Some practical aspects of. Magnetic Fields. Kai Schwarzwälder Institut für Physik, Universität Basel February 16 th, 2007

Transkript

1 Some practical aspects of Kai Schwarzwälder Institut für Physik, Universität Basel February 16 th, 2007

2 Outline Introduction: Examples of Magnetism Physical basics: Maxwell Equations Physical Quantities Technological Porperties Generating Superconductivity Quenching Sources Dangers and Risk Handling and Running a Magnet System Magnet design Coil design Electronic design Circuitry Magnet Types 2

3 Examples of Magnetism 3

4 Maxwell Equations div ~B =0 rot ~B = 4π c ~ j + ~E t div ~D = ρ rot ~D = 1 c ~B t ~D = ε 0 ε r ~E = ε 0 ~E + ~P ε r =1+χ e ³ ~B = µ 0 µ r ~H = µ 0 H ~ + ~M µ r =1+χ m c r = 1 εr ε 0 µ r µ 0 ~B =rot~a ~E = gradφ t ~A 4

5 Physical Quantities Quantity Symbol SI-Unit Further units Magnetic field, magnetic flux density, magnetic induction B T 1 Gauss= 10-4 T, 1γ=10-9 T magnetic field (strength), auxiliary field H A/m 1Oe=79.6A/m=10-4 T Magnetisation M A/m Magnetic Flux φ Wb 1Wb=1Vs=10 8 Mx Magnetic Moment µ Am 2 Inductivity/Inductance L H 1H=1Vs/A=Wb/A Permeability µ H/m Susceptibility χ - 5

6 Technological Properties Property Maximum field Homogeneity /10mm dsv Operating current Design quench field Sample access Magnet type Stability B B t Charging/ramping time Persistant switch resistance Compensating coils reduce stray field in MC to <0.03T Power Supply Properties B B x Solenoid 8T 0.1% <120A 9T 52mm solenoid; NbTi Split Coil 4T 1% <200A 4.8T 52mm split coil; NbTi 6

7 Field Profile Magnetic Field 100A R=2cm Magnetic Field 100A B / T Bz Br B / T Bz R=0 Br R=0 Bz R=2cm Br R=2cm Distance from centre of field /cm Distance from centre of field /cm Profile of the 8T solenoid at a current of 100A 7

8 Generating Permanent Magnet Homogeneity: ±25ppm at 0,2T Temperature dependent Ferromagnetica...0,3T heavy Electromagnet Homogeneity: ±25ppm at 0,2T Water cooling Yokes...0,3T High power dissipation Supercond. Magnet Homogeneity: ±0,5ppm at 4T Cryo techniques High fields quenching 8

9 Superconductivity General: Resistance vanishes at T c Type-I-SC: Magnetic field completely depleated Type-II-SC Magnetic fields penetrates in multiples of φ 0 =2x10-15 Vs 9

10 Flux Motion Fluxoids are driven by Lorentz-force caused by current density This motion itself causes Lorentz-force that counter-acts current (Lenz-law) Voltage drop / flux flow resistance Impurities hold fluxoids ( pinning ) Threshold force, Type-III- / hard SC 10

11 Quenching Sources Running above T C,B C,j C (cooling system failure) Flux flow / jump Eddy current Two coil sections move with respect to each other( if high and low field area are close to each other, during charging/ramping) C heat of many materials much lower than at RT 11

12 Quenching Dangers Damage by high voltage, high temperatures, strong forces (and gradients thereof) Damage of magnet, dewar, refridgerator E = 1 2 LI2 50kJ L 10H,I =100A I(t)R(t) =L I(t) 12

13 Running a Magnet System Charge rate: 0,1A/s Charge time: min Persistent mode: suderconductor switches 13

14 Magnet design: The Coil NbTi: state-of-the-art, good persistent mode, time-stable field, long charging times, lower fields (<9T) Nb3Sn: high fields(>9t), expensive, brittle, difficult to wind Fine filaments: less diamagnetism, better linearity Embedding in metal (Cu, Al) matrix, higher heat capacitance, current shortcut Trim coils and shim plates improve homogeneity Twisted wires 14

15 Magnet design: Electronics AC peaks effect persistent switch Quench detection (voltage on magnet input) Warning system (operating staff) No automatic recharge Programmable charge rate, voltage and current limits Fast ramp down Energy dissipation device /energy dump 15

16 Magnet design: circuitry Diodesorresistorsas shunt Heating circuit to spread power transformation 16

17 Magnet Types Solenoid Most frequent Bore: Homogeneity: 1%...0.1% 100mm dsv Versatile Robust High field magnets Inner coil (high field) Outer coil (background) Bore: Homogeneity: 0.1% 100mm dsv 17

18 Magnet Types Split coils Lower fields the solenoids of comparable size Lower homogeneity Higher fields on/in windings Gradient coils, MOT, ion traps Gradient coils 30mT/m 18

19 Magnet Types Multi-axis magnet One solenoid (main field) 1 or 2 split coils in or outside solenoid for additional directions Extremely large forces due to interacting coils Spin based physics 19

20 Magnet Types Demagnetisation set-ups High field in large volume Take switchable heat conductor into account Temperature measurement with NMR 20

21 Magnet Types: Misc 21

22 Thank you for your attention. 22