The Superconducting Linear Accelerator

Preparation of the superconducting cavities

Preparation of the superconducting cavities in the DESY cleanroom:
A string of eight cavities, each welded into its own liquid-helium tank, is being assembled and prepared for installation in an acceleration module. On the right: a single nine-cell cavity equipped with vacuum flanges and a radio-frequency input coupler for the performance test in a liquid-helium bath cryostat.

The electron injector section is followed by five 12-meter-long accelerator modules containing eight superconducting cavities each. The cavities are made from pure niobium and consist of nine cells. An important property of superconductors is that their resistance does not vanish in alternating electromagnetic fields, in contrast to the direct current case. In a microwave cavity the oscillating magnetic field of the RF wave penetrates into the superconductor to a depth of about 50 nm and induces forced oscillations of those electrons which are not bound in superconducting pairs. The microwave surface resistance is many orders of magnitude smaller than in normal copper cavities but is nevertheless responsible for non-negligible Ohmic heat generation at the inner cavity surface. The heat must be guided through the cavity wall into the liquid-helium bath. It constitutes a significant heat load on the helium refrigerator.

The exponential dependence of the resistance on temperature, predicted by the Bardeen-Cooper-Schrieffer (BCS) theory, is observed over a wide temperature range. Below 2 K one observes a residual resistance of a few nΩ caused by surface impurities. In the XFEL an accelerating field of more than 15 MV/m is needed to reach an FEL wavelength below 0.1 nm. Although the quality factor exceeds the excellent value of 1010, the RF energy dissipation in the cavity walls would be far too large for the liquid-helium plant if the cavities were operated in continuous mode. The necessary reduction of the cryogenic load by about a factor of 100 is the only – and unfortunate – reason to operate the cavities in pulsed mode with a duty cycle of 0.01.