99 research outputs found
The Quadrupole Resonator, Design Considerations and Layout of a New Instrument for the RF Characterization of Superconducting Surface Samples
A disk-shaped superconducting sample is welded onto an Nb support cylinder and exposed to the magnetic RF field of a four-wire transmission line resonator. The fields on the cylinder wall decay in a c ut-off like fashion in such a way that they perturb the measurement very little. RF dissipation of the disk is determined by substitution with a d.c. heater on the back of the sample which is made to produce the same temperature rise, controlled by thermometers
The quadrupole resonator: Construction, RF System Field Calculations and First Applications
The quadrupole resonator allows measurement of the RF properties of superconducting (sc) films deposited on disk-shaped metallic substrates. We describe the construction of the apparatus, the brazing and electron-beam welding procedures, the arrangements for compensating mechanical tolerances of samples and for assuring reproducible sample illumination. We explain the special features of the RF sy stem and give the results of field calculations with a 3D cavity code. Finally we present first measurements of Nb on Cu film samples and compare them with calibrations done with a bulk Nb sample
Status of RF power couplers for superconducting cavities at CERN
For LEP2 fixed RF power couplers of the open-ended coaxial line type with d.c. bias are used. The nominal power under matched conditions is about 120 kW at 352 MHz. However, to avoid ponderomotive instabilities, the cavities may not be detuned, i.e. the reactive beam loading cannot be compensated. The coupler is therefore exposed to standing waves with an equivalent power (travelling-wave (TW) producing the same field as the peak fields on the coupler line) of more than 200 kW. The final design of these couplers, their conditioning sequence and their actual performance are presented. For LHC a motor-driven mobile coupler is required to change the external cavity Q by a factor of four between beam injection and storage. During injection the forward power levels at 400 MHz are about 120 kW CW (for approximately 20 minutes) and 180 kW peak (for several milliseconds). Since practically all this RF power is reflected the equivalent travelling power is 480 kW and 720 kW, respectively. These couplers will be also provided with d.c. bias to suppress multipacting and ³deconditioning²
The LHC superconducting cavities
The LHC RF system, which must handle high intensity (0.5 A d.c.) beams, makes use of superconducting single-cell cavities, best suited to minimizing the effects of periodic transient beam loading. There will be eight cavities per beam, each capable of delivering 2 MV (5 MV/m accelerating field) at 400 MHz. The cavities themselves are now being manufactured by industry, using niobium-on-copper technology which gives full satisfaction at LEP. A cavity unit includes a helium tank (4.5 K operating temperature) built around a cavity cell, RF and HOM couplers and a mechanical tuner, all housed in a modular cryostat. Four-unit modules are ultimately foreseen for the LHC (two per beam), while at present a prototype version with two complete units is being extensively tested. In addition to a detailed description of the cavity and its ancillary equipment, the first test results of the prototype will be reported
The Higher-Order Mode Dampers of the 400 MHz Superconducting LHC Cavities
The accelerating system of the LHC consists of two structures per beam, each composed of four superconducting single-cell cavities. Their higher-order modes have to be damped sufficiently in order to prevent coupled-bunch instabilities and to limit parasitic mode losses. The first two higher-order modes do not propagate into the beam tubes between the cells. However, strong damping can be obtained with a special dipole mode coupler resonant at both modes. Because of the restricted space, a compact design is used. The other higher-order modes propagate and form coupled modes with unequal field distributions. They are damped by broadband couplers positioned on either side of each cavity cell. We present the design of the higher-order mode couplers together with measurements on a real cavity
Self-consistent solution of the Schwinger-Dyson equations for the nucleon and meson propagators
The Schwinger-Dyson equations for the nucleon and meson propagators are
solved self-consistently in an approximation that goes beyond the Hartree-Fock
approximation. The traditional approach consists in solving the nucleon
Schwinger-Dyson equation with bare meson propagators and bare meson-nucleon
vertices; the corrections to the meson propagators are calculated using the
bare nucleon propagator and bare nucleon-meson vertices. It is known that such
an approximation scheme produces the appearance of ghost poles in the
propagators. In this paper the coupled system of Schwinger-Dyson equations for
the nucleon and the meson propagators are solved self-consistently including
vertex corrections. The interplay of self-consistency and vertex corrections on
the ghosts problem is investigated. It is found that the self-consistency does
not affect significantly the spectral properties of the propagators. In
particular, it does not affect the appearance of the ghost poles in the
propagators.Comment: REVTEX, 7 figures (available upon request), IFT-P.037/93,
DOE/ER/40427-12-N9
Meson Form Factors and Non-Perturbative Gluon Propagators
The meson (pion and kaon) form factor is calculated in the perturbative
framework with alternative forms for the running coupling constant and the
gluon propagator in the infrared kinematic region. These modified forms are
employed to test the sensibility of the meson form factor to the
nonperturbative contributions. Its is a powerful discriminating quantity and
the results obtained with a particular choice of modified running coupling
constant and gluon propagator have a good agreement with the available data,
for both mesons, indicating the robustness of the method of calculation.
Nevertheless, nonperturbative aspects may be included in the perturbative
framework of calculation of exclusive processes.Comment: 18 pages, 7 figures. Discutions added, clarifing figures. Accepted to
be published in Phys. Rev.
Parasitic Energy Loss in the LEP Superconducting Cavities
The energy loss of bunches in the LEP superconducting (SC) cavities has been determined by measuring the closed orbit as a function of current with the beam position monitors located at finite dispersion. This method has already been used in earlier experiments to determine the distribution of the longitudinal impedance of different parts of LEP. In the present experiment the energy loss in two straight sections, containing only SC cavities, was compared with that in sections having both copper cavities and SC cavities. The results confirm the impedance calculations for the two types of cavities. The accuracy of the measurements was considerably improved by determining simultaneously the orbits of bunches with different currents. At the same time with these beam-based impedance measurements, the power dissipation was observed directly by local temperature monitors in different elements: the inter-cavity bellows inside the cryostat, the warm intermodule bellows, and Ferrite absorbers which were installed in two places to reduce the energy leaking out of cavities. These observations were correlated with the change of cryogenics power consumption, and showed an unexpected dependence of energy loss on beam energy
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