A density rise experiment on PLT

The evolution of the density profile in PLT during intense gas puffing is documented and analysed. Measurements of the spectrum of low-energy edge neutrals and of the change in central neutral density, indicate that charge-exchange processes alone cannot account for the central density rise. The transient density profile changes can be reproduced numerically by a diffusivity of of - 10⁴cm². s^-1and a spatially averaged inward flow of 10³cm.s^-1. These transport coefficients are 10-10²times larger than neoclassical. The ion energy confinement is reduced, the small-scale density fluctuations are increased, and runaway electron losses are increased during the density rise.