The influence of plasma-edge properties on high confinement discharges with a radiating plasma mantle at the tokamak TEXTOR-94

Abstract

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma boundary (with a fraction of the radiated power with respect to the total input power up to 90% with neon or argon cooling) with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (line-averaged central-electron density equal to or even above the Greenwald density limit ) in quasi-stationary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels . These changes are characterized by a reduction of the plasma-edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth of deuterium and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. An internal transport barrier is observed in the bulk of RI-mode plasmas (at ) characterized by an increase of the pressure gradient and of the shear of the toroidal velocity compared to discharges without additional impurity seeding. The dilution at the plasma boundary is strongly increased by the seeded impurities whereas the central dilution is only weakly affected.

Recent progress in ICRF physics

“…This mode of operation was observed in the TEXTOR experiment [29]. By seeding the plasma with neon (or other inert gas) a radiating edge mantle was created (as much as 80-90% of the power was radiated from the edge) but with acceptable core radiation.…”

Review and present status of the TEXTOR radiative improved (RI) mode

“…The decrease of the deuterium concentration n D /n e on the axis at γ = 0.85 is very small because even the low central concentration of Ne (0.9%) is offset by a decrease on the central C concentration from 1.4% at γ = 0.3 to 1% at γ = 0.85. On the other hand, at the plasma boundary, n D /n e decreases from 0.9 to 0.65 or 0.4, depending on the measurement method, confirming that the Ne concentration at the edge is higher; see Unterberg et al (1997) for more details.…”

“…An overview of the substantial changes of the plasma edge properties and of some bulk properties occuring when the radiated power fraction γ is increased has been presented recently (Unterberg et al 1997). These changes are characterized by reductions in the plasma edge density and temperature, a reduction of particle transport out of the confined plasma volume, and an increase in the penetration depth of deuterium and impurity atoms.…”

“…7. Unterberg et al (1997) also investigated the causality between the variation of plasma edge properties and the improvement in energy confinement by examining the temporal evolution of the physical parameters describing the formation of the radiating boundary and the onset of improved confinement. Figure 8 shows the time traces of the electron temperature at the last closed magnetic surface (LCMS), the steepening of the electron density profile characterized by the ratio between the line-integrated density measured by the central (r/a = 0) and an outer (r/a ≈ 0.8) channel of the HCN interferometer, the radiated power fraction γ and the energy confinement with respect to the ELM-free H-mode scaling expressed by f H93 .…”

“…2.3 Figure 11. Deuterium concentration nD/ne versus radiated power fraction γ: in the plasma centre (obtained by charge exchange recombination spectroscopy, CXRS) and at the plasma edge (obtained by using (i) the impurity flux ratios and (ii) neutral deuterium flux and ion sound velocity, as explained in Unterberg et al 1997).…”