Bitter electromagnet

Diamagnetic forces acting upon the water within its body levitating a live frog inside the 3.2 cm vertical bore of a Bitter solenoid at the Nijmegen High Field Magnet Laboratory, Nijmegen, Netherlands. The magnetic field was about 16 teslas. Video is available.[1]

A Bitter electromagnet or Bitter solenoid is a type of electromagnet used in scientific research to create extremely strong magnetic fields. Bitter electromagnets have been used to produce some of the strongest continuous manmade magnetic fields on earth (up to 45 teslas - as of 2011).

Advantages

Bitter electromagnets are used where extremely strong fields are required. The iron cores used in conventional electromagnets saturate and cease to provide any advantage at fields above a few teslas, so iron core electromagnets are limited to fields of about 2 teslas. Superconducting electromagnets can produce stronger magnetic fields but are limited to fields of 10 to 20 teslas, due to flux creep, though theoretical limits are higher. For stronger fields resistive solenoid electromagnets of the Bitter design are used. Their disadvantage is that they require very high drive currents, and dissipate large quantities of heat.

Construction

Plate from a 16 T Bitter magnet, 40 cm diameter, made of copper. In operation it carries a current of 40 kiloamperes

Bitter magnets are constructed of circular metal plates and insulating spacers stacked in a helical configuration, rather than coils of wire. The current flows in a helical path through the plates. This design was invented in 1933 by American physicist Francis Bitter. In his honor the plates are known as Bitter plates. The purpose of the stacked plate design is to withstand the enormous outward mechanical pressure produced by Lorentz forces, which increase with the square of the magnetic field strength. Additionally, water circulates through holes in the plates as a coolant, to carry away the enormous heat created in the plates due to resistive heating by the large currents flowing through them. The heat dissipation also increases with the square of the magnetic field strength.

Record Bitter magnets

The most powerful electromagnet in the world, the 45 T hybrid Bitter-superconducting magnet at the US National High Magnetic Field Laboratory, Tallahassee, Florida, USA

The strongest continuous magnetic fields on Earth have been produced by Bitter magnets. As of 2011 the National High Magnetic Field Laboratory in Tallahassee, Florida, USA, houses the world's strongest resistive magnet. This system has a maximum field strength of 36.2 teslas and consists of hundreds of separate Bitter plates. The system consumes 19.6 megawatts of electric power and requires about 139 litres of water pumped through it per second for cooling. This magnet is mainly used for material science experimentation. For similarly designed examples of bitter coils see the external links below.

The strongest continuous manmade magnetic field, 45 T, was produced by a device consisting of a Bitter magnet inside a superconducting magnet.

See also

• Superconducting magnet

External links

• National High Magnetic Field Laboratory Magnet Projects Page at Florida State University
• Magnets at Nijmegen High Field Magnet Laboratory
• The Frog That Learned to Fly and a ball of water inside a Bitter solenoid at the High Field Magnet Laboratory
• Diagrams and description of the Bitter solenoid used in the frog levitation demonstration
• Bitter magnet designs: NHMFL Bitter Magnet and Radbound University Bitter Solenoid