Design, fabrication, and low-power rf measurement of an $X$-band dielectric-loaded accelerating structure

This paper presents the design, fabrication, and low-power rf measurement of an externally powered $X$-band dielectric-loaded accelerating (DLA) structure with a dielectric constant ${ϵ}_{\mathrm{r}}=16.66$ and a loss tangent $\tan \delta =3.43\times {10}^{-5}$. A dielectric matching section for coupling the rf power from a circular waveguide to an $X$-band DLA structure consists of a compact dielectric disk with a width of 2.035 mm and a tilt angle of 60°, resulting in a broadband coupling at a low rf field which has the potential to survive in the high-power environment. Based on simulation studies, a prototype of the DLA structure was fabricated. Results from low-power rf measurements and the comparison with simulated values are presented. A significant discrepancy was found between measurements and simulations. The detailed analysis on the fabrication errors which may cause the discrepancy is also discussed.

Figure 1

Conceptual illustration of an externally powered DLA structure connected with two matching sections and two ${\mathrm{TE}}_{10}$-${\mathrm{TM}}_{01}$ mode converters with choke geometry.

Figure 2

Front view and longitudinal cross section of a cylindrical DLA structure. ${ϵ}_{\mathrm{r}}$, $R_{\mathrm{in}}$, $R_{\mathrm{out}}$, and $L$ represent dielectric constant, inner radius, outer radius, and length for the DLA structure.

Figure 3

Measurement setup of the dielectric properties of the material sample.

Figure 4

Longitudinal cross section of a circular waveguide, a compact dielectric matching section, and a DLA structure.

Figure 5

(a) Electric field distribution for the compact dielectric matching section with a chamfered corner. (b) Electric field magnitude along path $AFC$ which denotes a section of lines and arcs connected by the points $A$, $F$, and $C$, as shown in (a), where the distance of point $A$ is taken as 0 mm.

Figure 6

Simulated $S_{11}$ and $S_{21}$ as a function of frequency for the compact dielectric matching section.

Figure 7

Longitudinal cross section of a circular waveguide, a compact dielectric matching section with a design gap, and a DLA structure.

Figure 8

Simulated $S_{11}$ and $S_{21}$ as a function of design gap $d_2$.

Figure 9

(a) Electric field distribution for the dielectric matching section with a design gap, where the thin metallic coating is denoted by the white lines. (b) Electric fields magnitude along path $AGC$ for different design gaps $d_2$. Here path $AGC$ denotes a section of lines and arcs connected by the points $A$, $G$, and $C$, as shown in (a), where the distance of point $A$ is taken as 0 mm.

Figure 10

Electric field distribution for the full-assembly structure with a quarter geometry, where line $MN$ is located on the center along the $z$ axis.

Figure 11

Simulated $S_{11}^{\prime }$ and $S_{21}^{\prime }$ as a function of frequency for the full-assembly structure shown in Fig. 10.

Figure 12

Electric field magnitude along the line $MN$ shown in Fig. 10. The distance of point $M$ is taken as 0 mm.

Figure 13

Full-assembly mechanical design for the whole structure, including a center part and two end parts.

Figure 14

Prototypes of the mode converter with half-choke geometry (a) and conflat flange (b).

Figure 15

(a) The coated dielectric tube is put into the outer copper jacket. (b) The assembly center-part prototype with the vacuum housing.

Figure 16

The mechanical assembly (a) and simulation modeling (b) of two end-part prototypes and an aluminum waveguide (see first row of Table 2).

Figure 17

Comparison between simulated and measured S-parameters [(a) $S_{11}^{\prime }$ and (b) $S_{21}^{\prime }$] for two end-part prototypes connected with an aluminum waveguide.

Figure 18

The mechanical assembly (a) and simulation modeling (b) of two end-part prototypes and center-part prototype (see second row of Table 2).

Figure 19

Comparison between simulated and measured S-parameters [(a) $S_{11}^{\prime }$ and (b) $S_{21}^{\prime }$] for the full-assembly structure.

Figure 20

Measured electric field magnitude for injecting rf power from ports 1 and 3 (a), and from ports 2 and 4 (b), respectively.

Figure 21

The assembly of two end-part prototypes and an aluminum waveguide connected with two power splitters (see third row of Table 2).

Figure 22

Comparisons between measured (a) $S_{11}^{\prime }$ and $S_{22}^{\prime }$, (b) $S_{12}^{\prime }$ and $S_{21}^{\prime }$ for two end-part prototypes connected with an aluminum waveguide and two power splitters.

Figure 23

The full-assembly structure connected with two power splitters (see fourth row of Table 2).

Figure 24

Comparisons between measured (a) $S_{11}^{\prime }$ and $S_{22}^{\prime }$, (b) $S_{12}^{\prime }$ and $S_{21}^{\prime }$ for the full-assembly structure with two power splitters.

Figure 25

The choke gap at both sides changes with the moving of the two-halves copper jacket.

Figure 26

Comparisons between measured (a) $S_{11}^{\prime }$ and $S_{22}^{\prime }$, (b) $S_{12}^{\prime }$ and $S_{21}^{\prime }$ for the full-assembly structure with a sliding distance $d=1.0\mathrm{mm}$.

Figure 27

Comparison between simulated and measured electric field magnitude, for injecting rf power from ports 1 and 3 (a), from ports 2 and 4 (b), respectively.

Figure 28

Comparison between simulated and measured S parameters [(a) $S_{11}^{\prime }$ and (b) $S_{21}^{\prime }$] for the full-assembly structure with $R_{\mathrm{in}}=2.99\mathrm{mm}$, $W_1=2.08\mathrm{mm}$ at one side and $W_1=2.12\mathrm{mm}$ at the other side.

Figure 29

Comparison between simulated and measured S parameters [(a) $S_{11}^{\prime }$ and (b) $S_{21}^{\prime }$] for the full-assembly structure with $R_{\mathrm{in}}=2.99\mathrm{mm}$, $W_1=2.08\mathrm{mm}$ at one side and $W_1=2.12\mathrm{mm}$ at the other side, and an offset of 1.0 mm for the two-halves copper jacket in both horizontal ($x$ axis) and vertical ($y$ axis) directions, a microgap of 1.0 mm with an alignment angle $\beta ={45}^{\mathrm{o}}$ [see Fig. 15].