To measure inter and intra-Jb we applied transport current as a function of perpendicular magnetic field on chemically polished single and bi-crystals on as-received, high-purity (RRR>250), large-grain niobium sheet [1]. After measurement, we were able to use magneto-optical imaging to observe preferential flux penetration at some Nb grain boundaries, and finally we observed the microstructure of the GB in the same sample by scanning transmission electron microscopy (STEM), using EELS (Electron Energy Loss Spectroscopy) for oxygen analysis. The TEM sample was prepared using a FIB (Focused Ion Beam) tool, which can create cross sectional TEM lamellae and preserve the topmost layer of the sample. The STEM and EELS study was accomplished at ORNL as part of a SHaRE collaboration [2].

The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of Eacc ≥ 25 MV/m at a quality factor Q0 ≥ 5 · 10 . The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of Eacc ≥ 15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q0 = 5 · 10 9 was measured to be 20.1 ± 6.2 MV/m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities additional quality control measures were introduced, in particular an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at Q0 = 5·10 9 amounts to 25.0±3.2 MV/m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing R&D towards higher gradients is briefly addressed.

The 3rd harmonic 3.9GHz accelerating cavity was proposed to improve the beam performance of the electron/positron linear accelerators. In the frame of a collaborative agreement, Fermilab will provide DESY with a cryomodule containing a string of four cavities. Seven 9-cell Nb cavities were tested and six of them did reach accelerating gradient up to 24 MV/m almost twice more than design value of 14 MV/m. Two of these cavities are with new HOM couplers with improved design. In this paper we present all results of the vertical and horizontal tests.

Kneisel, Ciovati, Myneni and co-workers at TJNAF have recently fabricated two superconducting cavities from the center of a large grain Nb billet manufactured by CBMM. Both cavities had excellent properties with one attaining an accelerating gradient of 45 MV/m (2 K) after a 48 h and 120 °C bake [P. Bauer et al., An investigation of the properties of BCP niobium for superconducting RF cavities, in: K.-J. Kim, C., Eyberger (Eds.), Proceedings of the Pushing the Limits of RF Superconductivity workshop, Argonne National Laboratory Report ANL-05/10, March 2005, pp. 84–93]. An investigation is underway to use magneto-optical (MO) imaging to observe the flux penetration behavior of a sheet sliced from this billet. The large grain size (some larger than 50 mm) allowed us to isolate multiple bi-crystals and tri-crystals. In the first stage of the present study we have taken the as-received sheet (RRR ∼280), which has been etched to reveal the grain structure. By magneto-optical examination we observed preferential flux penetration at some grain boundaries of a bi-crystal where the grain boundary was almost perpendicular to the sample surface and there was <1 μm surface step across the boundary. At other grain boundaries, with large steps or where the grain boundaries were not normal to the surface, we observed no preferential flux penetration. Preliminary transport measurements on a bi-crystal showed greater normal state resistance and lower superconducting critical current at the grain boundary.

The FNAL A0 Photoinjector is being reconfigured to test the principal of transverse to longitudinal emittance exchange as proposed by Cornacchia and Emma,Kim and and Sessler, and others. The ability to perform such an exchange could have major advantages to FELs by reducing the transverse emittance. Several schemes to carry out the exchange are possible and will be reported separately. At the Fermilab A0 Photoinjector we are constructing a beamline to demonstrate this transverse to longitudinal emittance exchange. This beamline will consist of a dogleg, a TMn110 5 cell copper cavity, and another dogleg. The beamline is designed to reuse the bunch compressor dipoles of the photoinjector, along with some existing diagnostics. Beamline layout and simulations are presented. Emittance dilution effects are also discussed.

We report results on the performance of a free-electron laser operating at a wavelength of 13.7 nm where unprecedented peak and average powers for a coherent extreme-ultraviolet radiation source have been measured. In the saturation regime, the peak energy approached 170 µJ for individual pulses, and the average energy per pulse reached 70 µJ. The pulse duration was in the region of 10 fs, and peak powers of 10 GW were achieved. At a pulse repetition frequency of 700 pulses per second, the average extreme-ultraviolet power reached 20 mW. The output beam also contained a significant contribution from odd harmonics of approximately 0.6% and 0.03% for the 3rd (4.6 nm) and the 5th (2.75 nm) harmonics, respectively. At 2.75 nm the 5th harmonic of the radiation reaches deep into the water window, a wavelength range that is crucially important for the investigation of biological samples.

An experiment is being developed at the FNAL Photoinjector Lab to demonstrate the exchange of longitudinal emittance with a transverse horizontal emittance. The longitudinal electric field of a TM110 cavity vanishes on axis and increases linearly with transverse displacement. This "shearing" electric field is pivotal to the exchange. The design of this TM110 cavity is a variant of the Fermilab 3.9 GHz superconducting deflecting mode cavity; however, the cavity was constructed of OFHC copper. The authors report on the construction, field flatness, polarization and high power testing of a TM110 cavity.

Many scientific disciplines ranging from physics, chemistry and biology to material sciences, geophysics and medical diagnostics need a powerful X-ray source with pulse lengths in the femtosecond range [1-4]. This would allow, for example, time-resolved observation of chemical reactions with atomic resolution. Such radiation of extreme intensity, and tunable over a wide range of wavelengths, can be accomplished using high-gain free-electron lasers (FEL) [5-10]. Here we present results of the first successful operation of an FEL at a wavelength of 32 nm, with ultra-short pulses (25 fs FWHM), a peak power at the Gigawatt level, and a high degree of transverse and longitudinal coherence. The experimental data are in full agreement with theory. This is the shortest wavelength achieved with an FEL to date and an important milestone towards a user facility designed for wavelengths down to 6 nm. With a peak brilliance exceeding the state-of-the-art of synchrotron radiation sources [4] by seven orders of magnitude, this device opens a new field of experiments, and it paves the way towards sources with even shorter wavelengths, such as the Linac Coherent Light Source [3] at Stanford, USA, and the European X-ray Free Electron Laser Facility [4] in Hamburg, Germany.

Fermilab is developing third a harmonic 3.9 GHz superconducting cavity to improve performances of A0 and TTF photoinjectors. In the frame work of this project we have built and tested two nine-cell copper models and one 3-cell niobium cavity. Properties of the high order modes were carefully studied in a chain of two copper cavities at room temperature. In this paper we discuss results of cold tests of the 3-cell cavity before and after surface treatment.

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.

We present experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 107 in the vacuum ultraviolet (VUV) region at wavelengths between 80 and 120 nm. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. The radiation pulse length can be adjusted between 30 fs and 100 fs. Radiation spectra and fluctuation properties agree with the theory of high gain, single-pass free-electron lasers starting from shot noise.

The facility at Fermilab was used to develop, fabricate, and commission an RF gun for the TeSLA Test Facility Linac. A time dependence is observed in the photo-emission and field emission from the photo-injector at Fermilab. The focussing solenoids can be adjusted to cause the QE and dark current to increase or decrease with time. The observed variation is in excess of a factor of 5 in the QE and a factor of 10 in the dark current, with time constants measured in hours. The evidence suggests that the source of the time-dependent behaviour is the Cs2Te cathode

When a relativistic electron bunch traverses a structure, strong electromagnetic fields are induced in its wake. For a 12 nC bunch of duration 4.2 ps FWHM, the peak field is measured >0.5 MV/m. Time resolution of approximately 5 ps is achieved using electro-optic sampling with a lithium tantalate (LiTaO3) crystal and a short-pulse infrared laser synchronized to the beam. We present measurements for both the longitudinal and radial components of the field and relate them to the wall impedance.

The A0 Photoinjector at Fermilab can produce high charge (10-14 nC) electron bunches of low emittance (20 pi mm-mrad for 12 nC). We have undertaken a study of the optimal compression conditions. Off-crest acceleration in the 9-cell capture cavity induces an energy-time correlation, which is rotated by the compressor chicane (4 dipoles). The bunch length is measured using streak camera images of optical transition radiation. We present measurements under various conditions, including the effect of the laser pulse length (2 ps sigma Gaussian vs. 10 ps FWHM flat top). The best compression to date is for a 13.2 nC bunch with sigma = 0.63 mm (1.89 ps), which corresponds to a peak current of 2.8 kA.

When a relativistic electron bunch traverses a structure, strong electric fields are induced in its wake. We present measurements of the electric field as a function of time as measured at a fixed location in the beam line. For a 12 nC bunch of duration 4.2 ps FWHM, the peak field is measured >0.5 MV/m. Time resolution of ~5 ps is achieved using electro-optic sampling with a lithium tantalate (LiTaO_3) crystal and a short-pulse infrared laser synchronized to the beam. We present measurements under several different experimental conditions and discuss the influence of mode excitation in the structure.

We present the first observation of self-amplified spontaneous emission (SASE) in a free-electron laser (FEL) in the vacuum ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.