Production of superconducting 1.3-GHz cavities for the European X-ray Free Electron Laser

The production of over 800 1.3-GHz superconducting (SC) cavities for the European X-ray Free Electron Laser (EXFEL), the largest in the history of cavity fabrication, has now been successfully completed. In the past, manufacturing of SC resonators was only partly industrialized; the main challenge for the EXFEL production was transferring the high-performance surface treatment to industry. The production was shared by the two companies RI Research Instruments GmbH (RI) and Ettore Zanon S.p.A. (EZ) on the principle of “build to print”. DESY provided the high-purity niobium and NbTi for the resonators. Conformity with the European Pressure Equipment Directive (PED) was developed together with the contracted notified body TUEV NORD. New or upgraded infrastructure has been established at both companies. Series production and delivery of fully-equipped cavities ready for cold rf testing was started in December 2012, and finished in December 2015. More than half the cavities delivered to DESY as specified (referred to “as received”) fulfilled the EXFEL specification. Further improvement of low-performing cavities was achieved by supplementary surface treatment at DESY or at the companies. The final achieved average gradient exceeded the EXFEL specification by approximately 25%. In the following paper, experience with the 1.3-GHz cavity production for EXFEL is reported and the main lessons learned are discussed.

Figure 1

(a) Fully-equipped cavity. (b) Inside view of the cavity in the HT.

Figure 2

Overview of the treatment procedures including “final EP” and “BCP flash.”

Figure 3

The acceptance and release procedure for AL1 as example.

Figure 4

Cavity nonconformity life-cycle.

Figure 5

Flange assembly in the ISO 4 clean room at RI (courtesy of RI).

Figure 6

ISO 7 clean room adjoining the EP facility at RI (courtesy of RI).

Figure 7

ISO 7 clean room of EZ (courtesy of EZ).

Figure 8

Furnace for $800°\mathrm{C}$ annealing at EZ (courtesy of EZ).

Figure 9

Integral of weekly shipment of EXFEL cavities to DESY for the rf cold test.

Figure 10

Cavity length for the EXFEL cavities produced at RI, red lines—tolerance. The goal was to deliver the cavities in the lower tolerance range of length.

Figure 11

Fundamental-mode frequency for the cavities produced at RI, red lines—tolerance.

Figure 12

Measurements results of the ${\mathrm{Q}}_{\mathrm{load}}$ of the TM011 zero-mode at 2K for the EXFEL cavities produced at EZ; green line is a trend, red line—tolerance.

Figure 13

Measurements results of the ${\mathrm{Q}}_{\mathrm{load}}$ of the TM011 zero-mode at 2K for the EXFEL cavities produced at RI, violet and blue lines are trends before and after damping correction, red line—tolerance.

Figure 14

OBACHT image of a typical weld spatter found close to the equator welding seam. A maximum ${\mathrm{E}}_{\mathrm{acc}}$ of $35\mathrm{MV}/\mathrm{m}$ has been reached after the repair of the cavity.

Figure 15

OBACHT image (left) and laser scanning microscopy result on a replica sample (right) of an etching erosion on the equator welding seam. A maximum ${\mathrm{E}}_{\mathrm{acc}}$ of $15.9\mathrm{MV}/\mathrm{m}$ due to quench could not be overcome even after tank removal, grinding, $40\mu \mathrm{m}$ EP, and $10\mu \mathrm{m}$ final BCP.

Figure 16

OBACHT image of a scratch at the iris. The maximum ${\mathrm{E}}_{\mathrm{acc}}$ of this cavity was $22.5\mathrm{MV}/\mathrm{m}$ (limited by FE), but could be improved after repair to $33.0\mathrm{MV}/\mathrm{m}$ (limited by RF power).

Figure 17

Average value per month of the maximum and usable accelerating gradient (${\mathrm{E}}_{\mathrm{acc}}$) for the “as received” EXFEL cavities produced at RI. The red line indicates the specified requirement.

Figure 18

Average value per month of the maximum and usable accelerating gradient (${\mathrm{E}}_{\mathrm{acc}}$) for the “as received” EXFEL cavities produced at EZ. The red line indicates the specified requirement.

Figure 19

Examples of the ${\mathrm{Q}}_0$ (${\mathrm{E}}_{\mathrm{acc}}$) curves of some of the best cavities, either treated at RI using “EP final”, or at EZ using “BCP flash.”

Figure 20

Breakdown of cavity acceptance.

Figure 21

Histograms and yield plots of the usable gradient in the “as received” and “after retreatment” test for EXFEL cavities.