The magnetic field of pulsars and the gravito-magnetic theory

宇宙知识英语作文Title: Exploring the Vast Expanse of the Universe;Introduction:The universe, with its infinite expanse and countless wonders, has fascinated humanity for centuries. Exploring the mysteries of space has been a continuous endeavor, and our understanding of the cosmos has grown exponentially over time. This essay aims to provide a glimpse into the vastness of the universe and the remarkable knowledge we have acquired about it.The Expanding Universe:One of the most fundamental discoveries in cosmology is the fact that the universe is expanding. Scientists have observed that galaxies are moving away from each other, indicating that the universe is constantly growing. This led to the formulation of the Big Bang theory, suggesting that the universe originated from a single, incredibly dense point billions of years ago.Galaxies and Stars:The universe is comprised of billions of galaxies, each containing billions of stars. Galaxies come in various shapes and sizes, from spiral and elliptical to irregular formations. They are held together by gravity and play a crucial role in shaping the cosmic landscape.Stars, the building blocks of galaxies, are fascinating celestial bodies. They range in size, temperature, and luminosity. Our own star, the Sun, is just one of billions in the Milky Way galaxy. Stars are born from vast clouds of gas and dust, and their lifecycle involves fusion reactions that release tremendous amounts of energy.Black Holes and Neutron Stars:Among the most enigmatic entities in the universe are black holes and neutron stars. Black holes are regions of spacetime with such intense gravitational forces that nothing, not even light, can escape their grasp. They are formed from the remnants of massive stars that have collapsed under their own gravity.Neutron stars, on the other hand, are incredibly dense stellar objects composed mainly of neutrons. They are the remnants of massive stars that have undergone supernova explosions. Neutron stars possess powerful magnetic fields and emit beams of radiation, which can be observed as pulsars.Cosmic Microwave Background Radiation:The discovery of cosmic microwave background radiation (CMB) has provided strong evidence for the Big Bang theory. CMB is faint radiation that permeates the entire universe and is a remnant of the intense heat generated during the early stages of the universe's formation. By studying the characteristics of CMB, scientists have gained valuable insights into the evolution of the universe. Dark Matter and Dark Energy:Despite our significant progress, there are still some mysteries surrounding the universe. Dark matter and dark energy, for instance, remain elusive. Dark matter is believed to make up a significant portion of the universe, exerting gravitational forces but not interacting with light. Dark energy, on the other hand, is thought to be responsible for the accelerated expansion of the universe.Conclusion:The universe is an awe-inspiring realm, filled with endless wonders waiting to be explored. Our understanding of the cosmos has come a long way, thanks to the relentless pursuit of knowledgeand advancements in technology. However, there is still much more to learn and discover. As we delve deeper into the mysteries of the universe, we gain a greater appreciation for its grandeur and our place within it.contribute to its preservation. Join us on and embark on an exciting journey to explore and appreciate the beauty and importance of our natural environment.。

你听说过脉冲星吗英语作文In the vast universe, there exists a mysterious and fascinating celestial object - the pulsar. Have you ever heard of it?A pulsar is a highly distinctive and intriguing object. It is a type of star that exhibits unique properties and behaviors.Pulsars emit regular pulses of radiation, which make them stand out in the universe. These pulses can be detected and studied using specialized instruments.They are incredibly compact and have extremely high densities. Their gravitational pull is immense, making them some of the most powerful objects in the cosmos.The study of pulsars provides valuable insights into various aspects of astronomy. It helps us understand the nature of matter, the behavior of stars, and the properties of extreme environments.Pulsars also play a crucial role in our understanding of the universe's evolution. They offer clues about the formation and evolution of stars, as well as the processes occurring in deep space.Discovering and studying pulsars requires advanced technological tools and expertise. Astronomers use radio telescopes and other sophisticated instruments to detect and analyze their signals.。

麦婷婷，高梦祥，李利，等. 磁场调控丝状真菌生长发育及代谢产物合成的研究进展[J]. 食品工业科技，2023，44（13）：450−457.doi: 10.13386/j.issn1002-0306.2022090173MAI Tingting, GAO Mengxiang, LI Li, et al. Research Progress on the Regulation of Magnetic Field on the Growth and Development of Filamentous Fungi and the Synthesis of Metabolites[J]. Science and Technology of Food Industry, 2023, 44(13): 450−457. (in Chinese with English abstract). doi: 10.13386/j.issn1002-0306.2022090173· 专题综述 ·磁场调控丝状真菌生长发育及代谢产物合成的研究进展麦婷婷1，高梦祥1，李　利1，张佳兰2, *，王劲松3（1.长江大学生命科学学院，湖北荆州 434025；2.长江大学动物科学学院，湖北荆州 434025；3.荆楚理工学院生物工程学院，湖北荆门 444800）摘　要：磁场作为一种普遍存在的环境因子，影响着微生物的生长及代谢。

丝状真菌是一类重要的异养型真核生物，在食品工业和生物医药领域有广泛应用。

目前，采用磁场处理丝状真菌已成为工业重要研究目标，而红曲霉、黑曲霉和黄曲霉是丝状真菌常用的典型菌种。

本文分别介绍了磁场调控红曲霉、黑曲霉、黄曲霉等丝状真菌生长及代谢的研究进展，通过不同磁场类型、作用时间、磁场强度等多种磁场参数分析三种丝状真菌磁场磁效应，阐述了磁场对三种典型丝状真菌的生长发育及其代谢物影响规律和代谢关系。

a r X i v :a s t r o -p h /0601246v 2 26 O c t 2006Invited talk,47th APS DPP Meeting,Denver,Oct.24–28,2005;Phys.Plasmas 13,056501(2006)[astro-ph/0601246]Turbulence,magnetic fields and plasma physics in clusters of galaxiesA.A.Schekochihin 1,∗and S.C.Cowley 2,31DAMTP,University of Cambridge,Cambridge CB30WA,UK2Department of Physics and Astronomy,UCLA,Los Angeles,California 90095-15473Plasma Physics Group,Imperial College,Blackett Laboratory,Prince Consort Road,London SW72BW,UK(Dated:February 5,2008)Observations of galaxy clusters show that the intracluster medium (ICM)is likely to be turbulent and is certainly magnetized.The properties of this magnetized turbulence are determined both by fundamental nonlinear magnetohydrodynamic interactions and by the plasma physics of the ICM,which has very low collisionality.Cluster plasma threaded by weak magnetic fields is subject to firehose and mirror instabilities.These saturate and produce fluctuations at the ion gyroscale,which can scatter particles,increasing the effective collision rate and,therefore,the effective Reynolds number of the ICM.A simple way to model this effect is proposed.The model yields a self-accelerating fluctuation dynamo whereby the field grows explosively fast,reaching the observed,dynamically important,field strength in a fraction of the cluster lifetime independent of the exact strength of the seed field.It is suggested that the saturated state of the cluster turbulence is a combination of the conventional isotropic magnetohydrodynamic turbulence,characterized by folded,direction-reversing magnetic fields and an Alfv´e n-wave cascade at collisionless scales.An argument is proposed to constrain the reversal scale of the folded field.The picture that emerges appears to be in qualitative agreement with observations of magnetic fields in clusters.I.INTRODUCTIONClusters of galaxies are vast and varied objects that have long attracted the attention of both observers (in recent decades spectacularly aided by X-ray and radio telescopes)and theoreticians.The observed properties of clusters have proved far from easy to explain as new data has confounded many old theories.The overall budget of the cluster constituents is roughly as follows:∼75%of cluster mass is dark mat-ter,whose sole function is assumed to be to provide the gravitational well,∼20%of cluster mass is the diffuse X-ray-emitting plasma (the intracluster medium,or ICM),while the galaxies have an all but negligible mass.The plasma that makes up the ICM is made of hot and ten-uous ionized hydrogen:temperatures are in the range of 1−10keV,number densities ∼10−1−10−3cm −3,with colder,denser material found in the cool cores and hotter,more diffuse one in the outlying regions.70Necessarily,the first observations and the first the-oretical models of clusters concerned what one might call large-scale features,such as the overall profiles of mass and temperature,the structure formation and the role of the central objects.As observations increased in accuracy and resolution,the ICM was revealed to be much richer than simply a dull cloud of X-ray glow smoothly petering out with distance from the center.A great panoply of features has been detected:bub-bles,filaments,ripples,edges,shocks,sound waves,etc.,as well as very chaotic density,temperature and abun-dance distributions.1,2,3,4,5It is particularly the presence of chaotic fields and the evidence that this chaos exists in a range of scales 4,6,7that makes one expect that ICM,like so many other astrophysical plasmas,is in a turbulent state.It is essential to know the properties of this turbu-lence in order to predict the current and future statisticalmeasurements of the plasma and magnetic fields (spectra,correlation and distribution functions,etc.)and to model correctly the transport processes in the ICM that deter-mine,for example,the overall temperature profiles.8,9We shall assume that the physics of small scales is,at least to some degree,independent of large-scale cir-cumstances and that we can,therefore,gain some use-ful understanding of the turbulence in clusters by ignor-ing large-scale features and considering a homogeneous subvolume of the ICM.In what follows,after reviewing briefly what is known about fluid motions (Sec.II)and magnetic fields (Sec.III)in clusters,we describe,mostly in qualitative terms,what we consider to be the essential aspects of the small-scale physics of the ICM (for plasma physics,see Sec.IV).This will lead us to a tentative overall picture of the structure of the ICM turbulence (Sec.VI)as well as of the origin of its magnetic compo-nent (Sec.V).We must emphasize that the current state of the debates on the nature of turbulence in clusters (and,indeed,on its very existence 10)is such that even a fundamental,conceptual view of the problem has not yet been agreed,and,therefore,a quantitative theory —even an incomplete one such as exists for hydrodynamic turbulence —remains a matter of future work.II.TURBULENCEThe failure of one of the instruments on the ASTRO-E2satellite has set offthe planned direct detection of cluster turbulence 11,12into the (probably not very dis-tant)future.However,indirect evidence of turbulent gas motions does exist:three recent examples are the broad spectrum of pressure fluctuations measured in the Coma cluster 4,detection of subsonic gas motions in the core of the Perseus cluster,13and,again for the Perseus cluster,2TABLE I:Cluster ParametersParameter Expression Cool cores a Hot ICMT observed3×107K108Kn observed6×10−2cm−310−3cm−3v th,i(2T/m i)1/2700km/s1300km/sνii1.5nT−3/2b5×10−13s−12×10−15s−1λmfp v th,i/νii0.05kpc30kpcµ v th,iλmfp1028cm2/s1031cm2/sη3×1013T−3/2b200cm2/s30cm2/sU inferred250km/s300km/sL inferred10kpc200kpcL/U inferred4×107yr7×108yrRe UL/µ 702Rm UL/η4×10276×1029t visc(L/U)Re−1/25×106yr5×108yrl visc L Re−3/40.4kpc100kpcl res L Rm−1/25000km8000kmΩi,eq eB eq/cm i0.3s−10.04s−1ρi,eq v th,i/Ωi,eq3000km30,000kmB0B eqρi,eq/λmfp5×10−17G2×10−19G B1Eq.(13)c3×10−14G2×10−17G B2Eq.(15)c8×10−7G2×10−7GB visc B eq Re−1/49×10−6G4×10−6GB eq(8πm i nU2/2)1/23×10−5G4×10−6Gβeq8πnT/B2eq820l⊥(B2/B eq)L0.2kpc7kpcl B observed1kpc10kpca These numbers are based on the parameters for the Hydra A cluster given in Ref.20.b In these expressions,n is in cm−3,T in Kelvin.c We usedα=3/2to get specific numbers,but the outcome is not very sensitive to the value ofα.the broadening(assumed to be caused by turbulent dif-fusion)of abundance peaks associated with the brightestcluster galaxies.14These and other studies and models based on observational data appear to converge in ex-pecting turbulentflows with rms velocities in the range U∼102−103km/s at the outer scales L∼102kpc. The energy sources for this turbulence are probably the cluster and subcluster merger events and/or,especiallyfor the turbulence in cool cores,the active galactic nu-clei(AGN).The aforementioned observational estimates of the strength and scale of the turbulence are in order-of-magnitude agreement with the outcomes of numerical simulations of cluster formation11,15,16and of the buoy-ant rise of radio bubbles generated by the AGN.17,18Fur-ther discussion and references on the stirring mechanisms for cluster turbulence can be found in Refs.19,20,21. While estimates of the turbulence parameters appear robust roughly to within an order of magnitude,a more quantitative set of numbers is elusive,partly because of the indirect and difficult nature of the observations,partly because the conditions vary both in different clus-ters and within each individual cluster.Here we shalladopt twofiducial sets of parameters:one for cool cores and one for the bulk of hot cluster plasma.These aregiven in Table I(along with some theoretical quantities that will arise in Sec.V and Sec.VI).They will allowus to make estimates that will have the virtue of being consistent and systematic but must not be interpreted asprecise quantitative predictions.They are representative of the range of conditions that can be present in clus-ters.The turbulence is assumed to be stirred at the outer scale L(with rms velocity U at this scale)and to havea Kolmogrov-type cascade below this scale.The small-scale cutoffis determined by the microphysical proper-ties of the cluster plasma.In Table I,we give the value of the particle mean free pathλmfp,which can be usedin a naive estimate of viscosity:µICM∼v th,iλmfp,where v th,i=(2T/m i)1/2is the ion thermal speed.We seethat this gives fairly low values for the Reynolds number, Re∼UL/µICM.It is this feature of the ICM that con-tinues to fuel doubts about its ability to support turbu-lence,at least in the strict,hydrodynamic high-Reynolds-number sense.71However,one should be cognizant of the fact that whatever type of turbulence might exist in the cluster plasma,it is certainly not hydrodynamic,because this plasma is highly electrically conducting72and mag-netized.The presence of the magneticfields not only has a dynamical effect on the turbulence(due to the action of the Lorentz force,the medium acquires a certain elas-tic quality),but also changes the transport properties of the plasma itself:the viscosity,in particular,becomes strongly anisotropic.22These issues will constitute the main subject of this paper,butfirst let us briefly de-scribe what is known about magneticfields in clusters.III.MAGNETIC FIELDSThefirst observed signature of cluster magneticfields was the diffuse synchrotron radio emission in the Coma cluster detected in1970.23Starting from early1990’s, increasingly detailed measurements of the Faraday Ro-tation in the emission from intracluster radio sources have made possible quantitative estimates of the mag-neticfield strength and scales in a large number of clusters.24,25,26Randomly tangled magneticfields with rms strength of order B rms∼1−10µG are consistently found,with thefields in the cool cores of the cooling-flow clusters somewhat stronger than elsewhere.This is fairly close to the value B eq that corresponds to magnetic energy equal to the energy of turbulent motions(see Ta-ble I).Thus,the magneticfield must be dynamically important.The estimates for the tangling scale l B of the field are usually arrived at by assuming that direction reversals along the line of sight(probed by the Faraday Rotation measure)can be described as a random walk with a single step size equal to l B(the estimate of B rms is obtained in conjunction with this model).This gives3 l B∼1−10kpc.The single-scale model is almost certainly not a correctdescription on any but a very rough level.Fortunately,much more detailed information on the spatial structureof the clusterfields is accessible.First,using certain sta-tistical assumptions(most importantly,isotropy),it ispossible to compute magnetic-energy spectra from themaps of the Faraday Rotation measure associated withextended radio sources(the radio lobes of the jets emerg-ing from the AGN—these can be as large as∼102kpcacross).6,27This has been done most thoroughly for a ra-dio lobe located in the cool core of the Hydra A cluster.7The spectrum has a peak at k≃2kpc−1followed bywhat appears to be a power tail consistent with k−5/3down to the resolution limit of k≃10kpc−1.The rmsmagneticfield strength is B rms=7±2µG.The second source of information on the clusterfieldstructure is the polarized synchrotron emission,whichprobes the magneticfield in the plane perpendicular tothe line of sight.28Such data,while widely used for Galac-tic magneticfield studies,29has until recently not beenavailable for clusters.This is now changing:thefirstanalysis of polarized emission from a radio relic in thecluster A2256reveals the presence of magneticfilamentswithfield reversals probably on∼20kpc scale,which,however,is dangerously close to the resolution scale.30 This data is representative of the situation in the bulk of the ICM,rather than in the cores.Statistical analysis of such data will make possible quantitative diagnosis of thefield structure and its dynamical role.31Thus,our knowledge of the magneticfields in clusters, while far from perfect,is more direct and more detailed than that of the turbulent motions of the ICM.It is also due to improve dramatically with the arrival of new radio telescopes such as LOFAR and SKA.73IV.PLASMA PHYSICSThe key property of the ICM as plasma is that it is only weakly collisional and magnetized:given the ob-served values of the magneticfield,the ion gyroradius isρi∼104km,which is much smaller than the mean free path.Asρi≪λmfp already for dynamically veryweakfields(B≫B,see Table I),this is true both inthe observed present state of the ICM and during mostof its hypothetical past,when the magneticfield was be-ing amplified from some weak seed value.In a plasmawithρi≪λmfp,the equations for theflow velocity u and for the magneticfield B may be written in the followingform,valid at time scales≫Ω−1i(Ωi=eB/m i c is the ion cyclotron frequency)and spatial scales≫ρi=v th,i/Ωi,ρd u2 +∇· ˆbˆb p⊥−p +B2 ,(1)d B4πhas been absorbed intoB,and the resistive term has been omitted in Eq.(2)inview of the tiny value of the resistivity.The turbulentmotions in clusters are subsonic(U<v th,i),so we maytake∇·u=0and setρ=1.The magneticfield is,thus,in units of velocity,pressure in units of velocity squared.The proper way to compute p⊥and p is by a kineticcalculation.In the collisional limit,this was done in Bra-ginskii’s classic paper.22It is instructive to obtain hisresult in the following heuristic way that highlights thephysics behind the formalism.32Charged particles mov-ing in a magneticfield conserve theirfirst adiabatic in-variantµ=m i v2⊥/2B.Whenλmfp≫ρi,this conserva-tion is only weakly broken by collisions.As long asµisconserved,any change in thefield strength causes a pro-portional change in p⊥:summing up thefirst adiabaticinvariants of all particles,we get p⊥/B=const.Then1dt∼1dt−νiip⊥−pBdBdt u2 2=−µ |ˆbˆb:∇u|2=−µ 1dt 2 .(5)Thus,the Braginskii viscosity only dissipates such mo-tions that change the strength of the magneticfield.Mo-tions that do not affect B are allowed at subviscous scales.In the weak-field regime,these motions take the form ofplasma instabilities.When the magneticfield is strong,acascade of Alfv´e n waves can be set up below the viscousscale.Let us elaborate.The simplest way to see that pressure anisotropies leadto instabilities is as follows.32Imagine that the large-scaleenergy sources stir up a“fluid”turbulence with u,p⊥,p ,B at time and spatial scales above viscous.Wouldsuch a solution be stable with respect to much higher-frequency and smaller-scale perturbations?Linearizing4 Eq.(1)and denoting perturbations byδ,we get−iωδu=−i k(δp⊥+BδB)+ p⊥−p +B2 δK+iˆb k δp⊥−δp − p⊥−p −B2 δBβ αΩi,(8)whereα=α1+α2>0.This changes the characteristicsof the turbulence:the effective mean free path of theparticles isλmfp,eff∼v th,i/νeff,the effective(parallel)viscosity of the ICM isµ ,eff∼v th,iλmfp,effand,therefore,the effective Reynolds number isRe eff∼UL v2νeff.(9)th,iOn the other hand,using Eq.(4)with effective viscosity,we getµ ,effand|∇u|∼(U/L)Re1/2eff|∆|∼ UαBρi,eq U√B2eqFIG.2:The mechanism of the fluctuation dynamo.Equation (11)models qualitatively the assumed out-come of an as yet inexistent proper theory of the viscos-ity of magnetized ICM.Its solution is plotted inFig.1.Weshalluse this model shortly in our discussion of the fluctuation dynamo in clusters.V.FLUCTUATION DYNAMOIn Sec.III,we reviewed the observational evidence that testified to the presence of a dynamically significant ran-domly tangled magnetic field in clusters.What is the origin of this field?There are numerous physical rea-sons to expect that a certain amount of seed magnetic energy predates structure formation and was,therefore,already present at the birth of clusters.40,41Typical val-ues given for the strength of such field are in the range of B seed ∼10−21−10−17,although this may be an underestimate.42It then falls to the random motions of the cluster plasma to amplify the field to its observed magnitude of a few µG.77This,indeed,they should be able to do by means of the fluctuation (or small-scale)dynamo mechanism:the random stretching of the field.It is a fundamental property of a succession of random (in time)linear shears that it leads on the average to expo-nential growth of the energy of the magnetic field frozen into the medium.43,44,45The rate of growth is roughly equal to the rate of strain (shear,or stretching rate)of the random flow.While the mathematical theory of this process can be nontrivial,46the physics of it is basically illustrated by Fig.2.In Kolmogorov turbulence,the rate of strain is domi-nated by the viscous scale,so |∇u |∼t −1visc∼(U/L )Re 1/2.In fact,what is relevant for the growth of the magnetic field is not the full rate-of-strain tensor but its “paral-lel”component,ˆbˆb :∇u .Since this is exactly the type of motion damped by Braginskii viscosity [see Eqs.(4–5)],we can,for the purposes of the fluctuation dynamo,ignore any subviscous-scale velocity fluctuations.Thus,the magnetic field should grow according to 781dt=ˆbˆb :∇u ∼U Lv th ,i(1−t/t c )2+α,(14)where B (0)∼the greater of B seed and B 1and t c =(2+α)(L/U )Re −1/2[B 1/B (0)]1/(2+α)is at most (for B seed <B 1)the viscous turnover time t visc associated with the collisional Re.The explosive stage contin-ues until the amplified field starts suppressing the in-stabilities,i.e.,when βdrops to values comparable to(v th ,i /U )2Re 1/2eff.This happens at B ∼B 2,whereB 2=B eqv th ,iL1/(5+2α).(15)Thus,we have a mechanism that amplifies the field by many orders of magnitude from any strength above B 1to B 2in finite,cosmologically short,time ∼t c .The value of B 2turns out to be only just over an order of magnitude below B eq (see Table I).Further growth of the field is algebraically slow (B ∼t 1/2),but it does not have to go on for a very long time because B 2is already quite close to the observed field strength.To be precise,there are two algebraic regimes.During the first,Re effis still controlled by thesecond term in Eq.(11)as B hovers just below B eq Re −1/4effwhile B is increasing and Re effis decreasing.Eventually,B ∼B visc =B eq Re −1/4and Re effis returned back to Re (plasma instabilities are suppressed).79As this is also the field strength at which the field has energy comparable to the energy of the viscous-scale motions,any further growth of the field is a nonlinear process,in which the back reaction of the field on the flow has to be taken intoFIG.3:Evolution of the magnetic-field strength for the cool-core parameters of Table I.account.This can be done by assuming that,as thefield grows,it can no longer be stretched by motions whose energy it exceeds and that,therefore,at any given time, the dominant stretching is exerted by motions whose en-ergy is equal to that of thefield.Denoting their velocity and scale by u l and l and using u2l∼B2,we have58,59 dB2∼u3ll1−A 1FIG.5:A schematic illustration of the structure of cluster turbulence proposed in Sec.VI.ratio l /l⊥,the larger is the contrast between the strong field in the straight segments of the folds and the weakfield in the corners.We assume that the rms value of thefield is determined by the straight segments becausethese are thefields that are stretched by turbulence.It istheir growth that we studied in Sec.V.In the saturated state,we expect B straight∼B eq.For such a strongfield, the plasma instabilities are suppressed.It is intuitively clear that they must be suppressed in the regions of theweakfield as well,i.e.,thefield there cannot be weaker than B2[Eq.(15)].Indeed,as we saw in Sec.V,the explosive dynamo mechanism brings anyfield up to this value nearly instantaneously(for B>B2,the growth is much slower).We may conjecture that the maximum aspect ratio of the folds is set by the maximum contrast in thefield strength l /l⊥∼B eq/B2.81Substituting the numbers,wefind(Table I)that this prediction gives the field reversal scale no smaller than a few per cent of the outer scale L(taking l ∼L).This is in passable agree-ment with the observational evidence,which is the best one can expect,given the highly imprecise nature of our argument,of most observational inferences,and of the definitions of such quantities as l B,l⊥,l and L.For comparison of our model with observational data for a number of individual clusters,see Ref.20.It is fair to acknowledge that the above argument,while providing a useful constraint,falls short of a sat-isfactory explanation of thefield structure.One might argue that if,in the course of the turbulent stretch-ing/shearing of thefield,a region offield strength below B2appears(as explained above,in the corner of a fold), Re effthere becomes very large and a localized spot of high-Reynolds-number turbulence is formed.This should have two principal effects.Thefirst is akin to that of a locally enhanced turbulent resistivity,so thefield that violates our constraint is continuously destroyed.The second is a burst of explosivefluctuation dynamo in the spot,which produces more foldedfield with B>B2 and thus shuts itself down.These folds are then further stretched,sheared,etc.,again subject to the constraint that they are destroyed and replaced by new ones wher-ever a spot of weakfield appears.We do not currently have a more detailed mechanistic scenario of how exactly the folded structure with l /l⊥∼B eq/B2is established. It may be feasible to test these ideas numerically by solv-ing MHD equations with viscosity locally determined by the magnetic-field strength according to Eq.(11).Let us assume that clusterfields do indeed have a folded structure with a direction reversal scale l⊥∼(0.01...0.1)L,possibly determined by the argument given above.82The magnetic-energy spectrum then peaks at k∼1/l⊥.What is the structure of the turbulence above and below this scale?At scales l≪l⊥,the magnetic field reversing at the scale l⊥will appear uniform and,in accordance with the old idea of Kraichnan63could sup-port a cascade of Alfv´e n waves.This cascade can rig-orously be shown to be described by the equations of Reduced MHD at collisionless scales all the way down to the ion gyroscale.37The currently accepted theory of such a cascade,primarily associated with the names of Goldreich and Sridhar,64is based on the conjecture that at each scale,the Alfv´e n frequency is equal to the turbu-lent decorrelation rate.The result is a k−5/3spectrum of Alfv´e nicfluctuations—this possibly explains what ap-pears to be a k−5/3tail in the observed spectrum of mag-netic energy for the Hydra A core.7We cannot embark on a detailed discussion of the theory of the Alfv´e n-wave cascade here,so the reader is referred to Ref.65for a re-view and to Ref.37for the theoretical basis of extending this theory to collisionless scales.Above the reversal scale,l≫l⊥,the cluster turbu-lence should resemble the saturated state of isotropic MHD turbulence:a magnetic-energy spectrum with a positive spectral index corresponding to foldedfields and a kinetic-energy spectrum populated in the inertial range by a peculiar type of Alfv´e n waves that propagate along the folds(i.e.,simultaneously perturbing the antiparallel magneticfield lines).58,59This type of turbulence is also reviewed in Ref.65.It is probably of limited relevance for clusters because the Reynolds number in the ICM is not large enough to allow a well-developed inertial range. Fig.5summarizes the—admittedly,rather specula-tive—picture of cluster turbulence proposed above.We offer this sketch in lieu of conclusions.While we believe that the set of physical arguments that has led to it is not without merit,it is clear that much analytical,numeri-cal and observational work is needed before a conclusion can truly be reached in the study of turbulence,magnetic fields and plasma physics in clusters of galaxies.AcknowledgmentsHelpful discussions with T.Enßlin,G.Hammett, R.Kulsrud,E.Quataert,and P.Sharma are gratefully acknowledged.This work was supported by a UKAFF Fellowship,a PPARC Advanced Fellowship,King’s Col-lege,Cambridge(A.A.S.)and by the DOE Center for Multiscale Plasma Dynamics. 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武汉2024年06版小学六年级上册英语第六单元自测题考试时间：90分钟（总分:110）A卷考试人：_________题号一二三四五总分得分一、综合题(共计100题)1、填空题：I saw a ________ flying high in the sky.2、What is the name of the famous American author known for "The Adventures of Tom Sawyer"?A. Mark TwainB. Ernest HemingwayC. F. Scott FitzgeraldD. John Steinbeck答案: A3、填空题：The turtle hides in its _________. (壳)4、填空题：She is a lawyer, ______ (她是一名律师), who helps people.5、填空题：My ________ (玩具名称) is a treasure from my childhood.6、填空题：The __________ (历史的讨论) can lead to greater understanding.7、听力题：The chemical symbol for lithium is _______.8、填空题：The __________ (颜色) of a solution can indicate its chemical properties.9、听力题：The Great Red Spot is a giant storm on the planet ______.I bring a towel to dry off after swimming in the ______ (河).11、填空题：I enjoy watching ________ (比赛) at the stadium.12、What is the first month of the year?A. FebruaryB. JanuaryC. MarchD. April13、选择题：What do we call the act of taking care of someone?A. CaringB. NurturingC. SupportingD. Assisting14、听力题：The _____ (desk/table) is made of wood.15、What do you call the line separating day and night?A. EquatorB. HorizonC. MeridianD. Axis答案:B16、填空题：The elephant is known for its ______ (记忆).17、填空题：We have a ______ (愉快的) gathering for birthdays.18、How many months have 30 days?A. 4B. 5C. 6D. 7答案: A19、填空题：My pet hamster loves to explore its ______ (笼子).The chemical formula for arsenic trioxide is ______.21、听力题：We eat ______ (snacks) during recess.22、填空题：I find it ________ (有趣) to explore nature.23、What do you call a person who studies the effects of chemicals on living organisms?A. ChemistB. ToxicologistC. PharmacologistD. All of the above答案: D24、Which country is famous for kangaroos?A. CanadaB. AustraliaC. BrazilD. India答案: B25、community resource exchange) shares local assets. 填空题：The ____26、What is the capital of Mongolia?a. Ulaanbaatarb. Erdenetc. Darkhand. Choibalsan答案:a27、选择题：What do we call a person who studies the Earth?A. GeographerB. GeologistC. CartographerD. All of the above28、填空题：The ________ (杏树) in our garden gives us sweet fruit.29、What do you use to write on paper?a. Brushc. Scissorsd. Ruler答案:B30、What is the name of the famous mouse created by Walt Disney?A. TomB. JerryC. MickeyD. Donald答案:C31、听力题：She _____ (enjoy/enjoys) dancing.32、What do we call a young kangaroo?A. CalfB. JoeyC. KidD. Pup33、听力题：The chemical symbol for tungsten is ________.34、听力题：A rabbit's strong back legs help it to ______ quickly.35、听力题：I want to ___ an adventure. (have)36、听力题：The ____ is known for its agility and speed.37、填空题：A ____(refugee) flees their home due to conflict or danger.38、What do you call the process of removing the outer layer of fruit?A. SlicingB. PeelingC. ChoppingD. Dicing39、What is the weather like when it snows?A. HotB. ColdC. Humid答案:B40、填空题：The ______ (根系) structure of plants is complex and important.41、选择题：Which animal is known for its long neck?A. ElephantB. GiraffeC. ZebraD. Rhino42、What shape has four equal sides?A. TriangleB. RectangleC. SquareD. Circle答案: C43、填空题：The capital of Bangladesh is _____.44、听力题：My ______ loves to engage in discussions.45、听力题：A saturated fat is typically found in ______.46、填空题：Herbs are often grown in ______ (窗台) pots.47、填空题：The _____ (pistachio) tree produces nuts.48、填空题：The _______ (老虎) is striped.49、填空题：The three states of matter are solid, liquid, and _______. (气体)50、听力题：The nucleus of an atom contains protons and _____.51、What is the chemical symbol for water?A. H2OB. O2C. CO2D. NaCl答案:A52、Which planet is known as the Red Planet?A. EarthB. MarsC. JupiterD. Venus答案: B53、听力题：The ________ (activity) enhances learning.54、填空题：A ________ (冰川) can create valleys as it moves.55、What do you call a person who flies airplanes?A. PilotB. NavigatorC. EngineerD. Steward56、听力题：The study of Earth’s history helps us understand ______ changes.57、填空题：The __________ is a famous city known for its ancient history. (开罗)58、听力题：A mixture that does not settle over time is called a ______.59、填空题：The capital of Cuba is ________ (哈瓦那).60、填空题：We planted ________ in our garden.61、听力题：Plastics are made from long chains of _____, called polymers.62、填空题：When a solid dissolves in a liquid, it is called _______. (溶解)The ________ was a time of great change in Europe.64、What is the capital city of Hungary?A. BudapestB. DebrecenC. SzegedD. Miskolc65、How many colors are in a standard rainbow?A. 5B. 6C. 7D. 8答案:C66、听力题：They are _____ (watering) the plants.67、听力题：A solution where no more solute can dissolve is called ______.68、听力题：The main gas we breathe is ______.69、选择题：What is 6 + 4?A. 10B. 11C. 12D. 1370、What is the capital of Argentina?A. SantiagoB. MontevideoC. Buenos AiresD. Lima答案:C71、填空题：The ______ (植物的营养需求) varies greatly between species.72、听力题：We are going ________ a trip.The __________ (古埃及的象形文字) was used for writing on temple walls.74、Which animal is known for its ability to change colors?a. Chameleonb. Dogc. Catd. Rabbit答案:a75、填空题：_____ (土壤) quality is essential for plant growth.76、听力题：He is playing with his ___. (friends)77、What color is the sky?天空是什么颜色的？A. RedB. BlueC. GreenD. Yellow答案: B78、填空题：The flowers in the garden are _______ and cheerful, spreading happiness.79、What is 25 ÷ 5?A. 3B. 4C. 5D. 6答案:C80、听力题：A __________ is a natural elevation of the Earth's surface.81、What is the hardest natural substance on Earth?A. GoldB. IronC. DiamondD. Silver答案：C82、填空题：The __________ is a large archipelago in Southeast Asia. (印度尼西亚)Glacial deposits can create __________ features.84、How many days are there in a leap year?A. 364B. 365C. 366D. 367答案:C. 36685、What do you call a book with stories?A. NovelB. DictionaryC. EncyclopediaD. Journal86、听力题：The Earth's crust is divided into continental and ______ crust.87、听力题：The ____ has a unique way of moving and can hop very high.88、听力题：The Earth's surface is influenced by natural and ______ factors.89、ts can live in ______ (盐碱地). 填空题：Some pla90、选择题：What is the capital of Chile?A. SantiagoB. ValparaisoC. ConcepcionD. La Serena91、How many legs does a typical arachnid have?A. SixB. EightC. TenD. Twelve答案: B92、填空题：The dolphin jumps high out of the _________. (水)93、Which fruit is red and round?A. BananaB. AppleC. OrangeD. Grape答案:B94、填空题：The ______ (狐狸) is very clever and sly.95、听力题：The sky is _______ (clear) with no clouds.96、听力题：The ________ (lobster) is a type of seafood.97、What is the color of a typical mint leaf?A. GreenB. YellowC. BlueD. Red答案:A98、What is the capital of Somalia?a. Mogadishub. Hargeisac. Kismayod. Baidoa答案:a99、填空题：The _____ (草原) is home to many wildflowers.100、听力题：__________ change involves a change in state, not in composition.。

Magnetic D evices for a Beam Energy RecoveryTHz Free Electron LaserR. R. S. Caetano¹, G. Cernicchiaro², R. M. O. Galvão³1 2Universidade Federal do Rio de Janeiro,Macaé, R J, Bra z il, rcaetano@macae.ufrj.brCoordenação de Física Aplicada, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, RJ, Bra z il, geraldo@cbpf.br3Instituto de Física USP, São Paulo, SP, Bra z il.Abstract:This paper presents a numerical analysis of magnetic devices, dipole, quadrupole and undulator and a THz Free Electron Laser (FEL) electron-beam recovery system. Free Electron Laser are an important source of coherent radiation being used in the study of chemical properties of substances, thus being an important tool for various fields of science such as condensed matter physics, chemistry, biology and medicine. The magnetic device of this simulation is to contribute to the proposed deployment of a national laboratory for multiuser application and development of a recovery system FEL to operate in the far infrared range between 0.3 to 1.2 THz.Keywords: Free Electron Laser THz, magnetic device, simulation in COMSOL.1.IntroductionFree electron laser can be used are in spectroscopy thus have applications in scientific fields such as medicine, chemistry, condensed matter and biology [1]. Thus the Brazilian Center for Physics Research (CBPF) proposed a construction project of a Free Electron Laser (FEL) using the components of a Free Electron Laser of the College of Optics & Photonis (CREOL). The free-electron laser operating in the far infrared range working with wavelengths in the range of 200-600 micrometers. The equipment consists of a linear accelerator that generates electrostatic energy up to 1.7 MeV, a magnetic undulator, which is designed with permanent magnets made from neodymium iron boron (NdFeB) with 185 periods with a wavelength of 8 mm length of 1486 mm and a distance between the undulator cassette (gap) of 6 mm. Has magnetic dipoles and quadrupoles working in optics from the electron beam. The vacuum system is formed by mechanical, ionic pumps and turbomolecular [2], control is being developed for the LabView platform.This paper focuses on the numerical analysis using the COMSOL software of magnetic elements that are the dipole, quadrupole and undulator. These components work changing the trajectory and the size of the electron beam.2.Theory2.1 DipoleT he magnetic dipole is an element that has the function deflect the electron beam [3]. The dipole magnetic field is generated by the electric current passing through the coils. This current in solenoid generates a magnetic flux in the iron core creating a unidirectional magnetic field in between irons given by the right hand rule. The magnitude of the magnetic field can be extracted by Ampere's law gives us the relationship between the electric current and the magnetic field.Figure 1: Magnetic dipole.The dipole magnetic field is given by equation 1 where n is the number of turns, the electrical current I, is the distance between the irons and is the permeability of air.( 1)2.2 QuadrupoleT his component consists of four poles formed by rectangular hyperbolas with alternating magnetic fields, your objective is to change the diameter of the electron beam. The geometry of the quadrupole makes the magnetic field in the core is zero and the magnetic field is generated by modulo magnitude of the field in the x-axis and y-axis [3].Figure 2: Magnetic quadrupole.The quadrupole magnetic field can be calculated from the integration by Ampere's law.( 2)The magnetic field consists of two quadrupole components in the x and y axis, there is a resulting gradient g [T/ m]. Thus, the magnetic field in the x and y axes can be given by the equations , where , and the equation of the resulting magnetic field is equal to:( 3)2.3 UndulatorThe undulator is a mechanical structure consisting of periodic magnets with alternating poles, separated by a distance called (gap). These magnets are made of magnetic material pure permanent magnetic (PPM). This structure causes synchronous radiation be concentrated in a beam, thus reducing the radiation loss.Figure 3: Description of the undulator magnetic.The magnetic undulator field is perpendicular to the x axis, and the direction of the z-axis. Because the poles are alternating magnetic field in the undulator has a sinusoidal behavior as can be seen in equation 4. [4]. Where is the wavelength of the undulator and is the initial magnetic field. In equation 5 we have the initial magnetic field which is a ratio between the gap and the wavelength.(4)(5) e of COMSOL MultiphysicsThe numerical analysis of the dipole, quadrupole and undulator were built using the module AC/DC COMSOL multiphysics. To calculate the magnetic field in the dipole and quadrupole magnetic field the tool was used with the equation 6. In the undulator, the calculation of the initial magnetic field was carried out with the magnetic field in the current tool with the equation 7.(6)(7)3.1 DipoleThe geometry of the dipole was constructed in 3D and has two types of materials are iron, which comprises all the dipole and charge air by the cylindrical shell. Infinite element domain was used for the cylindrical shell for emulating an infinite open space causing all numerical analysis considers the limited space as being infinite.Figure 4. 3D geometry of the magnetic dipole.The interface was used for the dipole Magnetic Field <Ampere Law for the iron structure and for calculating coil was used in Magnetic Field <muti-turn coil <Coil Current Calculation which calculate the magnetic field in the coil according to the current electrical and number of turns. The dipole has 877 turns and the maximum current is 2.5 A.Figure 5. Coil Current Calculation in dipole magnetic.Dipole in the fine mesh, which corresponds to 61114 domain element, boundary element 9728, 1039 edge element was used. The study used for the calculation of the dipole was the Stationary Parametric Sweep and the electric current was applied in order to have values of -2.5 A to +2.5 A.Figure 6: mesh magnetic Dipole.3.2 QuadrupoleThe quadrupole was developed in 3D for its construction it was done primarily in 2D and using the Work Plane tool, after it was extruded to 3D format.Figure7. Quadrupole magnetic geometric in 2D.Figure 8. 3D geometry of the magnetic quadrupole.The materials used in the quadrupole are air and iron and infinite element domain was appliedto the cylindrical shell. To calculate the magneticfield of the quadrupole physical Magnetic Field <Ampere Law, which calculates the magnetic field from the magnetization and uses equation was used , where N is the number of turns, I is the electric current, A is the area and V is the volume of the coil. In the case of quadrupole have 344 turns and the electrical current worth 2.5 A. The magnetization in the quadrupole is oriented in the x and z axes have that each pole has a different orientation.Quadrupole in the mesh extremely fine,which corresponds to 749885 domain element, boundary element 58352, 2772 edge element was used. The study used for the calculation of the quadrupole was the Stationary Parametric Sweep and the electric current was applied in order to have values of -2.5 A to +2.5 A.Figure 9: mesh magnetic quadrupole.3.3 UndulatorFor this work was a session of 3D undulator built with 400 mm in length and can change the wavelength and the distance of the gap. Undulator possessed these variations to resemble the original equipment. The initial magnetic field generated in the undulator does not depend on the distance of the gap but wavelength (equation 5)and thus it is possible to make a comparison between the simulation and the experiment.The materials used were a NdFeB magnetic material is (33SH) and those manufactured with the air gaps 1010 steel, their averages are: x = 10.5 mm, y = 1.3 mm and z = 25 mm and x = 30mm, y = 2 7 mm and z = 13 mm, respectively [5]. On the external surface that is a spherical vacuum volume 220 mm radius of 5mm layer was used.Figure 10. 3D geometry of the magnetic Undulator.T he magnetic field of the undulator was obtained using the Magnetic Field, No current Physical<Magnetic Flux. When applying a magnetic field in a material, the resulting field B is the sum of the applied field H and the field of the magnetized material, as in Equation 8.( 8)Magnetic materials have hysteresis magnetization curves called reduce to zero the magnetic field applied, as can be seen in equation 9 [6].( 9)The magnetization is added in Magnetic flux Conservation, as the magnetization of the magnets are oriented alternately is necessary to indicate the direction of magnetization in this case, the direction is along the x axis. The mesh used was extra fine, possessing 5842814 domain boundary elements 681 250 and 80 793 edge andthe study used was stationary.Figure 11. Mesh Undulator magnetic.6.0 Results 6.1 DipoleThe simulation dipole was constructed to compare the value of the modulus of the magnetic field with the experimental value in Figures 12 and 13 have the simulation results.Figure 12. Magnetic Flux density in an dipolemagnetic device.Figure 13. Graph magnetic field x current electric indipole.Table 1: Comparison of the magnetic field obtained from the simulation and experimental analysis in the dipole magnetic device.The magnetic field values are obtained for the maximum value of electric current of 2.5 A. Thus the table 1 it can be seen that the threevaluesarecloseto a percentage error of 3% between experiment and COMSOL. 6.2 QuadrupoleThe simulation quadrupole was constructed to compare the value of the modulus of the magnetic field with the experimental value in Figures 14 and 15 have the simulation results.Figure 14. Magnetic Flux density in an quadrupolemagnetic device.Figure 15. Graph magnetic field x current electric inquadrupole.Table 2: Comparison of the magnetic field obtained from the simulation and experimental analysis in the quadrupole magnetic device.The magnetic field values are obtained for the maximum value of electric current of 2.5 A. Thus the table 2 it can be seen that the three values are close to a percentage error of 1,9 % between CREOL and experiment.6.3 UndulatorThe simulation study of the undulator was developed for the x axis and y axis. In Figures 16, 17, 18 and 19 have a magnetic field initial described in the two axes respectively.Figure 16. Magnetic Flux density in an undulator magnetic device (front view). Figure17. Magnetic field of the undulator gap measured in the z axis direction with respect to distance in the y direction.Figure 18. Magnetic Flux density in an undulator magnetic device (side view).In Figure 19 we have the magnetic field in the undulator which is a sine function according to equation 10, so this simulation is in agreement with theory. In Table 3it canbe observedthat the values stipulated by the project, experiment and COMSOL, so it is possible that there is an error 1% compared to COMSOL and experiment.Figura 19.Undulator magnetic field measured at the gap in the z direction as a function of distance in the x direction.Table 3: Comparison of the magnetic field obtained from the simulation and experimental analysis in the undulator magnetic device.7.0 ConclusionIn this paper we present 3D simulations of the dipole, quadrupole and undulator magnetic elements that are owned by a Thz Free Electron Laser. The results have to numerical simulation results are in agreement with experiment validating the paper. To the next module using the practical tracing in COMSOL, an electron beam will be added with the aim of studying the behavior of the electron beam in magnetic elements.8 Reference[1] S rinivas Krishnagopal*, Vinit Kumar†,Sudipta Maiti, S. S. Prabhu and S. K. Sarkar, "Free-electron lasers.," CURRENT SCIENCE, VOL. 87,, pp. NO. 8, 25, OCTOBER 2004.[2] M. Tecimer, Time –Domanin analysis andtechnology of THz Free Electron Lasers, Tel –Aviv University. : Faculty of Engineering.Departamento f Electical Engineering –Physical Electronics. , 2005.[3] J. Tanabe, Iron Dominated ElectromagnetsDesign, Fabrication, Assembly and Measurements, 2006.[4] M. D. J. R. Peter Schmüser, Ultraviolet andSoft X-Ray Free-Electron Lasers - Introduction to Physical Principles, Experimental Results, Technological Challenges, Springer, 2009.[5] J. G. L. R. E. Paul P. Tesch, "Finalconstruction of the CREOL 8 mm period hybrid undulator," Nuclear Instruments and Methods in Physics Research A375, pp. 504-507, 1996. [6] R. N. Faria, L.F.C.P. Lima, Introdução aomagnetismo dos materiais, São Paulo: Livraria da Fisica, 2005.。

&KDSWHU+DOO (IIHFW 6HQVRUVIntroductionThe Hall effect was discovered by Dr. Edwin Hall in 1879 while he was a doctoral candidate at Johns Hopkins University in Baltimore. Hall was attempting to verify the theory of electron flow proposed by Kelvin some 30 years earlier. Dr. Hall found when a magnet was placed so that its field was perpendicular to one face of a thin rectangle of gold through which current was flowing, a difference in potential appeared at the opposite edges. He found that this voltage was proportional to the current flowing through the conductor, and the flux density or magnetic induction perpendicular to the conductor. A l-though Hall’s experiments were successful and well received at the time, no applications outside of the realm of theoretical physics were found for over 70 years.With the advent of semiconducting materials in the 1950s, the Hall effect found its first applications. However, these were severely limited by cost. In 1965, Everett Vorthmann and Joe Maupin, MICRO SWITCH Sensing and Control senior d e-velopment engineers, teamed up to find a practical, low-cost solid state sensor. Many different concepts were examined, but they chose the Hall effect for one basic reason: it could be entirely integrated on a single silicon chip. This breakthrough resulted in the first low-cost, high-volume application of the Hall effect, truly solid state keyboards. MICRO SWITCH Sensing and Control has produced and delivered nearly a billion Hall effect devices in keyboards and sensor products.Theory of the Hall EffectWhen a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. This principle is known as the Hall effect.Figure 2-1 illustrates the basic principle of the Hall effect. It shows a thin sheet of semicon-ducting material (Hall element) through which a current is passed. The output connections are perpendicular to the direction of current. When no magnetic field is present (Figure 2-1), current distribution is uniform and no potential difference is seen across the output.When a perpendicular magnetic field is present,as shown in Figure 2-2, a Lorentz force is exerted on the current. This force disturbs the currentdistribution, resulting in a potential difference (voltage) across theoutput. This voltage is the Hall voltage (V H ). The interaction of the magnetic field and the current is shown in equation form as equ a-tion 2-1.Hall effect sensors can be applied in many types of sensing devices. If the quantity (parameter) to be sensed incorporates or can incorporate a magnetic field, a Hall sensor will perform the task.V H ∝ I × BFormula (2-1)Figure 2-4 Basic Hall effect sensorA differential amplifier with these characteristics can be readily integrated with the Hall element using standard bipolar transistor technology. Temperature compensation is also easily i n tegrated.As was shown by equation 2-1, the Hall voltage is a function of the input current. The purpose of the regulator in Figure 2-4 is to hold this current constant so that the output of the sensor only reflects the intensity of the magnetic field. As many systems have a regulated supply available, some Hall effect sensors may not include an internal regul a tor.Figure 2-12 NPN (Current sinking) . . . Digital output sensor。