ITERに係わる原子分子過程 Atomic and Molecular Processes in ITER SHIMADA, Michiya ITER International Team Annual Meeting of Japan Society of Plasma Science and Nuclear Fusion Research 28 Nov. 2006

Introduction ITER - burning plasma The plasma is characterized by autonomy. With significant alpha particle heating, the plasma determines its profiles. In the steady state operation, the plasma determines its current profile. The reactor plasma will be operated in the proximity of the MHD stability limit. Successful control of the core plasma requires accurate projection of the plasma behavior. A&M processes determine the source terms of plasma transport equations. Accurate A&M reaction rates are the key for the projection.

Contents High Z materials (impurities) Opacity (photon trapping) Photoionisation in the divertor Dust Summary

High Z materials Good candidates for plasma-facing materials of fusion reactors due to their low sputtering yield and long lifetime However, their allowance in the core plasma is low ~10-5 ASDEX-Upgrade experiments [Neu, NF (2005) 209] show accumulation of tungsten in the core for some discharge conditions High Z materials, such as tungsten, are good candidates for the plasma facing components (PFC) in fusion reactors, due to their low sputtering yield (hence long lifetime) and low tendency to trap tritium. However, their effect on radiative cooling is large, making the concentration allowable in the plasma core very low; e.g. 10-5 for tungsten. That is why high Z materials have been rarely used in experiments until recently. Motivated by the potential use of tungsten in fusion reactors, the use of high-Z material for PFC is slowly expanding. PFCs of C-MOD are completely covered by molybdenum. ASDEX-Upgrade has been conducting experiments with PFCs mostly covered by tungsten. JT-60U is using some tungsten coated tiles near the divertor strike points and JET is planning to convert its whole divertor to tungsten in the near future. ITER will use tungsten to cover the dome and divertor baffle areas. Why peaking concentration decrease? 2 reasons: increase in anomalous transport and reduction of neoclassical inward drift of impurity ions.

Validation of W spectrum modelled by ADAS While good agreement is obtained, some discrepancies exist [Neu NF (2005) 209]

Opacity reduces the radiative cooling rate and increases the current quench time at disruption [Lukash, IAEA 2006] Joule heating Without opacity Radiative cooling rate With opacity Current quench time ~ Te3/2

Photoionisation in the divertor A parasitic plasma (ne ≤ 1018 m-3, Te ~ 15 eV) is observed under the divertor roof baffle of ASDEX-Upgrade[V. Rohde et al. J. Nucl. Mat. 313-316 (2003) 337]. The density ne scales very strongly with radiation power Prad (ne ~ Prad2.7 x Po0.7, Po is the neutral pressure), suggesting that the plasma is created by photoionization. Intense radiation in the divertor can result in photoionisation of hydrogen and impurities and could form parasitic plasmas. Actually, parasitic plasmas are observed under the dome of ASDEX-Upgrade. The assessment of parasitic plasmas under the dome is important, since the ionisation of hydrogen and impurities by photoionisation, and the existence of parasitic plasmas could change the pattern of carbon codeposition, which might affect the efficiency of tritium removal.

Sharpe, PSI-2006

Dust Dust originating from plasma-facing materials is expected to have a particle size in the range of 10 nm-10 m. Dust can be radioactive and chemically reactive. The formation mechanisms, in which A&M processes should play some role, need to be understood. Dust made of PFC materials is expected to have a particle size in the range of 10 nm-10 m. It can be radioactive and chemically reactive. The formation mechanisms need to be understood.

summary Atomic and molecular processes play a crucial role in the particle, momentum and power balance in the present plasma confinement devices. In this presentation, I have discussed atomic and molecular processes that will increase their importance in ITER, but their reaction rates often have large uncertainty. Further work in theoretical studies of these processes and experimental validation should be encouraged.