Improved confinement with edge radiative cooling at high densities and high heating power in TEXTOR

Abstract

Improved confinement is achieved on TEXTOR under high power conditions (up to 4 MW of additional heating with NBI-CO+ ICRH, NBI-co +counter or NBI-co +counter +ICRH) with edge radiative cooling employing silicon or neon as the radiating impurities. It is shown that in quasi-stationary conditions up to 85% of the input power can be radiated. Such high power fractions offer the possibility of utilizing these techniques to facilitate the power exhaust problem for a tokamak reactor. Discharges with edge radiative cooling exhibit enhanced confinement properties at high densities, e.g. at a central line averaged electron density of 7.5 X 10l3 cm", an enhancement factor of 1.7 over ITER L89-P confinement scaling is obtained with an edge q value as low as 2.7. Stable discharges have been obtained even with the q = 2 surface located inside the radiating zone. Furthermore, for radiatively cooled discharges heated with balanced NBI-co+counter with or without ICRH, supershot-like peaked electron density profiles, with central density values above 1 .O x lOI4 are observed. The present results show that there is no impurity accumulation in the centre and the Ne and/or Si concentration is so low that the reactivity of the plasma remains unaffected.

The history of research into improved confinement regimes

“…In 1994 the Textor team reported on the development of the "radiative improved (RI)-mode" which recovered the LOC density scaling but with auxiliary heating [18]. The RI-mode could be accessed by puffing small amounts of neon into the limiter discharge or unintentionally by Si erosion from freshly siliconized plasma walls.…”

Plasma surface interactions in impurity seeded plasmas

“…Nevertheless, a few remarks are required since sufficient confinement is a necessary boundary condition for each radiative scenario. Originally, in small-medium size devices and at quite high collisionalities, impurity seeding often caused electron density peaking and improved energy confinement [2], [3]. This behaviour has been attributed to the reduction of ion temperature gradient mode (ITG) turbulence caused by a rise in the effective ion charge in combination with steepening of the density gradients [31].…”

“…Originally, a major focus was the improvement of energy confinement, which was often observed in radiative regimes in combination with electron density profile peaking [2], [3], [4]. However, this topic calmed down after these improved confinement regimes could not be fully reproduced in larger devices under low collisionality conditions [5] [6] [7].…”

Density limits in toroidal plasmas

“…Peaked density profiles have long been associated with the suppression of anomalous transport [131][132][133][134][135][136][137] resulting in selfsustained regimes with improved confinement and very high central densities. Studies of pellet fueled discharges yielded densities 1.5 to 2 times the limit compared to those fueled at the edge by gas [43,44,46,47].…”

“…In these experiments, a small amount of neon was puffed in neutral beam heated discharges resulting in peaked density profiles and somewhat improved energy confinement. These early results have been extended and explored extensively on many devices, particularly by the TEXTOR group [163,132,45,150] which has referred to the regime as RI-mode, for radiative improved confinement mode.…”