Clarke J A

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POSSIBLE INSERTION DEVICES FOR THE DIAMOND LIGHT SOURCEJ. A. Clarke, Daresbury Laboratory, Warrington, WA4 4AD, UKAbstractDIAMOND is a 3 GeV synchrotron light source that isplanned to replace the 2 GeV SRS at Daresbury.  It mustsatisfy a very broad range of future users including theUK academic community, research funded by theWellcome Trust, significant requirements of the Frenchgovernment supported science and other applications,including industrial and commercial ones. As with all 3rdgeneration light sources the primary beamlines will bebased on state of the art insertion devices. These deviceswill need to provide very high brightness from below 100eV to up to 20 keV and moderate brightness but very highflux beyond 20 keV and up to at least 100 keV. To meetthese demands several types of insertion device will needto be provided - planar undulators, multipole wigglers,wavelength shifters as well as more exotic devices forproviding variable photon polarisation. This paperdescribes the devices currently being assumed.1  INTRODUCTIONDIAMOND is a 3rd generation light source project thatwill be built in the UK [1]. The project has three fundingpartners; the British and French Governments and theWellcome Trust, a UK based life sciences charity. Thescientific needs of all three partners must be met by theinsertion devices and so they have to span a very broadphoton range with  internationally competitive photonbeam qualities. The insertion devices will need to provideextremely high brightness from below 100 eV to up to 20keV and moderate brightness but very high flux beyond20 keV and up to at least 100 keV. To meet thesedemands several types of insertion device will need to beprovided - planar undulators, multipole wigglers andwavelength shifters. There is also a major requirement forvariable photon polarisation over this whole wavelengthrange.2  DIAMOND PARAMETERSThe present DIAMOND design has an energy of 3 GeVand 24 cells. Six of the cells have a free straight length of9.36 m of which it is expected that 8 m will be availablefor IDs, the other 18 cells have a free straight length of5.86 m with 4.5 m of this available for IDs. Since thelattice is of the double bend achromat type there are 48dipoles (each of 1.4 T). However, due to engineering andbeamline layout constraints only approximately half ofthese will be available to be used for synchrotron radiationexperiments. The lattice has an exceptionally smallemittance of 2.5 nm rad, the consequent beam sizes anddivergences at the major source points are given in Table1. More details on the lattice can be found in [2].4.5 mStraight8.0 mStraightCentre ofDipolexσ  (µm) 79.9 166.0 33xσ ′ (µrad) 35.0 15.6 89yσ  (µm) 7.8 15.6 23yσ ′  (µrad) 3.1 1.6 4.5Table 1. DIAMOND electron beam size ( yx,σ ) anddivergences ( yx,σ ′ ) at the centre of the ID straights andthe dipole. A coupling of 1% has been assumed.Lifetime calculations have indicated that a verticalmagnet gap of 15 mm should be possible for the fulllength of the 4.5 m straight. Short period devices will beable to have a minimum gap of 10 mm, but only over thecentral 2 m of the straight section [3]. It is envisaged atpresent that the 8 m straight will be used for low field,longer period devices and so the gap will not be socritical. At present a gap of 20 mm is being assumed forthe 8 m straights. 3  INSERTION DEVICESNo beamlines have been formally decided yet for theDIAMOND light source and so it is not possible tofinalise the design of any of the IDs. However, a portfolioof undulators, multipole wigglers and wavelength shiftershas been assembled using justifiable assumptions so thatthe potential users of the new project are well informed ofwhat it will be capable of.3.1 Very High Brightness PhotonsOne of the major requirements of the DIAMOND lightsource is that very high brightness photons are availablefor protein crystallography. Typical photon energyrequirements for this thriving community are in the range10 - 15 keV and beyond. The high brightness required isonly achievable in a light source from undulators. In theDIAMOND project, short period undulators with smallgaps and high harmonic output are required to reach muchbeyond 10 keV. An international workshop was held atDaresbury in 1999 to discuss the feasibility of such small2319Proceedings of EPAC 2000, Vienna, Austriagap, high harmonic devices [4]. The workshop concludedthat operating with gaps in the range of 5 to 10 mm wasfeasible and that harmonics up to and beyond the 11thcould usefully be exploited. This important conclusionenabled the project to state with confidence that very highbrightness photons would be available from DIAMONDin the range 10 - 15 keV.Figure 1 shows the photon brightness for the presentcomplement of undulators. It can be seen that brightnessbeyond 1020 photons/s/mm2/mrad2/0.1% will be achievedover the wide operating range of 400 eV to 7000 eV.Table 2 gives details of the undulators that are presentedin figure 1. The useful photon range that is given in thetable is defined (somewhat artificially) as being thatwhich has a photon brightness of greater than 1018photons/s/mm2/mrad2/0.1%.Name U24 U36 U48 U80 U200Period(mm) 24 36 48 80 200Nperiods 83 125 93 56 40Length (m) 2.0 4.5 4.5 4.5 8.0Min. Gap(mm) 10 15 15 15 20Bmax (T) 0.60 0.60 0.84 0.67 0.27Kmax1.4 2.0 3.8 5.0 5.1PhotonRange(keV)1.8 - 3.6,5.6 - 22 0.8 - 19 0.2 - 14 0.08 - 8 0.03 - 2.7Table 2. DIAMOND Undulator Parameters. Nperiods isthe number of periods, Bmax  is the maximum on-axis fieldand Kmax is the maximum deflection parameter.1.0E+171.0E+181.0E+191.0E+201.0E+2110.0 100.0 1000.0 10000.0 100000.0Photon Energy (eV)U36U24U48U80U200U24Figure 1. Photon brightness (units of photons/s/mm2/mrad2/0.1%) for the present complement of DIAMONDundulators with a beam current of 300 mA.3.2 High Flux Beyond 20 keVThere is a requirement for medium brightness but veryhigh flux photons beyond 20 keV and up to as high asperhaps 100 keV. This will be met by a combination ofmultipole wigglers and wavelength shifters. Again, theexact requirements for particular beamlines is unknown atpresent so different possibilities are being explored.Figure 2 gives the photon flux that would be availablefrom two of the undulators presented earlier, two furtherIDs and the bending magnet. The 1.6 T multipole wigglerhas 45 periods, each 100 mm long. The wavelength shifteris a single pole 6 T superconducting magnet. There isconsiderable interest at present in having a higher fieldmultipole wiggler of between about 3 and 4 T. Possibledesigns will need to be explored in the near future toassess this requirement.Proceedings of EPAC 2000, Vienna, Austria23201.0E+121.0E+131.0E+141.0E+151.0E+1610 100 1000 10000 100000Photon Energy (eV)1.6 T Multipole WigglerBending Magnet6 T Wavelength ShifterU48U80Figure 2. Photon flux (units of photons/s/0.1%) for DIAMOND with a beam current of 300 mA. The bending magnet,multipole wiggler and wavelength shifter flux is given per horizontal mrad.3.3 PolarisationThere is a significant demand already for light ofvariable polarisation to be available over a wide range ofthe spectrum. At present it seems that the greatest demandfor variable polarisation will be at the low energy end ofthe spectrum (below 100 eV to 2 keV) and this will bebest met by helical undulators, probably of the Apple IItype [5]. However, it is likely that an asymmetric wigglerwill be needed for high energy photons of left and righthelicity.3.4 Two IDs Per StraightAlthough DIAMOND will have 22 free straightsections available for IDs the user demand is expected tobe so high that serious consideration is already beinggiven to having two independent IDs in some straights.This will allow 2 simultaneous experiments per straightand could effectively double the capacity of the source.The electron beam within the straight section would becarefully steered so that the photon beams from the twoIDs would be separated by a small angle. Several‘chicane’ schemes are possible and these will be assessedin the future design study to decide which is the mostadvantageous.Separate ID modules of approximately 2 m in lengthare being assumed at present as this fits in nicely with therequirement to have 2 independent IDs in the 4.5 m driftspaces and also with having ‘single’ 8 m IDs in the longerstraights (made up of 4 modules).REFERENCES[1] M.W. Poole et al, “Evolution of the DIAMONDLight Source Project”, these proceedings.[2] S.L. Smith and A. Wolski, “Developments in theDIAMOND Storage Ring Lattice”, theseproceedings.[3] A.A. Chesworth et al, “DIAMOND: A UK NationalLight Source Project”, PAC ’99, New York, 1999,p2433.[4] “Realising Small Gap Insertion Devices in 3rdGeneration Light Sources”, Workshop Record,Daresbury Laboratory, April-May 1999.[5] S. Sasaki, “Analyses for a planar variably-polarisingundulator”, NIM A 347 (1994) p83.2321Proceedings of EPAC 2000, Vienna, Austria

Possible insertion devices for the DIAMOND light source

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