Recent DIII-D divertor research

Abstract

DII-D currently operates with a single-or double-null open divertor and graphite walls. Active particle control with a divertor cryopump has demonstrated density control, efficient helium exhaust, and reduction of the inventory of particles in the wall. Gas puffing of D2 and impurities has demonstrated reduction of the peak divertor heat flux by factors of 3-5 by radiation. A combination of active cryopumping and feedback-controlled D2 gas puffing has produced similar divertor heat flux reduction with density control. Experiments with neon puffing have shown that the radiation is equally-divided between a localized zone near the X-point and a mantle around the plasma core. The density in these experiments has also been controlled with cryopumping. These experimental results combined with modeling were used to develop the new Radiative Divertor for DIII-D. This is a double-null slot divertor with four cryopumps to provide particle control and neutral shielding for high-triangularity advanced tokamak discharges. UEDGE and DEGAS simulations, benchmarked to experimental data, have been used to optimize the design.

Chapter 4: Power and particle control

“…6(a). Similar results have been obtained from all other divertor experiments [92][93][94][95]. On the left hand side of Fig.…”

“…The general aspects of the physical model of detached divertor regimes described in Section 4.2 have been supported by a series of theoretical and experimental advances over the last several years. Although the theoretical basis and prediction of ionneutral effects leading to pressure loss existed long ago [90,91], experimental measurements of pressure loss on a number of experiments [92][93][94][95] engendered initial 'detachment' theories [48,49,96] that focused on explaining those results. Examples of experimental measurements of momentum (pressure) loss and accompanying particle flux reductions to the divertor plate are shown in Fig.…”

Chapter 3: MHD stability, operational limits and disruptions

“…This physics is more appropriate to the analysis of divertor behaviour presented in Chapter 4 and a detailed discussion is deferred until then. Experimentally, it is observed (see, for example, [222,[247][248][249][250]) that as the bulk density rises, the plasma density at the divertor target initially rises. With increasing edge density, however, both the density and electron temperature at the target fall.…”

Transition to supersonic flows in the edge plasma

“…Given the large values of the flows that are reported experimentally [21], and given the constraint that the ion plasma flow must be supersonic in the sheath boundary layer, the so-called Bohm criterion, it is mandatory to properly describe the transition from subsonic to supersonic flows [22]. In advanced divertor operation [23], the electron temperature drops below the ionization energy so that a new regime is reached where the neutral density can build up and where plasma recombination can take place if the electron temperature drops below the 1 eV range. This so-called detached plasma regime combines low ionization capability, plasma momentum losses and volumetric plasma recombination.…”