Calculation and Testing Method of Gap Impedance for X-Band Klystron Multi-Gap Output Cavity

GUO Xin; ZHANG Zhiqiang; GU Honghong; LIANG Yuan; SHEN Bin

doi:10.11999/JEIT250002

Volume 47 Issue 8

Aug. 2025

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GUO Xin, ZHANG Zhiqiang, GU Honghong, LIANG Yuan, SHEN Bin. Calculation and Testing Method of Gap Impedance for X-Band Klystron Multi-Gap Output Cavity[J]. Journal of Electronics & Information Technology, 2025, 47(8): 2953-2962. doi: 10.11999/JEIT250002

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GUO Xin, ZHANG Zhiqiang, GU Honghong, LIANG Yuan, SHEN Bin. Calculation and Testing Method of Gap Impedance for X-Band Klystron Multi-Gap Output Cavity[J]. Journal of Electronics & Information Technology, 2025, 47(8): 2953-2962. doi: 10.11999/JEIT250002

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Calculation and Testing Method of Gap Impedance for X-Band Klystron Multi-Gap Output Cavity

doi: 10.11999/JEIT250002 cstr: 32379.14.JEIT250002

GUO Xin^{1, 2
,
,},
ZHANG Zhiqiang^{1, 2},
GU Honghong¹,
LIANG Yuan¹,
SHEN Bin^{1, 2}

1.
Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China
2.
School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 101408, China

Received Date: 2025-01-06
Rev Recd Date: 2025-05-29

Available Online: 2025-06-14

Publish Date: 2025-08-27

Abstract

Abstract

Objective From a structural standpoint, the klystron is a narrowband device that realizes beam-wave interaction through a sequence of independent resonant cavities. Advances in simulation techniques and computational methods for klystrons, as well as advancements in electric vacuum materials, manufacturing processes, and related technologies, have continuously enhanced their power and bandwidth performance. For example, high-power wideband klystrons are now widely applied in radar and communication systems. X-band wideband klystrons have achieved megawatt-level output pulse power, offering substantial utility in a range of radar applications. To satisfy the bandwidth demands of microwave electronic systems, research into wideband klystron technologies is increasingly prioritized. Expanding the bandwidth of the klystron output section is therefore a critical technology in the development of broadband klystrons. Methods Current approaches to expanding the frequency bandwidth of klystrons primarily rely on techniques such as staggered tuning of resonant cavities, integration of waveguide filters at the output cavity, and utilization of overlapping mode configurations in Multi-Gap Output Cavity (MGOC). The output section of high-frequency structures typically adopts a double-gap coupled cavity, for which gap impedance testing methods are relatively well established. Building on this foundation, a triple-gap coupled output cavity structure is developed, enabling further bandwidth enhancement. The flatness of the gap impedance across the operating band of an MGOC directly determines the gain and bandwidth performance of the klystron. Therefore, accurate calculation and testing of gap impedance are essential. This study proposes a method for calculating MGOC impedance based on cavity equivalent circuit theory. The MGOC is modeled as a resonant circuit comprising capacitive and inductive elements, and the gap impedance matrix is derived using the mesh current method. Based on microwave network theory, a corresponding experimental method for measuring MGOC impedance is also proposed. By analyzing the phase of the reflection coefficient at the output coupling port under various conditions—including all gaps open, single-gap short-circuits, and localized perturbations at individual gaps—the gap impedance within the frequency band of the cold test sample is determined. Using this theoretical framework, an X-band four-gap output cavity structure is designed. The gap impedance of the fabricated sample is measured to verify the validity of the proposed method. Results and Discussions The form of the MGOC impedance derived using equivalent circuit theory is presented (Equation 6). The experimental model of the X-band four-gap output cavity is constructed, and the optimized electrical parameters for each cavity are listed (Table 1). The calculated frequency bandwidth over which the internal impedance exceeds 3300 Ω in the X-band reaches 1200 MHz (Fig. 3). This represents a 30% increase compared to the triple-gap cavity and a twofold improvement over the double-gap cavity, meeting the expected design performance. The structural dimensions of the X-band four-gap output cavity are summarized (Table 2). A schematic of the MGOC modeled as an (n+1)-port microwave network is shown (Fig. 5). By solving for the impedance at the output coupling port, the relationship between the output port and other ports is obtained (Equation 13). The impedance for the all-gaps-open condition is given (Equation 14). The impedance for the case where a single gap is short-circuited and all other gaps remain open is derived (Equation 15), and the impedance corresponding to a perturbation in any single gap capacitance, with the remaining gaps open, is expressed (Equation 16). Based on transmission line theory, the impedance at each gap is calculated using these three sets of expressions (Equations 26～29). Using this theoretical framework, the X-band four-gap cavity prototype is fabricated and tested. To support structural optimization, the four fundamental mode field distributions of the four-gap cavity are first analyzed (Figs. 6 and 7). The parameters obtained via the equivalent circuit method are refined and adjusted for the cold test component. The final measured impedance distribution of the X-band four-gap cavity is presented (Fig. 9). The measured bandwidth, with a gap impedance exceeding 3400 Ω, reaches 1185 MHz, which closely agrees with the calculated result based on the equivalent circuit model. Conclusions This study proposes a method for calculating the gap impedance of a klystron MGOC based on the mesh current approach within the cavity equivalent circuit framework. A design scheme for an X-band four-gap output cavity is presented, and its impedance bandwidth is compared with those of triple-gap and double-gap cavities. The calculated bandwidth of the four-gap cavity is 33% greater than that of the triple-gap design and twice that of the double-gap counterpart. Building on this, a measurement method for MGOC gap impedance is developed using microwave network theory. Cold test experiments are conducted on an X-band four-gap cavity prototype. The measured results closely match the theoretical predictions, with the impedance exceeding 3400 Ω across nearly 1.2 GHz of bandwidth. Moreover, the proposed cold measurement technique enables the estimation of mutual impedance between cavity gaps by measuring impedance with any two gaps in a short-circuited state. This capability offers important insights into the coupling behavior among cavity modes. These findings provide a robust theoretical and experimental foundation for advancing broadband klystron technologies.
- Klystron,
- Multi-Gap Output Cavity (MGOC),
- Equivalent circuit,
- Microwave network

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References(20)

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