Development of a movable standing wave resonant test system for fundamental power couplers with an extraordinary power gain

Off-line high-power tests of the fundamental power couplers prior to their on-line operations are of importance for ensuring their operating reliability and stability. To test the couplers using a limited power supply effectively, a movable standing wave (SW) resonant test system with an extraordinary power gain has been developed at Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). The system consists of a movable resonator having two movable shorts for the enhancement and movement of the SW field, and a tunable secondary coupler for feeding power into the resonator without reflection. The proof-of-principle structure of the system has been built and tested both at the low-power and at the high-power levels. The low-power test demonstrates that the moving range of the SW resonant field is over half a wavelength which ensures that the fundamental couplers can be tested by SW field at all reflected phases, and the system can provide a power gain ranging from 50 to 94, corresponding to 200–376 ($4\times 50–4\times 94$) of power gain in the case of traveling wave resonant ring system. Two types of multipacting inside the couplers were observed during the high-power tests and the mechanism of their influences on the power gain was analyzed. This movable and high-power-gain solution can be beneficial for the promotion of SW resonant test systems for fundamental couplers.

Figure 1

Outline of the movable SW resonant test system.

Figure 2

The normalized power gain of the test scheme, with (a) at $Q_0=1000$, and (b) at $Q_0=10000$.

Figure 3

Mechanical drawings of the movable SW resonant test scheme: (a) the fundamental power couplers; (b) the coaxial connector; (c) the conjunction of two fundamental power couplers; and (d) the mechanical diagram of the test system.

Figure 4

The electric field distributions in the resonant cavities with four and five antinodes at 162.5 MHz. The movable shorts are at the limited positions, and the axis of the tunable secondary coupler lies at $z=0\mathrm{cm}$. (a) shows the resonant field in the cavities having four field antinodes, with the left short located at $z=-30$ and $-101\mathrm{cm}$; (b) shows the resonant field in the cavities having five field antinodes, with the left short located at $z=-30$ and $-127\mathrm{cm}$.

Figure 5

The normalized electric field magnitude in the cavities with four and five aninodes. These magnitude results correspond to the field distributions shown in Fig. 4, and are evaluated along the longitudinal lines at the same transverse position in the cavities. (a) shows the normalized electric field magnitude in the cavities with four field antinodes; (b) depicts the normalized electric field magnitude in the cavities with five field antinodes.

Figure 6

The mechanical drawings of the secondary coupler. (a) shows the coupler with electric probe; (b) depicts the enlarged view of the electric probe; (c) is the enlarged view of the magnetic loop; and (d) shows the coupler with magnetic loop.

Figure 7

The antenna is adjacent to the electric field node and the magnetic field antinode, with the field frequency at 162.5 MHz. The left short is at the position of $z=-88\mathrm{cm}$. (a) shows the electric field distribution in the cavity; (b) shows the magnetic field in the cavity.

Figure 8

The Smith Charts of the test system with the electric probe (a: under coupling, minimum $S_{11}=-2.5\mathrm{dB}$) and the magnetic loop (b: over coupling, minimum $S_{11}=-8.1\mathrm{dB}$). The left short is at $z=-88\mathrm{cm}$, and $h=1\mathrm{mm}$.

Figure 9

Connection of the two fundamental couplers to be tested. (a) presents the coaxial connector between the two couplers; (b) depicts the gaps preserved on both sides of the core insert.

Figure 10

The SW electric field distributions in fundamental couplers at different reflected phases. (a) presents the electric field with the left short at $z=-30\mathrm{cm}$; (b) shows the field with the left short at $z=-76\mathrm{cm}$.

Figure 11

Temperature distributions of the fundamental couplers. (a) depicts the temperature rises with the left short at $z=-30\mathrm{cm}$; (b) presents the temperature rises when the left short locates at $z=-76\mathrm{cm}$. The ambient temperature is around $27°\mathrm{C}$, and the power in cavity is at 2.5 kW.

Figure 12

Thermal stress distributions of the fundamental couplers with 2.5 kW of power in the cavity. The left short in (a) locates at $z=-30\mathrm{cm}$, and (b) at $z=-76\mathrm{cm}$.

Figure 13

Secondary electron yield curves for copper of the coaxial line and ceramic of the windows.

Figure 14

Multipacting distribution in the fundamental couplers when the left short is at $z=-30\mathrm{cm}$ (a) and $-76\mathrm{cm}$ (b), with 2.5 kW of power in the cavity.

Figure 15

The proof-of-principle structure of the test system: (a) and (b) show the tunable secondary coupler with the electric probe and magnetic loop; (c)–(f) present the conjunction processes of the two fundamental power couplers; (g) is the overall view of the proof-of-principle structure of the test system.

Figure 16

Proof-of-principle structure of the test system under the low-power tests. (a) gives the overall view of the resonant test system; (b) depicts the secondary coupler; (c) shows the VNA utilized in the low-power tests.

Figure 17

The achievable $S_{11}$ of proof-of-principle structure for the test system within the entire moving range.

Figure 18

Low-power test results of the proof-of-principle structure for the test system. (a) shows the power gain and $S_{21}$ parameter within the entire moving range of the system, and (b) gives the $Q_0$ and $k$ of the system.

Figure 19

The influences of the secondary coupler’s coupling factor $\beta$ and the cavity’s detuning $\delta$ on the power gain of the proof-of-principle structure. (a) shows the influence of the coupling factor with the cavity detuning $\delta$ kept at 0; (b) presents the influence of the detuning $\delta$ under different $\beta$, with the left short at $z=-70\mathrm{cm}$.

Figure 20

The proof-of-principle structure of the test system with the vacuum pumping systems. During the high-power tests, region between the two cold windows and that between the cold and warm windows in the right coupler were kept in vacuum, and the data of vacuum gauge 1 and 2 were recorded. P1 to P5 are the positions at which the temperatures were monitored.

Figure 21

Temperature rise at the monitored positions of the proof-of-principle structure with 2.5 kW of power inside the cavity. The ambient temperature is around $27°\mathrm{C}$.

Figure 22

High-power test results with the left short at $z=-30\mathrm{cm}$.

Figure 23

High-power test results with the left short at $z=-52\mathrm{cm}$.

Figure 24

High-power test results with the left short at $z=-76\mathrm{cm}$.

Figure 25

Different behaviors of the proof-of-principle structure caused by (a) the sustainable multipacting and (b) the unsustainable multipacting. (a) corresponds to the faded area in Fig. 23 and (b) corresponds to the faded area in Fig. 24.

Figure 26

Secondary electron yield as a function of the impact energy.