Field emission studies in vertical test and during cryomodule operation using precise x-ray mapping system

Field emission is one of the most serious issues in the degradation of superconducting cavity performance. However, surveying field emission sources during the cavity performance test (called the “vertical test”) and cryomodule operation is difficult. Therefore, to precisely investigate electron emissions from the field emission source, we developed a diagnostic system for the energy recovery linac 1.3 GHz nine-cell superconducting cavity for both vertical tests and the cryomodule operation. The developed system is comprised of two types of sensors: a carbon sensor and a Si $p$-$i$-$n$ diode, that measure the temperature rise and the radiation produced by electron emissions, respectively. Rotating the sensor array around the cavity axis in the vertical test provides detailed information on the entire cavity surface. The precise x-ray mapping profile measured by the system in the vertical test enables identification of the local emission source. This paper describes how the field emission source is identified in the vertical test from the standpoint of not only experimental results obtained with the newly developed rotating mapping system but also the detailed approach based on precise simulations. In addition, field emission studies with the optimized instrumentation have been extended to cryomodule tests, both standalone and in the beam line. These developments allow us to monitor changes in the cavity field emission signatures introduced by cryomodule assembly and beam operation.

Figure 1

Conceptual design (left) and picture (right) of the KEK-ERL model-2 cavity.

Figure 2

Schematic diagram (left) and picture (right) of the main-linac cryomodule [6].

Figure 3

Setup of the vertical test with the rotating mapping system of KEK-ERL model-2 cavity. (Left) Pictures of the overall mapping system and expanded view around the sensors. (Right) Sensor positions of the mapping system around one cell. Four carbon resistors per cell were set on the equator points every 90°. In addition, six carbon resistors (solid black), eight Si $p$-$i$-$n$ diodes per cell, and one $p$-$i$-$n$ diode per iris (light-blue) were arranged along the cavity at 0° cavity angle. The 0° cavity angle represents the initial point of the rotating mapping system.

Figure 4

Results of the fourth vertical test of the No. 1 ERL cavity. The horizontal axis shows $E_{\text{acc}}$ of the measured accelerating field. The left (right) vertical axis shows the measured unloaded $Q$ (measured x-ray on the top flange). The dynamic range of the radiation monitor is from 0.0001 to $100\mathrm{mSv}/\mathrm{h}$.

Figure 5

(Top) X-ray mapping data in the fourth vertical test of the No. 1 ERL cavity under $13.9\mathrm{MV}/\mathrm{m}$ accelerating field of the $\pi$ mode. The horizontal axis shows the rotating angle of the mapping system. The vertical axis shows the location of the $p$-$i$-$n$ diodes along the cavity axis, as shown in the left drawing. The color profile shows the voltages of the $p$-$i$-$n$ diodes. The brighter color indicates a higher voltage, as shown in the right bar. Unfortunately, we could not obtain the data of $p$-$i$-$n$ diodes set at the “6–7 iris” and “8cell down4,” as indicated in Fig. 3, owing to the break of signal lines in the vertical test. (Bottom left) Three-dimensional plot of the x-ray mapping data as in the top figure. (Bottom right) Expanded view of the stacked x-ray mapping data near the sharp radiation spot on the eighth cell and the ninth cell around a 330° cavity angle. The colored profiles of the figure show the stacked data of the sensors defined by Fig. 3.

Figure 6

Picture of the inner surface of the No. 1 ERL cavity taken with the inspection camera [22] after the fourth vertical test. The niobium tip was located at a 150° cavity angle at the 8–9 iris (the dotted orange circle).

Figure 7

Results of the final vertical test of the No. 2 ERL cavity. The horizontal axis shows $E_{\text{acc}}$ of the measured accelerating field. The left (right) vertical axis shows the measured unloaded $Q$ (measured x-ray on the top flange). The dynamic range of the radiation monitor is from 0.0001 to $100\mathrm{mSv}/\mathrm{h}$.

Figure 8

Temperature mapping data (left) and x-ray mapping data (right) obtained in the final vertical test of the No. 2 ERL cavity at a $23\mathrm{MV}/\mathrm{m}$ accelerating field. Both horizontal axes of the figures show the rotating angle of the mapping system. Both vertical axes of the figures show the location of the temperature sensors and $p$-$i$-$n$ diodes along the cavity axis. The color profile shows the voltages of the $p$-$i$-$n$ diodes (temperature rise) on the right (left) figure, respectively. The brighter color shows a higher voltage (temperature rise), as shown in the right bar of the right (left) figure.

Figure 9

Expanded views of the stacked temperature mapping data (left) and the stacked x-ray mapping data (right) near the sharp radiation spot on the seventh cell and the eighth cell at a cavity angle of approximately 50°. The colored profiles of both figures show the stacked data of the sensors defined by Fig. 3.

Figure 10

Temperature rise (left) and $p$-$i$-$n$ diode output (right) as a function of the accelerating field. The inner figure of the left figure shows the expanded view around the quench field. The inner figure of the right figure shows the plot in which the vertical axis is logarithmically represented. All data near the seventh cell and the eighth cell were taken at a 50° cavity angle.

Figure 11

Typical examples of electron trajectories from field emission sources near the iris position of the KEK-ERL model-2 cavity under an accelerating field of $15\mathrm{MV}/\mathrm{m}$ of the $\pi$ mode. The emitter positions of A, B, and C are located 6.8, 3, and 0 mm from the iris center along the cavity axis, respectively.

Figure 12

(Bottom) Simulated electron trajectories (red lines) from the field emission source for the KEK-ERL model-2 cavity under an accelerating field of $13.9\mathrm{MV}/\mathrm{m}$ of the $\pi$ mode for every 0.5° rf phase. The emitter is located at a 1 mm distance on the LBP side from the 8–9 iris center. (Top) The impact energies (blue dots) from the simulated electron trajectories inside the cavity along a 330° cavity angle (opposite the emitter location) for every 0.05° rf phase. The horizontal (vertical) axis shows the length along the cavity axis with iris and SBP end positions (the impact energies of electrons on the cavity surface including the end cap), respectively. The impact energies surrounded by the pink dotted area reached the SBP end position denoted by the pink dotted area in the bottom figure.

Figure 13

(Left) Power landing on the cavity surface ($P_{\mathrm{FE}}$) of the seventh cell, eighth cell, and ninth cell from the electron trajectories under the same conditions as shown in Fig. 12. The horizontal (vertical) axis shows the length along the cavity axis with iris positions [the power of $P_{\mathrm{FE}}$ calculated by . (2)], respectively. (Right) Power of $P_{\mathrm{FE}}$ as a function of the rf phase $\theta$. The horizontal (vertical) axis shows the rf phase (the power of $P_{\mathrm{FE}}$ and $E_{\text{acc}}$), respectively. The blue line (red line) in Fig. 13 (right) shows the power landing near the 8–9 iris (7–8 iris), and the pink line shows the power on the SBP end. The green dotted line shows the accelerating field.

Figure 14

X-ray mapping data in the fourth vertical test of the No. 1 ERL cavity under a $13.1\mathrm{MV}/\mathrm{m}$ accelerating field at the end cell of the $7/9\pi$ mode. The horizontal axis shows the rotating angle of the mapping system. The vertical axis shows the location of the $p$-$i$-$n$ diodes along the cavity axis. The color profile shows the voltages of the $p$-$i$-$n$ diodes. The simulated electron trajectories (red lines) for every 1° rf phase from the same emitter location as shown in Fig. 12 with the $7/9\pi$ mode under a $13.1\mathrm{MV}/\mathrm{m}$ accelerating field at the end cell are shown on the left side.

Figure 15

Explanation of how the field emission creates radiation patterns seen in the experimental results of the $\pi$ mode of the fourth vertical test under $13.9\mathrm{MV}/\mathrm{m}$ accelerating field with the No. 1 ERL cavity.

Figure 16

Results of the final vertical test of the No. 3 ERL cavity (left) and No. 4 ERL cavity (right). The horizontal axis of each plot shows $E_{\text{acc}}$ of the measured accelerating field. The left (right) vertical axis of each plot shows the measured unloaded $Q$ (measured x-ray on the top flange). The dynamic range of the radiation monitor is from 0.0001 to $100\mathrm{mSv}/\mathrm{h}$.

Figure 17

Setup of the $p$-$i$-$n$ diodes at helium jacket ends. Pink circles show the setting position of the $p$-$i$-$n$ diodes. Thirty-two $p$-$i$-$n$ diodes were set to detect the angular distribution of x-ray profiles of both sides.

Figure 18

(Top) Measured x-ray mapping data taken in the first vertical test of the No. 4 ERL cavity under a $22\mathrm{MV}/\mathrm{m}$ accelerating field of the $\pi$ mode. (Bottom left) Measured x-ray profiles at the 1–2 iris and 8–9 iris locations of the top figure. (Bottom right) Measured x-ray angular profiles with both the $p$-$i$-$n$ profile monitors. The horizontal axis shows the rotating angle of the mapping system. The vertical axis shows the voltage of the $p$-$i$-$n$ diode. Blue squares (red circles) show the measured voltage of each $p$-$i$-$n$ diode set at the LBP (SBP) side, respectively.

Figure 19

(Top) Measured x-ray mapping data taken in the final vertical test of the No. 3 ERL cavity (a) and No. 4 ERL cavity (c) under a $22\mathrm{MV}/\mathrm{m}$ accelerating field of the $\pi$ mode. (Bottom) Measured x-ray angular profiles with both the $p$-$i$-$n$ profile monitors taken in the final vertical test of the No. 3 ERL cavity (b) and No. 4 ERL cavity (d) under a $22\mathrm{MV}/\mathrm{m}$ accelerating field of the $\pi$ mode. The horizontal axis shows the rotating angle of the mapping system. The vertical axis shows the voltage of the $p$-$i$-$n$ diode. Blue squares (red circles) show the measured voltage of each $p$-$i$-$n$ diode set at the LBP (SBP) side, respectively.

Figure 20

(Left) Positions of the $p$-$i$-$n$ profile monitors of the cryomodule operation of the cERL main linac. The $p$-$i$-$n$ profile monitor had 16 $p$-$i$-$n$ diodes, and four $p$-$i$-$n$ profile monitors were set at both ends of the No. 3 and No. 4 ERL cavities as shown in the red squares. (Right) Pictures of the $p$-$i$-$n$ profile monitors at the SBP side (upper) and LBP side (lower).

Figure 21

Results of the high-power test of the No. 3 ERL cavity (left) and the No. 4 ERL cavity (right). The blue squares (red circles) in the right figure show the results before (after) the radiation-increasing event, respectively. The horizontal axis shows the accelerating field ($E_{\text{acc}}$), and the vertical axis shows the measured unloaded $Q$ in each plot.

Figure 22

(a) Arrangement of the $p$-$i$-$n$ diode position at the LBP and SBP sides on the cavity. Zero degree and the forward direction of the rotation of the cavity angle in this figure are the same as in Fig. 17. (b) The green triangles (red circles) of the x-ray angular profile with the $p$-$i$-$n$ profile monitor set on both ends of the No. 3 ERL cavity of the SBP (LBP) side under $9.6\mathrm{MV}/\mathrm{m}$ during the high-power test, respectively. Pink arrows show the expected x-ray peak from the previous vertical test. (c),(d) The pink triangles (blue circles) of the x-ray angular profile with the $p$-$i$-$n$ profile monitors set on both ends of the No. 4 ERL cavity of the SBP (LBP) side during the high-power test under $13.5\mathrm{MV}/\mathrm{m}$ before the unexpected radiation-increasing event (c) and under $11.5\mathrm{MV}/\mathrm{m}$ after the unexpected radiation-increasing event (d), respectively. The horizontal axis of each plot shows the $p$-$i$-$n$ diode position in (a). The left and right vertical axes show the output voltages of the $p$-$i$-$n$ diodes of the LBP and SBP sides, respectively.

Figure 23

Results of the measurements of the unloaded-$Q$ value of the No. 3 and No. 4 ERL cavities. The red (blue) circles show the final measurement results of the high-power test of the No. 3 (No. 4) ERL cavity as shown in Fig. 21. The pink (green) squares show the measurement results of the unloaded-$Q$ values of the No. 3 (No. 4) ERL cavity after the two-month beam operation in the cERL. The horizontal (vertical) axis shows the accelerating voltage (unloaded $Q$), respectively.

Figure 24

Measured x-ray angular profile at the No. 3 ERL cavity before beam operation (a) and after two-month beam operation (b). Measured x-ray angular profile of the No. 4 ERL cavity before beam operation (c) and after two month beam operation (d). All angular profiles were obtained under a $9.6\mathrm{MV}/\mathrm{m}$ accelerating field with the $p$-$i$-$n$ profile monitors at the LBP sides. The horizontal (vertical) axis shows the $p$-$i$-$n$ diode position (the measured voltages of the $p$-$i$-$n$ diodes) around the cavity, as shown in Fig. 22.

Figure 25

The left (right) figure shows the time trend of the measured $p$-$i$-$n$ diode output at the No. 3 (No. 4) cavity, respectively. The horizontal axis of each figure shows the time axis. The left (right) axis of each figure show the $p$-$i$-$n$ diode output (accelerating field), respectively. The solid lines of each figure show the $p$-$i$-$n$ diode output of each $p$-$i$-$n$ diode position at the LBP side around the cavity as denoted in Fig. 22. The red (blue) dotted lines show the accelerating field of the No. 3 (No. 4) cavity, respectively; these two dotted lines are superposed in the left and right figures.