Encyclopedia of Physical Science and Technology - Atomic and Molecular Physics

276 Pages·2001·12.896 MB·English

by  Robert Allen Meyers (Editor)

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Accelerator Physics
and Engineering
Frank T. Cole, deceased
Maury Tigner
CornellUniversity
Alexander W. Chao
StanfordLinearAcceleratorCenter
I. Introduction
II. History
III. ApplicationsofAccelerators
IV. TypesofAccelerators
V. PhysicalPrinciplesofBeamMotion
VI. AcceleratorsoftheFuture
GLOSSARY Cyclic accelerator Particle accelerator in which each
particle passes many times through a small potential
Betatron Circular induction accelerator for electrons. droptobeacceleratedtohighenergy.
The magnetic guide field rises during acceleration to Cyclotron Circularacceleratorinwhichprotonsorheavy
keepparticlesonacircleofconstantradius. ionsspiraloutwardfromthecenterastheyareaccel-
Circular accelerator Cyclic accelerator in which parti- eratedbyaradio-frequencyvoltage.
cles are bent by magnetic fields around closed paths, Electron volt (eV) Unit used to describe the energy of
passing many times through the same accelerating particlesinanaccelerator.
system. Emittance Areainthetransversephasespace(inunitsof
Colliding beams System in which the fixed target is millimeter–milliradians) occupied by the distribution
replaced by a second beam of accelerated parti- ofparticlesinabeam.
cles moving in the opposite direction. The collisions Fixed target Target fixed in location that the beam
of moving particles produce very high-energy phen- strikesafteraccelerationtoproducephysicalchanges
omena. ofinterest.
27
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28 AcceleratorPhysicsandEngineering
Focusing system System for confining divergent parti- Aparticleacceleratorisadevicethatmanipulatesthe
clesinabeamclosetotheidealorbitduringthecourse motionofchargedparticlebeams.Themostcommonma-
ofaccelerationandstorage. nipulation is to increase the energy of the particles, thus
High-voltage accelerator Particle accelerator in which theterm“accelerators.”Inmoderntimes,however,other
each particle passes once through a high potential. variations which do not accelerate or for which acceler-
Examples are Cockcroft–Walton accelerators, Van de ation plays only a minor role have been introduced. All
Graaffgenerators,andMarxgenerators. thesedevicesarealsocustomarilyconsideredasparticle
Inductionaccelerator Acceleratorinwhichparticlesare accelerators.
acceleratedbyelectricfieldsthataregeneratedfroma Therearealsodevicesthatmanipulateneutralparticles.
changingmagneticfieldbyFaraday’slawofinduction. Thephysicsofthesedevicessometimesisconsideredas
Linearaccelerator Cyclicacceleratorinwhichacceler- partofacceleratorphysicsbecausetheycanbedescribed
ationtakesplacealongastraightlineasparticlespass byverysimilarphysicalprinciples.Atomicbeamdevices
sequentiallythroughrepeatedacceleratingunitsinsyn- andparticletrapsareexamplesofthiscategory.
chronismwithanelectromagneticwave.
Microtron Circularacceleratorinwhichelectronsmove
incirclesthatarealltangentatonepointwherearadio- I. INTRODUCTION
frequencyvoltageacceleratesthem.
Radio-frequencysystem Systemwhereparticlesareac- A. ParametersCharacterizingtheBeam
celerated in cyclic accelerators. Accelerating field is
Other than the particle species, the final energy of the
provided by an electromagnetic wave at a microwave
accelerated particles is the most important parameter of
frequency.Theoscillatingfieldmustsynchronizewith
a particle accelerator. The particles have electric charge
thetimeofarrivaloftheparticlestobeaccelerated.
equaltotheelectronchargeeoramultipleofit,andthey
Storage ring Circular accelerator with magnetic field
areacceleratedbypotentialsmeasuredinvolts(V).There-
fixed in time so that one or more beams of particles
fore,anaturalunitofenergyistheelectronvolt(eV),the
cancirculatecontinuouslyforalongperiodoftime.
energyacquiredbyoneelectronchargeinpassingthrough
Strong-focusing system System of alternating focusing
apotentialdifferenceof1V.
anddefocusinglenseswhichproduceastrongnetfo-
The electron volt is a very small unit of energy (1 eV
cusingeffect. =1.6×10−19 Joule),moredirectlyapplicabletoenergy
Synchrotron Circularacceleratorinwhichparticlesare
levels in atoms than to accelerators. There are therefore
keptinacircleofconstantradiusbyamagneticguide
multiples of the electron volt that are used to describe
fieldthatrisesintimeastheyareacceleratedbyaradio-
accelerators.
frequencysystem.
Wakefield Electromagneticmicrowavefieldsexcitedin 1keV=103eV
thevacuumchamberbyapassingbeam.
1MeV=106eV
1GeV=109eV
A PARTICLE BEAM is an ensemble of particles that
1TeV=1012eV
movetogetherincloseproximity.Abeamischaracterized
byafewbasicparameterssuchastheparticlespeciesand Theenergiesofparticleacceleratorsnowbeingoperated
theaverageenergyoftheparticles.Thebeamofparticles rangefromafewhundredkeVto1TeV.Thesizesofpar-
mustbewellcollimatedsothatallparticlesstayinclose ticleacceleratorsrangefromtable-topdevicestodevices
proximitythroughoutitsmotion.Deviationofthemotion stretchingoverseveralmiles.Oneofthelargest,theFermi
ofindividualparticlesfromtheaveragemotionofthebeam NationalAcceleratorLaboratorynearChicago,isshown
mustbekeptsufficientlysmall. in an aerial view in Fig. 1.
Thespeciesofparticlesinthebeamismostcommonly Asecondimportantparameterusedtocharacterizean
electricallycharged. Most common examples of particle acceleratorisitsintensity,usuallythenumberofparticles
speciesareelectronsandprotons.Lesscommonparticles movingtogetherinabunch,or,inthecaseofacontinuous
includechargedparticlessuchasmuons,pions,orneutral flowofparticles,thenumberofparticlesacceleratedper
particles such as some specific atoms or molecules. The second.
physicsofacceleratorsalsosignificantlyoverlapswiththe Other parameters characterizing a particle beam in-
physics of light optics and lasers, as light can be treated clude parameters that specify the degree of collimation
asabeamofphotonparticles. ofthebeam.Inparticular,thespreadofparticleenergies
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AcceleratorPhysicsandEngineering 29
along. For some high-performance accelerators, such as
inanelectronmicroscopeorinastoragering,ithasbeen
necessarytoconsiderverycleverelementarrangementsin
suchawaythatnonlinearopticalaberrationsarecompen-
satedorminimized.Inastoragering,thebeamisstored
fortypicallymuchlongerthan1010 revolutions,ormany
moretimesthantheearthhascirculatedaboutthesun.Par-
ticlemotionwillhavetobestableforthislongastorage
time.Thisbranchofacceleratorphysicsthereforerequires
ahighlysophisticatedknowledgeofnonlineardynamics
andchaosphysics.
Asthebeamintensityisincreased,theselfelectromag-
neticfieldsgetsstronger.Thebeaminteractswithitssur-
roundingstocreateaperturbingwakefield,whichinturn
maycausethebeamtobecomeunstable.Alargenumber
ofvarioustypesofcollectivebeaminstabilitiesoccurdue
FIGURE1 Aerialviewoftheworld’shighestenergyaccelerator,
tothehighbeamintensity.Understandingandanalysisof
theFermiNationalAcceleratorLaboratory.Theacceleratorisinan
undergroundtunnel.Theacceleratedbeamisextractedforusein this branch of beam physics is closely related to plasma
experimentsintheareasstretchingtowardthetopleftofthepho- physics.
tograph.[CourtesyofFermilabNationalAcceleratorLaboratory; Some high-performance accelerators require colli-
Batavia,IL.]
mated beams with very small energy spreads and emit-
tances. Still another branch of accelerator physics ad-
arounditsaveragevalueisonesuchparameter.Thisenergy dressesthisissuebyinnovationsofseveraltypesofbeam
spread,ormomentumspread,whichisrelated,isdenoted coolingtechniquesusedtoreducetheenergyspreadand
by(cid:3)E/E,andtypicallyrangesfrom10−2 to10−4.Two emittances.
moreparameters,calledtransverseemittances,specifythe Acceleratorphysicsisbothafundamentalresearchand
degreethebeamisbunchedintoatightbundlethroughout anappliedresearch.Theabove-mentionedaspectsareex-
itsmotion.Theemittancesaredenotedby(cid:5) and(cid:5) ,and amples of fundamental research in accelerator physics.
x y
areinunitsofmillimeter–milliradians.Atightlybundled Applied research in accelerator physics concentrates on
beam will require small values of the emittances. These the development of accelerator technology, which con-
threeparameters,(cid:3)E/E,(cid:5) ,and(cid:5) ,willhavetomeetthe stitutes a research area in its own right because of the
x y
requirementsoftheacceleratorapplicationinhand. complicationanddepthinvolved.Asacceleratorapplica-
Still other parameters may characterize special beam tions put forward increasingly demanding requirements
properties.Oneexampleisthebeampolarization,which onthebeam,researchinacceleratortechnologybecomes
characterizes the degree of alignment of all the spins of increasinglyspecializedandsophisticated.
theparticlesinthebeam.Ahighdegreeofpolarizationis
a very useful tool in analyzing some of the high-energy
C. AcceleratorTechnology
physicsexperiments.
Technologyprovidesthemeanstomanipulatethebeams
in accelerators. Accelerator research therefore also cov-
B. AcceleratorPhysicsResearch
ers areas of the physics and engineering of accelerator
Accelerator physics is a branch of physics that studies technology. Notable examples include the technologies
thedynamicsofthebeamsinaccelerators.Sometimesit ofhigh-powermicrowavedevices,room-temperatureiron
is also called beam physics, although, strictly speaking, magnets,superconductingmagnets,large-scaleultra-high
acceleratorphysicsstudiesonlythepartofbeamphysics vacuum, intense particle sources, computer control and
encounteredinaccelerators,whilebeamphysicsmayalso networking, fast electronics, high-power switches, and
containthestudyofneutralparticledevices,particletraps, materialsdevelopments.
andcosmicraymechanisms. Microwave (also referred to as radio-frequency wave)
Particle motion in an accelerator has a close analogy technologyisthemainwaytoaccelerateparticlesinac-
withlightoptics.Onebranchofacceleratorphysics,called celerators. The microwave involved typically has a fre-
beamoptics,studiesthemotionofparticlesinthechannel quencyrangingfromafewhundredmega-Hertz(MHz)to
ofacceleratorelements.Theacceleratorelementsarear- afewtensofgiga-Hertz(GHz),whereHertzisafrequency
rangedinanoptimalmannertoguideandfocusthebeam unit of one cycle per second. Developing high-power
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30 AcceleratorPhysicsandEngineering
microwave sources is one of the major technology re-
searchactivities.
Superconductingmagnetsprovidehighmagneticfields
for the purpose of guiding and focusing particle motion
in accelerators. The higher the magnetic field can reach,
the more compact the accelerator can be made. In addi-
tion,superconductingmagnetsalsosaveoperatingpower
comparedwithroom-temperatureironandcoppercoun-
terparts.Developinghigh-fieldsuperconductingmagnets
isoneimportanttechnologyissueinacceleratorphysics.
Superconductivity also benefits radio-frequency de-
vices. In particular, by replacing the room-temperature
copperradio-frequencycavitiesbysuperconductingcavi-
ties,onecanreachhighacceleratingfieldsandsaveoper-
atingpower.Thedevelopmentofsuperconductingradio-
frequency cavities has made substantial progress in the
pasttwodecades.
There are also accelerator applications using high-
temperature superconductors. Research in this direction
hasalsomadeprogressinrecentyears.
II. HISTORY
FIGURE2 ThevacuumchamberofthefirstLawrencecyclotron.
During the nineteenth century, physicists experimented [CourtesyofLBNL.]
with Crookes tubes, evacuated glass systems containing
internalelectrodes.Acurrentofelectronswillflowwhena
voltageisappliedbetweentheseelectrodes.J.J.Thomson (at1.2MeV)in1932.Asuccessionofcyclotronswasbuilt
used a Crookes tube in his discovery of the electron in inLawrence’slaboratorythroughthe1930sandreached
1890. Ro¨ntgen discovered X-rays using a Crookes tube 5 to 10 MeV. Their original cyclotron is shown in Fig. 2.
in 1896. The X-ray tube was later made into a practical Lawrence’s cyclotrons and electrostatic generators,
deviceforuseinmedicinebyCoolidge. which had been conceived and demonstrated by Van de
Rutherfordspurredthedevelopmentofparticleacceler- Graaff in 1931, were used for nuclear physics research
atorsforuseinnuclearphysicsresearchinafamouslecture throughout the 1930s. Both were limited to energies of
in1920.Hepointedouttheneedforhigherenergyparti- 15MeVorless,andreachingenergiesbeyondthislimit
clesforthefurtherunderstandingoftheatomicnucleus. was a major topic of research in the 1930s. Thomas
Duringthenextdecade,bothhigh-voltageandcyclicac- proposed the azimuthally varying-field (AVF) cyclotron
celeratorswereinvented.Manydifferentmethodsofpro- in 1938, but his work was not understood until much
ducinghighvoltageweredemonstrated,buttherewasal- later. Kerst, with the help from an orbit dynamist, Ser-
ways great difficulty in avoiding sparks at high voltage. ber,builtthefirstsuccessfulbetatronin1941andbuilta
Cockcroft and Walton, in Rutherford’s Cavendish Lab- second20-MeVmachinebeforeWorldWarIIintervened.
oratory, developed a successful accelerating tube. They A 100-MeV betatron was built in 1945, and a 300-MeV
used an existing voltage-multiplying circuit and built a betatronafewyearslater.Thismodelofbetatronwasbuilt
300-keV proton accelerator, which they used in 1932 to inlargequantitiesforuseinX-raytestingoflargecastings,
do the first nuclear physics experiment with accelerated particularlyforthearmorofmilitarytanks.
particles. Thenextgreatstepinenergybeganin1944and1945
Isingwasthefirsttoconceive(in1925)acyclicaccel- when Veksler in the U.S.S.R. and McMillan in the U.S.
erator,adrift-tubelinearaccelerator.Widero¨eexpanded independentlyconceivedtheprincipleofphasestability,
theideaandbuiltaworkingacceleratorin1928,andac- permittingfrequencymodulationoftheacceleratingvolt-
celeratedmercuryions. age in a cyclotron to overcome effects of relativity and
PerhapsthemostimportantconsequenceofWidero¨e’s making the synchrotron possible. In 1946, the first syn-
workwastostimulateLawrencetoconceivethecyclotron. chrocyclotron was operated and a number of 300-MeV
LawrenceandLivingstonbuiltthefirstoperatingcyclotron electronsynchrotronscameintooperationinthenextfew
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AcceleratorPhysicsandEngineering 31
years.Theresearchdonewiththeseacceleratorswasim- configuration.Theintensestackedbeamcanthenbeused
portantinstudyingthepropertiesofthepion. incolliding-beamexperiments.Theconceptofcolliding
World War II radar work had stimulated the develop- beams had been known for many years (it was patented
ment of radio-frequency power sources and these were in 1943 by Widero¨e), but beam stacking is essential to
usedinlinearaccelerators.Traveling-waveelectronlinear achieve useful rates of collisions. Shortly afterwards, a
accelerators,atfrequenciesofapproximately3GHz,were numberofpeople(Newton,Lichtenberg,Ross,and,inde-
extensivelydevelopedbyHansen,Ginzton,Panofsky,and pendently,ONeill)proposedtheconceptofastoragering
their collaborators. This effort led over the years to the separatefromtheacceleratingdevice.Thestorageringis
50-GeVStanfordLinearAcceleratoroftoday.Alvarezex- abettercolliding-beamsystemthantheFFAGaccelerator
tendedtheconceptofthedrift-tubeacceleratorforheavier becauseitislesscostlyandbecauseitprovidesmorefree
particlesandbuiltthefirstofmanydrift-tubeaccelerators spacefordetectors.
used in physics and chemistry research and as injectors Experimental confirmation of these ideas did not lag
forsynchrotrons. farbehind.Thefirststrong-focusingelectronsynchrotron
Workalsobeganinthelate1940stobuildprotonsyn- wasa1-GeVsynchrotronoperatedbyWilsonandhiscol-
chrotrons. The first proton synchrotron, the 3-GeV cos- laboratorsin1955atCornellUniversity,followedbysev-
motronatBrookhaven,NY,cameintooperationin1952, eralotherelectronsynchrotrons.TheFFAGprincipleand
andthe6-GeVBevatronatBerkeley,CA,cameintoop- beam stacking were demonstrated by Kerst and his col-
erationin1954.Aninterestingprecursor,a1-GeVproton laborators in the 1950s. Traveling-wave electron linear
synchrotron,conceivedin1943independentlyoftheprin- acceleratorsreached1GeVinthissameera,andaseries
ciple of phase stability, was built in Birmingham, U.K. of important experiments on electron-proton scattering
However,theprojectwastooambitiousfortheresources wasdonethatelucidatedthestructureoftheproton.The
availableatthetime,andthemachinedidnotcomeinto firstelectronstorageringswerebuiltandoperatedinthe
operationuntilJuly1953.Theprotonsynchrotronswere early1960satStanford,CA;Frascati,Italy;Novosibirsk,
usedforresearchinheaviermesons,the“strange”parti- then-U.S.S.R.; and at Cambridge, MA. Two large pro-
cles that had been observed in cosmic-ray experiments, ton synchrotrons of 28- and 33-GeV energy were built
andinantiprotons. byCERN,anewinternationallaboratoryinEurope,and
Speculative discussions aimed toward increasing the by the Brookhaven Laboratory. These became the foun-
highest accelerator energy led to the conception of the dationofmajoradvancesinhigh-energyphysics,withthe
strong-focusing principle by Courant, Livingston, and discovery of many new particles and the beginning of a
Snyder in 1952. It was later found that Christofilos had conceptual ordering among them and understanding of
developedtheprincipleindependentlyin1950.Strongor them.TheelectronsynchrotronsandtheStanfordLinear
alternating-gradientfocusingkeepstheoscillationsofpar- Accelerator,whichreached20GeVin1966,addedtothis
ticlesabouttheidealorbitsmallandmakespossiblecom- understanding.
pactandeconomicalmagnets. In the late 1960s, the first major proton storage ring,
The discovery of strong focusing led to an explo- the ISR, was built at CERN. It stored and collided two
sionofideas.In1953,KitigakiandWhiteindependently protonbeamsof28GeVeach.Collidingthesetwobeams
conceived the separated function strong-focusing syn- isequivalenttoafixed-targetacceleratorofover1500GeV
chrotron, which made possible higher guide fields and energy.
moreeconomicaldesigns.In1958,Collinsconceivedthe Thesecondgenerationofstrong-focusingsynchrotrons
long straight section, which made possible economical alsobegantobebuiltinthelate1960s.Theseincorporated
configurations with space for injection, acceleration, ex- themoreefficientseparated-functionmagnetsystemand
traction, and detection equipment. As well as making it longstraightsections.Aprotonsynchrotronthatreached
possibletogotoahigherenergywithprotonsynchrotrons, 400 GeV was completed at the new Fermilab in Illinois
thestrong-focusingprinciplegaveimpetustonewthinking in 1972. A similar synchrotron was later built at CERN.
inmanyotherdirections.Linearacceleratorsweregreatly Theseworkhorsesincorporatednewbeam-sharingmeth-
improvedinperformancebytheadditionofstrongfocus- odsandeachcouldprovidebeamssimultaneouslytosev-
ingalongtheorbit,firstconceivedbyBlewett.TheAVF eraltargetsandadozenmajorexperiments.Animportant
principlewasrediscoveredbyanumberofpeople,among featurethathasmadethismultipleusepossibleisthevery
them Kolomensky, Ohkawa, Snyder, and Symon. It was high degree of precision in beam handling and manipu-
extendedtospiral-sectorfocusingbyKerstin1954.Kerst lation. The data from these and ISR experiments led to
thenproposedthatsuccessivelyacceleratedbeamscould the development of quantum chromodynamics and elec-
be “stacked” in circulating orbits in a fixed-field alter- troweaktheory,largeadvancesinourunderstandingofthe
natinggradient(FFAG)accelerator,avariantoftheAVF basicbuildingblocksofnature.
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32 AcceleratorPhysicsandEngineering
Important experimental evidence also came from the 0780381-003
100TeV
second generation of electron storage rings, now always
ENERGY GROWTH
with positrons as the second beam, the first of which
OF ACCELERATORS
reached3GeVineachbeamatStanfordin1972.Electron- 10TeV (PRIOR TO 1979)
positron storage rings have now reached 100 GeV per
beam. As in the case of proton synchrotrons, electron– ISR
1TeV Storage Ring
positronstorageringshavebeendevelopedtoahighart. (Equiv. Energy) FNAL, SPS
Radio-frequencysystemstoreplacetheenergylostinsyn- AG
chrotronradiationareamajorfactorinthedesignandcost 100GeV
y
oftheserings,butatthesametimethesynchrotronradia- nerg ProWtreoank SFyonccuhsriontgron SLED
tionalsoprovidesabeamcoolingmechanismthatmakes E
m 10GeV Cornell
thebeamverysmallinsize,makingprecisemanipulations ea Electron Linac
praotses.ibInlefaacntd, siynncrceharsoitnrgonthraedciaotliloidninhga-sbebaemcominetearavctailoun- erator B 1GeV EleWcteroank SFoyncuchsirnogtron AG SynchPrrooctoyncl oLtirnoanc
el
able experimental tool in its own right for use in atomic c
c
A
physicsandmaterialsscienceresearch,andanumberof 100MeV Betatron
Sector-focused
Cyclotron
single-beamelectron(orpositron)storageringshavebeen Cyclotron
builtasdedicatedsynchrotronradiationfacilities,provid-
10MeV
inginparticularX-raybeamsmanyordersofmagnitude Electrostatic
Generator
stronger than can be obtained from a laboratory X-ray
tube. 1MeV
Rectifier
Thetwolargeprotonsynchrotronsweredevelopedfur- Generator
therinquitedifferentdirections.AtFermilab,supercon- 100keV
1930 1940 1950 1960 1970 1980
ducting magnets underwent long, arduous development,
and a superconducting magnet ring was built and in- FIGURE3 Livingstonchart.
stalledinthetunnelofthe400-GeVaccelerator.Itreached
800GeVin1983.TheCERNsynchrotronwasconverted
ofeachnewtypeofacceleratorgaveanenergyincrease.
to a proton-antiproton storage ring by the addition of a
Eachacceleratortypeiseventuallyreplacedbyanotheras
small ring to accumulate antiprotons, making use of the
itreachesitslimitforproducinghigherenergybeams.It
newtechniqueofstochasticbeamcoolinginventedbyvan
is evident from the chart that the field of accelerator re-
derMeer.Colliding-beamexperimentshavebeencarried
searchhasbeenactiveandproductiveoverthelastseveral
out there, culminating in the discovery of the W and Z
decades.
particlesin1983.
Thespectacularsuccessesoftheseacceleratorsandstor-
ageringshaveledtoanumberofnewinitiatives.Alarge
electron–positron(LEP)ringtoinitiallyreachenergiesof III. APPLICATIONS OF ACCELERATORS
50 GeV, and later upgraded to reach over 100 GeV, has
beenbuiltatCERN.Anelectron–protonring(HERA)has Many types of accelerators are being used today. Exam-
beenbuiltinGermany.Asingle-pass,colliding-beamsys- plesrangefromdailyappliancessuchastelevisionsetsand
temSLChasbeenbuiltatStanford.Here,thetwo50-GeV microwaveovenstomedium-sizedacceleratorsformed-
beamscollideonlyonceinalinearsystem,notacirculat- icalandindustrialusestogiganticdevicessuchasthose
ingconfiguration.Auseableeventrateisachievedbyvery usedinhigh-energyphysicsandnuclearphysicsresearch.
smallbeamsizes(thusincreasingthedensity).Construc-
tionisinprogressatCERNonaproton–protoncollider,
A. HouseholdAppliances
alargehadroncollider,with7TeVineachbeam.
Inparallelwiththeaboveeffortsbasedonmoretradi- Some household appliances are miniature accelerators.
tionalapproaches,researchworkhasbeencarriedoutfor Mostnotableexamplesarevacuumtubes,televisionsets,
manyyearsonnewmethodsofparticleacceleration,mak- andmicrowaveovens.
inguseofplasmasandlasers,withthegoalofachieving The recent electronic revolution has been based on
substantiallyhigheracceleratingfields. semiconductors. However, the first electronic revolution
The historical development of the energy of particle wasmadepossiblebytheinventionofthevacuumtubes.A
accelerators is plotted in Fig. 3, the famous Livingston vacuumtubeisaminiatureacceleratorconsistingbasically
chart. One can see from the chart that the development ofaelectricheater,anelectron-emittercalledthecathode,
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AcceleratorPhysicsandEngineering 33
Anode Screen high-energyphysicsexperiment,particleswithveryhigh
Cathode
energiesaredirectedtobombardatarget.Intheinterac-
tionbetweenaprojectileparticleandthetargetparticleit
Vacuum Tube strikes,newkindsofparticlescanbeproducedthatpro-
vide clues to the nature of matter. Systematic study of
Heater theseinteractionsrequirescontrolled,copiousproduction
Deflection
Voltage Plates using accelerators. A similar requirement holds for the
10-2000 researchofnuclearphysics.Comparedwithaccelerators
8575A1
of the household, these accelerators for high-energy and
FIGURE4 Schematicofacathoderaytube.Actualdevices,es-
nuclearphysicsresearcharemuchlargerinsizeaswellas
peciallyattheanode,aretypicallymorecomplexwithadditional
incomplexity.
considerationofcollimationandprovidingaccuratefocusingofthe
beamimageonthescreen. Whenanacceleratedparticlestrikesastationaryparticle
inatargetinafixedtargetexperiment,alargefractionof
theenergysolaboriouslyputintotheparticlegoestomove
andanelectron-collectorcalledtheanodewhichismain- alltheproductsforwardinthedirectionofmotionofthe
tainedatapositivepotentialrelativetothecathode.Such beam,becausemomentumisconservedinallcollisions.
avacuumtubecanbeusefulinrectifyingoscillatingcur- Thus, a 100-GeV proton accelerator has available only
rentsintoadccurrent,butbyaddinganadditionalcontrol approximately 27 GeV for making new particles. When
gridbetweenthecathodeandtheanode,thevacuumtube theprotonenergyisincreasedto1000GeV,theavailable
becomes an amplifier, a wide application of which trig- energyincreasestoonly43GeV.
geredthefirstelectronicrevolution.Mostfunctionsofthe Awaytomakealltheenergyusefulistoutilizeasec-
vacuum tubes have been replaced by semiconductor de- ond accelerated beam as a target. If the two beams are
vices. However, important applications remain in areas movinginoppositedirections,thetotalmomentumofthe
wherehighbeampowerisinvolved. system is zero and none of either beam’s energy need
Acathoderaytubeconsistsofaheater,acathode,and be used in moving products downstream. This colliding
ananode,justlikeavacuumtube,buttheanodehereserves beams method is a more economical method of creating
onlytoprovidetheacceleratingvoltagefortheelectrons new,higherenergyinteractionsandnewparticles,asde-
andnotastheelectroncollector.Electronsaremadetopass picted schematically in Fig. 5.
theanodethroughthepassagehole,andstrikeafluorecent Thedifficultyofacollidingbeamconfigurationisthat
screen downstream to produce an image on the screen. thecollisionrateincollidingbeamsismuchlowerthanina
Thedirectionofmotionoftheelectronsiscontrolledby fixedtargetbecausetheparticledensityinabeamismuch
asetofdeflectingplates.Cathoderaytubesarethebasic
deviceforscientificinstrumentssuchastheoscilloscope
andthestreakcamera,butmostcommonlytheyareused (a) Secondary
in the television set. Figure 4 illustrates the schematic of Beams
acathoderaytube. Primary Beam
Theminiatureacceleratorusedinamicrowaveovenis
amagnetron,whichconsistsofacylindricalvacuumtube
Target
surroundedbyamagneticsolenoid.Acylindricalvacuum
tube has a cathode at the center of the cylinder and the
anodeattheoutercylindricalsurfaceofthecylinder.By
imposingonthetubewithanoscillatingsolenoidalmag-
(b) Secondary
neticfield,thecathodeelectronswillmoveinanoscilla- Beams
tory pattern, which in turn generates microwaves. High-
powermicrowavesareusedinradars,butmorecommon
householduseisinthemicrowaveoven.Inamicrowave Primary Beam Primary Beam
oven,themicrowavefromthemagnetronisdirectedinto
theovenfromtheanode.
Secondary
B. High-EnergyandNuclearPhysics 10-2000
Beams
8575A2
Particle accelerators are essential tools of high-energy,
FIGURE5 Schematicdiagramoffixed-targetandcolliding-beam
or elementary-particle, physics, as it is also called. In a experimentalmethods.
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34 AcceleratorPhysicsandEngineering
lessthaninasolidtarget;therefore,therearemanyfewer
particles to interact with in a beam. This difficulty can
be partially overcome by storing two circulating beams
ofparticlesinastoragering.Theretheypassmanytimes
throughoneanothertoincreasetheeffectivecollisionrate.
Inalltheseexperiments,higherenergiesareneededto
probetosmallerdistanceswithintheatomicnucleus.As
a result, there has been a constant drive toward higher
energy.
Thehighestenergiesnowusedforphysicsexperiments
are approaching 1 TeV in fixed-target research and two
beamseachof1-TeVenergyincolliding-beamresearch.
Atthistime,acollider,thelargehadroncollider(LHC),
withbeamsup7TeVeachisbeingconstructedatCERN.
FIGURE7 SpectrometeratEndStationAofSLAC.[Courtesyof
Reaching higher and higher beam energies is not the
StanfordLinearAcceleratorCenter.]
only frontier in high-energy physics accelerators. One
modernclassofstorage-ringcollidersaimsforextremely
highrateofevents,evenatmoderateenergies.Thesecol- ploratorydesignresearchofamuoncolliderareexamples
liders, called factories, demand a deeper understanding ofcurrenteffortsbeingcarriedoutaroundtheworld.
oftheacceleratorphysicsinvolved.Examplesincludethe
φ-meson factory at Frascati, and the B-meson factories
C. Spectrometer
at Tsukuba, Japan, and Stanford, CA. Both B-factories
startedoperationin1999. Spectrometersaredevicesfortheprecisemeasurementof
A large nuclear physics accelerator, the relativistic energies or masses of the particles in a beam. These de-
heavy ion collider (RHIC), is newly commissioned at vices are used, for example, in the secondary beams to
BNL.Bycollidingtwobeamsofheavyions,suchasgold sortouttheproductsofhigh-energyornuclearreactions.
ions,at100GeV/nucleonperbeam,theRHICisintended Chargedparticlesproducedinthesereactionsareguided
to create a new type of matter, the quark–gluon plasma, through a spectrometer to analyzing stations. Although
by the violent collision. Figure 6 shows a glimpse of the no acceleration is performed on these particles, acceler-
RHICasseeninitstunnel. atorphysicsisneededtomanipulatethem.Theworking
The development of particle accelerators for high- principleissimilartothatoftheprismsinlightoptics.In
energy and nuclear physics research is continuing ac- ordertobeabletodetectrareevents,ofparticularconcern
tively.Theabove-mentionedLHC,theelectron–positron are issues of high resolution, large angular and energy
linearcolliderresearchinseverallaboratories,andtheex- acceptance, and being able to handle a wide variety of
beamsandtargets.Forshort-livedproducts,itisalsonec-
essarytomakethespectrometerpathasshortaspossible.
Aspectrometer,usedinthefixed-targetexperimentatthe
Stanford Linear Accelerator Center, is shown in Fig. 7.
D. MedicalAccelerators
Acceleratorshavebeenextensivelyusedinmedicine.For
example,acommonX-raymachineisaparticleaccelera-
tor(typicallya5-to30-MeVelectronlinearaccelerator).
Init,electronsareacceleratedandmadetostrikeaheavy-
metaltargettoproduceX-rays.
Recently, charged particle beams from accelerators
have been used to treat cancers directly instead of being
used to produce X-rays first. The main limitation of the
useofX-raystotreatcancersisthattheydepositmostof
theirenergywheretheyoriginallyenterthebodyand,in
ordernottodamagehealthytissue,theoveralldosehasto
FIGURE6 Insidethetunneloftherelativisticheavyioncollider
(RHIC)duringinstallationwork.[CourtesyofBNL.) beratherlimited.Ontheotherhand,particlebeamsoffer
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AcceleratorPhysicsandEngineering 35
theadvantageofdepositingmostoftheenergyinarather generate microwaves useful under a variety of circum-
narrowregionjustbeforetheyarestoppedinthetissue. stances.Onenotableexampleofsuchacceleratorsisthe
Particle accelerators are also used in medicine to pro- klystron. Compared with the magnetron, a klystron has
duceradioactiveisotopes,whicharethenusedtotracethe a linear architecture instead of a cylindrical one, and it
movementofchemicalsthroughthehumansystemasan typically is capable of generating microwave power in
aidindiagnosis. themultipletensofmegawattrangeinapulsedoperation
mode.
E. ElectronMicroscope
H. Industry,MaterialScience
Electron microscopes are small accelerators of high op-
ticalprecisionandmechanicalstability.Thewavenature Theuseofacceleratorsinindustryhassomesimilarityto
of the electrons allows them to be used in a microscope their use in medicine. The particle energies are usually
to replace optical light. The very short wave length of low,hundredsofkeVto10MeVinmostcases.Amajor
the elctrons makes it possible to achieve resolutions not industrial use is in diagnosis and testing. Pressure ves-
achievable with the optical microscopes. With close at- sels,boilers,andotherlargemetalcastingsareroutinely
tentionpaidtoitsbeamoptics,includingcompensationof X-rayed to search for internal flaws and cracks. Particle
theirhigh-orderaberrations,modernelectronmicroscopes energies of 20 MeV or more are often used for greater
now achieve unprecedented resolutions in the Angstrom penetrationofthickcastings.Suchacceleratordevicesare
range,permittingviewsofsingleatoms. alsousedtodetectcontrabandatair-andseaports.
Particle accelerators are also used in materials treat-
ment. Precise concentrations of impurity ions are im-
F. SynchrotronRadiationandFree
plantedinmetalsurfacesforsolid-stateelectronicsmanu-
ElectronLasers
facture.Particlebeamsareusedtoetchmicrochipsinthe
Anotherscientificapplicationofacceleratorsthathaswide productionofintegratedcircuits.
use is in the production of synchrotron radiation in the Manymanufacturedobjectsaresterilizedbyaccelera-
form of intense ultraviolet light or X-rays. This syn- tors. Such sterilization is the preferred method for ban-
chrotronradiationisproducedbyanelectron(orpositron) dagesandsurgicalinstrumentsbecauseitdamagesthem
beamstoredinastoragering.Theradiationhastheprop- lessthanheatsterilization.Theacceleratorenergiesused
ertyofhighintensityandexcellentcollimation.TheX-ray for sterilization are low enough that no radioactivity is
intensityfromasynchrotronradiationfacility,forexam- inducedintheobjectbeingsterilized.
ple, is typically several orders of magnitude higher than Materials are also changed chemically by accelerator
commonX-raysources.Availabilityofthisintensesource radiation. A notable application is in the polymerization
has opened up many new areas of research, including ofplastics.Transparentshrinkwrappingistreatedbyac-
atomicandmolecularphysics,biology,chemistry,surface celerators to produce the desired shrinkability with the
andmaterialsciences,andmicromachining. application of heat. Cables are radiated to increase their
Apotentiallyevenmoreintenselightsourceisprovided durability.
by free electron lasers (FELs). Infrared and ultraviolet Foodpreservationbyacceleratorradiationisalsobeing
FELsarebeingdevelopedallovertheworld.X-rayFELs carried out, mostly on a trial basis at this time, but with
based on electron linear accelerators are being designed somelarge-scaleapplicationbythemilitaryservices.
at Stanford and Deutsches Elecktronen Synchrotron in A recent accelerator application is the destruction of
Germany. These high-performance accelerators are be- harmful bacteria in sewage by accelerator beams so the
comingtheradiationsourcesofthefuture. treatedsewagecanthenbeusedasfertilizer.
Somemilitaryusesofacceleratorshavebeensuggested
butnonehasasyetbeenputintoactualpractice.
G. MicrowaveSources
Accelerators are used to generate microwaves, which
I. Chemistry
are electromagnetic radiation whose wavelength is in
the range between one millimeter to many meters. Mi- In addition to industrial chemical applications, accelera-
crowaves are useful for radar and for long-distance torsarealsousedforresearchinchemistry.Typicallythis
communication purposes. They are also used as the ac- involves low-energy (keV or lower) cold beams of neu-
celerationmechanisminallcyclicaccelerators.Asmen- tralorchargedmoleculesofspecificspecies.Interactions
tionedearlier,magnetronsareamicrowave-generatingac- betweenthemoleculesinthebeamwithagastargetoran-
celerator. There are many variation of accelerators that otherbeamgivesinformationoftheinteractionpotential,
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36 AcceleratorPhysicsandEngineering
thedynamics,andtherotationorexcitationelectronicen- Inordertoavoidscatteringofbeamparticlesbyresid-
ergylevelstructuresofthemolecules. ualgas,theremustbeanacceleratingtubethatisevacu-
atedtolowpressure.Thevoltagedropmustbedistributed
somewhat uniformly along this tube to avoid sparking.
J. NeutronSource,FusionDriver Forvoltagesaboveapproximately1MV,theaccelerating
tubeisalmostalwaysinsulatedoutsidethevacuumbya
Anintensebeamofprotonsorheavyions,ledtobombard
pressurized gas of high dielectric strength, often sulfur
atarget,canserveasanintensesourceofneutrons,which
hexafluoride.
areusefulformaterialresearchofindustrialormilitaryap-
To produce a dc voltage, electric charge is brought to
plications.Suchadeviceisanalternativeorcomplemen-
the terminal. Charge may be brought electronically by
tarytoresearchreactors.Anacceleratorthatiscapableof
voltage-multiplying circuits (e.g., the Cockcroft–Walton
producingsuchintenseprotonbeamsistechnicallyvery
setandtheMarxgenerator),ormechanicallybyamoving
demanding. A project called Spallation Neutron Source
belt(e.g.,theVandeGraaffaccelerator).
(SNS),atOakridgeNationalLaboratory,TN,withthede-
signgoalof1MWbeampower,isunderconstruction.
Research and development is being carried out to 1. VoltageMultiplyingDevices
test the applicability of accelerators to confined fusion
The first successful high-voltage accelerator, the
as energy source. In the systems envisaged, beams of
Cockcroft–Walton accelerator, made use of a voltage-
high-energyparticleswouldbeusedtobombardasmall
doubling circuit, the Greinacher circuit, shown in Fig. 8.
deuterium–tritium pellet. The inertia of the beams im-
Tworectifiersactonoppositesidesoftheacsinewaveto
plodesthepellet,andintheprocess,thepelletisheatedto
charge a capacitance to twice the voltage. The principle
thepointatwhichdeuteriumandtritiuminthepelletwould
can be extended to many stages. Cockcroft and Walton
fuse.Averylargeacceleratorsystemwouldbeneededto
acceleratedprotonsto300keVandin1932demonstrated
producefusionenergyeconomically.Ideashavealsobeen
thefirstnuclearreactionwithartificiallyacceleratedpar-
proposedtousesuchasystemtodisposeofnuclearwaste
ticles.ModernCockcroft–Waltongeneratorsareavailable
whileproducingenergy.
commercially with voltages up to approximately 1 MV.
Special pressurized systems have been built to 3 MV.
Cockcroft–Walton generators are used often as the first
IV. TYPES OF ACCELERATORS
stage of higher energy accelerator systems because they
produce beams with very good energy regulation. This
Accelerators can be divided into two classes, those in
application however has nowadays been replaced by the
whichaccelerationiscarriedoutbyuseofahighdcvolt-
radio-frequencyquadrupoles.
age, and those in which acceleration is carried out by a
The Marx generator is similar in principle. The recti-
lowerbutoscillatingvoltage,whicharecalledcyclicac-
fiersystemisexternaltothecapacitorstack.Inessence,
celerators.Cyclicacceleratorsarefurtherdividedintolin-
thecapacitorsarechargedinparallel,thendischargedin
earacceleratorsandcircularaccelerators.
series through spark gaps. Marx generators were origi-
nallyusedinthe1920stoproducesurgesofhighvoltage
A. High-voltageAccelerators
In a high-voltage accelerator, a terminal or electrode is
charged to high dc voltage and particles are accelerated
from it to ground potential. If the terminal is charged to
avoltage V,asinglychargedionwillgainenergyeV in
theaccelerator.Themaximumpossiblevoltageislimited
bysparkingtoground.Itisalsolimitedbylessdramatic
coronadischarge.Withamplespacetogroundandscrupu-
lousattentiontodetailindesign,terminalshavebeenbuilt
thathold25MV.
The simplest way to produce high voltage is with an
acelectricalstep-uptransformersystem.X-raymachines
producevoltagesupto1MVbythismethod.Thebeam
isonlyacceleratedonone-halftheaccycleandvariesin
energythroughoutthepulse. FIGURE8 Schematicdiagramofavoltage-multiplyingcircuit.