1 EX / P 5 - 24 Observation of a Natural Particle Transport Barrier in HL - 2 A Tokamak

In this paper we report the observation of a natural generated internal particle transport barrier ( pITB ) in purely Ohmically heated plasmas in the HL-2A tokamak. The pITB have been observed when the line average electron density exceeds a threshold, which is nc=2.2×10m. The phenomenon is perfectly reproducible and identified by profile analysis, perturbative transport studies with Supersonic Molecular Beam Injection (SMBI) modulation and the Doppler reflectometry. The influence of the ECRH heating on pITB will be also presented in this paper. The barrier is generally located around ρ= 0.65-0.7 with a width of 1-2 cm. Especial analysing the propagation of a particle wave generated by SMBI modulation across the barrier, the particle diffusivity and the convective velocity have been separately determined. The diffusivity is rather will-like than step-like and lower in the zone of barrier than outside. The convection is inward outside of the barrier, while the convection is outward inside the barrier. The experimental results have been compared to an analytical model for particle transport to quantitatively characterize this barrier. A satisfactory agreement has been found between the experimental points and the analytical calculation. The change of sign for the convective velocity has been explained by the turbulence TEM/ITG system. The density threshold in pure Ohmic discharge may be correlated to the TEM/ITG transition via the collisionality. Some possibilities of the mechanism leading to the pITBs formation have been discussed and need further experiments to be confirmed.