B343 Confinement

Confinement in spheromaks is supposed to be heavily affected by plasma turbulence, which produces the dynamo effects, just as in reversed field pinches. The largest value for energy confinement time («0.2 ms) was obtained many years ago on the CTX reactor [Jarboe, 1994]. Local diffusivity is consistent with the expression, derived first by [Rechester and Rosenbluth, 1978], for the collisionless diffusion of a test particle in a stochastic magnetic field.

At this stage it is not clear whether poor energy confinement is an inherent feature of spheromaks. As noted above, relaxed configurations are sustained by the generation of magnetic fields through the dynamo process. Such a mechanism can produce stochastic magnetic field lines and very poor confinement. The main issue to keep energy confinement at an acceptable level is therefore to keep the dynamo mechanism running with the minimum amount of turbulence. It should be noted that in recent years encouraging results have been obtained in several reversed field pinch experiments where transport has been successfully reduced by controlling the level of plasma turbulence [Sarff et al., 2002]. For example, when the so-called "quasi-single-helicity" states are produced (i.e., when turbulence with a given helicity component dominates) the volume of plasma filled with stochastic magnetic field lines is reduced and, as a consequence, confinement is improved.

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