A review of recent developments in atomic processes for divertors and edge plasmas

Abstract

The most promising concepts for power and particle control in tokamaks and other fusion experiments rely upon atomic processes to transfer the power and momentum from the edge plasma to the plasma chamber walls. This places a new emphasis on processes at low temperatures (1-200 eV) and high densities (10 20 -10 22 m -3 ). The most important atomic processes are impurity and hydrogen radiation, ionization, excitation, recombination, charge exchange, radiation transport, molecular collisions, and elastic scattering of atoms, molecules and ions. Important new developments have occurred in each of these areas. New calculations for impurity and hydrogen ionization, recombination, and excitation rate coefficients for low temperature plasmas that include collisional radiative effects and multi-configuration interactions indicate that earlier estimates of these rate coefficients for plasmas with low Z impurities such as beryllium are too high in selected regions of interest [1]. Transport effects and charge exchange recombination are also key elements for determining the ionization-recombination balance and radiation losses. Collisional radiative effects for hydrogen are an essential ingredient for understanding and modelling high recycling divertors [2,3]. Since the opacity in a high recycling divertor can be large for many of the strongest lines of recycling hydrogen atoms and low Z impurities, the transport of this radiation determines where the energy is deposited and strongly influences the recombination and ionization rate coefficients [4,5]. Molecular collisions play a large role in recycling and in the energy and particle Shortened PSI Review Paper 6 2 balance at the plasma edge [6]. Charge Exchange and elastic collisions are also important for determining the neutral gas pressure and the transfer of plasma energy and momentum to the chamber walls [7,8]. The best available data for these processes and an assessment of their role in plasma wall interactions are summarized, and the major areas where improved data are needed are reviewed.

Physics of ultimate detachment of a tokamak divertor plasma

“…The reason for this is the 'multi-step' effects in the processes of excitation and ionization of the neutrals and ions (Post 1995). Such processes involve both radiative decay (which can be considered a 'one-body' process) and binary excitation/de-excitation.…”

Runaway generation during disruptions in JET and TEXTOR

“…At constant I P , the electric field is increasing with impurity concentration, here carbon density. The carbon density, indicated by the dotted lines, is calculated from corona equilibrium [11] assuming P rad = P OH , with P rad ∼ n e n C L C (T e ) and P OH ∼ E 2 /η. Above the maximum emissivity of carbon at around 6 eV, the corona model might not be applicable in this context and/or radial transport has to be taken into account instead of the 0-D assumption P rad = P OH .…”

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

“…[29][30][31] As a result, the effective cooling rate becomes a function of the product of this time and electron density. The stronger the impurity transport, the smaller the residence time and the more significant the increase of the cooling rate compared to the coronal level should be expected.…”

“…The stronger the impurity transport, the smaller the residence time and the more significant the increase of the cooling rate compared to the coronal level should be expected. In some papers, 31,32 an extraordinary importance of impurity chargeexchange with hydrogen neutrals, recycling from the wall, had been anticipated. Charge-exchange leads to the conversion of ''dim'' highly ionized particles into radiant ions of a low charge and, finally, to an enhancement of the cooling rate.…”