Brianti, Giorgio

Chimenti, V

Clozza, A

Delle Monache, Giovanni O

Rossi, S

Sanelli, C

Sgamma, F

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN - PS DIVISIONPS/DI/Note 99-23 (Tech)DESIGN OF THE DIPOLE VACUUM CHAMBERFOR THE CNAO SYNCHROTRON(Based on the PIMM Study)G. Brianti1, V. Chimenti2,  A. Clozza2,  G. Delle Monache2,  S. Rossi1, C. Sanelli2,F. Sgamma2AbstractThe thin walled vacuum chamber of the CNAO synchrotron dipole has been studied in theframework of the Proton Ion Medical Machine Study (PIMMS). Sixteen curved chambers,one for each dipole, about 1.9 m long and with inner aperture 130 mm x 64 mm areneeded. The rapidly changing magnetic field in the dipole (maximum field rate of 3 T/s)imposes a stainless steel wall of thickness not bigger than 0.4 mm. Different constructionschemes of the chamber have been investigated and two prototypes have been simulated,realised and measured.1 TERA Foundation2  Laboratori Nazionali di Frascati dell ‘INFN, Frascati (Roma), Italy.Geneva, Switzerland17 November 1999open-2000-16710/Aug/200011. INTRODUCTIONThe thin walled vacuum chamber of the CNAO synchrotron dipole has been studied in theframework of the Proton Ion Medical Machine Study (PIMMS) aiming at an optimiseddesign of a synchrotron for radiation therapy. The mean required pressure in thesynchrotron ring is about 10-9 Torr. At this pressure level, in situ bakeout is not absolutelynecessary and it is ruled out by the complications of bakable diagnostics and of othercomponents, by the cost increase of the magnets (larger gap size) and by the increasedcomplexity of the control system and operating procedures [I].The synchrotron ring is composed of 16 dipoles shown in Fig. 1 [2]. Sixteen curvedchambers, one for each dipole, are needed. A chamber is composed of two major parts: athin part inside the dipole yoke (length of the core equal to 1553 mm) and a thicker partthat makes the transition between the thin part and the flange which is positioned outsidethe coil return path. In this latter part, comprised between the end of the yoke and the endof the coil along the beam trajectory, will be placed, the welded manifold of a pumpingstation. The second paragraph of this paper presents the design characteristics of the thinvacuum chamber inside the iron yoke of the dipole.Fig. 1 Geometry of the CNAO dipole (all dimensions in mm) [2].The dipole vacuum chamber is designed trying to satisfy the following requirements:- to produce a minimum influence and distortion of constant magnetic field, i.e. byusing a chamber material with very low magnetic permeability;- to reduce the effects of the eddy currents on the beam due to pulsed magnetic field,i.e. small thickness of the vacuum chamber wall. In the case of CNAO, the rapidlychanging magnetic field in the dipole (maximum field rate of 3 T/s) imposes astainless steel wall of thickness not bigger than 0.4 mm;- to have the smallest possible external chamber dimensions for a required beamaperture, to reduce the impact on the magnet aperture;- to be low in cost and easy to handle, therefore excluding large scale use of expensivematerials (e.g. ceramic chambers).2Different construction schemes of the chamber, presented in the third paragraph, havebeen investigated and two prototypes have been simulated, realised and measured.2. DESIGN CHARACTERISTICSIn the CNAO synchrotron the horizontal aperture and good field region are determined bythe length of the separatrices at extraction, while the vertical aperture and good fieldregion are fixed by the vertical dimensions of the carbon beam at injection. To fix theaperture of the vacuum chamber the following assumptions have been made [3]:1.2.3.4.a beam model with uniform distribution in 3D and beam envelope at + 45 o (  beingthe rms value of the distribution);a maximum margin for the horizontal closed orbit distorsion of f 10 mm, scaling withthe horizontal betatron Twiss function;a maximum margin for the vertical closed orbit distorsion of rt 7.5 mm, scaling withthe vertical betatron Twiss function;a security margin for collimation equal to half the maximum closed orbit margin, i.e.& 5 mm both horizontally and vertically, scaling with the related betatron Twissfunctions.In Table 1 the aperture of the vacuum chamber and the good field region, inside thedipoles, are summarised together with the available space in the dipole aperture. In Fig. 2the various contributions to the vertical aperture of the dipoles are presented: beside thechamber aperture and thickness, twice 2.6 mm are foreseen for the supports to givemechanical strength to the thin chamber and twice one extra millimetre for alignment.Table 1 Apertures and good-field region.Aperture of the dipole H x V [mm] 280 x 72Chamber aperture in the dipole H x V [mm] 130 x 64Good field region H x V [mm] 114x56As shown in the Figure 2, from the edge of the beam to the pipe there are 11 mm at thecentre, position where the buckling deformation is maximum. It seems reasonable toaccept a maximum buckling under vacuum within few percent of this margin (e.g. 0.5 mmof total deformation correspond to 2% of this margin).During extraction the separatrices have a constant length and extend across the horizontalaperture of the vacuum chamber. Moreover the circulating beam is displaced inside thevacuum chamber at extraction to build the required spiral step and during injection toavoid losses on the extraction septa. Therefore, in a medical synchrotron with resonantslow extraction, the form of the vacuum chamber have to guarantee a sufficient verticalclearance at the extremes of the horizontal aperture.In PIMMS a super-ellipse cross section of the dipole vacuum chamber (equation of thechamber contour (x/a)3 + (y/l~)~ = 1) has been proposed [3]. For the prototypes describedin the next paragraph the racetrack design has been chosen; as shown in Fig. 3, it gives alarger clearance and fully accommodate the good field region.li.-.-.lil;i.l._,Horizontal axisFig. 2 Vertical half aperture of synchrotron dipoles. - Cubic super-ellipse- Racetrack- Good field region- i :- Pole faceJ-80 -60 -40 -20 0 20 40 60 80Horizontal aperture [mm]Fig. 3 Cross sections comparison.The cycle of the dipoles gives a maximum field rate of 3 T/s. This rather fast variation ofthe magnetic field gives rise to induced current in the yoke laminations and in the vacuumchamber, that tend to reduce and distort the magnetic field in the good field region. Thetime constant of these effects and the consequent field distortions have been estimated [4and 51.The effect of the eddy currents in the vacuum chamber is reduced increasing the resistanceto their flow in the material: this is obtained reducing the wall thickness and also choosinga material with high resistivity. The purpose is to reduce the effects of eddy currents in thevacuum chamber below the level of the lamination effects. These effects are here4estimated in terms of response time constant and consequent field distortion. For astainless steel chamber (resistivity equal to 72 pR cm), 0.4 mm thick, the principal timeconstant is approximately of the order of 40 p.s. The corresponding field distortion isexpected to be of the order of 14 x 10-4 for proton injection at 7 MeV. This value isobtained considering a constant ramp rate of the field of 3 T/s and is given by the ramprate times the time constant. It is lower that the field distortion produced by the eddycurrents in the dipole laminations (thickness of the carbon steel lamination of 1.5 mm),estimated to be 24 x 10--40-4 [5].3. CONSTRUCTION TECHNIQUES AND PROTOYPESDue to the reduced thickness, a way of strengthening the chamber against buckling isnecessary. Two different designs have been analysed: corrugation and the reinforcementusing ribs. Other less conventional methods, like for example a thin layer deposit ofstainless steel on a glass or carbon support, have been discarded, since reliability and easymaintenance are of major concern for a radiation therapy facility.Both techniques have been already developed and applied in rapid cycling magnetic fields:the corrugated chamber in the PS Booster synchrotron of CERN [6], the reinforcedchamber in the switching magnets of Daphne in Frascati and in the Booster of the ESRF inGrenoble [7].In the case of the PS Booster of CERN the chamber has a thickness of 0.4 mm, outerdimensions of 69 mm and 138 mm for the vertical and the horizontal plane, respectively.The corrugation height is 3 mm and the material is the high nickel alloy Inconel X - 750.The material has been chosen for its high resistivity (121 j_Q cm) compared to stainlesssteel AISI 316 LN (72 uR cm) and for its mechanical characteristics, high module ofelasticity (21’100 kg/mm2 19’100 kg/mm2 for AISI 316 LN), and good machinability. Inthe PSB were installed 128 corrugated chambers, each 1822 mm long. The high number ofchambers (realised by Calorstat, France) justified the high cost of the tooling that wasneeded for the chamber realisation. The tooling cost represents the major drawback of thissolution, because of the limited number of chambers of the CNAO synchrotron.Reinforced thin chambers were realised by Ansaldo, Italy for the switching magnet inFrascati, see Table 2, and by Calorstat for the Booster of Grenoble. In both casesreinforcement ribs have been brazed to the thin vacuum tube. In the case of Frascati, thechamber thickness is 0.3 mm. It is inserted in a fast switching magnet and it is designed tostand ramping fields of several tens of Tesla per second.Table 2 Characteristics of the chamber for Frascatiswitching magnet.Internal dimensions H x V [mm] 54x 18 IExternal dimensions H x V [mm]Chamber length [mm]1 74 x 24.31397GeometryRibs pitch [mm]Ellipse20Rib thickness [mm]Pipe thickness [mm]10.35The chambers in Grenoble are in stainless steel with a nominal wall thickness of 0.3 mm(0.275 mm minimum, 0.325 mm maximum). Internal form and dimensions are varying, togive an idea of the dimensions, one portion is elliptical 59.7 x 36.1 mm2 The ribs have athickness of 1 mm and a typical pitch of 20 mm. The shape, dimension and frequency ofribs is different for Frascati and Grenoble and it is adapted to minimise the chamberdeformation under vacuum. With this construction technique the brazing materialconstitute the most expensive part of the chamber realisation.A first prototype, 4 cm long, of the CNAO vacuum chamber has been realised at Frascati.The thickness of the chamber is 0.28 mm and the transversal dimensions are 145 x 64mm2; the profile is racetrack and the material is stainless steel. The ribs, with thickness of2 mm, pitch of 18 mm and realised with a milling machine, have been brazed with a silver- copper - titanium alloy. The measured total buckling under vacuum has been of 0.4 mm.Even if this measure cannot be taken into account because of the particular geometry ofthe prototype, it was very helpful in order to demonstrate that a brazed solution can befeasible.A second prototype has been realised still in Frascati (Figures 4 to 6 and Table 3) adoptinga different construction technique. It has been obtained from a solid block of stainless steel(AISI 316 L forged) which has been first machined to get the external profile of thechamber and the reinforcement ribs; the inner hole has been obtained by electrondischarge machining. In the case of this prototype, electron discharge machining has beenapplied twice with decreasing granularity to get a smooth inner surface. The smoothness ofthe surface could be further improved by electrochemical polishing.For this prototype, pipe and ribs thickness were fixed at 0.4 mm and 2 mm, respectively.The rib pitch has been evaluated by means of the ANSYS code (finite element analysis --fea), on a basis of 0.5 mm pipe maximum deflection.Fig. 4 The second thin chamber prototype: ribs thickness and pitch (dimensions in cm).6RO. 5-,I/4Fig. 5 Cross section of the second thin chamber prototype (dimensions in cm).Fig. 6 The second thin chamber prototype realised in Frascati.Table 3 Characteristics of the second prototype.1 Internal dimensions H x V [mm] 1 145 x 64 1External dimensions H x V [mm]Prototype length [mm]1 175 x 7074.51 Geometry 1 Racetrack 11 Ribs Ditch [mm] I 18 IRib thickness [mm]Pipe thickness [mm]20.3 - 0.47The machining process limits the maximum length of each piece to about 150 mm. Theribs at the two extremities could be of half thickness and tilted, so that it is possible toweld consecutive pieces along the rib profile and also give the necessary curvature to thedipole chamber.Fig. 7 Vacuum chamber geometry for ANSYS fea.This second construction technique has the advantage of being completely automated andthus with very precise reproducibility. No brazing, no heating in furnace and no specialtooling are necessary. The second prototype have been tested under vacuum too and theresults are summarised in Table 4.l-i--,-Fig. 8 Vacuum chamber buckling, ANSYS fea results.Table 4 Buckling tests under vacuum.Test pressure [mbar]Maximum total buckling [mm]Upper face buckling [mm]Lower face buckling [mm]0.50.250.25The total buckling measured on the prototype has been equal to 0.5 mm and it is within thedesign limits set in the previous paragraph. The deformation is elastic and agrees quitewell with the simulations.9The fea of the prototype has been made considering the 3 planes of symmetry of thestructure. The tolerance of the pipe thickness between 0.3 mm and 0.4 mm obtained withthe technique mentioned above, has not be considered, so that a mean value of 0.35 mmwas adopted (see Figure 7). Moreover being the structure not affected by bucklingbehaviour, the modelling of geometric imperfection is not effective on the results.The analysis performed is a non linear type. The result shows a reduction in height of thepipe cross section of 0.61 mm (Fig. 8).4. CONCLUSIONSThe characteristics of the thin walled vacuum chamber of the CNAO synchrotron havebeen presented. The necessity to reduce the effects of the eddy currents imposes a stainlesssteel wall of thickness not bigger than 0.4 mm.Various construction techniques have been compared and simulations and prototypes havebeen realised. The thin walled chamber reinforced with ribs represents the most attractivesolution. In particular the technique adopted for the second prototype has the advantage ofbeing completely automated and thus with very precise reproducibility. No brazing, noheating in furnace and no special tooling are necessary and also the costs seem to becompetitive.The simulations and tests under vacuum have shown that the chamber prototype presents atotal buckling within the design limits.5. ACKNOWLEDGEMENTSThe authors have benefited from discussions with R. Veness, C. Hauviller (CERN),J.M. Filhol (Grenoble) and F. Gramatica (TERA). They also want to thank V. Lo110 (INFNLNF) for drawings and M. Di Virgilio (INFN LNF) for vacuum oven brazing.10REFERENCES[1][2]F. Gramatica, The vacuum system for the TERA synchrotron, presented in a ProjectAdvisory Committee seminar at CERN (23 March 1998).G. Brianti, M. Giesch and S. Rossi, Dipole of the CNAO Synchrotron (Based on thePIMM Study), PS/DI/Note 98-15 (Tech.) and TERA 98/8 ACC 28 (November1998).[3][4 ][5]161PIMMS Group, PIMMS Aperture Specifications, presented in a PIMMS seminar atCERN (February 1997).P. Bryant, Eddy currents in accelerator magnet vacuum chambers, presented in anextended PIMMS Meeting at CERN (December 1997).P. Bryant, Eddy currents in the PIMMS main magnets, PIMMS vol II, to bepublished.G. Brianti and K. Schindl, Computation of stress and deflections of elliptical vacuumchambers with corrugated thin wall profile of variable amplitude, SI/Int. DL/69-5(1969).[7] J.M. Filhol (Grenoble), private communication.  

Design of the dipole vacuum chamber for the CNAO synchrotron (based on the PIMM study)

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