Breakdown study based on direct in situ observation of inner surfaces of an rf accelerating cavity during a high-gradient test

We have developed normal-conducting accelerating single-cell cavities with a complete higher-order-mode (HOM) heavily damped structure, into which we feed a 508.9-MHz continuous wave. During a high-gradient test of the second production version of the cavity, we performed a breakdown study based on direct in situ observation of the inner surfaces of the cavity. This paper presents our experimental findings obtained from this observation.

Figure 1

DR cavity.

Figure 2

Photographs of the end plate (a) before and (b) after electropolishing with approximately $40\mu \mathrm{m}$ of etching. The outer diameter is approximately 500 mm. The inner diameter of the beam port is 150 mm.

Figure 3

(a) Diagram of DR cavity No. 2. (b) Photograph of the test stand used for the high-gradient test of DR cavity No. 2 in a radiation-controlled area. The bold arrows indicate the flow of high rf power generated by a 1.2-MW continuous-wave klystron. The klystron is not included in this photograph. The turbo-molecular pump is behind the sputter-ion pump.

Figure 4

Mirror chamber used for the high-gradient test of DR cavity No. 2. (a) Aluminum elliptical mirror mounted on an ICF203 blank flange. The surface was finished by lapping, yielding $R_{\mathrm{a}}\approx 1\mathrm{nm}$. (b) Stainless steel vacuum chamber containing the mirror. The inner surface of this chamber was electropolished. (c) Mirror chamber attached to DR cavity No. 2.

Figure 5

(a) Schematic of the experimental devices. Red cylinders and yellow regions represent the three TV cameras and their fields of view, respectively. During the high-gradient test, the inside of the cavity (blue) was a vacuum, and the mirrors (green) were contained in the mirror chambers, as shown in Fig. 4. (b) Cross section along the horizontal plane including the center of the cavity.

Figure 6

Waveforms of the oscilloscope displayed for a time span of $20\mu \mathrm{s}$ ($=2\mu \mathrm{s}/\mathrm{div}$) when the interlock system was activated. The red dashed curves show the envelope of the 508.9-MHz pickup signal from DR cavity No. 2, and the red solid lines show the zero level. (a) Waveforms when the rf switch was turned off for a reason related to the klystron. (b) Example of cavity breakdown events.

Figure 7

Conditioning histories until $V_c$ reached (a) 0.90 MV for DR cavity No. 1 and (b) 0.95 MV for DR cavity No. 2 with data points recorded every 10 s. Data with no input rf power, data while the input rf power was kept constant, and data used to tune the control system or klystron are excluded here. The light blue lines indicate the reference vacuum pressure specified in the automatic conditioning by computer control. If the vacuum pressure is higher than the reference pressure, the rf power is stepped down until the vacuum pressure becomes lower than the reference pressure, and then the power is gradually stepped up as long as the vacuum pressure is lower than the reference pressure. The step-up slope is proportional to the difference between the reference and vacuum pressures.

Figure 8

Surface field of the accelerating mode at $V_c=0.90\mathrm{MV}$ ($E_{\mathrm{acc}}=3.5\mathrm{MV}/\mathrm{m}$). The region inside the white dashed ellipse on the FEP was observable with TV camera 3, and the observable region on the TEP was almost the same. The maximum strengths of the electric and magnetic fields are $13\mathrm{MV}/\mathrm{m}$ on the nose cone and $34\mathrm{kA}/\mathrm{m}$ at the aperture for the HOM waveguide, respectively. (a) Electric field on the FEP. (b) Electric field on the TEP with a vertical cut plane. (c) Magnetic field on the FEP. (d) Magnetic field on the TEP with a vertical cut plane.

Figure 9

Example of spot-type explosion events with a stable bright spot. (a), (b), and (c) are snapshots, in chronological order, of the FEP recorded with TV camera 3 during the rf conditioning. The insets are magnified views of the region outlined by a red square. The bright spot at 3 o’clock [inside the red square in (a)] exploded at $V_c=0.95\mathrm{MV}$, as shown in (b), and then disappeared, as shown in (c). A clear spike was observed in the intensity of the region outlined by a red square at the moment of this cavity breakdown, as shown in (d).

Figure 10

Snapshots of the FEP recorded with TV camera 3 at $V_c=0.90\mathrm{MV}$.

Figure 11

Snapshots of the TEP recorded with TV camera 2 at $V_c=0.90\mathrm{MV}$.

Figure 12

Example of spot-type explosion events without a stable bright spot. (a), (b), and (c) are snapshots, in chronological order, of the FEP recorded with TV camera 3 during the rf conditioning. The insets are magnified views of the region outlined by a red square. A spot-type explosion occurred at 11 o’clock [inside the red square in (b)] at $V_c=0.65\mathrm{MV}$; however, no bright spot was present in this region until one frame before this cavity breakdown, as indicated by the red square in (a). A clear spike was observed in the intensity of the region outlined by a red square at the moment of this cavity breakdown, as shown in (d).

Figure 13

Example of spot-type explosion events without a stable bright spot, accompanied by the sudden appearance of a new bright spot. (a), (b), and (c) are snapshots, in chronological order, of the FEP recorded with TV camera 3 during the rf conditioning. The insets are magnified views of the region outlined by a red square. There was an explosion at 1 o’clock [inside the red squares in (b) and (c)] at $V_c=0.56\mathrm{MV}$; however, no bright spot was present in this region until 2 frames ($2/30\mathrm{s}$) before this cavity breakdown, as indicated by the red square in (a). A clear spike was observed in the intensity of the region outlined by a red square at the moment of this cavity breakdown for two frames, as shown in (d).

Figure 14

Example of spot-type explosion events without a stable bright spot, accompanied by the sudden appearance of a new bright spot. (a)–(f) are snapshots, in chronological order, of the TEP recorded with TV camera 2 during the rf conditioning. The insets are magnified views of the region outlined by a red square. There was an explosion at 3 o’clock [indicated by the red square in (b)–(e)] at $V_c=0.80\mathrm{MV}$; however, no clear bright spot was present in this region until 4 frames ($4/30\mathrm{s}$) before this cavity breakdown, as indicated by the red square in (a). A new small bright spot appeared in this region 3 frames (0.1 s) before this cavity breakdown, as indicated by the red square in (b). A clear spike was observed in the intensity of the region outlined by a red square at the moment of this cavity breakdown for four frames, as shown in (g).

Figure 15

Example of spot-type explosion events without a stable bright spot, accompanied by the sudden appearance of a new bright spot. (a)–(f) are snapshots, in chronological order, of the FEP recorded with TV camera 3 during the rf conditioning. The insets are magnified views of the region outlined by a red square. In this case, a new bright spot appeared at 11 o’clock 45 frames (1.5 s) before this cavity breakdown at $V_c=0.54\mathrm{MV}$. A clear enhancement was observed in the intensity of the region outlined by a red square over the 46 frames, starting at the moment when the new bright spot appeared, as shown in (g).

Figure 16

Examples of non-spot-type breakdown events. (a) and (b) Cavity breakdown event at $V_c=0.76\mathrm{MV}$ accompanied by a flash. This event is included in the “Non-spot-type flash” column in Table 4. (c) and (d) Snapshots $2/30\mathrm{s}$ before a cavity breakdown at $V_c=0.90\mathrm{MV}$ accompanied only by lightning. No other phenomena were observed. This event is included in the “Non-spot-type lightning only” column in Table 4.

Figure 17

Distribution of explosion spots for spot-type explosion events, shown as circles. The plot colors of blue, green, yellow, and red indicate cavity voltage ranges of $V_c\le 0.40\mathrm{MV}$, $0.40\mathrm{MV}<V_c\le 0.60\mathrm{MV}$, $0.60\mathrm{MV}<V_c\le 0.80\mathrm{MV}$, and $0.80\mathrm{MV}<V_c\le 0.95\mathrm{MV}$, respectively, at the moment of cavity breakdown. The small holes, two each for the FEP and TEP, are monitor ports for pickup antennas. The horizontal and vertical dashed lines define the quadrants used in Fig. 18. This partitioning yields the smallest possible sections having the same geometry. Quadrant $Q_1$ of the TEP, shown in red, is where the brazing filler metal leaked (Appendix pp2), and the effect of the leakage was at a visual maximum.

Figure 18

(a) Fraction of spot-type explosion events with an explosion spot in each quadrant of the FEP (blue squares) and TEP (magenta circles), where the binomial errors are shown with a $1\sigma$ confidence interval. Quadrants $Q_1$, $Q_2$, $Q_3$, and $Q_4$ are defined in Fig. 17. A fraction of 0.25 (perfect uniformity) is shown by the dashed line. (b) Statistical significance of the deviation from perfect uniformity.

Figure 19

Radiation dose rates measured approximately 2 m from the cavities in the radiation-controlled area. The background level was approximately $0.1\mu \mathrm{Sv}/\mathrm{h}$.