8 research outputs found
3D Simulation of the Effects of Surface Defects on Field Emitted Electrons
The ever-growing demand for higher beam energies has dramatically increased the risk of RF breakdown, limiting the maximum achievable accelerating gradient. Field emission is the most frequently encountered RF breakdown where it occurs at regions of locally enhanced electric field. Electrons accelerated across the cavity as they tunnel through the surface in the presence of microscopic defects. Upon Impact, most of the kinetic energy is converted into heat and stress. This can inflict irreversible damage to the surface, creating additional field emission sites. This work aims to investigate, through simulation, the physics involved during both emission and impact of electrons. A newly developed 3D field model of an 805 MHz cavity is generated by COMSOL Multiphysics. Electron tracking is performed using a Matlab based code, calculating the relevant parameters needed by employing fourth Order Runge Kutta integration. By studying such behaviours in 3D, it is possible to identify how the cavity surface can alter the local RF field and lead to breakdown and subsequent damages. The ultimate aim is to introduce new surface standards to ensure better cavity performance
THE EFFECTS OF FIELD EMITTED ELECTRONS ON RF SURFACE
The ever-growing demand for higher RF gradients has
considerably increased the risk of breakdown in
accelerating structures. Field emission is the most
common form of RF breakdown that generates free
electrons capable of inflicting irreversible damages on the
RF surface. This paper presents a systematic experimental
and simulation programme to understand possible sources
and their influence on RF cavity operatio
Experimental analysis of surface finish in normal conducting cavities
A normal conducting 805 MHz test cavity with an in built button shaped sample is used to conduct a series of surface treatment experiments. The button enhances the local fields and influences the likelihood of an RF breakdown event. Because of their smaller sizes, compared to the whole cavity surface, they allow practical investigations of the effects of cavity surface preparation in relation to RF breakdown. Manufacturing techniques and steps for preparing the buttons to improve the surface quality are described in detail. It was observed that even after the final stage of the surface treatment, defects on the surface of the cavities still could be found
Robust cognitive beamforming for cell-edge coverage in multicell networks with probabilistic constraints
In this paper, we introduce a downlink beamforming strategy in a cognitive cell located at the boarder of two adjacent cells of a multicell network to support the local cell-edge users of both cells. The proposed strategy is formulated as an optimization problem to minimize a linear combination of total transmit power of the cognitive base station (BS) and the resulting total interference on the other users located outside of the cognitive cell, so that the signal-to-interference-plus-noise ratio (SINR) targets of the cell-edge users are maintained. In a realistic scenario where CSI may be imperfect, the beamforming design for the cognitive BS based on perfect channel state information (CSI) can easily end up violating the tolerable interference levels of the users falling outside of the cognitive cell. We reformulate the proposed strategy as a robust optimization problem with outage-probability based constraints to account for the imperfection in CSI. Using the S-Procedure, we transform the intractable probabilistic constraints to a computationally tractable set of conservative deterministic constraints. Finally, applying the rank relaxation, we rewrite the resulting problem in semidefinite programming (SDP) form that can be solved using the standard convex optimization packages. The simulation results confirm the effectiveness of the proposed robust scheme in power-efficiently expanding the range of achievable SINR targets for the cell-edge users
A power-efficient coverage scheme for cell-edge users using cognitive beamforming
This paper addresses the problem of strong intercell interference on cell-edge users in conventional cellular networks by deploying cognitive cells within the vicinity of primary cell borders. The cognitive base stations serve primary cell-edge users within the cognitive cells. In return, the cognitive base stations are rewarded by the same spectrum allocated to the primary base stations to serve secondary users. We propose a strategy that is formulated as an optimization problem for the cognitive cell to minimize the total transmit power of the cognitive base station. This optimization problem is subjected to maintain a controlled level of interference at the primary outer-cell users falling outside of the cognitive cell and to assure required levels of signal-to-noise-plus-interference-ratio (SINR) at all primary cell-edge and secondary users within the cognitive cell. Simulation results confirm that the beamforming scheme in conjunction with the proposed cognitive structure lead to a significant reduction in overall power transmitted in the network
The effects of field emitted electrons on RF surfaces
The ever-growing demand for higher RF gradients has
considerably increased the risk of breakdown in
accelerating structures. Field emission is the most
common form of RF breakdown that generates free
electrons capable of inflicting irreversible damages on the
RF surface. This paper presents a systematic experimental
and simulation programme to understand possible sources
and their influence on RF cavity operation
Power-efficient downlink transmission in multicell networks with limited wireless backhaul
This article shows that division of a cell into tiers of smaller cells reduces power consumption. However, using the same frequency-time resources within multiple divided cells causes strong intercell interference. Given this circumstance, three beamforming techniques for multicell networks are presented to tackle the resultant challenging intercell interference environment. The schemes minimize the total transmit power across the coordinating base stations while simultaneously considering the quality of service of each user so that the latter is not unduly affected. Since the beamforming approaches require the circulation of information, an energy-efficient backhaul protocol is demonstrated