Magnet quench
According to Hirsch, High-temperature superconducting magnets can accidentally quench, which means suddenly “go normal” with a large release of stored energy. During a quench, a large S/C magnet can be damaged by high voltage, high temperature, and sudden large forces. Although magnets are designed to withstand an occasional accidental quench, repeated quenches can shorten their useful lives.
Small S/C magnets are widely used in magnetic resonance imaging machines, nuclear magnetic resonance equipment, and mass spectrometers. These systems are routinely stable and well behaved. Larger S/C magnets are used in particle accelerators, where difficulties have occurred and are considered a “fairly routine event.”
At the Fermilab particle accelerator, “a quench generates as much force as an exploding stick of dynamite. A magnet usually withstands this force and is operational again in a few hours after cooling back down. If repair is required, it takes valuable time to warm up, fix, and then cool down the magnet—days or weeks in which no particle beams can be circulated, and no science can be done.”
Events like these in accelerators are often caused by particle beams striking chamber walls, creating sudden, localized heating. Disruptions in tokamaks might provide similar triggers, but they are not the only events that can initiate quenching. To date, quenches have occurred on at least 17 occasions in tokamak experiments constructed with S/C magnets, due a number of factors including fast current variations, vacuum loss, subsystem failures, operator errors, and mechanical failure. Some failures can be avoided relatively easily, whereas others can require costly magnet and magnet casing replacements.
If S/C magnets are to be used, configurations that are inherently more stable should be favored.
Disruptions
As the European Fusion Network acknowledged:
Tokamaks operate within a limited parameter range. Outside this range sudden losses of energy confinement can occur. These events, known as disruptions, cause major thermal and mechanical stresses to the structure and walls.
Columbia University researchers wrote:
disruptions could cause catastrophic destruction to the vacuum vessel and plasma-facing components.
Heat load
Tritium breeding
Induced radioactivity and neutron-induced damage
A tokamak power system will very quickly become highly radioactive and contaminated with tritium. Researchers are looking into reduced-activation ferritic/martensitic (RAFM) steel, vanadium (V), and silicon carbide (SiC) with reduced induced radioactivity.