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DESY activities on 1.3 GHz tesla shape single cell and nine-cell large grain (LG) resonators are presented; results of the past five years are covered. The R&D program explores the potential for production of elliptical superconducting cavities. The main efforts have been devoted to material investigation, development of LG disk production, cavity fabrication from this material, and a search for appropriate treatment. More than 250 LG disks are manufactured; several single cell and 11 nine-cell resonators are produced and rf tested after buffered chemical polishing and after additional electropolishing. A maximum accelerating gradient of approximately 45 MV=m for this type of cavity was achieved in two resonators. Two of the LG cavities have been installed and are currently being used in the FLASH accelerator operation. Assembly of a cryomodule, consisting of LG cavities only, is in the works. Perspectives of the LG cavity application are discussed.

The R&D program on superconducting cavities fabricated from electron beam melted large grain/single crystal (LG/SC) niobium discs explores it's potential for production of approximately 1000 cavities for the European XFEL. Thermal, electrical, mechanical properties, crystal orientation and structure are investigated with the aim to make the fabrication procedure more efficient. In opposite to fine grain niobium the thermal conductivity of LG/SC has a pronounced maximum at 2K. Calculation found a correlation between thermal conductivity enhancement and phonon scattering at the grain boundaries. Detected enhancement is very susceptible to plastic deformation that can cause the complete elimination of the low temperature peak. The final annealing at 800°C of cavities made from large grain niobium is necessary for hydrogen outgassing, as well as for the thermal conductivity enhancement due to stress relaxation and recovery of crystal defects introduced at the cavity fabrication. The effects of annealing temperature up to 1200°C, heating rate, and holding time on the structure recovery after rolling are also established. Total elongation at the uniaxial tensile tests for LG is very high (50-110%) and depends significantly on the load direction, because only very few grains are in the gage length. The elongation after fracture by bi-axial testing (bulging test) for LG is lower (<15%) yet sufficient for deep drawing of half-cells. Metallographic investigation of and electron beam welding tests on, niobium single crystals show that an appropriate disc enlargement and annealing can be done without destruction of the single crystal. These tests showed that a cavity can be produced without grain boundaries even in the welding area. On base of the results a fabrication method of single crystal cavities is proposed.

The accelerating gradient Eacc of X-ray Free Electron Laser (XFEL) prototype cavities manufactured at the industry and treated at DESY demonstrates wide-range scattering from 15 to 41 MV=m. Most cavities satisfy the XFEL specification. Few cavities with low performance (15-17 MV=m) are limited by quench without field emission. The T-map analysis detected quench areas mainly close to the equator. Optical control by a high resolution camera has been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to understand the cause of reduced performance and get more detailed information of the origin of defects, some samples have been extracted from two cavities and investigated by light microscope, digital light microscope with 3D profile measurement, scanning electron microscope SEM, energy dispersive x-ray analysis, and Auger spectroscopy. The electron backscattered diffraction method in a SEM is used to make localized measurements of the lattice curvature. Several surface flaws with sizes from a few m to hundreds of m detected by microscopy. The defects can be separated into two categories. The first category of defects consists of foreign elements (often an increased content of carbon). Inclusions with increased content of carbon adhere on the surface and presumably have a hydrocarbon nature. Deviation from a smooth surface profile characterizes the second type of defects (holes, bumps, and pits). Some holes and bumps were found directly in the welding seam. The hot spots in the heat-affected zone (HAZ) of the equator welds have been partially associated with pits too. The study correlates the location of pits with the presence of plastic strain found to remain after welding. Pits near the HAZ were found either coincident with or near areas of high strain. Pits away from the weld were often found at grain boundary triple junctions.

We describe Cerenkov absorption by plasma electrons of electromagnetic waves that have a frequency considerably smaller than the gyro frequency of electrons and considerably larger than the gyro frequency of ions, the so-called “whistling atmospherics” that propagate along the inhomogeneous plasma cylinder. An expression has been deduced for the energy absorbed by a length unit of the plasma cylinder per unit time, and a coefficient has been found for the damping of free oscillations propagating along the plasma cylinder. If the phase velocity of the wave is of the order of the thermal velocity of the electrons and the wavelength is of the order of the plasma radius, the energy gained per unit time on the average by one plasma electron equals dW/dt ≈ (Hz2/Ho2)ωTe (Hz is the amplitude of the magnetic field of the wave; ω is its frequency; H0 is the steady magnetic field strength; Te is the electron temperature). The limits of applicability of the linear theory are discussed. For fields Hz larger than some critical value Hc ≈ H0 (ωτ)−2/3, where τ is the frequency of collisions between electrons and ions, a plateau forms for the background distribution function, Under conditions when distortion of the distribution function becomes large, dW/dt ≈ Tc/τ. Since W ≈ Tc, electron temperature increases in this case according to the law Te ≈ T0 [l + at/τ]2/3, where T0 is the initial elecgron temperature and a ≈ 1. Since the field of whistles penetrates well into the plasma, their Cerenkov absorption can be used for the heating of the electron component of a plasma with large density (n0 ≈ 1014—1015cm−3) and with a sufficiently large initial temperature (T0≳ 100 eV).