Energy Distribution Of Incident Ions Research Articles

Study of the erosion and redeposition of W considering the kinetic energy distribution of incident ions through a semi-analytical model

In today’s nuclear fusion devices, erosion of high-Z metallic plasma-facing materials (PFMs) is mainly caused by physical sputtering. That is, by the exchange of energy between plasma ions and the atoms in the walls. In most of the numerical codes currently in use impinging plasma is approximated as a fluid. By averaging the incident particles’ energy distribution the high-energy population of the eroded material is underestimated. For heavy materials such as W, high-energy eroded particles tend to ionize far from the wall and they are less affected by the sheath electric field hence, not being attracted back to the wall, they have a higher chance to contaminate the core plasma. This could in turn result in an underestimation of the net erosion sources. In this work, a semi-analytical model was developed to include the energy distribution of the incident particles. Then, by Monte Carlo method, the net erosion of tungsten from a smooth PFM was calculated. The results show that the kinetic description in energy is important only for incident particles ionized once. For instance, it is particularly important for plasma ions such as Deuterium. It is seen that Deuterium contribution to the W net sources is not always negligible if compared to light impurities or to tungsten self-sputtering in the range of plasma parameters tested. Finally, results show that the difference between the fluid and kinetic models becomes more pronounced for high-screening plasma conditions.

Experimental and theoretical study of ion reflection for new diagnostic method of ion energy distribution in edge plasma

Abstract A new diagnostic method to measure ion energy distribution using ion reflection on the solid surface, one of the processes of plasma-wall interactions, is proposed. It is well known that the energy distribution of reflected particles is related to the energy distribution of incident ions. This method is used in relatively Iow ion temperature region such as edge plasma in fusion experimental devices. Incident ions to target plate which is inserted in the edge plasma are mainly reflected as neutral particles, and time-of-flight method is utilized to measure energy distribution of reflected particles. Energy distribution of incident ions is deduced by fitting with calculation using Monte Carlo simulation code TRIM.SP. The sheath potential in front of the target can also be obtained with this method.

Ponderomotive force and ion energy distributions in an rf sheath.

The incident-ion energy distribution in a collisionless radio-frequency (rf) sheath is derived analytically for a general electric-field configuration in the high-frequency regime. The analysis is based on a two-time-scale asymptotic expansion of the ion equation of motion, where the ratio of the ion transit frequency ${\mathrm{\ensuremath{\omega}}}_{\mathrm{tr}}$ to the rf frequency \ensuremath{\omega} is assumed to be small. It is shown that the ``ponderomotive force'' due to the rf modulation of the electric field exerts a retarding effect on the ion motion, counteracting the dc-bias field. The results obtained here are applicable to rf-discharge-based process tools used in microelectronics fabrication, where the conditions of low collisionality and high rf frequency are usually satisfied.

A model for hydrogen isotope backscattering, trapping and depth profiles in carbon and amorphous silicon

A model for low-energy hydrogen trapping and backscattering in carbon and amorphous silicon is described. Depth profiles are calculated and numerical results presented for various incident angular and energy distributions. The calculations yield a relation between depth profiles and the incident-ion energy distribution. The results obtained for reflection coefficients and for saturation doses are in good agreement with those obtained experimentally as described in the companion paper. The use of the model for tokamak plasma diagnosis is discussed.