Manufacture of superconducting niobium cavity parts by hydropercussion punching

A complete cycle of manufacturing all parts of superconducting niobium cavities which comprise two half-cells, two drift tubes, and two flanges each has been developed. The cavity half-cells are manufactured by hydropercussion punching, which has a number of advantages over the instrumental stamping technique. For the first time, the diagram of formability of ultrapure niobium has been experimentally obtained for hydropercussion punching and the key parameters of the process have been determined that ensure complete deep-drawing of workpieces, viz., the value of the limit drawing ratio of ultrapure niobium that is 1.92 at a specific impact energy of 0.42 MJ/m². The deviations of the half-cell dimensions from the rated values do not exceed 0.1 mm. Production tools required for all manufacturing steps have been created. The parameters of machining of the niobium cavity parts prior to welding have been experimentally established.     

Improvement of the polishing treatment for niobium surfaces of superconducting cavity resonators

For superconducting cavities made from niobium sheet, which are interesting in the field of accelerator applications, smooth and defect free inner surfaces are needed to achieve a high unloadedQ along with high accelerating field strength. These can be obtained using chemical polishing procedures. Whereas normally a mixture of HF, HNO₃, and H₃PO₄ is applied, we chose the known bath containing HF, HNO₃, and H₂SO₄ and worked above 50 °C. The surface quality was judged by visual inspection using a light microscope as well as by microwave measurements of one spherical resonator. The new method gave less grain boundary etching and, with high field levels, an enlarged unloadedQ.     

Preliminary study of a niobium quarter-wave prototype cavity for a heavy-ion superconducting linac

Superconducting quarter-wave resonators,due to their compactness and their convenient shape for tuning and coupling,are very attractive for low-β beam acceleration.In this paper,two types of cavities with different geometry have been numerically simulated the first type with larger capacitive load in the beam line and the second type of lollipop-shape for 100 MHz,β=0.06 beams then the relative electromagnetic parameters and geometric sizes have been compared.It is found that the second type,whose structural design is optimized with the conical stem and shaping drift-tube,can support the better accelerating performance.At the end of the paper,some structural deformation effects on frequency shifts and appropriate solutions have been discussed.     

Preliminary study of a niobium quarter-wave prototype cavity for a heavy-ion superconducting linac

Superconducting quarter-wave resonators,due to their compactness and their convenient shape for tuning and coupling,are very attractive for low-β beam acceleration.In this paper,two types of cavities with different geometry have been numerically simulated the first type with larger capacitive load in the beam line and the second type of lollipop-shape for 100 MHz,β=0.06 beams then the relative electromagnetic parameters and geometric sizes have been compared.It is found that the second type,whose structural design is optimized with the conical stem and shaping drift-tube,can support the better accelerating performance.At the end of the paper,some structural deformation effects on frequency shifts and appropriate solutions have been discussed.     

Nuclear excitation by electrons and positrons

Existing measurements of the ratio $\text{σ}{}^{\mathrm{?}}\text{σ}{}^{\mathrm{+}}$, the nuclear excitation cross sections produced by electrons and positrons, are compared with the results of distorted wave calculations. The behaviour as a function of atomic number Z and electron energy E₀ is shown to be principally due to Coulomb distortion. The value of $\text{σ}{}^{\mathrm{?}}\text{σ}{}^{\mathrm{+}}$ displays no clear structure corresponding to resonances in the photoabsorption cross section, except that it should be quite sensitive to the presence of quadrupole strength.     

Design and simulation of a new type of 500 MHz single-cell superconducting RF cavity

This paper illustrates the design and simulation of a unique 500 MHz single-cell superconducting radio frequency cavity with a fluted beam pipe and a coaxial-type fundamental power coupler. The simulation results show that the cavity has a high r/Q value, a low peak surface field and a large beam aperture, so it can be a candidate cavity for high current accelerators. With the help of a fluted beam tube, almost all the higher order modes can propagate out of the cavity, especially the first two dipole modes, TE₁₁₁ and TM₁₁₀, and the first higher monopole mode, TM₀₁₁. The external quality factor of the coaxial fundamental power coupler is optimized to 1.2×10⁵, which will be useful when it is applied in the light source storage ring.     

Electromagnetic radiation of channeling electrons and positrons

General formulae for the spectral and angular distributions of radiation of relativistic channeled particles have been derived. These formulae are valid both for γ-quanta radiation and for radiation in the optical region. The problem of experimental detection of this radiation is discussed.     

Elastic-scattering cross-section for electrons and positrons from atomic hydrogen

The differential cross-sections of atomic hydrogen for elastic scattering of electrons and positrons have been rederived with the help of a method using a single parameter-dependent unitary shift operator for the calculation of the direct contribution. When the parameter approaches zerc the new method leads to the well-known conventional Glauber results. The numerical calculations include polarization effects and the exchange corrections obtained according to alternative approximation methods. Results calculated with Franco’s exchange show a definite improvement over the earlier results for medium energy electrons at large angles of scattering. Total elastic cross-sections have been calculated for 50 and 100eV electrons and positrons.     

Thermal conductivity of superconducting niobium

The phonon scattering term in the superconducting state of the electronic conduction has been obtained for niobium which is regarded as an intermediate coupling superconductor. The limiting slope for the phonon scattering term of the reduced thermal conductivity against reduced temperature has been found to be 2.8, as compared with 1.5 for weak coupling superconductors.     

Intermediate state of superconducting niobium

A domain pattern characteristic for an attractive interaction between flux lines and a correlation between the flux-line lattice and the crystal lattice of niobium crystals is observed.     

Streaking at high energies with electrons and positrons

A detection scheme for characterizing high-energy γ-ray pulses down to the zeptosecond timescale is proposed. In contrast to existing attosecond metrology techniques, our method is not limited by atomic shell physics and therefore capable of breaking the MeV photon energy and attosecond time-scale barriers. It is inspired by attosecond streak imaging, but builds upon the high-energy process of electron-positron pair production in vacuum through the collision of a test pulse with an intense laser pulse. We discuss necessary conditions to render the scheme feasible in the upcoming Extreme Light Infrastructure laser facility.     

Surface studies of niobium chemically polished under conditions for superconducting radio frequency (SRF) cavity production

The performance of niobium superconducting radiofrequency (SRF) accelerator cavities is strongly impacted by the topmost several nanometers of the active (interior) surface, especially as influenced by the final surface conditioning treatments. We examined the effect of the most commonly employed treatment, buffered chemical polishing (BCP), on polycrystalline niobium sheet over a range of realistic solution flow rates using electron back scatter diffraction (EBSD), stylus profilometry, atomic force microscopy, laboratory XPS and synchrotron (variable photon energy) XPS, seeking to collect statistically significant datasets. We found that the predominant general surface orientation is (1 0 0), but others are also present and at the atomic-level details of surface plane orientation are more complex. The post-etch surface exhibits micron-scale roughness, whose extent does not change with treatment conditions. The outermost surface consists of a few-nm thick layer of niobium pentoxide, whose thickness increases with solution flow rate to a maximum of 1.3-1.4 times that resulting from static solution. The standard deviation of the roughness measurements is ±30% and that of the surface composition is ±5%.     

Elastic scattering of electrons and positrons by cadmium atoms

ABSTRACT

The differential, integrated elastic, total and momentum transfer cross sections along with Sherman function for the elastic scattering of electrons and positrons by cadmium atoms have been evaluated from the partial wave solution of the Dirac relativistic scattering equations for a projectile-atom complex potential at the energy range 6.4 eV < E < 1.0 keV. For various scattering quantities, a comparison of our results exhibits better agreement with the experimental data than the other available theoretical values.     

超导直线增能器低温恒温柜的研制

文中详细介绍了超导直线增能器用低温恒温柜的研制过程.采用液氮冷屏保护,液氦作冷源,冷却四只QWR超导腔;静态热损耗小于10W,高频功率负载条件下总热损耗小于50W.该装置还具有机械调谐和功率调谐功能,设备的测量、控制和数据采集采用壹套PLC和工控机来完成,设备的自动化程度和可靠性较高.