1098 C. Llewellyn Smith and S. Cowley 10 1 ) s e r e h p s o m t (a 0.1 E t i P t c u d o r p no0.01 i s u f 0.001 power plant conditions self-burning plasma ITER JET 1980 1968 1 10 100 1000 central ion temperature Ti (million degrees Celsius) Figure 3. Selected results from different tokamaks demonstrate substantial progress over recent decades from the low temperature, low energy gain points at the bottom left. Temperatures above 100 M◦C are now routinely achieved and an energy gain of around one has been reached. A power plant needs an energy gain (Q) above 10, and this should be achieved in ITER. shear layer develops at the edge of the plasma—in this layer the turbulence is suppressed by shear in the plasma rotation. Theory conﬁrms that when the shearing rate is faster than the eddy turnover the sheared rotation rips apart the turbulent eddies and suppresses heat transport. Just how the plasma spontaneously creates shear layers is not fully understood. Nonetheless, ITER will rely on the H mode for its highest performance. If turbulence could be suppressed throughout the plasma, tokamak reactors could be made considerably smaller (and cheaper) than currently predicted. The magnetic ﬁeld provides a force (J × B) to contain the plasma pressure— roughly speaking this is a magnetic pressure. In atmospheres the magnetic pressure is PMagnetic ∼ 4B2, where B is measured in tesla and PMagnetic is in atmospheres. JET operates at up to 4 T and ITER will operate at up to 5.2 T. The cost of the coils can be (6.2) Phil. Trans. R. Soc. A (2010)