Performance in the vertical test of the 832 nine-cell 1.3 GHz cavities for the European X-ray Free Electron Laser

The successful production and associated vertical testing of over 800 superconducting 1.3 GHz accelerating cavities for the European X-ray Free Electron Laser (XFEL) represents the culmination of over 20 years of superconducting radio-frequency R&D. The cavity production took place at two industrial vendors under the shared responsibility of INFN Milano–LASA and DESY. Average vertical testing rates at DESY exceeded 10 cavities per week, peaking at up to 15 cavities per week. The cavities sent for cryomodule assembly at Commissariat à l’énergie atomique (CEA) Saclay achieved an average maximum gradient of approximately $33\mathrm{MV}/\mathrm{m}$, reducing to $\sim 30\mathrm{MV}/\mathrm{m}$ when the operational specifications on quality factor (Q) and field emission were included (the so-called usable gradient). Only 16% of the cavities required an additional surface retreatment to recover their low performance (usable gradient less than $20\mathrm{MV}/\mathrm{m}$). These cavities were predominantly limited by excessive field emission for which a simple high pressure water rinse (HPR) was sufficient. Approximately 16% of the cavities also received an additional HPR, e.g. due to vacuum problems before or during the tests or other reasons, but these were not directly related to gradient performance. The in-depth statistical analyses presented in this report have revealed several features of the series produced cavities.

Figure 1

3D model of the series XFEL cavity equipped for delivery to DESY.

Figure 2

Overview of the surface treatment at RI (final EP scheme, left) and EZ (final flash BCP scheme, right).

Figure 3

Workflow of the vertical acceptance test. A simplified list of “decisions” (applied after each vertical test) is given in the gray box and their related branch points are indicated by the red numbered circles.

Figure 4

Section diagram of the vertical test cryostat. Note that only three of the four cavities are visible.

Figure 5

Left: vertical test inserts ready for test. Right: 3D view drawing of a test insert supporting four equipped cavities.

Figure 6

Magnetic field distribution inside the cryostats XATC1 (top) and XATC2 (bottom) of the bottom 2-m vertical section of the cryostat. The vertical extent of the cavities is indicated in gray.

Figure 7

The vacuum system of the vertical insert: $\mathrm{V}1/\mathrm{V}2$: main valves; V3.1–V3.4: all-metal angle valves placed on the top of the insert (room temperature); V4.1–V4.4: all-metal angle valve on the cavities (2 K); FRG 1–FRG 4: full range $\text{gauge}+\text{burst}$ disc; $\mathrm{I}1/\mathrm{I}2.1\text{−}\mathrm{I}2.4$: bellows.

Figure 8

Stacked bar chart showing the number of cold vertical tests by calendar month for RI (orange) and EZ (blue) cavities based on test date. The dashed line is the total average test rate taken from 1 October 2013 to 31 November 2015, which is considered the peak testing period (after ramp up and before ramp down).

Figure 9

Relative breakdown of the 831 cavities showing the fraction of cavities which have exactly 1, 2, 3, or 4 or more tests in the database.

Figure 10

Reasons for cavities requiring a second cold vertical test. Note that the percent figures indicate the fraction of the total number of cavities (831) and not the fraction of the chart itself (corresponding to the 28% of cavities with more than one test in Fig. 9).

Figure 11

Breakdown of the categories for the first cold vertical test in the database of the 831 cavities.

Figure 12

The distributions and yield curves for the maximum achieved gradient (blue) and the usable gradient (orange) for all cavities “as received.” Yield is defined as those cavities which have a gradient greater than or equal to the specified value. (The darker color represents the overlap of the two histograms.)

Figure 13

Scatter plot showing the maximum gradient and associated $Q_0$ achieved for the “as received” cold vertical tests, broken down by test limit reason. The dashed line indicates the ideal 200 W forward power limit. Nearly all power-limited cavities are below the $Q_0$ acceptance criterion of ${10}^{10}$. (BD: breakdown; $\mathrm{BD}+\mathrm{FE}$: breakdown with significant field emission; PWR: forward power limited; $\mathrm{PWR}+\mathrm{FE}$: forward power and significant field emission; other: e.g. HOM coupler heating; see text for more details.)

Figure 14

Breakdown of the limiting criteria for the usable gradient for “as received” cold vertical tests. Those cavities with the usable gradient technically limited by the $Q_0$ threshold (${10}^{10}$) have been further divided into those with no field emission [Q0 (No FE)] and those with field emission (Q0).

Figure 15

Distribution of the usable gradient in the “as received” cold vertical tests, separated out by field-emission limited ($\mathrm{FE}+\mathrm{Q}0$) and nonfield-emission limited [$\mathrm{BD}+\mathrm{Q}0$ (No FE)].

Figure 16

Distribution of usable gradient in the “as received” cold vertical tests for cavities showing no field emission, separated into breakdown (BD) and Q0(No FE) limited.

Figure 17

$Q_0$ distributions for the “as received” cold vertical tests, measured at $4\mathrm{MV}/\mathrm{m}$ (generally field emission free) and $23.6\mathrm{MV}/\mathrm{m}$. (Note that 15% of the cavities did not reach $23.6\mathrm{MV}/\mathrm{m}$ in the “as received” cold vertical test and therefore do not appear in the respective histogram.)

Figure 18

“As received” $Q_0$ measured at $23.6\mathrm{MV}/\mathrm{m}$ separated into cavities with and without field emission.

Figure 19

“As received” usable gradient performance by delivery month. The box-whisker symbol represents the gradient distribution for the respective calendar month. The dark blue joined dots indicate the monthly average gradient. The number of cavities delivered in each month is given at the base of the chart. The red dashed horizontal lines indicate the average performance over the first and second halves of the production, indicating improved average performance during the latter. (The last bin contains all remaining cavities arriving at DESY after 1 November 2015.)

Figure 20

The fractional numbers of cavities having “as received” usable gradient in the indicated ranges for quarterly production. A clear improvement (reduction) of the number of cavities with gradients less than $20\mathrm{MV}/\mathrm{m}$ can be seen in the second half of production.

Figure 21

“As received” low-field $Q_0$ performance (measured at $4\mathrm{MV}/\mathrm{m}$) by delivery month. (For an explanation of the plot see Fig. 19 caption.) The two red dashed horizontal lines indicating the average performance over the first and second halves of the production are indistinguishable, indicating no average improvement in the latter half. (The last bin contains all remaining cavities arriving at DESY after 1 November 2015.)

Figure 22

Breakdown of the reasons for the first cavity retreatment at DESY. Field emission, low $Q$, and quench are performance related (total of 66%) while leak and other are nonperformance driven. (Total chart represents 16% of the cavities.)

Figure 23

Fraction of cavities undergoing 1, 2, and 3 retreatments (after the first cold vertical test) broken down by category. (Note “1st” does not include the retreatments done before the first cold vertical test.)

Figure 24

Usable gradient before and after a retreatment for the specified retreatments. Those points above the dashed diagonal represent improvement.

Figure 25

Improvement in the usable gradient distribution after the application of HPR. To emphasize the effectiveness of HPR on low-performing cavities, only those cavities for which the initial (“before”) gradient was less than $20\mathrm{MV}/\mathrm{m}$ are shown.

Figure 26

Impact of HPR on the $Q_0$ measured at $4\mathrm{MV}/\mathrm{m}$. The average increases from $2.1\times {10}^{10}$ to $2.4\times {10}^{10}$ ($\sim 14\%$).

Figure 27

Breakdown of limiting criteria for the usable gradient of cavities before (left) and after (right) retreatment. Field emission is clearly significantly reduced.

Figure 28

Final maximum and usable gradient distributions of cavities accepted for string assembly (including retreatments).

Figure 29

Comparison of the usable gradient distributions for the first and final (accepted) test.

Figure 30

Breakdown of usable gradient limiting criterion for cavities accepted for string assembly (including retreatments).

Figure 31

Comparison of the distribution of $Q_0$ measured at $23.6\mathrm{MV}/\mathrm{m}$ for the first and final (accepted) tests. In both cases very few cavities were below the nominal specification of ${10}^{10}$.

Figure 32

Comparison of yield data for the first-pass maximum gradient taken from the ILC technical design report, and the European XFEL “as received” final EP treatment. The European XFEL data points were manually added to the original figure taken from [49] using the same error calculation formulas. Numbers in parentheses refer to cavity sample size.

Figure 33

Distribution and yield of the “as received” maximum gradient separated by vendor.

Figure 34

Distribution and yield of the “as received” usable gradient separated by vendor.

Figure 35

Distribution and yield of the “as received” low-gradient $Q_0$ (measured at $4\mathrm{MV}/\mathrm{m}$), separated by vendor.

Figure 36

$Q_0$ at the maximum achieved gradient for “standard” accepted cavities, separated by vendor. The gray dashed curve represents the 200 W power limit in the cold vertical test.

Figure 37

The distribution of the high-gradient ($\ge 25\mathrm{MV}/\mathrm{m}$) linear $Q$ slope expressed by the figure of merit ($s$) of the measured $Q_0(E)$ curves, separated by vendor.

Figure 38

Comparison of average maximum gradient (left) and average $Q_0$ at $4\mathrm{MV}/\mathrm{m}$ (right) of 814 cavities accepted for string assembly, separated by the vendors of the cell material. The bars give the 95% confidence level.

Figure 39

Schematic top view of both test cryostats ($\text{XATC}1/2$) showing the cavity positions inside the stands and the rf input power distribution. The orientation of the insert in the cryostat was always the same; it cannot be rotated. The insert structure is not visible.

Figure 40

Mean values of the quality factor measured at $4\mathrm{MV}/\mathrm{m}$ [$Q_0(4\mathrm{MV}/\mathrm{m})$] for all inserts. The error bars represent $\pm \mathit{\sigma }/\sqrt{\mathit{n}}$.

Figure 41

Mean values of the $Q_0(4\mathrm{MV}/\mathrm{m})$ distributions over the four possible insert positions (cf. Fig. 39). The error bars represent $\pm \mathit{\sigma }/\sqrt{\mathit{n}}$.

Figure 42

Relative change of usable gradient ($\mathrm{\Delta }$) with respect to the usable gradient as determined from the first power rise. Vertical tests with degradation or processing ($|\mathrm{\Delta }|>10\%$) are shown outside the horizontal dashed lines. Note the $1\mathrm{MV}/\mathrm{m}$ binning of the usable gradient causes overlaying data points. More than half of the vertical tests (573 out of 1101) showed no difference in the usable gradient within the vertical test (represented by the many overlaying dots at $\mathrm{\Delta }=0$).

Figure 43

Ratios of $Q_0$ measured at 1.8 K and 2 K at different accelerating gradients. The average ratio is $1.4\pm 0.2$.