes. In asynchronous tuning, the cavities are tuned to slightly different frequencies for a fixed-tuned broadband operation. Such tuning techniques are typically called \"stagger tuning.\" Staggered tuned bunching circuits with input and intermediate cavities can produce a well-bunched electron beam across a wide frequency range. The staggered tuning method is popularly used for multi-cavity Klystrons, generally employing up to five to six cavities. This paper illustrates the staggered tuning method applied to a V-band Klystron employing 11 cavities. A novel and systematic approach of stagger tuning has been used for a device employing a large number of cavities, reported in [27; 28]. The proposed device is simpler in design with a compact form factor and microfabrication compatibility, which are the main advantages of this device. This approach employs extensive three-dimensional simulation of the individual cavities (Eigen-mode simulations) and PIC simulation of the complete RF section at each frequency point of interest. The results of the PIC simulations for each tuning configuration of the device have been presented. Section II describes the design and parameters of the radial re-entrant squared cavities (RRSC).