Trends in high magnetic fields

Transcription

1 Trends in high magnetic fields Oliver Portugall Laboratoire National des Champs Magnétiques Intenses (LNCMI) Toulouse and Grenoble, France

2 Magnetic fields in science and engineering Magnetic fields act selectively on the motion of charges and the alignment of spins. In practice, this makes them useful to manipulate matter (levitation, deflection, separation, alignment, ) probe matter (NMR, ESR, MRI, Hall effect, ) induce new fundamental states (normal state of superconductors, field-induced superconductivity, magnetic phase transitions, quantum critical points, ) Research in high magnetic fields 15 Nobel prices in physics, chemistry and medicine

3 Magnetic field generation Generating a high magnetic field = Concentrating energy in the center of a magnet The energy tries to escape by dissipation into and expansion of the confining medium

4 The dissipation problem To generate 10 T Technical solutions: in a 20 x 20 cm² coil Up to 25 T superconducting magnets requires 20 A/mm². (dissipationless). Up to 35 T resistive magnets with Within 7.5(isothermal). s this would heat forced cooling a Cu-magnet Up to 45 T hybrid magnets from 300 to resistive 800 K. insert). (superconducting with Above 50 T pulsed magnets exposed less than 15 sduration to strongwithin currents with limited (adiabatic). the magnet would melt.

5 Magnetic pressure B2 pmag = 2μ0 Scientific research Technology Tensile strength limit (fibres) Tensile strength limit (conductors)

6 Basic techniques for generating high magnetic fields Speed limited Force limited Heat limited

7 Pulsed current generators LC circuits are simple, reliable and easy to operate

8 Pulsed current generators but electrical energy storage is less efficient than chemical! Generators for pulsed magnetic fields are large instruments. LNCMI 14 MJ generator.

9 Magnet technology What are the basic requirements for a pulsed magnet in order to be useful for scientific application?

10 Magnet technology High magnetic field Long pulse duration What most users require Large bore High duty cycle / repetition rate Low noise, high homogeneity Adapted geometry What some users require Mobility Long lifetime What the facilities want

11 Magnet technology High magnetic field Long pulse duration Large bore High duty cycle / repetition rate Low noise, high homogeneity Adapted geometry Mobility Long lifetime

12 How to increase the magnetic field To increase the magnetic field build a stronger magnet. If stronger materials are not available use more of the materials that are available. As the magnet becomes bigger use more energy. If your existing generator is too small

13 How to increase pulse duration To increase pulse duration first decrease the current density. To compensate for the loss of magnetic field add more windings on the outside. As the magnet becomes bigger use more energy. If your existing generator is too small

14 How to increase the bore To increase the bore remove the inner layers of the magnet. To compensate for the loss of magnetic field add more windings on the outside. As the magnet becomes bigger use more energy. If your generator is too small

15 Magnet technology Pulse shapes (March 2010)

16 Magnet technology High magnetic field Long pulse duration Large bore High duty cycle / repetition rate Low noise, high homogeneity Adapted geometry Mobility Long lifetime

17 Mobile generators 1 kj (left) 260 kj (bottom) 1 MJ (right) used at CLIO scheduled for LULI used at ESRF, LULI, ILL used at ESRF, ILL

18 Magnet technology High magnetic field Long pulse duration Large bore High duty cycle / repetition rate Low noise, high homogeneity Adapted geometry Mobility Long lifetime

19 Low noise / high homogeneity: Pulsed field NMR Nb NMR at 24 T! 93

20 Magnet technology High magnetic field Long pulse duration Large bore High duty cycle / repetition rate Low noise, high homogeneity Adapted geometry Mobility Long lifetime

21 Adapted geometries : Magnets for x-ray scattering Magnet with conical opening for X-ray scattering in Faraday geometry Magnet with radial opening for X-ray scattering in Voigt geometry

22 Adapted geometries : Magnets for x-ray scattering Magnet with radial opening for X-ray scattering in Voigt geometry

23 Adapted geometries : Idealized vs real magnets Ideal coils are rotationally symmetric. Real coils are helical. At the end of each layer the wire has to pass to the adjacent layer. The transition creates a mechanical discontinuity.

24 Adapted geometries : Magnets for x-ray scattering Magnet with radial opening for X-ray scattering in Voigt geometry

25 Adapted geometries : The X-coil Development of a pulsed dipole magnet, producing 30 T over a length of 50 cm

26 Adapted geometries : The X-coil The objective: Field Maximize the magnetic field along a transverse optical path. The constraints: 1. Limited energy. 2. Technical feasibility. The compromise: m A magnet forming a large X. Laser

27 Adapted geometries : The X-coil Force and tensile strength in a cylindrical magnet Force and tensile strength in a dipole magnet

28 Magnet technology High magnetic field Long pulse duration Large bore High duty cycle / repetition rate Low noise, high homogeneity Adapted geometry Mobility Long lifetime

29 How to make use of inertia: single-turn coils Consider a simple disposable coil being connected to a capacitor bank. The rise time of the field is determined by the circuit impedance and capacitance. The disintegration of the coil is determined by its mass and the applied force. The circuit impedance can be adjusted to make the rise time of the field faster than the disintegration.

30 How to make use of inertia: single-turn coils 300 T in 5 mm diameter 100 T in 20 mm diameter 5 μs duration Repetitive experiments Coil Cryostat

31 How to make use of inertia: electromagnetic flux compression Acceleration phase: Tphase: Inversion 10dB/dt - 20 inverts μs duration B is screened, the current B accelerates the liner as the liner implodes Single shot experiment Compression phase: db/dt prevents flux diffusion through the liner

32 How to make use of inertia: explosive flux compression