795 research outputs found
Simple model for the RF field amplitude dependence of the trapped flux sensitivity in superconducting RF cavities
The improvement of the performance of RF superconducting cavities has
recently motivated a considerable research effort in order to elucidate the
effect of trapped magnetic flux on the surface resistance . In this
paper we show that by introducing a non-linear pinning force in the
Gittleman-Rosenblum equations for the RF power dissipation due to a trapped
magnetic flux in a superconductor, we can empirically describe the linear
dependence on the RF field amplitude of the additional surface
resistance . We also show that the proportionality between the RF-field
dependent and independent terms and , and the
frequency dependence of follow naturally from this approach
A flexible space-variant anisotropic regularisation for image restoration with automated parameter selection
We propose a new space-variant anisotropic regularisation term for
variational image restoration, based on the statistical assumption that the
gradients of the target image distribute locally according to a bivariate
generalised Gaussian distribution. The highly flexible variational structure of
the corresponding regulariser encodes several free parameters which hold the
potential for faithfully modelling the local geometry in the image and
describing local orientation preferences. For an automatic estimation of such
parameters, we design a robust maximum likelihood approach and report results
on its reliability on synthetic data and natural images. For the numerical
solution of the corresponding image restoration model, we use an iterative
algorithm based on the Alternating Direction Method of Multipliers (ADMM). A
suitable preliminary variable splitting together with a novel result in
multivariate non-convex proximal calculus yield a very efficient minimisation
algorithm. Several numerical results showing significant quality-improvement of
the proposed model with respect to some related state-of-the-art competitors
are reported, in particular in terms of texture and detail preservation
Reflection of infrared radiation from thin aluminium layers
The thermal shielding of the LHC magnets cryostats will make use of Multi-Layer Insulation. This is a sandwich of several Mylar (polyester) foils 6 µm thick coated with a thin film of aluminium, having a thickness of some 30 nm. The thickness of the aluminium film must be kept at a minimum to minimise lateral thermal conduction. The outer layer of this sandwich stays at a temperature of 20 K or below, and receives IR radiation from surfaces at 77 K (wavelength of 37.6 µm at the peak of blackbody radiation), which should be reflected with the highest efficiency. The minimum thickness for the aluminium layer to avoid transmission of the radiation can be calculated by making use of the skin effect theory, taking into account the changes in electrical properties that are due to the extremely low thickness of the film
Measurement of the dynamic response of the CERN DC spark system and preliminary estimates of the breakdown turn-on time
The new High Repetition Rate (HRR) CERN DC Spark System has been used to
investigate the current and voltage time structure of a breakdown. Simulations
indicate that vacuum breakdowns develop on ns timescales or even less. An
experimental benchmark for this timescale is critical for comparison to
simulations. The fast rise time of breakdown may provide some explanation of
the particularly high gradients achieved by low group velocity, and narrow
bandwidth, accelerating structures such as the T18 and T24. Voltage and current
measurements made with the previous system indicated that the transient
responses measured were dominated by the inherent capacitances and inductances
of the DC spark system itself. The bandwidth limitations of the HRR system are
far less severe allowing rise times of approximately 12ns to be measured
Analysis and optimisation of a variational model for mixed Gaussian and Salt & Pepper noise removal
We analyse a variational regularisation problem for mixed noise removal that was recently proposed in [14]. The data discrepancy term of the model combines L1 and L2 terms in an infimal convolution fashion and it is appropriate for the joint removal of Gaussian and Salt & Pepper noise. In this work we perform a finer analysis of the model which emphasises on the balancing effect of the two parameters appearing in the discrepancy term. Namely, we study the asymptotic behaviour of the model for large and small values of these parameters and we compare it to the corresponding variational models with L1 and L2 data fidelity. Furthermore, we compute exact solutions for simple data functions taking the total variation as regulariser. Using these theoretical results, we then analytically study a bilevel optimisation strategy for automatically selecting the parameters of the model by means of a training set. Finally, we report some numerical results on the selection of the optimal noise model via such strategy which confirm the validity of our analysis and the use of popular data models in the case of "blind'' model selection
Diffusion of oxygen in niobium during bake-out
Bake-outs at temperatures between 100 C and 150 C for a duration up to two days have become customary for optimising the performance of bulk niobium cavities. This treatment results in the diffusion of oxygen, originating from the surface oxide, into the niobium. The theoretical oxygen profile has been simulated using the diffusion equations, and compared with some experimental results
New local field quantity describing the high gradient limit of accelerating structures
A new local field quantity is presented which gives the high gradient performance limit of accelerating structures due to vacuum rf breakdown. The new field quantity, a modified Poynting vector S_{c}, is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity S_{c} takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30Â GHz rf tests, both traveling wave and standing wave, and the value of S_{c} achieved in the experiments agrees well with analytical estimates
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