DESY, Hamburg
INFN/LASA, Segrate (MI)
The preparation phase for the European XFEL cavity production includes a number of actions. Material issues: qualification of high purity niobium vendors, verifying of large grain material as a possible option, construction of the scanning device for the niobium sheets. Mechanical fabrication issues: accommodation of the TESLA cavity design to the XFEL demands, device construction for RF measurement of components, integration of the helium tank and it's welding to the cavity into the fabrication sequence, documentation and data transfer, application of a new high resolution camera for inspection of the inside surface. Treatment and RF measurement: establishing the XFEL recipe, in particular the final surface treatment (final 40 μm EP or short 10 μm Flash BCP), and the cavity preparation strategy (vertical acceptance test with or without helium tank welded, with or without assembly of HOM antennas), construction of the cavity tuning machine. About 50 prototype cavities are produced at the industry, treated (partially in industry and partially at DESY) and RF-tested at DESY. The XFEL requirements are fulfilled with a yield of approx. 90%.
DESY, Hamburg
A straight drift-quadrupole system, though not being an achromat, can transport certain incoming beam ellipses without introducing first-order chromatic distortions. Several examples of such apochromatic beam transport are available in the literature. In this paper we show that the possibility of apochromatic focusing is a general property: For every drift-quadrupole system there exist an unique set of Twiss parameters (apochromatic Twiss parameters), which will be transported through that system without first order chromatic distortions. Moreover, we prove that at the same time the apochromatic Twiss parameters bring the second order effect of the betatron oscillations on the shift of the average bunch path length to the minimal possible value and also minimize the effect of betatron oscillations on bunch lengthening for Gaussian beam. As an example we consider the application of the apochromatic focusing concept to the design of matching sections and phase shifter of the post-linac collimation section of the European XFEL Facility.
DESY, Hamburg
An achromat is a focusing system, in which as large a number of higher order aberrations as possible is canceled by symmetries of the linear optics and the rest is corrected by the usage of third and higher order multipoles. The first achromats ever considered were repetitive achromats, in which the cancellation of higher order aberrations relies on appropriate selection of cell tunes. Later on achromats, employing mirror symmetry, were also developed. In this paper we remove one superfluous constraint on the linear optics in the theory of four cell mirror symmetric achromats, make an accurate consideration of two cell repetitive achromats, and compare the number of multipoles required for each of those achromats. Moreover, we contribute a point of view, from which both approaches to the achromat design become identical. As a practical application we consider the design of the arcs of the post-linac collimation section of the European XFEL Facility.