Section II Ex-Vessel Systems Magnet Systems

DOE-HDBK-6004-99

SECTION II

EX-VESSEL SYSTEMS

MAGNET SYSTEMS

Description

The magnet system, for a tokamak device, consists of the toroidal field coils, the poloidal field coils

and the central solenoid. Toroidal field (TF) coils are superconducting cables cooled with liquid

helium which are wound into D shapes. Each coil circumscribes the vacuum vessel cross-section, and

the set of toroidal field coils make a complete circle around the torus. The poloidal field (PF) coils

are also typically superconducting cables cooled with liquid helium and wound into horizontal rings

which are located above and below the vacuum vessel with typically some coil sets inside and outside

the toroidal field coils. The TF and PF coils provide the basic magnetic field geometry for plasma

confinement and position control. The central solenoid conductors are typically superconducting

cables wound horizontally and situated at the center of the vacuum vessel torus supported by, for

example, a bucking cylinder. The central solenoid set provides the transient field to induce all or part

of the plasma current.

Recommended Design Practice

The dielectric strength of the insulation should be provided either by materials with an intrinsic

dielectric strength, or by materials tested before assembly onto the magnet.

The mechanical integrity of the magnets should not depend on the shear strength of the insulating

materials or the shear bond between insulation and structural materials.

Since leaks at coolant connections are a common cause of magnet faults, such connections should

be kept away from mechanical load paths, placed outside the winding pack and, as far as possible,

in regions where some access is, in principle, possible for inspection or repair.

Manufacturing can allow many faults to occur. Machining chips left in the coil slowly abrade

insulation and then cause a failure after some years of machine operation. Very strict tests to

determine the cleanliness of finished units should be performed.

CRYOSTAT

Description

The cryostat is a metal chamber surrounding the fusion device which provides a thermal barrier to

conduction and thermal radiation between the superconducting coils and other cold structures and

the rest of the facility. It may also serve as the biological shield for radiation from the tokamak. The

chamber is usually cylindrical with a top and bottom. There are usually large penetrations in the top,

bottom, and sides of the cryostat, primarily for access to the vacuum vessel and magnets for

maintenance and inspection. The cryostat may be double-walled with an evacuated or filled annulus.

The cryostat itself is usually evacuated and it may be lined with cryogenic panels or insulating material

to reduce radiant heat transfer.