REVISED PROOF
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Temperature stability of magnetic field for periodic
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permanent-magnet focusing system
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Li Li*, Jian-Ya Chen, Yun-Jie Liu
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Received: 20 October 2013 / Revised: 19 November 2013 / Accepted: 12 December 2013
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Ó The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2014
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Abstract In this study, finite element analysis based on
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an Ansoft Maxwell software was used to reveal the tem-
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perature stability of a magnet ring and the equivalent
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structural periodic permanent-magnet (PPM) focusing
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system. It is found that with the temperature increasing, the
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decrease rate of magnetic induction peak (B
z
)
max
of single
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magnet ring is greater than that of remanence B
r
of magnet
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in the range from room temperature to 200 °C, however,
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the PPM focusing system do have the same temperature
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characteristics of permanent-magnet materials. It indicates
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that the magnetic temperature properties of the PPM sys-
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tem can be effectively controlled by adjusting the tem-
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perature properties of the magnets. Moreover, the higher
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permeability of the magnets indicates the less H
cb
, giving
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rise to lower magnetic induction peak ðB
z
Þ
0
max
: Finally, it
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should be noted that the magnetic orientation deviation
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angle h (\15°) of permanent magnets has little effect on
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the focusing magnetic field of the PPM system at different
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temperatures and the temperature stability. The obtained
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results are beneficial to the design and selection of per-
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manent magnets for PPM focusing system.
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Keywords Periodic permanent-magnets focusing system;
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Rare earth permanent magnets; Finite element simulation;
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Temperature stability; Wireless underground sensor
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network
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1 Introduction
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In wireless underground sensor network (WUSN) [1],
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energy saving and amplification must be considered as a
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key factor, because the transmission medium of soil results
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in great attenuation to the electromagnetic wave transfer-
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ring in it [2, 3]. A traveling wave tube (TWT), which can
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be used in big power devices for data collection in WUSN,
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is one kind of microwave tubes by modulating the electron
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beam continuously to achieve the amplification function [4,
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5]. In the TWT, the electron beam interacts with the
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traveling microwave field of slow-wave circuit. During
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6–40 wavelengths of slow-wave circuit, electron beam
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continuously delivers the energy to the field of microwave
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signal, so that the signal could be amplified. A periodic
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permanent-magnet (PPM) focusing system is an important
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part of TWT [6–9]. Periodic permanent-magnet focusing
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system has a lot of advantages, such as small size, light
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weight, no power consumed, small stray magnetic field,
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and ease for use, which is suitable for the whole ultra-high-
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frequency band and the large power TWT. The axial
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magnetic field distribution of magnetic ring is the key to
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achieve periodic magnetic field of the focusing system.
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To obtain high performance of PPM focusing system, a
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few researchers have done lots of work up to now [10–15].
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Bao [10] reported that magnetic field only within a certain
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range was helpful to focus electron beam for a PPM
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focusing system. Cai et al. [11] devised a Ku band small
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power TWT PPM focusing system. Deng et al. [12] pointed
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out a graduated transition structure PPM focusing system,
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to improve the laminarity of the beam and increase the
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stability of the electron optical system. Cheng and Xie [13]
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indicated how to eliminate the irreversible loss of the
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magnetic ring saturated magnetically. Peng and Li [14]
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reported that the nonaxisymmetric feature of axial
A1 L. Li*, J.-Y. Chen, Y.-J. Liu
A2 Beijing Key Laboratory of Network System Architecture
A3 and Convergence, Beijing University of Posts and
A4 Telecommunications, Beijing 100876, China
A5 e-mail: lili66@bupt.edu.cn
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Rare Met.
RARE METALS
DOI 10.1007/s12598-013-0202-2 www.editorialmanager.com/rmet
Journal : Large 12598 Dispatch : 6-1-2014 Pages : 5
Article No. : 202
h LE h TYPESET
MS Code : RMET-D-13-00302 h CP h DISK
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