4.3 Emission of Ions and Neutrals
61
4.3.4 Ion-induced Desorption and Radiation-Enhanced
Sublimation
Incident particles deposit their energy to surface constituent atoms so as impurity
atoms and molecules adsorbed on the surface. Ion-induced desorption is observed
when surface adsorbed or absorbed atoms or molecules get enough energy to desorb
from the surface. Although the energy of the released particles is lower than that
of sputtered atoms, it is much larger than that of the thermalized one at the surface.
The particle flux caused by the ion-induced desorption is much higher than thermally
desorbed ones and released particle flux becomes much larger than that expected from
the vapor pressure of the target material, which is referred to as radiation-enhanced
sublimation (RES). RES occurs in any materials and becomes appreciable far below
their melting temperature. In particular, carbon materials show large RES above
1200 K as seen in Fig. 4.11. For utilization of C as PFM at elevated temperatures,
RES should be taken into account.
Plasma-induced desorption or discharge-induced desorption of adsorbed
molecules typically water and hydrocarbons in a vacuum vessel is appreciable
as strong impurity radiation during the plasma ramp-up process, often resulting
in plasma collapse. The surface cleaning before plasma discharge has been very
important.
4.4 Emission of Electrons and Photons
4.4.1 Electron Emission
Energetic ion injection induces electron emission often referred to as secondary
electron emission. Because of the secondary electron emission from the target under
the ion injection, target current measurements to determine the incident ion flux often
result in overestimation. That is the reason to use the Faraday cup which suppresses
the secondary electron emission for the determination of the injected ion current. If
the surface is contaminated with some absorbents, the secondary electron emission
is more appreciable, and measured current often becomes two or three times large
than the incident ion current.
For secondary electron emission, there are several different mechanisms. They are
ion-induced kinetics emission, potential emission caused by ion injection, photoinduced emission by photon injection, and thermionic emission caused by the target
temperature rise.
The ion-induced kinetic emission is caused by the collision of the incident ion (or
its belonging electrons) with electrons of constituent elements of the target. The ioninduced Auger electron emission is included in the kinetic emission. The potential
emission is caused by a large electric field induced between the incident ion and
the target surface. Figure 4.13 shows the energy distribution of secondary electrons
61
4.3.4 Ion-induced Desorption and Radiation-Enhanced
Sublimation
Incident particles deposit their energy to surface constituent atoms so as impurity
atoms and molecules adsorbed on the surface. Ion-induced desorption is observed
when surface adsorbed or absorbed atoms or molecules get enough energy to desorb
from the surface. Although the energy of the released particles is lower than that
of sputtered atoms, it is much larger than that of the thermalized one at the surface.
The particle flux caused by the ion-induced desorption is much higher than thermally
desorbed ones and released particle flux becomes much larger than that expected from
the vapor pressure of the target material, which is referred to as radiation-enhanced
sublimation (RES). RES occurs in any materials and becomes appreciable far below
their melting temperature. In particular, carbon materials show large RES above
1200 K as seen in Fig. 4.11. For utilization of C as PFM at elevated temperatures,
RES should be taken into account.
Plasma-induced desorption or discharge-induced desorption of adsorbed
molecules typically water and hydrocarbons in a vacuum vessel is appreciable
as strong impurity radiation during the plasma ramp-up process, often resulting
in plasma collapse. The surface cleaning before plasma discharge has been very
important.
4.4 Emission of Electrons and Photons
4.4.1 Electron Emission
Energetic ion injection induces electron emission often referred to as secondary
electron emission. Because of the secondary electron emission from the target under
the ion injection, target current measurements to determine the incident ion flux often
result in overestimation. That is the reason to use the Faraday cup which suppresses
the secondary electron emission for the determination of the injected ion current. If
the surface is contaminated with some absorbents, the secondary electron emission
is more appreciable, and measured current often becomes two or three times large
than the incident ion current.
For secondary electron emission, there are several different mechanisms. They are
ion-induced kinetics emission, potential emission caused by ion injection, photoinduced emission by photon injection, and thermionic emission caused by the target
temperature rise.
The ion-induced kinetic emission is caused by the collision of the incident ion (or
its belonging electrons) with electrons of constituent elements of the target. The ioninduced Auger electron emission is included in the kinetic emission. The potential
emission is caused by a large electric field induced between the incident ion and
the target surface. Figure 4.13 shows the energy distribution of secondary electrons