Page "Tokamak" Paragraph 12

This device where a large toroidal current is established ( 15 Mega-amps in ITER ) suffers from a fundamental problem of stability.

The nonlinear evolution of magnetohydrodynamical instabilities leads to a dramatic quench of the plasma current on a very short time scale, of the order of the millisecond.

Very energetic electrons are created ( runaway electrons ) and a global loss of confinement is finally obtained.

A very high energy is deposited on small areas.

This phenomenon is called a major disruption .< ref > Kruger, S. E .; Schnack, D. D .; Sovinec, C. R., ( 2005 ).

" Dynamics of the Major Disruption of a DIII-D Plasma ".

Phys.

Plasmas 12, 056113..

< http :// www. scidac. gov / FES / FES_FusionGrid / pubs / kruger-phys-plasma-2005. pdf </ ref > The occurrence of major disruptions in running tokamaks has always been rather high, of the order of a few percent of the total numbers of the shots.

In currently operated tokamaks, the damage is often large but rarely dramatic.

In the ITER tokamak, it is expected that the occurrence of a limited number of major disruptions will definitively damage the chamber with no possibility to restore the device .< ref > Wurden, G., ( 2011 ) International Workshop " MFE Roadmapping in the ITER Era ", Princeton < http :// advprojects. pppl. gov / Roadmapping / presentations / MFE_POSTERS / WURDEN_Disruption_RiskPOSTER. pdf ></ ref >< ref > Baylor, L. R .; Combs, S. K .; Foust, C. R .; Jernigan, T. C.

; Meitner, S. J .; Parks, P. B .; Caughman, J.

B .; Fehling, D. T .; Maruyama, S .; Qualls, A. L .; Rasmussen, D. A .; Thomas, C. E., ( 2009 ).

" Pellet Fuelling, ELM Pacing and Disruption Mitigation Technology Development for ITER ".

Nucl.

Fusion 49 085013..

< http :// www-pub. iaea. org / MTCD / Meetings / FEC2008 / it_p6-19. pdf ></ ref > This critical issue is rarely debated by the promoters of the project.

In fact, the very large diameter of the next generation tokamak implies that it will be very difficult to mitigate the disruptions that pose a significant challenge in future tokamaks where the increased stored energy can lead to unacceptably large transient heat loads on plasma facing components.