Modelling of confinement degradation in the radiative improved mode caused by a strong gas puff

Abstract

Confinement deterioration and rollover from the radiative improved (RI) mode to the low (L) confinement mode in plasmas with a strong gas puff in the tokamak TEXTOR-94 are modelled by the one-dimensional transport code RITM (radiation from impurities in the transport model). The anomalous transport coefficients in the plasma core include contributions from the ion temperature gradient and dissipative trapped electron instabilities. This model allows us to reproduce the L-RI bifurcation with impurity seeding in good agreement with experimental observations. Whereas the transport at the plasma edge under the RI-mode conditions might be described by the electrostatic turbulence caused by electric currents in the scrape-off layer of the limiter, the present computations show that the level of the edge transport must be increased by at least a factor of three in order to reproduce the evolution of the plasma density profile and the effective ion charge during RI-to-L back transitions. An increase of this order is in agreement with reflectometer measurements and could tentatively be explained by the effect of neutrals on resistive drift modes.

Local effects of gas fuelling and their impact on transport processes in the plasma edge of the tokamak TEXTOR

“…Caused by the significant increase of the collisionality at the puffing location (m* increases by an order of magnitude to values as high as 250) we expect a substantial increase of drift resistive ballooning modes as predicted in [7]. Experimentally, a marked increase of density fluctuations measured by reflectometry just inside the LCFS is observed under conditions of strong gas fuelling [5] which at least qualitatively supports our picture of an increased edge transport. As a result, we also observe remote from the fuelling location an increase of the electron density (cf.…”

“…The increase of edge transport owing to the perturbation leads to a more effective impurity screening. If the impurity content in the plasma bulk is reduced too strongly, a confinement degradation back to L-mode levels takes place through the re-appearance of ITG modes as discussed in [5].…”

“…With the help of the RITM code the confinement degradation occurring with strong fuelling could be modeled and explained by a re-appearance of ion temperature gradient driven (ITG)-modes, which initially are reduced in the transition from L-to RI-mode, if the particle transport at the edge is notably increasing [5]. Such an enhanced transport has been associated with drift resistive ballooning modes growing during the formation of a cold and dense plasma cloud at the location of the strong gas injection [6,7].…”

Influence of the boundary conditions on the H-mode power threshold

“…Recently, the one-dimensional code RITM ͑radiation by impurities in the transport model͒ [1][2][3][4] has been significantly amended by including a model for the edge transport, which reproduces the formation of the edge transport barrier ͑ETB͒, and was applied for a self-consistent modeling of the H-mode plasmas in the tokamak JET ͑Joint European Torus 3,4 ͒. The present paper is an extension of studies done in Ref.…”

Numerical analysis of the influence of different kinds of transport on the edge plasma profiles in TEXTOR tokamak

“…2 we show the contributions to the anomalous radial transport due to different instabilities: the drift resistive ballooning (DRB), the drift Alfvén (DA) as well as the ion temperature gradient (ITG) and the dissipative trapped electron instability (DTE). It can be seen that the last two instabilities (ITG and DTE, we used the expressions for the diffusion coefficients found in [9] and the references cited therein) are usually negligible for the typical edge plasma parameters of TEXTOR tokamak. For lower input particle flux (lower plasma density) DA dominates whereas for higher fluxes (densities) DRB starts to be more important.…”