We report progress on the design of the Next Linear Collider (NLC) Damping Rings complexes. The purpose of the damping rings is to provide low emittance electron and positron bunch trains to the NLC linacs, at a rate of 120 Hz. As an option to operate at the higher rate of 180 Hz, two 1.98 GeV main damping rings per beam are proposed, and one positron pre-damping ring. The main damping rings store up to 0.8 amp in 3 trains of 190 bunches each and have normalized extracted beam emittances {gamma}{var_epsilon}x = 3 mm-mrad and {gamma}{var_epsilon}y = 0.02 mm-mrad. The optical designs, based on a theoretical minimum emittance lattice (TME), are described, with an analysis of dynamic aperture and non-linear effects. Key subsystems and components are described, including the wiggler, the vacuum systems and photon stop design, and the higher-order-mode damped RF cavities. Impedance and instabilities are discussed

Anderson, S.

Atkinson, D.

Corlett, J.

De Santis, S.

Hartman, N.

Kennedy, K.

Li, D.

Marks, S.

McKee, B.

Minamihara, Y.

Nishimura, H.

Pivi, M.

Raubenheimer, T.

Reavill, D.

Rimmer, R.

Ross, M.

Schlueter, R.

Sheppard, J. C.

Wolski, A.

UNT Digital Library

THE NEXT LINEAR COLLIDER DAMPING RING COMPLEX*J. Corlett†, D. Atkinson, S. De Santis, N. Hartman, K. Kennedy, D. Li, S. Marks, Y. Minamihara, H. Nishimura, M.Pivi, D. Reavill, R. Rimmer, R. Schlueter, A. Wolski, LBNL, Berkeley, CA, USAS. Anderson, B. McKee, T. Raubenheimer, M. Ross, J. C. Sheppard, SLAC, Stanford, CA, USA                                                * Work supported by the US DOE under contract DE-AC03-76SF00098 (LBNL), and DE-AC0376SF00098 (SLAC)† jncorlett@lbl.govAbstract We report progress on the design of the Next LinearCollider (NLC) Damping Rings complexes. The purposeof the damping rings is to provide low emittance electronand positron bunch trains to the NLC linacs, at a rate of120 Hz. As an option to operate at the higher rate of 180Hz, two 1.98 GeV main damping rings per beam areproposed, and one positron pre-damping ring. The maindamping rings store up to 0.8 amp in 3 trains of 190bunches each and have normalized extracted beamemittances γεx  = 3 mm-mrad and γεy  = 0.02 mm-mrad.The optical designs, based on a theoretical minimumemittance lattice (TME), are described, with an analysis ofdynamic aperture and non-linear effects. Key subsystemsand components are described, including the wiggler, thevacuum systems and photon stop design, and the higher-order-mode damped RF cavities. Impedance andinstabilities are discussed.1 INTRODUCTIONThe NLC damping ring complexes provide damping ofthe e+/e- beams from the sources, producing stable, low-emittance beams which pass on to bunch lengthcompressors and pre-acceleration before entering the mainlinacs [1]. The main damping rings of both e+/e- systemsare identical. Due to the large emittance of positronsproduced from the target, a pre-damping ring is required toreduce the e+ beam emittance to be accommodated by themain damping ring acceptance. Each ring will storemultiple trains of bunches at once, and a single fullydamped bunch train is extracted while a new train from thesource is injected. The repetition rate is 120 Hz, althoughoptions to operate at 180 Hz have been explored.2 DESIGNThe damping ring designs use Theoretical MinimumEmittance (TME) arc cells, with a racetrack layout inwhich damping wigglers, RF systems, feedback systems,a circumference adjustment chicane, and injection/extraction hardware are located in the long straights.Damping wigglers are required to provide the necessarydamping at 1.98 GeV, given a 120 Hz injection/extractionrate and the extracted beam emittance requirement. Generaldiscussion of design criteria are given in [2], and details ofthe current lattice designs in [3]. Principal parameters ofthe rings are listed in Table 1.The rings resemble third-generation synchrotron lightTable 1: Parameters for the Main Damping Ring andPositron Pre-Damping Ring.MDR PPDRCircumference (m) 299.792 218.336Max. Current (A) 0.8 0.75Bunch trains xBunches per train xBunch separation(ns)3 x 190 x 1.4 2 x 190 x 1.4νx, νy, ν s 27.26, 11.14,0.00358.91, 7.24,0.031γεx equilib. (mm-mrad) 2.17 103γεx extract. , γεy extract.(mm-mrad)2.18, 0.02 114, 69σε (%), σz  (mm) 0.09 , 3.6 0.09 , 7.2ξx uncorr., ξy uncorr -37.12, -28.24 -10.39, -12.23τx, τy, τε (ms) 4.85, 5.09,2.614.24, 5.13,2.87Usr (kV/turn) 777 561α p 2.95 x 10-4 7.05 x 10-3VRF (MV),Frequency (MHz)1.07, 714 3.4, 714Lattice 36 TME cells 16 TME cellssources and the B-Factories in that they must store highcurrent beams (~ 1A) while attaining small emittances,low vacuum pressure, and large synchrotron radiationpower loading. Many design and technology issues forFigure 1. Damping rings layouts, showing e+ and e-complexes at the ends of the main linacs.Positron complexElectron complexFigure 2. Main ring arc lattice functions.the damping rings have been successfully demonstrated atoperating light sources, and at the B-Factory colliders.For operational flexibility, the designs allow for anenergy range of ±5%, and bunch charge and separationvariation of a factor of two such that average current ismaintained.Layout of the damping rings enclosures are shown inFigure 1, in a configuration in which the damping ringscomplexes are located at the ends of the main linacs. Otherconfiguration options include a central injector complex inwhich the rings are close to the interaction point andtransfer lines connect to the big bends at the ends of thelinacs.Arc dipoles utilise gradient magnets to provideadditional focussing, and main ring arc lattice functionsare shown in Figure 2. Dynamic aperture is predicted to bein excess of 15 times the injected beam rms transversedimensions [3]. Obtaining the extracted emittance requiresbeam-based alignment to determine BPM position relativeto the quadrupole centers. Simulations show that extractedbeam normalized emittances of γεx † 3 mm-mrad and γεy† 0.02 mm-mrad may be produced with acceptabletolerance on magnet alignment and magnet positioncontrol [3]. Skew quadrupoles will be used to correctcoupling and minimise residual vertical dispersion.The damping times of the bare lattice are of the order of10 ms. To provide sufficiently rapid damping to allowextraction of small emittance beams at 120 Hz, asignificant fraction of the circumference is occupied bydamping wiggler magnets. The 2.15 T, 27 cm periodwiggler has integrated wiggler field of 106.9 T2m,providing two-thirds of the total energy loss.Injection and extraction systems require fast rise and falltimes of 65 - 100 ns to avoid perturbation of stored beamsin the ring. Extraction kickers and septa are located afterthe RF cavities and immediately before the injectionhardware, such that the RF system does not experiencelarge transients due to missing bunch trains.The positron beams from the source have a large edgeemittance of 30000 mm-mrad, requiring a pre-dampingring which reduces the emittance by approximately twoorders of magnitude. The pre-damping ring lattice is alsobased on TME arc cells, the acceptance is significantlylarger than the main rings and the circumference reduced toaccommodate only two trains [3].3 TECHNOLOGYIn addition to the demands of current generation lightsources and B-factories, the damping ring complexincludes additional challenges associated with fastdamping, high throughput injection from a powerfulinjector, and extraction of stable beams. We havefocussed design effort on critical systems including theHOM damped RF cavities, the vacuum systems, and thedamping wiggler.3.1 RF systemThe RF cavities are higher-order-mode damped single-cell cavities, based on the successful PEP-II design. Threedamping waveguides are used to externally load the cavityHOM's, reducing their impedance and thus coupled-bunchgrowth rates to a level at which feedback systems maycontrol residual instabilities [4,5]. The cavity is designedfor a frequency of 714 MHz, gap voltage of 500 kV andbeam current of 800 mA, and is a conventional reentrantshape except for the addition of the HOM damping ports.Three cavities are sufficient to provide the necessaryvoltage for the main damping rings, however we requireseven cavities to provide the voltage necessary formomentum acceptance in the PPDR.Designs for the RF window, HOM loads and tunershave been developed. Figure 3 shows a CAD drawing ofthe cavity; further details may be found in [4].Figure 3. HOM damped RF cavity.3.2 Wiggler magnetWigglers are required to provide most of the damping inboth the main rings and the pre-ring. The wiggler magnetdesign is a NdFeB and vanadium permandur hybrid. Themain ring design has a peak field of 2.15 T, pole gap 2cm, and period 27 cm. There are ten sections each oflength 4.6 m, giving an integrated field-squared of 106.9T2m. The design is similar to insertion devices used onexisting synchrotron light sources.The finite pole width of the wiggler magnet producesnonlinear field components that may deteriorate thedynamic aperture and reduce injection efficiency. Theeffects on beam dynamics of the field roll-of (By vs x)have been investigated and is not believed to be a limitingfactor for 13 cm pole width [6].The PPDR wiggler has peak field of 2 T, pole gap 4cm, and period 1 m.3.3 Vacuum systems The vacuum system in the arcs is based on analuminum structure with an antechamber in which photonstops and vacuum pumping is located. A narrow slotallows synchrotron radiation to pass from the beamaperture into the antechamber, allows for vacuumconductance to achieve a residual pressure of 10-9 Torr atthe beam, and electromagnetically isolates the beamchamber from the large antechamber to reduce theimpedance. Approximately 5.5 kW of synchrotronradiation power is emitted from each arc dipole, and isabsorbed on discrete photon stops, each with a vacuumpump mounted below.The wiggler occupies 46 m of the main ringcircumference, and produces a synchrotron radiation powerof 420 kW. The majority of this power is absorbed ondiscrete photon stops located between each of the tenwiggler sections, with a vacuum pump mounted below.The aluminum vacuum chamber has a central circularbeampipe aperture, with openings along each side to allowsynchrotron radiation to pass through to the photon stops.The wiggler vacuum chamber includes facility to installtitanium sublimation pumping units along either side ofthe beamline, as can be seen in figure 4. This distributedpumping along the length of the wiggler is required tomaintain a residual pressure of 10-9 Torr at the beam,given the limited conductance and large surface area of thisvacuum chamber. Experiments are under way to determinethe feasibility of producing very low outgassing vacuumchambers to eliminate the need for the distributed TSPsystem [7].Figure 4. Wiggler section showing vacuum chamber withsemi-circular features to hold distributed pumping.The wiggler photon stops have a contoured surfacewhich is designed to produce a uniform power density ofapproximately 7 Wmm-2 from the incident synchrotronradiation. Mis-steering of ±0.75 mm on the photon stopresults in a still-tolerable maximum power density of 20Wmm-2.  The photon stops are fabricated from Glid-Copcooled by a serpentine water channel, and the maximumstress is approximately 34,000 psi - a safety factor of twobelow yield.4  IMPEDANCE AND COLLECTIVEEFFECTSWakefields of individual vacuum chamber componentshave been produced using the 3-D time-domain MAFIAcode, and resonant modes have been calculated usingMAFIA modelling of the RF cavities.Calculations of single-bunch effects using the wakefieldgenerated from estimated components designs suggest thatinstability begins at a bunch population of 1.25x1011, afactor of eight greater than design value [4,5].Transverse multi-bunch stability is dominated by theresistive wall impedance, with small contributions fromthe cavity HOM's. A broad-band, bunch-by-bunchtransverse feedback system will be required to maintainbeam stability.Longitudinal coupled-bunch motion may be driven bythe fundamental mode of the accelerating cavities, as thecavities are detuned under beamloading. Notch filters inthe RF system may be used to damp these few modes.Simulations of the electron cloud build-up in thepositron main ring have been made with the simplegeometry of a circular beampipe without antechamber.Corresponding growth rate estimates are less than 10,000s-1, and the transverse feedback system will damp thismotion.5 REFERENCES [1] J. C. Sheppard et al, "Update on the NLC InjectorSystem Design", this Conference.[2] M. Ross et al, "The Next Linear Collider DampingRing Complex", Proc. PAC99, New York, 1999.[3] A. Wolski and J.N. Corlett, " The Next LinearCollider Damping Ring Lattices", this Conference.[4] R.A. Rimmer et al, "An RF cavity for the NLCdamping rings", this Conference.[5] J. Corlett et al, "Impedance and instabilities in theNLC damping rings", this Conference.[6] A. Wolski et al, "Effects of damping wigglers onbeam dynamics in the NLC damping rings", thisConference.[7] S. Shen et al, " Fabrication and measurement of low-outgassing surfaces for UHV vacuum chambers", thisConference.

The next linear collider damping ring complex

https://digital.library.unt.edu/ark:/67531/metadc717894/m2/1/high_res_d/783878.pdf

The next linear collider damping ring complex

Abstract

Similar works

Full text

Available Versions

UNT Digital Library