On secondary electron emission and its semi-empirical description

Abstract

The description of secondary electron emission, as presented by plasma-material interaction fusion compendia, is demonstrated to be outdated both in its theoretical and experimental aspects. As a consequence, the recommended treatment leads to a strong overestimation of the secondary electron emission yields for tokamak relevant materials. Reliable experimental data-sets, in fusion energy ranges, are identified after a detailed review of a recently updated electron-solid interaction database and previously published experimental results. A novel semi-empirical approach is proposed for the description of the secondary electron emission yield. Application of the approach for a large number of solids reveals an unprecedented agreement with experimental data. The present results can serve as a reliable input for future quantitative investigations of the effect of secondary electron emission on various aspects of scrape-off-layer physics.

Observation of reduction of secondary electron emission from helium ion impact due to plasma-generated nanostructured tungsten fuzz

“…Because the effect is small, the distribution function of reflected electrons can be approximated simply by the expected mean energy, ≈ 2/3 ! [31]. Fitting to the measured I-V curve for smooth W, we find !…”

“…Electron-impact SEE (which consists of secondary electron emission [31], electron backscattering [33], and electron reflection) does exhibit strong angle-of-incidence dependence in the subkeV energy range. The previous observation of a ~2× reduction in electron impact SEE due to W fuzz growth [11] could therefore e.g.…”

Survival and in-vessel redistribution of beryllium droplets after ITER disruptions

“…For impinging ions, no experimental data for multiply charged ions exists in the relevant energy range and a representative angle of 45 • is chosen. This choice is made by considering a simplified thin sheath model in which the sheath acceleration is included as a shift of the unperturbed ion Maxwellian distribution by a constant normal velocity component equal to the ion thermal speed.…”

“…• peak to 45 • , is meant to account for the fact that ion-related surface processes are significantly stronger for grazing impact angles.…”

Electron kinetics at the plasma interface

“…The crude description reflects simply the fact that in most cases the surface parameters have been neither worked out theoretically nor measured experimentally. In particular at low impact energies the data base is rather sparse or outdated as emphasized for the particular case of secondary electron emission by Tolias [20]. It is only recently that experimental work started anew to determine at energies relevant for low-temperature plasma applications surface parameters, such as, the electron backscattering probability [21] or the secondary electron emission coefficient [22,23].…”