Superconducting TESLA cavities Aune, B.; Bandelmann, R.; Bloess, D. ...
Physical review special topics. PRST-AB. Accelerators and beams,
09/2000, Letnik:
3, Številka:
9
Journal Article
Recenzirano
Odprti dostop
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≥25MV/m at a quality ...factor Q0≥5×109 . The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of Eacc≥15MV/m . In a first series of 27 industrially produced TTF cavities the average gradient at Q0=5×109 was measured to be 20.1±6.2MV/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=5×109 amounts to 25.0±3.2MV/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 research and development towards higher gradients is briefly addressed.
Coherent transition radiation (CTR) in the THz regime is an important diagnostic tool for analyzing the temporal structure of the ultrashort electron bunches needed in ultraviolet and x-ray ...free-electron lasers. It is also a powerful source of such radiation, covering an exceptionally broad frequency range from about 200 GHz to 100 THz. At the soft x-ray free-electron laser FLASH we have installed a beam transport channel for transition radiation (TR) with the intention to guide a large fraction of the radiation to a laboratory outside the accelerator tunnel. The radiation is produced on a screen inside the ultrahigh vacuum beam pipe of the linac, coupled out through a diamond window and transported to the laboratory through an evacuated tube equipped with five focusing and four plane mirrors. The design of the beamline has been based on a thorough analysis of the generation of TR on metallic screens of limited size. The optical propagation of the radiation has been computed taking into account the effects of near-field (Fresnel) diffraction. The theoretical description of the TR source is presented in the first part of the paper, while the design principles and the technical layout of the beamline are described in the second part. First experimental results demonstrate that the CTR beamline covers the specified frequency range and preserves the narrow time structure of CTR pulses emitted by short electron bunches.
The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) ...detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties.
High-gain free-electron lasers (FELs) are capable of generating femtosecond x-ray pulses with peak brilliances many orders of magnitude higher than at other existing x-ray sources. In order to fully ...exploit the opportunities offered by these femtosecond light pulses in time-resolved experiments, an unprecedented synchronization accuracy is required. In this Letter, we distributed the pulse train of a mode-locked fiber laser with femtosecond stability to different locations in the linear accelerator of the soft x-ray FEL FLASH. A novel electro-optic detection scheme was applied to measure the electron bunch arrival time with an as yet unrivaled precision of 6 fs (rms). With two beam-based feedback systems we succeeded in stabilizing both the arrival time and the electron bunch compression process within two magnetic chicanes, yielding a significant reduction of the FEL pulse energy jitter.
Precise measurements of the temporal profile of ultrashort electron bunches are of high interest for the optimization and operation of ultraviolet and x-ray free-electron lasers. The electro-optic ...(EO) technique has been applied for a single-shot direct visualization of the time profile of individual electron bunches at FLASH. This paper presents a thorough description of the experimental setup and the results. An absolute calibration of the EO technique has been performed utilizing simultaneous measurements with a transverse-deflecting radio-frequency structure that transforms the longitudinal bunch charge distribution into a transverse streak. EO signals as short as 60 fs (rms) have been observed using a gallium-phosphide (GaP) crystal, which is a new record in the EO detection of single electron bunches and close to the physical limit imposed by the EO material properties. The data are in quantitative agreement with a numerical simulation of the EO detection process.
Surface superconductivity in niobium for superconducting RF cavities Casalbuoni, S.; Knabbe, E.A.; Kötzler, J. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
02/2005, Letnik:
538, Številka:
1
Journal Article
Recenzirano
Odprti dostop
A systematic study is presented on the superconductivity (SC) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3
GHz cavities for the linear collider project TESLA. ...Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and DC magnetic fields, and also for different frequencies of the applied AC magnetic field. The bulk superconductivity parameters such as the critical temperature
T
c
=
9.26
K and the upper critical field
B
c
2
(
0
)
=
410
mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above
B
c
2
is found in all samples. The critical surface field exceeds the Ginzburg–Landau field
B
c
3
=
1.695
B
c
2
by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex AC conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) “coherent surface superconductivity”, allowing SC shielding currents flowing around the entire cylindrical sample, for external fields
B in the range
B
c
2
<
B
<
B
c
3
coh
, and (2) “incoherent surface superconductivity” with disconnected SC domains for
B
c
3
coh
<
B
<
B
c
3
. The “coherent” critical surface field separating the two phases is found to be
B
c
3
coh
=
0.81
B
c
3
for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.
The basics of superconductivity are outlined with special emphasis on the features which are relevant for the application in magnets and radio frequency cavities for high energy particle ...accelerators. The special properties of superconducting accelerator magnets are described in detail: design principles, magnetic field calculations, magnetic forces, quench performance and persistent magnetization currents. The design principles and basic properties of superconducting cavities are explained as well as the observed performance limitations and the countermeasures. The ongoing research efforts towards maximum accelerating fields are addressed and the coupling of radio frequency power to the particle beam is treated.
Study of the transverse coherence at the TTF free electron laser Ischebeck, R; Feldhaus, J; Gerth, Ch ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
07/2003, Letnik:
507, Številka:
1-2
Journal Article
Recenzirano
Odprti dostop
Double slits with different separations, crossed slits and circular apertures have been used to study the transverse coherence of the VUV light of the SASE Free Electron Laser at the TESLA Test ...facility at DESY. The resulting diffraction patterns are converted to visible light by a Ce:YAG crystal and imaged by a high-resolution CCD camera. The visibility of the diffraction patterns indicates a high degree of transverse coherence. Measurements have been taken at various operating modes and wavelengths of the FEL. A numeric FEL simulation code has been used to calculate the wavefronts of the light at the exit of the undulator. By propagating the wavefronts through the optical setup, the diffraction at the double slits is computed with the code GLAD. Good agreement with the measurements is found.
The main goal of this text is to provide a systematic and didactic approach to the physics and technology of free-electron lasers. Numerous figures are used for illustrating the underlying ideas and ...concepts, and many links to other fields of physics are provided.
PDF
