Magnet Design Joint ICTP-IAEA Workshop on Accelerator Technologies, Basic Instruments and Analytical Techniques 21 – 29 October 2019 Trieste Italy Lowry Conradie Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
MAGNETS 1. Introduction Magnets in everyday life, in history, Understanding magnetism, Glossary, Units 2. General Principles of magnets Type, number of poles, Field shapes Pole shape, Fringe fields, Saturation, Shims, Field quality, Magneto-motive force 3. Magneto-motive force Dipole and Quadrupole 4. Magnetization of iron Hysteresis, permeability, materials 6. Magnet design Computer programs Steps in designing a magnet Design a magnet (example - tutorial) Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Magnets in Everyday Life Rubber mat magnets Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Magnets in Everyday Life Accelerators – mainly electromagnets. • Dipoles for bending a charged particle beam • Quadrupoles for focusing a beam • Sextupoles, octupoles, etc for higher order beam corrections • Fast deflecting magnets for beam injection and extraction Permanent magnets in vacuum pumps, gauges and sweeping devices, but nowadays also as beam optical devices Particle detectors Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
How strong are magnets? Typical Values Here is a list of how strong some magnetic fields can be: 10 -14 Tesla Smallest value in a magnetically shielded room 10 -10 Tesla Interstellar space Earth's magnetic field 0.00005 Tesla = 0.5 Gauss Small bar magnet 0.01 Tesla Within a sunspot 0.15 Tesla Small NIB magnet 0.2 Tesla Big electromagnet 2 Tesla Surface of neutron star 100,000,000 Tesla Magstar 100,000,000,000 Tesla What is a Tesla? It is a unit of magnetic flux density. It is also equivalent to these other units: 1 weber per square meter 10,000 Gauss (10 kilogauss) 10,000 magnetic field lines per square centimeter 65,000 magnetic field lines per square inch. 1Gauss is about 6.5 magnetic field lines per square inch. If you place the tip of your index finger to the tip of your thumb, enclosing approximately 1 square inch, four magnetic field lines would pass through that hole due to the earth's magnetic field! Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Some Units and Conversion Numbers in Electromagnetism Charged Particle properties Particle energy : 1eV = (1.6x10 -19 C)(1V) = 1.6x10 -19 J Current i in ampere (A), current density j in (A/m 2 ) Number of conductor turns in a coil is N Magnetic Field Strength H : 1 Oe = (10 3 /4 ) A/m = 79.58 A/m (mmf) Magnetic Flux : 1 Wb = 1 Vs Magnetic Flux Density B : 10 4 G = 1 Wb/m 2 = 1 Vs/m 2 = 1 T Permeability of any material = = 0 r (unit = Vs/Am = H/m) Permeability of vacuum = = 0 r = (4 x 10 -7 ) x 1 = 4 x 10 -7 H/m Magnetic Flux Density in relation to its magneto-motive force (mmf) : B = H Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
TYPES OF MAGNETISM A. DIAMAGNETISM • due to the modification of the electron orbit magnetic moment by an external field (a pure orbit effect) • Present in all materials, independent of temperature • Shows no hysteresis • Very weak B. PARAMAGNETISM • atoms present a permanent magnetic moment, e.g. odd number of electrons (mostly an electron spin effect) • Incomplete inner electronic shells (transition and rare earth elements) • Can be orders of magnitude bigger than diamagnetism • No hysteresis effect C. FERROMAGNETISM • Larger inter-atomic distances • Electron spin effect – line up from atom to atom – polarization • “conduction electrons” from the 4s -shell free to wander between atoms Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
TYPES OF MAGNETS A. Permanent Magnets (magneto-motive force from intrinsic material properties) B. Electro-magnets (magneto-motive force generated from applied electric current) DC-current AC-current (pulsed, eddy-currents, laminations) Super-conducting electro-magnets and materials Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
VISUAL PERCEPTION - FIELD LINES N S GENERAL RULES FOR USING LINES TO VISUALIZE MAGNET FIELDS 1. Any line (all lines) must close on itself or end according to a specified boundary condition. .B = 0 Permanent Magnets : Field Shape 2. Lines may NOT cross or touch. 3. Lines usually cross an air/iron interface perpendicularly. 4. The higher the field density, the denser the line representation. N S
Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Electro-Magnets : n = 2, 4, 6, etc. poles Different Dipole geometries C Magnet Advantages: • Easy asses • Simple design Disadvantages: • Pole shims needed • Field asymmetric • Less rigid H Magnet • Advantages • Symmetric • Rigid Disadvantage: • Need shims • Difficult to access Dipole for bending/steering a beam Quadrupole for focussing/defocussing a beam Window frame Magnet Higher orders for creating magnetic bottles, Advantages: beam profile shaping and corrections to • No shims • Symmetric inadequate fields from other magnets • Compact • Rigid Combinations, active and passive components Disadvantages: • Access problems Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy • Insulation thickness
Electro-Magnets : Pole Shape For normal fields: y y Dipole: Y= ± g/2; (g is pole gap). xy=+R 2 /2 R Quadrupole: } g/2 0 xy= ± R 2 /2; x (R is radius of pole opening). Sextupole: 3x 2 y – y 3 =±R 3 ; Equations of ideal pole shape Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Electro-Magnets : Field Shape y y xy=+R 2 /2 R } g/2 0 x Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Electro-Magnets : Fringe Fields & Field Saturation Magnetic field distribution and Square ends: magnet ends • Display non linear effects due to saturation • Influence the radial distribution in the fringe field Chamfered ends: • Magnetic length better define • Prevent saturation Control of the longitudinal field at magnet ends Control of the longitudinal field at magnet ends 14
MAGNETO-MOTIVE FORCE : DIPOLE MAGNET l iron Ampere’s Law ∮ H .d l = N I (ampere-turns) NI = ∮ H .d l = ( H air .g air + H iron .l iron ) ; x H = B / B NI = ∮ B/ .d l = B air .g air / 0 + B iron .l iron / iron g air neglect 2 nd term with iron about 5000 larger then` 0 NI = B air .g / 0 x 0 = 4 x 10 -7 (webers/amperemeter) Electrical power P = I 2 R 0 g 2 R 0 = r L/A, with r = resistivity of conductor material Saturation effect : keep field in yoke < 1.5 T by L = length of the conductor and A the providing enough area of steel. crossectional area of the coil Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
MAGNETO-MOTIVE FORCE : QUADRUPOLE MAGNET Quadrupole with hyperbolic pole faces and with aperture a , such that the field at radius r from the axis is B(r) = K.r Ampere’s Law H .d l = N I (ampere-turns) 𝐶 𝑏𝑗𝑠 𝐶 𝑗𝑠𝑝𝑜 NI = H .d l 𝜈 0 𝜈 𝑠 l iron ) ; = ( 𝜈 0 g air + a NI = 0 ∫ B(r)/ 0 .dr + (iron path) + (path perpendicular to field) On the first path (red) B(r) = K.r/ μ 0 . The second integral (green) is very small for μ r >> 1. The third integral (blue) vanishes since B is perpendicular to the direction of integration, ds. So we get in good NI a NI = (1 / 0 ) 0 ∫ K r. d r NI = (1 / 0 ) Ka 2 /2 , but Ka = B poletip NI = (B poletip .a)/(2 0 ) Power ( I ) 2 a 4 Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
MAGNETIZATION CURVE and PERMEABILITY B = H saturation B = H = 0 r H r = 0 B/H Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
Magnetic Materials: relative permeability µ= µ 0 µ r =(4πx10 -7 ) µ r Relative permeabilities Substance Group type Relative permeability, µ r Bismuth Diamagnetic 0.99983 Silver Diamagnetic 0.99998 Lead Diamagnetic 0.999983 Copper Diamagnetic 0.999991 Water Diamagnetic 0.999991 Vacuum Nonmagnetic 1 + Air Paramagnetic 1.0000004 Aluminium Paramagnetic 1.00002 Palladium Paramagnetic 1.0008 2-81 Permalloy powder (2 Mo, 81 Ni) ‡ Ferromagnetic 130 Cobalt Ferromagnetic 250 Nickel Ferromagnetic 600 Ferroxcube 3 (Mn-Zn-ferrite powder) Ferromagnetic 1,500 Mild steel (0.2 C) Ferromagnetic 2,000 Iron (0.2 impurity) Ferromagnetic 5,000 Silicon iron (4 Si) Ferromagnetic 7,000 78 Permalloy (78.5 Ni) Ferromagnetic 100,00 Mumetal (75 Ni, 5 Cu, 2 Cr) Ferromagnetic 100,000 Purified iron (0.05 impurity) Ferromagnetic 200,000 Superalloy (5 Mo, 79 Ni) ‡ Ferromagnetic 1,000,000 Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
HYSTERESIS Permanent magnets defined by curve in 2 nd quadrant Joint ICTP-IAEA Workshop 21 – 29 October 2019 Trieste Italy
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