Chemistry Reference
In-Depth Information
The electrons from the high-energy tail of the Fermi function can leave the
solid if they are able to overcome the work function (energies greater than
F
+
W
). This process is known as thermionic emission; its current is derived
from the (temperature dependent) number of electrons at these energy levels.
Thevelocitydistributionof theemittedelectrons is Maxwellianbecausethey
are thermalized by collision in the solid [110]. Thermionic emission is quite
important in arcs and hot spots. Graphite has a work function of 4.6 eV.
Therefore, about 2500 K is needed for thermionic emission to occur (which
is very close to the melting temperature of carbon).
•
Field emission:
The effective work function of a solid can be reduced by the
electric fields (Schottky effect) and an additional emission enhancement can
be reached by the tunneling of electrons through the potential wall created
by the electric field. This process is especially important for the creation of
arcs in fusion plasmas. The field emission electron microscope relies on this
phenomenon.
•
Secondary electron emission:
Electrons hitting a surface are either reflected
elastically (usually a small amount) or ejected due to inelastic collisions with
other electrons. The energy distribution of the electron-induced secondary
electrons has two peaks: One at low energies and one close to the energy
of the impinging electrons. Ions produce a large electric field in front of
the surface and create a potential wall (as with field emission) through
which the electrons can tunnel and neutralize the ions. If the difference
between the energy of the electrons and the ionization energy of the ion is
larger than the work function, secondary electrons can be emitted.
•
Arcs:
The emission processes discussed before can locally result in strongly
varying electric potential conditions at a surface [107]. For example,
for field-emission at a tip, one can locally get a smaller potential drop:
A positive current flows into the surface at this position. The current can melt
the material and a unipolar arc is created, stabilized by thermo-electrons,
for which the vessel is the cathode and the plasma the anode.
9.3.2 I
NTERACTION OF
H
YDROGEN
P
LASMA WITH
C
ARBON
-B
ASED
M
ATERIALS
Typical high-quality graphite consists of granules (typically 1-10 μm, macroscale)
separated by voids that are typically a fraction of a micrometer wide. The granules
consist of graphitic micro-crystallites of 10-100 nm size separated by micro-voids
that are typically 1nm wide (mesoscale) [111,112]. These substructures, voids and
micro-voids, provide a large internal surface area inside the graphite where hydrogen
interstitial atoms can diffuse and react with each other. This will affect the hydrogen
isotope inventory and recycling behavior and also chemical erosion. Due to the large
internal surface area provided by the graphite, it acts like a sponge for hydrogen.
For carbon-based plasma-facing materials, essentially four mechanisms have
been identified for the retention and uptake of hydrogen [113] (see Figure 9.19).
1. Formation of a saturated surface layer
2. Surface diffusion due to internal porosity
