Wiggler and Undulator Magnets - PDF document

Wiggler and Undulator Magnets Two new devices are being added to synchrotron radiation sources to extend the spectral range and increase brightness. To scientists using vacuum ultraviolet and x rays the most important characteristics of an ideal radiation source would be a high intensity within a small solid angle and a high intensity within a small wavelength interval, both extending over a broad range of wavelengths. High spatial brightness rfxle$flux$finzmnt$f$ysfrr$ permits the delivery of a large number of Utrnpe$zme$ysuuzm%$hutzntuuuy$yvehzxfr$inyzxnguznut$uk$radiation produced by ring bending magnets and wigglers, interference eects in undulator radiation result in vefpy$fz$ute$ux$f$kefi$fif—eretlzmy"$2z$zmeye$zutfgre$wavelengths undulator radiation has high spatial and A wiggler is a magnetic structure with one or a few periis considerably greater than deviation produced by one ring bending magnet. A wiggler is usually designed so that the angular excursion of zme$xfinfznut$kxus$efhm$vure$fvvxuxnsfzery$firry$zme$ftgular acceptance of the synchrotron light beam pipe.2ryu%$nz$ny$uyufrry$ieynltei$yu$zmfz$zme$vefp$sflteznh$fieri$sfy$ge$lxefzex$zmft$zme$fieri$nt$zme$getintl$sflnets, resulting in a spectrum which may extend to higher photon energies. Thus, a wiggler magnet otended spectral range and overall enhancement of the radiation brightness compared to the ring bending mag2rzmuulm$zme$fixyz$hutyniexfznut$uk$finllrexy$fti$utiurfzuxy$luey$gfhp$14$ux$suxe$yefxy%$zmenx$uye$fy$xfinfznut$yuuxhey$kux$exvexnsetzy$ny$utry$xehetz"$Tme$fixyz$finllrex$magnet to be used routinely as a radiation source began operation in March 1979 at the Stanford Synchrotron Radiation Laboratory. The substantial extension of spectral range and intensity provided by this magnet and the demonstration of its compatibility with storageoperation have had a profound eect on the developratories.Two examples of this impact are:Uyex$ntzexeyz$nt$zme$fixyz$VVUM$finllrex$mfy$geet$yu$great that plans to build additional beam lines from bending magnets were suspended and the emphasis at SSRL is now on building more wigchrotron Radiation Facility originally conceived GeV, 565 mA machine to meet certain inzetynzy$fti$yvehzxfr$yvehnfihfznuty"$Tme$lxuuv$ny$now considering an “allwiggler machine”which virtually all of this radiation used by experimenters would be produced by wigglers up to . With wigglers, the ring would only have to operate at 3.5 GeV with 100 mA to meet the same intensity and spectraltions as the bendingmagnet beams on the higher energy, higherUtiurfzuxy$mf—e$tuz$yez$mfi$f$ynltnfihftz$nsvfhz$ut$yythmxuzxut$xfinfznut$xeyefxhm"$Muyz$uk$zme$fiuxp"$ifze$mfy$concentrated on producing visible undulator radiation . It is only very recently that undulators designed to serve as radiation sources for experiments in ray parts of the spectrum have been installed in storage rings nt$Pu—uyngnxyp%$UVVU<$Rxyfy%$France; and at Stanford. Based on the brief experience with these devices plus the many studies done at longer wavelengths, it is our opinion that undulators will have an impact on research with synchrotron radiation comparable to or exceeding that of wigglers, particularly for Figure 1 — Visible light from a superconducting undulator at the ACO storage ring operated at 240 MeV. The symmetric dark spots at the upper center and lower center of the photo The magnetic (Courtesy of Y. Farges and Y. Petroff, LURE, Orsay, UndulatorsThe earliest eonsideration of undulators luey$gfhp$zu$f$zmeuxeznh$vfvex$gy$a"$M"$Hntzguxl083-"$Tme$fixyz$utiurfzux$fify$sfie$gy$Ifty$Muzz$fti$hufiuxpexy in 1953. It was a permanent magnet with a vexnui$uk$3.$ss$fti$f$lfv$uk$3$ss<$nz$fhmne—ei$fieri$yzxetlzmy$uv$zu$fguuz$4$pH"$Uyntl$gefsy$kxus$rntefx$accelerators, Motz observed visible radiation when electron energies were in the range of 100 MeV and millimeter radiation at about 3 MeV. Physics Today, May 1981, Volume 34, Issue 5, pp. 5063, doi: 10.1063/1.2914568 cow. (a) Undulator off: The two lobes are synchrotron radiation from the bending magnets adjacent to the undulator. (b) Undulator on: The central spot is due to the undulator. The electron energy is 175 MeV. (Courtesy V. V. Mikhailin, MosIn the 1970s undulators were installed in two synchrotrons in the Soviet Union and systematic investigations of the properties of the radiation were carried out. One E"$G"$2rkexu—%$du$2"$Cfymsfpu—%$F"$H"$Ceyyutu—$fti$uufiuxpexy used the Pachra 1.3Ge V synchrotron of the Lebedev Physics Institute, Moscow. The other group rei$gy$M"$M"$PnpnzntGe V synchrotron at the Research Institute of Nuclear Physics of the Tusyp$Suryzehmtnh$Jtyznzuze"$ Figure 3 — Undulator radiation patterns for different polarization$and$varied$electron$energy.$The$angular$distribution$). The undulator had a period of 14 cm In both cases relatively lowup to a few hundred MeV were used, so that the undulator proiuhei$ntzexkexethe$vefpy$nt$zme$—nyngre$vfxz$uk$zme$yvehtrum undulator radiation is due in large part to the excellent fiuxp$uk$zmeye$lxuuvy"$Tmenx$xe—nefiy$fti$zme$fiuxp$uk$ provide a comprehensive description of the properties spectrum, intensity, angular distribu of undulator radiation and extensive references to the rapidly growing literature on the subRecently, eorts have begun to extend undulator radiation into the vuv and xray parts of the spectrum where, gehfuye$uk$zme$fgyethe$uk$rfyexy%$zmey$fxe$rnpery$zu$ge$important sources of radiation.A superconducting undulator has been operated on the MeV storage ring ACO, and permanentmagnet undulators have been operated on VEPPHe$a$yzuxfle$xntl$fz$zme$Pu—uyngnxyp$Smyynhy$Jtyzntute, and SPEAR,GeV storage ring at SLAC, (shown in the withdrawn position) could be inserted. The vacuum$chamber"$which$is$not$disturbed$in$the$changeover$from wiggler to undulator, is midway between the wiggler Undulators are being included in the new dedicated synchrotron sources now being completed around the world. The reduced electronvergence planned for some of these machines will enable undulators to produce even higher spectral and spatial The devices built to date produce highrfzux$xfinfznut$fz$vmuzut$etexlney$gerufi$fguuz$1$pea"$Higher photon energies can be reached by undulators finzm$ymuxzex$vexnuiy$fti$mnlmex$sflteznh$fieriy$ux$gy$Physics Today, May 1981, Volume 34, Issue 5, pp. 5063, doi: 10.1063/1.2914568 energy electrons. With electron energies of 4 GeV, such as are available with SPEAR, it should be possible to extend the highermfxsutnh$vefpy$kxus$zme$utiurfzux$zu$fguuz$7$pea"$We will describe the plans for such an undulator at SSRL later in this article. Even higher energies could attained if undulators were installed in recently completed, higherenergy collidingbeam storage rings in the Soviet Union and the US. Wigglers. The earliest suggestion of a wiggler magnet to produce synchrotron radiation was made by K. W. Robinson in an unpublished report at the Cambridge Electron Accelerator $nt$0845"$Tme$fixyz$finllrex$was built CEA in 1966, not as a source of synchrotron radiation, but to provide additional damping of betatron and synchrotron oscillations to convert the alternating and hence anti magnetic lattice of the CEA to a beam storage system. The PEP storage ring also contains wiggler magnets to control storedbeam size and polarization time constants.2$finllrex$sfltez$fify$fixyz$uyei$fy$f$yythmxuzxut$xfinftion source at SSRL in 1979.pH$sfltez$vxu—niei$zme$suyz$vufiexkur$gefs$uk$x$xfyy$ever produced and also made possible an increase in the interaction rate of colliding beams on SPEAR. This success prompted the installation in 1980 of a pair of longer magnets in SPEAR . With all eight coils powered in this magnet, the electron beam executes four full oscillations. The electrical circuit can be remotely changed to produce one, two or four oscillations to control the total power radiated. This ring is now operated for half the time as a beam synchrotron radiation source. A similar wig is now operating in the Adone storage ring at Frascati see the photo on page 30$fti$f$24$pH%$pole superconducting wiggler has been installed in $nt$Pu—uyngnxyp" Wigglers are in construction or being planned for most of the new dedicated synchrotron sources. Spencer and bntnhp review the characteristics of operating planned wigglers and undulators as of 1979. The simplest wiggler magnet consists of 3 poles and produces one oscillation of the electron beam. The total and hence the angular divergence of the radiation, although small The fan of radiation produced by the wiggler is thus large enough that two or more simultaneous experiments can share the beam. The center pole is generally uvexfzei$fz$f$fieri$mnlmex$zmft$zme$getintl$sfltezy$uk$the ring to extend the spectrum to higher photon energies. The spectral distribution is a smooth continuum as it is for the bending magnets. For wigglers and bending magnets the spectrum is characterized by the critical Physics Today, May 1981, Volume 34, Issue 5, pp. 5063, doi: 10.1063/1.2914568 c= 3hc 32 $ny$zme$sflteznh$fieri$ of the wiggler or bending magnet and is the radius of curvature; the hxnznhfr$etexly$ny$nt$pea%$zme$gefs$etexly$ny$nt$Hea"$Tme$wiggler’s eectiveness in extending the spectrum to higher photon energies is thus dependent on the ratio of zme$finllrex$fieri$zu$getintl$sfltez$fieri"$Cetintlsfltez$fieriy$fxe$zyvnhfrry$nt$zme$xftle$404$pH"$5ut—etznutfr$erehzxusfltez$finllrexy$mf—e$geet$uyei$finzm$fieriy$uv$zu$fguuz$1.$pH$fti$yuvexhutiuhzntl$finllrexy$iesigned to reach 405.$pH$fxe$nt$hutyzxuhznut"$Jz$fryu$sfltez$finllrexy$finzm$fieriy$uv$zu$fz$refyz$05$pH$ymuuri$ge$vuyyngre"$2t$exfsvre$uk$zme$ectiveness of a wiggler in extending the spectrum f—fnrfgre$kxus$zme$getintl$sfltezy$ny$ln—et$nt$filuxe$5$for three facilities. All wiggler spectra are shown for one pole. The SPEAR wiggler has eight eective full poles so the wiggler spectrum should be increased by a factor of 8. The Daresbury wiggler has a strong central pole and fiefpex$eti$vurey$yu$zme$u—exfrr$ntzetynzy$ny$etmfthei$gy$a factor that varies from 2 at low photon energy to 1 at the highest photon energy. The Hefei wiggler has not been designed yet. Multipole wigglers represent extended sources, which must be considered in the design rnte$uvznhy$zu$sfxnsnze$zme$flux$f—fnrfgre$fz$zme$exvexnsetz"$Tme$flux$ny$ln—et$vex$snrrnxfinft$uk$geti$integrated over all angles normal to the bending plane. 10001001010.01.1110100Photon Energy (keV) Intensity [1010 Wiggler (18 kG, 14.7 keV)Wiggler (50 kG, 2.1 keV)6IGG;FR$%-1$J