PSI Introduction

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Based on an article by Janev and Miyahara in the APID vol. 1 .[1]

Contents

General Description

The critical role that plasma-material interactions play in the achievement of the technical objectives of a reactor level fusion device can be understood from the fact that the effects of these interactions define the boundary conditions for both the plasma dynamics inside the torus and the thermal processes in the surrounding material structures. The plasma-surface interaction processes are a source of impurities in the plasma which through their powerful radiation become an essential factor in the plasma energy balance. The plasma-wall interaction processes, together with the gas phase atomic processes between plasma particles and released impurities, define the parameters of the boundary plasma (outside the separatrix, the last closed magnetic flux surface). The edge plasma, through its dynamical coupling with the main plasma (inside the separatrix), has a strong influence on the plasma transport processes and thereby on the gross energy confinement time. Moreover, the plasma edge conditions play an essential role in the transition from the low (L-) to the enhanced (H-) plasma confinement regime. Furthermore, the most dangerous, disruptive plasma instabilities develop at the singular magnetic flux surfaces at the plasma periphery, where also most of the neoclassical and anomalous transport is generated. Impurity radiation and hydrogen recycling in the plasma edge, together with the strong parallel (to the magnetic field lines) plasma flow in the scrape-off layer (SOL), are the major factors which determine the plasma parameters in this region.

On the other hand, the material plasma boundaries have to receive the entire thermal power and to conduct it to the background heat sink structures and coolants. The thermo-mechanical response of plasma facing components to plasma particle and heat fluxes defines components to plasma particle and heat fluxes defines not only the structural and functional integrity (lifetime) of these components but also the heat transport and heat extraction processes. The surface heating and temperature gradients in the plasma facing materials define the initial conditions for the thermal stress behaviour of the components and their thermo-mechanical and thermo-hydrodynamic compatibility with the supporting structures and the coolants. Plasma-material interactions have an impact on the reactor safety because of problems connected with the tritium inventory.

The most important effects of plasma-material interactions on the reactor performance are : impurity generation, erosion of plasma facing materials and thermal action on the plasma facing components. Impurities are generated by a number of processes induced by particle impact (physical sputtering, radiation enhanced sublimation, desorption, chemical erosion), by thermal processes (thermal sublimation, desorption, evaporation) as well as by electrical phenomena in the plasma-wall system, such as unipolar arching.

impurity shielding

Diffusion of impurities into the central plasma region may lead to intolerable radiation losses and prevent ignition. The radiation processes in the hot (T~ 15-20 eV) central plasma region are line emission, bremsstrahlung and dielectronic recombination. The radiative power losses due to these processes are proportional to q, q2 and q4 respectively, where q is the impurity ion charge. High-Z impurities can prevent ignition and also introduce an effective dilution of fuel density.

Minimization of impurity influxes from the walls is particularly important for long pulse (or steady state) operation regimes and the efficient shielding of the plasma from impurities is an important design issue. An approach to this problem in most tokamak reactor designs is the use of a poloidal divertor. Strong temperature gradients in the region outside the last closed magnetic flux surface divert the radially diffusion plasma into a rapid conductive longitudinal flow towards the divertor chamber, where it intercepts the divertor plates. The plasma particle and heat fluxes on the vessel wall are thereby significantly reduced. The parallel plasma flow in the scrape-off region also entrains the ionized wall impurities towards the divertor, thus providing a shield for the main plasma. However, the diverted particle and power fluxes striking the divertor plates induce very intense plasma-material interaction. Still, the divertor concept for impurity control is attractive since it localizes the problem. Moreover, this concept offers also a way for resolving the thermal power and particle exhaust problem.

erosion

Material erosion of plasma facing components is a serious design issue regarding not only plasma contamination but also the lifetime of these components. Excessive erosion rates may require frequent replacement of plasma facing components (particularly the divertor plates), which has an impact on the reactor engineering, the effective operation time and the cost. Under normal operating conditions, the main erosion mechanisms are physical sputtering, thermal sublimation, radiation enhanced sublimation (for carbon based materials only) and chemical sputtering (for materials chemically active with hydrogen, such as carbon based materials and metal oxides). During off-normal events, such as plasma disruptions and runaway electrons, the main erosion mechanisms are evaporation and cluster emission.

power and particle exhaust

The thermal power and helium ash exhaust from the reacting plasma volume is also one of the most serious reactor design issues. The plasma power consists of fusion alpha particle heating power, auxiliary heating power, associated with non-inductive current drive, and Ohmic heating power. The main design concern related to the thermal power exhaust are the high power loads on divertor plates and the divertor material should withstand such loads preferably for the entire integrated operation time and be able to efficiently transmit the received power to the background structures. Therefore, these materials should have adequate thermo-mechanical properties (high thermal conductivity, low thermal expansion coefficient, good thermal shock resistance, high yield strength and toughness, low elastic moduli, long fatigue lifetime) and thermo-physical characteristics (high melting point, high evaporation heat, low erosion and low tritium inventory) . In addition, the neutron irradiation effects (good neutron damage resistance, low helium production, swelling and embrittlement) and safety requirements (low neutron activation) should be considered. Thus, the thermal power exhaust problem strongly influences several reactor design aspects.

The plasma-material interaction processes have an indirect effect on the helium exhaust problem, mainly through the hydrogen recycling in the divertor region. The hydrogen recycling coefficient is determined, among other factors, by the particle reflection from the surface, hydrogen trapping and detrapping processes, particle impact induced desorption and other surface chemistry phenomena.

Research Activities in Plasma-Matter-Interaction


References

  1. R. K. Janev and A. Miyahara , Plasma-material interaction issues in fusion reactor design and status of the database, Atomic and Plasma-Material Interaction Data for Fusion, v.1 p.123 (1991)
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