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Magnets for FFAGs; Neil Marks FFAG School 2011 Magnets for FFAGs Neil Marks, STFC- ASTeC / U. of Liverpool, Daresbury Laboratory, Warrington WA4 4AD, U.K. Tel: (44) (0) Fax: (44) (0)

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Magnets for FFAGs; Neil Marks FFAG School 2011 ‘ Omnis Gallia in tres partes divisa est’ GAIVS IVLIVS CÆSAR (100BC - 15 March 44BC) De Bello Gallico, Book 1, Ch 1. Likewise, this presentation!

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Magnets for FFAGs; Neil Marks FFAG School 2011 Present a short overview of electro- magnetic technology as used in particle accelerators, considering only warm magnets (ie superconducting magnets excluded) in three main sections: i) DC magnets used in general accelerator lattices; ii) pulsed magnets used in injection and extraction systems; iii) specialised FFAG issues (including practical examples in determining pole face geometry); (this section will also be delivered in the main workshop so will only be presented here if time permits). Objectives

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Magnets for FFAGs; Neil Marks FFAG School 2011 Section 1: ‘General’ Accelerator Magnet Theory and Practice. Maxwell's 2 magneto-static equations; Fields in free space: Solutions in 2D with scalar potential (no currents); cylindrical harmonics; field lines and potential for dipole, quadrupole, sextupole; Introduction of steel: Ideal pole shapes for dipole, quad and sextupole, and combined function magnets, significance and use of contours of constant scalar potential; Introduction of currents: Ampere-turns in dipole, quad and sextupole; coil economic optimisation-capital/running costs; Practical Issues: FEA techniques - Modern codes- OPERA 2D; TOSCA; judgement of magnet suitability in design. Contents – Section 1.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Section 2: Injection and Extraction system pulsed magnets; Methods of injection and extraction; Septum magnets; Kicker magnets and power supplies. Section 3: Geometry of specialised FFAG magnets. Example of lattice magnet requirements for a 27 cell FFAG (Pumplet*); Development of a suitable pole for two lattice magnets; A recent development at DL to save space and operating cost of an of- centre multipole FFAG magnet. * Acknowledging design by Grahame Rees, ASTeC, RAL. Contents – Sections 2 & 3.

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Magnets for FFAGs; Neil Marks FFAG School 2011 SECTION 1 General Accelerator Magnet Theory and Practice

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Magnets for FFAGs; Neil Marks FFAG School 2011 Maxwell’s equations: .B = 0 ; H = j ; j = 0; So then we can put: B = - So that: 2 = 0 (Laplace's equation). Taking the two dimensional case (ie constant in the z direction) and solving for cylindrical coordinates (r, ): = (E+F )(G+H ln r) + n=1 (J n r n cos n +K n r n sin n +L n r -n cos n + M n r -n sin n ) No currents, no steel:

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Magnets for FFAGs; Neil Marks FFAG School 2011 The scalar potential simplifies to: = n (J n r n cos n +K n r n sin n ), with n integral and J n,K n a function of geometry. Giving components of flux density: B r = - n (n J n r n-1 cos n +nK n r n-1 sin n ) B = - n (-n J n r n-1 sin n +nK n r n-1 cos n ) In practical situations:

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Magnets for FFAGs; Neil Marks FFAG School 2011 This is an infinite series of cylindrical harmonics; they define the allowed distributions of B in 2 dimensions in the absence of currents within the domain of (r, ). Distributions not given by above are not physically realisable. Coefficients J n, K n are determined by geometry (remote iron boundaries and current sources). Note that this formulation can be expressed in terms of complex fields and potentials. Significance

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Magnets for FFAGs; Neil Marks FFAG School 2011 To obtain these equations in Cartesian coordinates, expand the equations for and differentiate to obtain flux densities; cos 2 = cos 2 – sin 2 ;cos 3 = cos 3 – 3cos sin 2 ; sin2 = 2 sin cos ;sin3 = 3sin cos 2 – sin 3 ; cos 4 = cos 4 + sin 4 – 6 cos 2 sin 2 ; sin 4 = 4 sin cos 3 – 4 sin 3 cos ; etc (messy!); x = r cos ; y = r sin ; and B x = - / x;B y = - / y Cartesian coordinates:

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Magnets for FFAGs; Neil Marks FFAG School 2011 Cylindrical:Cartesian: =J 1 r cos +K 1 r sin . =J 1 x +K 1 y B r = J 1 cos + K 1 sin ;B x = -J 1 B = -J 1 sin + K 1 os ;B y = -K 1 So, J 1 = 0 gives vertical dipole field: K 1 =0 gives horizontal dipole field. Dipole field n=1:

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Magnets for FFAGs; Neil Marks FFAG School 2011 Cylindrical: Cartesian: = J 2 r 2 cos 2 +K 2 r 2 sin 2 ; = J 2 (x 2 - y 2 )+2K 2 xy B r = 2 J 2 r cos 2 +2K 2 r sin 2 ; B x = -2 (J 2 x +K 2 y) B = -2J 2 r sin 2 +2K 2 r cos 2 ; B y = -2 (-J 2 y +K 2 x) J 2 = 0 gives 'normal' or ‘upright’ quadrupole field. K 2 = 0 gives 'skew' quad fields (above rotated by /4). Lines of flux density Line of constant scalar potential Quadrupole field n=2:

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Magnets for FFAGs; Neil Marks FFAG School 2011 Cylindrical;Cartesian: = J 3 r 3 cos 3 +K 3 r 3 sin 3 ; = J 3 (x 3 -3y 2 x)+K 3 (3yx 2 -y 3 ) B r = 3 J 3 r 2 cos 3 +3K 3 r 2 sin 3 ;B x = -3 J 3 (x 2 -y 2 )+2K 3 yx B = -3J 3 r 2 sin 3 +3K 3 r 2 cos 3 ;B y = -3 -2 J 3 xy +K 3 (x 2 -y 2 ) Line of constant scalar potential Lines of flux density +C -C +C -C +C -C J 3 = 0 giving 'normal' or ‘right’ sextupole field. +C -C +C -C +C -C Sextupole field n=3:

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Magnets for FFAGs; Neil Marks FFAG School 2011 Dipole; constant field: Quad; linear variation: Sextupole: quadratic variation: x By x x Summary: variation of By on x axis.

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Magnets for FFAGs; Neil Marks FFAG School 2011 magnet strengths are specified by the value of k n ; (normalised to the beam rigidity); order n of k is different to the 'standard' notation: dipole is n = 0; quad is n = 1; etc. k has units: k 0 (dipole) m -1 ; k 1 (quadrupole)m -2 ; etc. Alternative notation:

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Magnets for FFAGs; Neil Marks FFAG School 2011 What is the ideal pole shape? Flux is normal to a ferromagnetic surface with infinite : Flux is normal to lines of scalar potential, (B = - ); So the lines of scalar potential are the ideal pole shapes! (but these are infinitely long!) curl H = 0 therefore H.ds = 0; in steel H = 0; therefore parallel H air = 0 therefore B is normal to surface. Introducing iron yokes and poles.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Equations for Ideal (infinite) poles; (J n = 0) for normal (ie not skew) fields: Dipole: y= g/2; (g is inter-pole gap). Quadrupole: xy= R 2 /2; Sextupole: 3x 2 y - y 3 = R 3 ; R Equations of ideal poles

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Magnets for FFAGs; Neil Marks FFAG School 2011 'Combined Function Magnets' - often dipole and quadrupole field combined (but see next-but-two slide): A quadrupole magnet with physical centre shifted from magnetic centre. Characterised by 'field index' n, +ve or -ve depending on direction of gradient; do not confuse with harmonic n! is radius of curvature of the beam; B o is central dipole field Combined function magnets

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Magnets for FFAGs; Neil Marks FFAG School 2011 Combined dipole/quadrupole Combined function (large dipole & small quadrupole) : beam is at physical centre flux density at beam = B 0 ; gradient at beam = B/ x; magnetic centre is at B=0. separation magnetic to physical centre = X 0 magnetic centre, x’ = 0 physical centre x = 0 X0X0 x x’

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Magnets for FFAGs; Neil Marks FFAG School 2011 Pole for a combined dipole and quad.

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Magnets for FFAGs; Neil Marks FFAG School 2011 i)dipole, skew quad, sextupole, octupole ( in the SRS) ii) dipole & sextupole (for chromaticity control); iii) dipole, quadrupole sextupole and octupole (and more!); Other combinations: # i) generated by multiple coils mounted on a yoke; amplitudes independently varied by coil currents. # ii) and iii) generated by pole shapes given by sum of correct scalar potentials, hence amplitudes built into pole geometry (but not variable); important for FFAG magnet design – see section 3. Combined function magnets.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Vector potential in 2D By definition:B = curl A (A is vector potential); anddiv A = 0 Expanding:B = curl A = ( A z / y - A y / z) i + ( A x / z - A z / x) j + ( A y / x - A x / y) k; where i, j, k, are unit vectors in x, y, z. In 2 dimensionsB z = 0; / z = 0; SoA x = A y = 0; and B = ( A z / y ) i - ( A z / x) j In a 2D problem, A is in the z direction, normal to the Plane of the problem. Note: div B = 2 A z / x y - 2 A z / x y = 0;

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Magnets for FFAGs; Neil Marks FFAG School 2011 Total flux between 2 points. In a two dimensional problem the magnetic flux between two points is proportional to the difference between the vector potentials at those points. B A1 A2 (A 2 - A 1 ); for proof see next slide.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Proof Consider a rectangular closed path, length in z direction at (x 1,y 1 ) and (x 2,y 2 ); apply Stokes’ theorem: x y z (x 1, y 1 )(x 2, y 2 ) B A ds dS = B.dS = ( curl A).dS = A.ds But A is exclusively in the z direction, and is constant in this direction. So: A.ds = { A(x 1,y 1 ) - A(x 2,y 2 )}; = { A(x 1,y 1 ) - A(x 2,y 2 )};

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Magnets for FFAGs; Neil Marks FFAG School 2011 Practically, poles are finite, introducing errors; these appear as higher harmonics which degrade the field distribution. However, the iron geometries have certain symmetries that restrict the nature of these errors. Dipole: Quadrupole: The practical pole in 2D

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Magnets for FFAGs; Neil Marks FFAG School 2011 Lines of symmetry: Dipole: Quad Pole orientation y = 0; x = 0; y = 0 determines whether pole is normal or skew. Additional symmetry x = 0;y = x imposed by pole edges. The additional constraints imposed by the symmetrical pole edges limits the values of n that have non zero coefficients Possible symmetries.

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Magnets for FFAGs; Neil Marks FFAG School + -- Type Symmetry Constraint Pole orientation ( ) = - (- )all J n = 0; Pole edges ( ) = ( - )K n non-zero only for: n = 1, 3, 5, etc; So, for a fully symmetric dipole, only 6, 10, 14 etc pole errors can be present. ++ ++ Dipole symmetries

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Magnets for FFAGs; Neil Marks FFAG School 2011 Type Symmetry Constraint Pole orientation ( ) = - ( - ) All J n = 0; ( ) = - ( - ) K n = 0 all odd n; Pole edges ( ) = ( /2 - ) K n non-zero only for: n = 2, 6, 10, etc; So, a fully symmetric quadrupole, only 12, 20, 28 etc pole errors can be present. Quadrupole symmetries

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Magnets for FFAGs; Neil Marks FFAG School 2011 Type Symmetry Constraint Pole orientation ( ) = - ( - )All J n = 0; ( ) = - (2 /3 - )K n = 0 for all n ( ) = - (4 /3 - ) not multiples of 3; Pole edges ( ) = ( /3 - )K n non-zero only for: n = 3, 9, 15, etc. So, a fully symmetric sextupole, only 18, 30, 42 etc pole errors can be present. Sextupole symmetries.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Summary of ‘allowed harmonics’ in fully symmetric magnets with no dimensional errors: Fundamental geometry ‘Allowed’ harmonics Dipole, n = 1n = 3, 5, 7, ( 6 pole, 10 pole, etc.) Quadrupole, n = 2n = 6, 10, 14,.... (12 pole, 20 pole, etc.) Sextupole, n = 3n = 9, 15, 21,... (18 pole, 30 pole, etc.) Octupole, n = 4n = 12, 20, 28,.... (24 pole, 40 pole, etc.) Summary: allowed harmonics.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Now for j 0 H = j ; To expand, use Stoke’s Theorum: for any vector V and a closed curve s : V.ds = curl V.dS Apply this to: curl H = j ; then in a magnetic circuit : H.ds = N I; N I (Ampere-turns) is total current cutting S Introduction of currents

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Magnets for FFAGs; Neil Marks FFAG School 2011 B is approx constant round the loop made up of and g, (but see below); But in iron, >>1, and H iron = H air / ; So B air = 0 NI / (g + / ); g, and / are the 'reluctance' of the gap and iron. Approximation ignoring iron reluctance ( / << g ): NI = B g / 0 Excitation current in a dipole

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Magnets for FFAGs; Neil Marks FFAG School 2011 For quadrupoles and sextupoles, the required excitation can be calculated by considering fields and gap at large x. For example: Quadrupole: Pole equation: xy = R 2 /2 On x axes B Y = gx; where g is gradient (T/m). At large x (to give vertical lines of B): N I = (gx) ( R 2 /2x)/ 0 ie N I = g R 2 /2 0 (per pole). The same method for a Sextupole, ( coefficient g S,), gives: N I = g S R 3 /3 0 (per pole) Excitation current in quad & sextupole

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Magnets for FFAGs; Neil Marks FFAG School 2011 In air (remote currents! ), B = 0 H B = - Integrating over a limited path (not circular) in air:N I = ( 1 – 2 )/ o 1, 2 are the scalar potentials at two points in air. Define = 0 at magnet centre; then potential at the pole is: o NI Apply the general equations for magnetic field harmonic order n for non-skew magnets (all Jn = 0) giving: N I = (1/n) (1/ 0 ) B r /R (n-1) R n Where: NI is excitation per pole; R is the inscribed radius (or half gap in a dipole); term in brackets is magnet strength in T/m (n-1). = 0 = 0 NI General solution-magnets of order n

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Magnets for FFAGs; Neil Marks FFAG School 2011 Standard design is rectangular copper (or aluminium) conductor, with cooling water tube. Insulation is glass cloth and epoxy resin. Amp-turns (NI) are determined, but total copper area (A copper ) and number of turns (N) are two degrees of freedom and need to be decided. Current density: j = NI/A copper Optimum j determined from economic criteria. Coil geometry

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Magnets for FFAGs; Neil Marks FFAG School 2011 Advantages of low j: lower power loss – power bill is decreased; lower power loss – power converter size is decreased; less heat dissipated into magnet tunnel. Advantages of high j: smaller coils; lower capital cost; smaller magnets. Chosen value of j is an optimisation of magnet capital against power costs. running capital total Current density - optimisation

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Magnets for FFAGs; Neil Marks FFAG School 2011 The value of number of turns (N) is chosen to match power supply and interconnection impedances. Factors determining choice of N: Large N (low current) Small N (high current) Small, neat terminals.Large, bulky terminals Thin interconnections-hence lowThick, expensive connections. cost and flexible. More insulation layers in coil,High percentage of copper in hence larger coil volume andcoil volume. More efficient use increased assembly costs.of space available High voltage power supplyHigh current power supply. -safety problems.-greater losses. Number of turns per coil-N

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Magnets for FFAGs; Neil Marks FFAG School 2011 From the Diamond 3 GeV synchrotron source: Dipole: N (per magnet):40; I max1500A; Volts (circuit):500V. Quadrupole: N (per pole)54; I max200A; Volts (per magnet):25V. Sextupole: N (per pole)48; I max100A; Volts (per magnet)25V. Examples-turns & current

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Magnets for FFAGs; Neil Marks FFAG School 2011 Dipoles can be ‘C core’ ‘H core’ or ‘Window frame’ ''C' Core: Advantages: Easy access; Classic design; Disadvantages: Pole shims needed; Asymmetric (small); Less rigid; The ‘shim’ is a small, additional piece of ferro-magnetic material added on each side of the two poles – it compensates for the finite cut-off of the pole, and is optimised to reduce the 6, 10, pole error harmonics. Magnet geometry

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Magnets for FFAGs; Neil Marks FFAG School 2011 Cross section of the Diamond storage ring dipole. Typical ‘C’ cored Dipole

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Magnets for FFAGs; Neil Marks FFAG School 2011 ‘H core’: Advantages: Symmetric; More rigid; Disadvantages: Still needs shims; Access problems. ''Window Frame' Advantages: High quality field; No pole shim; Symmetric & rigid; Disadvantages: Major access problems; Insulation thickness H core and window-frame magnets

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Magnets for FFAGs; Neil Marks FFAG School 2011 Providing the conductor is continuous to the steel ‘window frame’ surfaces (impossible because coil must be electrically insulated), and the steel has infinite , this magnet generates perfect dipole field. Providing current density J is uniform in conductor: H is uniform and vertical up outer face of conductor; H is uniform, vertical and with same value in the middle of the gap; perfect dipole field. J H Window frame dipole

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Magnets for FFAGs; Neil Marks FFAG School 2011 Insulation added to coil: B increases close to coil insulation surface B decrease close to coil insulation surface best compromise Practical window frame dipole.

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Magnets for FFAGs; Neil Marks FFAG School 2011 ‘Diamond’ storage ring quadrupole. The yoke support pieces in the horizontal plane need to provide space for beam-lines and are not ferro-magnetic. Error harmonics include n = 4 (octupole) a finite permeability error. Open-sided Quadrupole

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Magnets for FFAGs; Neil Marks FFAG School 2011 To compensate for the non-infinite pole, shims are added at the pole edges. The area and shape of the shims determine the amplitude of error harmonics which will be present. Dipole: Quadrupole: The designer optimises the pole by ‘predicting’ the field resulting from a given pole geometry and then adjusting it to give the required quality. When high fields are present, chamfer angles must be small, and tapering of poles may be necessary Typical pole designs

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Magnets for FFAGs; Neil Marks FFAG School 2011 A first assessment can be made by just examining B y (x) within the required ‘good field’ region. Note that the expansion of B y (x) y = 0 is a Taylor series: B y (x) = n =1 {b n x (n-1) } = b 1 + b 2 x + b 3 x 2 + ……… dipolequad sextupole Also note: B y (x) / x = b b 3 x + …….. So quad gradient g b 2 = B y (x) / x in a quad But sext. gradient g s b 3 = 2 2 B y (x) / x 2 in a sext. So coefficients are not equal to differentials for n = 3 etc. Assessing pole design

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Magnets for FFAGs; Neil Marks FFAG School 2011 A simple judgement of field quality is given by plotting: Dipole: {B y (x) - B y (0)}/B Y (0) ( B(x)/B(0)) Quad: dB y (x)/dx( g(x)/g(0)) 6poles: d 2 B y (x)/dx 2 ( g 2 (x)/g 2 (0)) ‘Typical’ acceptable variation inside ‘good field’ region: B(x)/B(0) 0.01% g(x)/g(0) 0.1% g 2 (x)/g 2 (0) 1.0% Is it ‘fit for purpose’?

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Magnets for FFAGs; Neil Marks FFAG School 2011 Computer codes are now used; eg the Vector Fields codes -‘OPERA 2D’ and ‘OPERA 3D’. These have: finite elements with variable triangular mesh; multiple iterations to simulate steel non-linearity; extensive pre and post processors; compatibility with many platforms and P.C. o.s. Technique is iterative: calculate flux generated by a defined geometry; adjust the geometry until required distribution is achieved. Design computer codes.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Pre-processor: The model is set-up in 2D using a GUI (graphics user’s interface) to define ‘regions’: steel regions; coils (including current density); a ‘background’ region which defines the physical extent of the model; the symmetry constraints on the boundaries; the permeability for the steel (or use the pre- programmed curve); mesh is generated and data saved. Design Procedures – OPERA 2D

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Magnets for FFAGs; Neil Marks FFAG School 2011 Model of Diamond storage ring dipole

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Magnets for FFAGs; Neil Marks FFAG School 2011 With mesh added

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Magnets for FFAGs; Neil Marks FFAG School 2011 Pole profile, showing shim and Rogowski side roll-off for Diamond 1.4 T dipole.: Close-up of pole region.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Diamond s.r. dipole: B/B = {By(x)- B(0,0)}/B(0,0); typically 1:10 4 within the ‘good field region’ of -12mm x +12 mm.. 2 D Dipole field homogeneity on x axis

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Magnets for FFAGs; Neil Marks FFAG School 2011 Transverse (x,y) plane in Diamond s.r. dipole; contours are 0.01% required good field region: 2 D Dipole field homogeneity in gap

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Magnets for FFAGs; Neil Marks FFAG School 2011 OPERA 3D model of Diamond dipole.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Diamond dipole poles

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Magnets for FFAGs; Neil Marks FFAG School 2011 Diamond quadrupoles have an angular cut at the end; depth and angle were adjusted using 3D codes to give optimum integrated gradient. Simplified end geometries - quadrupole

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Magnets for FFAGs; Neil Marks FFAG School 2011 It is not usually necessary to chamfer sextupole ends (in a d.c. magnet). Diamond sextupole end: Sextupole ends

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Magnets for FFAGs; Neil Marks FFAG School 2011 SECTION 2 Injection and extraction magnets.

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Magnets for FFAGs; Neil Marks FFAG School 2011 The Injection/Extraction problem. Single turn injection/extraction: a magnetic element inflects beam into the ring and turns-off before the beam completes the first turn (extraction is the reverse). Multi-turn injection/extraction: the system must inflect the beam into the ring with an existing beam circulating without producing excessive disturbance or loss to the circulating beam. Accumulation in a storage ring: A special case of multi-turn injection - continues over many turns (with the aim of minimal disturbance to the stored beam). straight section injected beam magnetic element

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Magnets for FFAGs; Neil Marks FFAG School 2011 Single turn – simple solution A ‘kicker magnet’ with fast turn-off (injection) or turn-on (extraction) can be used for single turn injection. injection – fast fallextraction – fast rise Problems: i) rise or fall time will always be non-zero loss of beam; ii) single turn inject does not allow the accumulation of high current; iii) in small accelerators revolution times can be << 1 s. iv) magnets are inductive fast rise (fall) means (very) high voltage. B t

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Magnets for FFAGs; Neil Marks FFAG School 2011 Multi-turn injection 1 - general Beam can be injected by phase-space manipulation: Inject into an unoccupied outer region of phase space with non-integer tune which ensures many turns before the injected beam re-occupies the same region (electrons and protons): eg – Horizontal phase space at Q = ¼ integer: x x’ septum turn 1 – first injection turn 2turn 3 turn 4 – last injection 0 field deflect. field Then the beam has to be moved back from the septum magnet!

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Magnets for FFAGs; Neil Marks FFAG School 2011 Multi-turn injection – 2 Lepton storage rings : Inject into outer region of phase space – damping (slow?) coalesces beam into the central region before re-injecting. dynamic aperture injected beamnext injection after 1 damping time stored beam Protons: Inject negative ions through a bending magnet and then ‘strip’ to produce a positive ion after injection (eg H - to p).

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Magnets for FFAGs; Neil Marks FFAG School 2011 Multi-turn extraction solution ‘Shave’ particles from edge of beam into an extraction channel whilst the beam is moved across the aperture: beam movement extraction channel Points: some beam loss on the septum cannot be prevented; efficiency can be improved by ‘blowing up’ on 1/3rd or 1/4 th integer resonance. septum

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Magnets for FFAGs; Neil Marks FFAG School 2011 Magnet requirements Magnets required for injection and extraction systems. i) Kicker magnets: pulsed waveform; rapid rise or fall times (usually << 1 s); flat-top for uniform beam deflection. ii) Septum magnets: pulsed or d.c. waveform; spatial separation into two regions; one region of high field (for injection deflection); one region of very low (ideally 0) field for existing beam; septum to be as thin as possible to limit beam loss. Septum magnet schematic

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Magnets for FFAGs; Neil Marks FFAG School 2011 Kicker Magnet & Power Supplies Because of the demanding performance required from these systems, the magnet and power supply must be strongly integrated and designed as a single unit. Two alternative approaches to powering these magnets: 1) Distributed circuit: magnet and power supply made up of delay line circuits. 2) Lumped circuits: magnet is designed as a pure inductance; power supply can be use delay line or a capacitor to feed the high pulse current.

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Magnets for FFAGs; Neil Marks FFAG School 2011 High Frequency Kicker Magnets Kicker Magnets: used for rapid deflection of beam for injection or extraction; usually located inside the vacuum chamber; rise/fall times << 1µs. yoke assembled from high frequency ferrite; single turn coil; pulse current 10 4 A; pulse voltages of many kV. Typical geometry:

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Magnets for FFAGs; Neil Marks FFAG School 2011 Kickers - Distributed System Standard (CERN) delay line magnet and power supply: Power Supply Thyratron Magnet Resistor The power supply, interconnecting cables and terminating resistor are matched to the surge impedance of the delay line magnet:

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Magnets for FFAGs; Neil Marks FFAG School 2011 Distributed System -mode of operation the first delay line is charged to by the d.c. supply to a voltage :V; the thyratron triggers, a voltages wave: V/2 propagates into magnet; this gives a current wave of V/( 2 Z ) propagating into the magnet; the circuit is terminated by pure resistorZ, to prevent reflection.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Kickers – Lumped Systems. The magnet is (mainly) inductive - no added distributed capacitance; the magnet must be very close to the supply (minimises inductance). I = (V/R) (1 – exp (- R t /L)

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Magnets for FFAGs; Neil Marks FFAG School 2011 Improvement on above C The extra capacitor C improves the pulse substantially.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Resulting Waveform Example calculated for the following parameters: mag inductance L = 1 H; rise timet = 0.2 s; resistorR = 10 ; trim capacitor C = 4,000 pF. The impedance in the lumped circuit is twice that in the distributed! The voltage to produce a given peak current is the same in both cases. Performance:at t = 0.1 s, current amplitude = of peak; at t = 0.2 s, current amplitude = 1.01 of peak. The maximum ‘overswing’ is 2.5%. This system is much simpler and cheaper than the distributed system.

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Magnets for FFAGs; Neil Marks FFAG School 2011 EMMA kicker magnet – ferrite cored lumped system.

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Magnets for FFAGs; Neil Marks FFAG School 2011 EMMA Injection Kicker Magnet Waveform

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Magnets for FFAGs; Neil Marks FFAG School 2011 Septum Magnets – ‘classic’ design. Often (not always) located inside the vacuum and used to deflect part of the beam for injection or extraction: The thin 'septum' coil on the front face gives: high field within the gap, low field externally; Problems: The thickness of the septum must be minimised to limit beam loss; the front septum has very high current density and major heating problems

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Magnets for FFAGs; Neil Marks FFAG School 2011 Septum Magnet – eddy current design. uses a pulsed current through a backleg coil (usually a poor design feature) to generate the field; the front eddy current shield must be, at the septum, a number of skin depths thick; elsewhere at least ten skin depths; high eddy currents are induced in the front screen; but this is at earth potential and bonded to the base plate – heat is conducted out to the base plate; field outside the septum are usually ~ 1% of field in the gap.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Comparison of the two types. Classical:Eddy current: Excitationd.c or low frequency pulse;pulse at > 10 kHz; Coilsingle turn includingsingle or multi-turn on front septum;backleg, room for large cross section; Coolingcomplex-water spiralsheat generated in in thermal contact with shield is conducted to septum;base plate; Yokeconventional steel laminationshigh frequency material (ferrite or very thin steel lams).

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Magnets for FFAGs; Neil Marks FFAG School 2011 Example Skin depth in material: resistivity ; permeability ; at frequency is given by:d = (2 / µµ 0 ) Example: EMMA injection and extraction eddy current septa: Screen thickness (at beam height): 1 mm; " " (elsewhere) – up to10 mm; Excitation25 µs, half sinewave; Skin depth in copper at 20 kHz0.45 mm

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Magnets for FFAGs; Neil Marks FFAG School 2011 Location of EMMA septum magnets

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Magnets for FFAGs; Neil Marks FFAG School 2011 Design of the EMMA septum magnet Inner steel yoke is assembled from 0.1mm thick silicon steel laminations, insulated with 0.2 m coatings on each side.

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Magnets for FFAGs; Neil Marks FFAG School 2011 SECTION 3 FFAG Pole Design.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Are FFAG magnets ‘complex’? Yes – often complex – but no more difficult than ‘conventional’ synchrotron magnets! Consideration in pole design: i) what is the lattice specification – either variation on axis of By vs x, or, better still, the harmonic components of the By? ii) then (for the magnet designer) – what basic type of magnet is it (dipole, quadrupole, sextupole, etc). iii) finally, what procedure should be used to establish the pole profile? See next slide.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Detailed Procedure To determine the ‘perfect’ pole: i) establish coefficients of Taylor series (harmonic amplitudes) up to 3 rd or 4 th order (or higher) to ‘fit’ the specified By(x) curve; ii) develop the equation for scalar potential (x,y) by summing the scalar potentials for all harmonics (equations for (x,y) shown in slide 10,11 and 12); iii) generate a table of (x,y) values corresponding to a fixed value of (an iso-potential line) – this is one of the (infinite number) of‘perfect’ pole shapes (not taking account of pole edge or end effects). Procedure illustrated-the determination of poles for ‘complex’ magnets in a FFAG – ‘Pumplet'

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Magnets for FFAGs; Neil Marks FFAG School 2011 FFAG ‘Pumplet’ (*) (*) as specified by Grahame Rees.

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Magnets for FFAGs; Neil Marks FFAG School 2011 By(T) and x (T) specifications

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Magnets for FFAGs; Neil Marks FFAG School 2011 Magnet bd – By curve fitting Fit: Series: b 0 + b 1 x + b 2 x 2 + b 3 x 3 ; Coefficients: b 0 = ; b 1 = E-4; b 2 = E-6; b 3 = E-7; RMS fitting error: 3.67 E-5; 8:10 4 of mean (need to be better for actual project).

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Magnets for FFAGs; Neil Marks FFAG School 2011 bd - lines of iso scalar potential Pairs of (x,y) and (x,-y) to give = ± T mm; this gives the poles shapes.

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Magnets for FFAGs; Neil Marks FFAG School 2011 bd poles and vac vessel. What By distribution does this give? Model using OPERA 2D.

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Magnets for FFAGs; Neil Marks FFAG School 2011 OPERA 2D model

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Magnets for FFAGs; Neil Marks FFAG School 2011 By vs x at y = 0. How does this compare with the specified data?

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Magnets for FFAGs; Neil Marks FFAG School 2011 bd -comparison of OPERA 2D with defined By RMS error (fitting + determining potentials + OPERA FEA) : 3.75 E-5

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Magnets for FFAGs; Neil Marks FFAG School 2011 Magnet BF curve fit What sort of magnet is this? dipole/quadrupole/sextupole? See next slide.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Magnet BF curve fit – extended. Extrapolated down to x = – 55 mm. It’s a sextupole (with dipole, quadrupole and octupole components) magnetic centre at - 40 mm !!!

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Magnets for FFAGs; Neil Marks FFAG School 2011 Magnet BF pole arrangement with vac vessel. Poles have: = ± 0.35 T mm; Note all that wasted space at -70 < x < -25? See next slide but one!

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Magnets for FFAGs; Neil Marks FFAG School 2011 OPERA Model of magnet BF

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Magnets for FFAGs; Neil Marks FFAG School 2011 Comparison between OPERA B Y prediction and defined data; top pole at full potential

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Magnets for FFAGs; Neil Marks FFAG School 2011 Solution to wasted space problem. Side poles have: = ± 0.35 T mm; BUT: Central poles have: = ± 0.01 T mm; Central poles: i) are much closer to median line; ii) now require only 1/35 of the coil excitation current. STFC have now applied for a patent for this arrangement. = 0.35 =

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Magnets for FFAGs; Neil Marks FFAG School 2011 OPERA model of BF; (low potential central poles).

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Magnets for FFAGs; Neil Marks FFAG School 2011 Comparison of By: between OPERA & defined data; reduced on top pole.

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Magnets for FFAGs; Neil Marks FFAG School 2011 Finis Thank you for listening; Any questions?

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