Micro Reentrant Cavity for 100 GHz Klystron

Mauro Mineo and Claudio Paoloni Department of Electronic Engineering

University of Rome Tor Vergata Via del Politecnico 1, 00133, Rome, Italy

Abstract: A micro reentrant rectangular cavity for klystrons operating at 100 GHz is proposed. The cavity is designed for higher order mode operation. This permits relative larger dimensions compatible with a realization by mechanical micromachining or high-aspect ratio lithographic fabrication process. Three-dimensional electromagnetic simulations demonstrate the properties of the cavity suitable to be used in a 100 GHz klystron.

Keywords: klystron; reentrant cavity; high order mode; micromachining.

Introduction Klystron is a well-known, high efficiency amplifier, with simple structure and scalable dimensions. It is typically designed with cylindrical reentrant cavities in fundamental mode [1 - 3]. When the operating frequency approaches the 100 GHz value, fabrication challenges become relevant. Different klystrons up to 200 GHz are reported in literature [4 -6].

The radius of a 100 GHz cavity is in the range of about one thousand microns, that makes the fabrication of a klystron structure a key issue. Mechanical fabrication techniques, due to the quality of the metal surface roughness, critical for losses, could be inadequate, when such dimensions are required, for a reliable and repeatable fabrication. The lithographic fabrication is a viable technique, but it allows only two-dimension patterns. Therefore, no geometry variation in the third dimension is possible, unless of using multi-step complex procedure. This makes unpractical the realization of cylindrical cavities.

In this paper, a micro reentrant rectangular cavity, purposely designed to be suitable for fabrication, operating at a higher order mode, is proposed. The larger dimensions, with respect to a cavity in fundamental mode working at the same frequency, and the rectangular shape, make the cavity compatible with the most advanced fabrication processes, enabling the realization of klystrons above 100 GHz.

Micro reentrant cavity A micro reentrant rectangular cavity working in high order mode is proposed and designed for 100 GHz operating frequency. A square transversal section was chosen to assure the best interaction with a cylindrical electron beam. The drift tube is included in the cavity geometry for a design oriented to application in klystron. A rendering of the micro rectangular cavity, including the

drift tube, is shown in Fig.1a. A schematic of the cavity is shown in Fig.1b.

a) b)

Figure 1 a) Square reentrant cavity; b) Schematic

The analysis and design of the cavity were performed by eigenmode 3-D simulations (CST – MWS). Firstly, it was defined the initial dimension of the cavity, on the basis of the fundamental mode dimensions, compatible to the constraints of the fabrication process. The parameters that mainly affect the operating frequency of the cavity are the transverse dimension, a + b, and the cavity length d.

An analysis of higher order modes to highlight the mode whose field distribution assures a relevant Ez component on the drift tube cross-section was performed. Higher order modes symmetrical with respect to both x and y Cartesian planes are considered. The fifth symmetrical mode is chosen due to its Ez-field distribution on the section of the drift tube, showing a field distribution similar to the fundamental mode. Potential mode competition was evaluated and it is assumed that can be easily controlled by a proper coupler design. The quality of the interaction of the RF with the electron beam is a function of the z -component of the electric field on the cross-section of the drift tube, where the electron beam flows.

Then, the dimensions were computed, as shown in Table I, to fix the frequency of the chosen high order mode at the desired value of about 100 GHz. From the point of view of fabrication, it could be convenient to increase the transversal dimension a + b of the cavity, decreasing d to maintain the frequency fixed. In Table II the fundamental and the chosen high order modes parameters are compared. The value of the unloaded Q = 1183 is comparable to the one for the cavity in fundamental mode. The R/Q = 25.3 Ω is lower than in case of fundamental mode, but in the typical range of higher order mode cavities [7]. The dimensions of the high order mode cavity, in particular the

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transverse dimension a + b = 2.43 mm, is more than twice than in case a cavity working at the fundamental mode at 100 GHz. The larger dimensions fully satisfy the constraints of micromechanics fabrication processes.

Figure 2 a) Fundamental mode; b) High order mode

The distribution of the z-component of the electric field of the fundamental mode and the fifth symmetrical mode are compared by 3-D electromagnetic simulation. The transverse section contour plots (a) and normalized amplitude plot (b) of the magnitude of the z-component of the E-field are shown in Fig.2 for the fundamental and the fifth symmetrical mode. For both the modes, a high level of electric field is present on the drift tube section. It can be noticed that, for the same amount of stored energy, the normalized level of the Ez field magnitude in the drift tube region is even higher for the higher order mode than the fundamental mode. The distribution of the Ez field on the longitudinal section of the drift tube region is shown for the fundamental (a) and the high order mode (b) in Fig.3. The Ez field distribution demonstrates that an effective interaction with the electron beam is assured and the operation in fifth higher order symmetrical mode is effective as in fundamental mode.

a) b)

Figure 3 a) Fundamental mode; b) High order mode

Conclusions A micro reentrant rectangular cavity to be used in 100 GHz klystrons is presented. The dimensions, derived for higher order mode operation, and the rectangular shape, are compatible with available mechanical micromachining and high-aspect ratio fabrication processes. Three-dimensional electromagnetic simulations demonstrate the quality of the electrical performance, in particular the high level of z-component of the electric field on the drift tube and of the other parameter fundamental for klystron performance.

Acknowledgements MIUR PRIN Project 2008 funded this work.

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Table 1. Cavity Dimensions

a 1.09 mm

b 1.34 mm

c 0.24 mm

d 1.8 mm

s 0.2 mm

Table 2. Cavity Mode Parameters

Mode Frequency (GHz) Q R/Q

Fundamental 18.16 1228 49.3

High order mode 101.1 1100 23.7

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