Plasma Energy

DOE-STD-6003-96

threaten confinement. Excessive motion of magnets or associated supports could break tritium

lines or diagnostic penetrations into the vacuum vessel. In general, due to the developmental

nature of this system, it is desirable that the fusion magnets not be classified as a safety-class

or safety-significant system in the performance of their primary design function of plasma ion

and electron confinement. Therefore, magnet systems in fusion facilities should be designed

such that faults in the magnets and the associated ancillary systems (power supply, electrical

systems) should not threaten safety functions. Where feasible, a design goal should be to

design for symmetrical fault conditions to minimize loads.

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. The dielectric

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

strength, or by materials tested before assembly into the magnet. Since leaks at coolant con-

nections 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 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.

6.3.3.4 Plasma Energy

For the next several generations of magnetic confinement devices, the plasma will be

part of the experimental program, and there will be a need to decouple plasma physics issues,

where possible, from facility safety issues, especially public safety. Where there is overlap

between facility safety and the plasma system, such as during VDEs or strong disruptions, it

is recommended that plasma-related consequences be confined to the interior of the vacuum

vessel or cryostat to minimize potential public safety concerns. Several considerations regarding

how the plasma is operated may affect the overall device safety. In particular, the issue of

plasma stability is the primary concern. In the domain of stability, there are two primary

categories: (1) thermal stability of the plasma and (2) plasma disruptions.

The disruption area concerns the sudden loss of thermal and /or magnetic energy from

the plasma. This category of events can produce undesirable transient heat and/or mechanical

loads on fusion island components.

a. Thermal Stability

The thermal stability area concerns a prevention of a plasma transient to higher fusion

powers than provided by the facility design. In the event of uncontrolled thermal runaway,

plasma-facing components could be subjected to higher heat loads than during normal opera-

tions. The plasma can be operated in either a thermally stable or unstable regime. There are

several options to ensure a stable level of fusion power. One option is to operate at the high-

temperature, thermally stable operational point. Another option is to operate in a driven mode.

This is the case when auxiliary power must be injected into the plasma to drive currents in

the plasma or to simply maintain the plasma power balance. With a driven plasma the only

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