  The understanding of the mechanisms responsible for particle transport is of
the utmost importance for magnetized fusion plasmas. A peaked density profile
is attractive to improve the fusion rate, which is proportional to the square
of the density, and to self-generate a large fraction of non-inductive current
required for continuous operation. Experiments in various tokamak devices (AUG,
DIII-D, JET, TCV, TEXT, TFTR) have indicated the existence of an anomalous
inward particle pinch. Recently, such an anomalous pinch has been unambiguously
identified in Tore Supra very long discharges, in absence of toroidal electric
field and of central particle source, for more than 4 minutes [1]. This
anomalous particle pinch is predicted by a quasilinear theory of particle
transport [2], and confirmed by non-linear turbulence simulations [3] and
general considerations based on the conservation of motion invariants [4].
Experimentally, the particle pinch is found to be sensitive to the magnetic
field gradient in many cases [5, 6, 7, 8], to the temperature profile [5, 9]
and also to the collisionality that changes the nature of the microturbulence
[10, 11, 12]. The consistency of some of the observed dependences with the
theoretical predictions gives us a clearer understanding of the particle pinch
in tokamaks, allowing us to predict more accurately the density profile in
ITER.
