Validation of the superconducting 3.9 GHz cavity package for the European X-ray Free Electron Laser

A full test of the cavity package concept under realistic operating condition was a necessary step before the assembly of the European XFEL (EXFEL) 3.9 GHz superconducting system and its installation in the accelerator. One cavity, equipped with magnetic shielding, power coupler and frequency tuner has been tested in a specially designed single cavity cryostat in one of the test benches of the DESY Accelerator Module Test Facility (AMTF). The cavity was operated at high pulsed power up to an accelerating field of $24\mathrm{MV}/\mathrm{m}$, above the quench accelerating field of $21\mathrm{MV}/\mathrm{m}$ achieved during the continuous wave (CW) vertical qualification test and with a large margin with respect to the EXFEL maximum operating specification of $15\mathrm{MV}/\mathrm{m}$ for the 3.9 GHz system. All subsystems under test—coupler, tuner, waveguide tuners, low level radio-frequency (LLRF) system—were qualified to their design performances.

Figure 1

The two EXFEL 3.9 GHz cavity packages in the cavity string, with opposing coupler orientations.

Figure 2

The cavity package installed in the cryoadapter before connection to the XATB1 cryogenic feedbox. The cavity is supported by a sliding mechanism to the stainless steel suspension support, which reproduces the mechanical installation in the accelerating module cryostat.

Figure 3

Evolution of the relative HOM transmission spectra of the fundamental passband modes, from cavity reception (inset a), FPC installation (inset b, showing the detuning of the HOM1 notch filter), HOM retuning (inset c) and after installation of all ancillaries (inset d). In each plot, diamonds show the FPC-HOM2 passband, circles the PU-HOM1 passband and the triangle indicates the fundamental mode FPC-PU transmission.

Figure 4

The cryostat adapter during its connection to the XATB1 cryogenic feedbox.

Figure 5

Cooldown/warmup cycle of the cryoadapter for the 3.9 GHz horizontal tests. The cryogenic circuits are cooled and warmed with a standard procedure requiring approximately 14 h. More than 48 hours were needed for complete thermalization of the cold mass before the 2 K pump down.

Figure 6

Cavity frequency offset (left) and frequency offset per motor turn (right) during the tuning process.

Figure 7

Cavity transmitted power (shown in arbitrary units) using a rectangular low power rf pulse with 1.3 ms length. The bold (blue) region shows the portion of the pulse used for decay time and $Q_L$ evaluation.

Figure 8

Typical pulse measured during the long stable operation at $20\mathrm{MV}/\mathrm{m}$, 10 Hz operation with a 0.5 ms fill time and 0.6 ms flat top pulse in feed forward mode. $E_{\mathrm{acc}}$ (top plot) and forward power (bottom plot) are displayed in arbitrary units. Power readings and computed $Q_{\mathrm{ext}}$ values are shown in the left part of the control system panel.

Figure 9

Comparison between the vertical test measurements of the cavity at INFN-LASA and the qualification test in the horizontal cryostat at the AMTF.