Nuclear Fusion Research and Development

4-11 Toward Clarifying Complex Plasma Behavior=Development of an Integrated Simulation Code for Entire Tokamak Plasma=

18

without pellet
Perturbation
by pellet
Collapse byinstability

Pedestalregion

002.50Normalized plasma radius 1

40010000.88

1 0.88

Normalized plasma radius

1

Normalized plasma radiusElectron temperature(eV)

Diffusivity Normalized(m2/s)
gradient

Plasma pressure(kPa)

Pedestal region

Ion temperature(keV)

10In front ofdivertor plate0-5015Distance along divertor plate (cm)Fig.4-26 Profile evolution when a pellet is injected during anincrease in pressure gradient in pedestal region (left) and when nopellet is injected (right)
An ablated pellet absorbs plasma energy, makes perturbation in→) and localized pressure gradientpressure profile (top,(middle), and triggers an instability that enhances diffusivity (bottom) andcollapses the profile (). Without a pellet, the pressure gradientincreases in the entire pedestal region and triggers an instability. Pelletinduced diffusivity is so localized that energy loss is reduced.

In fusion plasmas, many phenomena interact, resulting incomplex behavior that must be understood and controlled. To
do so, we divide the tokamak plasma into three regions: thecore, the pedestal and its peripheral divertor regions. Then,for each region, we developed physical models for plasmaheat/particle transport, instabilities, and so on. We then
combined the models to obtain an integrated model thatcovers the entire region from the core to the divertor; such acomplete model is required for self-consistent evaluation of
the entire plasma. By using this model, we clarified thevarious behaviors discussed below, thereby contributing to
the design of ITER, JT-60SA, and DEMO.
(1) Core model development and integration. The
radial transport of alpha particles, enhanced by instabilities
induced by alpha particles themselves, was modeled andincluded in a core integrated code. We evaluated thereduction of fusion performance in ITER. We developed aplasma rotation model and clarified the mechanism ofrotation driven by alpha particles. Then, we evaluated theplasma rotation in DEMO. We developed a pinch model ofhigh-Z impurity and found large inward pinch when theplasma rotates in the direction opposite to that of the plasma

Fig.4-27 Temperature profile evolution incore/pedestal regions (top) and in front ofdivertor plate (bottom)
While transport is reduced in the pedestal region
and core temperature increases (), temperature
decreases once () and then increases () in
front of the divertor plate. Integrated code
covering entire plasma can clarify this type ofdynamics over the entire plasma.

current. Using this model, we succeeded in explaining
Tungsten accumulation in JT-60U experiments, which had
been an unresolved issue.
(2) Pedestal model integration. We developed apedestal integrated code by coupling the core integrated codewith an instability code, a model of solid pellet injection, anda simple divertor model. This model clarified the onset
mechanism of the instability induced by the pellet, and it
showed reduced energy loss by the pellet-induced instability(Fig.4-26). These results can serve as a guide to reduce theheat load on the divertor plate in ITER.
(3) Integration of entire plasma region. We coupled
the core integrated code with a divertor integrated code
composed of plasma fluid code and neutral/impurity Monte
Carlo code, instead of the above simple divertor model, anddeveloped an integrated code that can treat the entire plasma
region self-consistently. The code enabled us to clarify thedynamics of the entire plasma (Fig.4-27) and to study
detailed operational scenarios that would yield highperformance in the core plasma compatible with low heatload on the divertor plate.

Reference
Hayashi, N. et al., Integrated Modeling of Whole Tokamak Plasma, Plasma and Fusion Research, vol.6, 2011, p.2403065-1-2403065-8.

64 JAEA R&D Review 2012