Overview of radiative improved mode results on TEXTOR-94

Abstract

The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

Recent progress toward high performance above the Greenwald density limit in impurity seeded discharges in limiter and divertor tokamaks

“…This regime has been demonstrated for quasistationary periods of several seconds, equivalent to more than 160 energy confinement times or several skin times. 4 In fact, it is very similar to the Z mode 5,6 of ISX-B ͑Impurity Study Experiment B͒ which was, however, observed in a narrower parameter range.…”

Quasistationary High Confinement Discharges with trans-Greenwald Density on TEXTOR-94

“…A subject of intense investigations in present day fusion research is the development of an operational scenario, providing at the same time (i) confinement with at least H-mode quality and (ii) high densities around or above the empirical Greenwald density value n GW I P ͞pa 2 [1] [with I P the plasma current (MA), the minor plasma radius (m), and n GW in units 10 20 m 23 ]. Success in this direction has been obtained in the past years on limiter [2,3] and divertor tokamaks by pellet injection or adjusting deuterium fueling and pumping [4,5]. Recently, high energy confinement at a Greenwald numbern e,0 ͞n GW 1.4 (n e,0 being the central averaged chord density) was obtained at DIII-D by optimizing divertor pumping in gas puffed H-mode discharges with edge localized modes (ELM) [6,7].…”

“…2, where the normalized confinement time t E P 2͞3 tot ͞I P (P tot is the total heating power in MW) is plotted versus the Greenwald numbern e,0 ͞n GW as introduced in Ref. [3]. The data in this diagram, which have been obtained after a fresh siliconization, illustrate three types of behavior depending on the deuterium (D) gas fueling rate.…”

Asymmetric radiation-induced toroidal flow and improved confinement in tokamaks

“…This new operational regime was discovered in the Impurity Study Experiment ͑ISX-B͒ tokamak 5 and then confirmed and thoroughly reinvestigated on the Tokamak Experiment for Technology Oriented Research ͑TEXTOR-94͒. 6,7 Compared to the L ͑low͒ confinement mode, the RI mode is characterized by more peaked density and temperature profiles, larger operating densities ͑N e may be well above the Greenwald limit͒, and larger confinement times ͓ E may be close to edge localized mode ͑ELM͒ H-mode values͔. The mechanisms considered presently to lead to confinement improvement in the RI mode are largely based on the reduction of the growth rate of the toroidal ion temperature gradient ͑ITG͒ mode when the plasma Z eff increases.…”

“…[6][7][8] We may thus conclude that flow generation due to radiation asymmetry effects may not be important for hot edge plasmas. We now present a semiquantitative explanation of how a small poloidal asymmetry in the radiated power can modify the basic characteristic of L-mode discharges and then lead to a L-RI mode transition.…”