Design, fabrication, and high-gradient testing of an $X$-band, traveling-wave accelerating structure milled from copper halves

A prototype 11.994 GHz, traveling-wave accelerating structure for the Compact Linear Collider has been built, using the novel technique of assembling the structure from milled halves. The use of milled halves has many advantages when compared to a structure made from individual disks. These include the potential for a reduction in cost, because there are fewer parts, as well as a greater freedom in choice of joining technology because there are no rf currents across the halves’ joint. Here we present the rf design and fabrication of the prototype structure, followed by the results of the high-power test and post-test surface analysis. During high-power testing the structure reached an unloaded gradient of $100\mathrm{MV}/\mathrm{m}$ at a rf breakdown rate of less than $1.5\times {10}^{-5}\mathrm{breakdowns}/\mathrm{pulse}/\mathrm{m}$ with a 200 ns pulse. This structure has been designed for the CLIC testing program but construction from halves can be advantageous in a wide variety of applications.

Figure 1

The HFSS design of the structure demonstrating the structure half on the left and how each half will be joined together [7, 9].

Figure 2

HFSS model of a single cell quarter, a flatbed region representing gap added to house the metal joining surface [7, 9].

Figure 3

Surface field simulations for the CLIC-G-OPEN design compared to the CLIC-G structure. [9].

Figure 4

Rounding profile used on the CLIC-G-OPEN compared to previous CLIC structures [7].

Figure 5

A cross section of the cell geometry with parameters $F_x$ and $F_y$ which represent size of the flat region of the racetrack design [9].

Figure 6

Optimization of the straight sections, $F_x$ and $F_y$, in the racetrack geometry. [9].

Figure 7

CLIC-G-OPEN structure half with 24 regular cells and 2 matching cells. The waveguide coupler is also displayed on the right [11].

Figure 8

Photos of the CLIC-G-OPEN after milling and after installation [11].

Figure 9

S-parameters and results from bead-pull measurement of the brazed structure before and after tuning [11].

Figure 10

Photos of the CLIC-G-OPEN after milling and after installation [11].

Figure 11

Example of the rf signals for the input, transmitted, and reflected signals.

Figure 12

Summary of the CLIC-G-Open high power test history.

Figure 13

Breakdown distributions inside the structure normalized to the total number of breakdowns.

Figure 14

Diagram of a half of the CLIC-G-OPEN structure demonstrating the EDM cut in blue. Post-test observations with a borescope looked at the iris 2 (red) and iris 4 (green) before cutting. After cutting, SEM images of the first iris looked at the left (yellow) and right (cyan) nose, and the center (magenta). An X-Y cross section demonstrating structure nomenclature.

Figure 15

A comparison of the rf tests before and after high power testing.

Figure 16

Borescope images from the structure after conditioning. The location of the irises is described in Fig. 14.

Figure 17

Leaked brazing material visible at the input of the structure circled in blue.

Figure 18

Scanning electron microscope images of various segments along the first iris. Overlaid is a field map of the surface electric field for a comparison of the breakdown location to the field strength. The location of each scan is described in Fig. 14.