Page 203 - Fundamentals of Magnetic Thermonuclear Reactor Design

Vacuum and Tritium System Chapter | 6 185

or blow up, leaving cavities and voids. This is accompanied by the injection of
metal particles and He atoms into the plasma.
As the energy spectrum of α-particles bombarding the wall, and the irra-
diation temperature range are very wide, there may be different generations of
swells and voids in a metal. A critical He infiltration, at which bubble blowing
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18
2
up and void formation begins is 10 –10 atom/cm . The temperature range
(0.35–0.6) T is most risky as metal strength is already deteriorated, while the
m
rates of implanted atoms’ diffusion and bubble migration are not high enough at
∗
these temperatures. The critical concentration of implanted He, C (expressed CHe∗
He
through the He-metal atomic ratio in the T/T ≤ 0.4 range), at which surface
m
deformation begins, is determined empirically as
T
C * He = 0.5 − .
T m CHe*=0.5−TTm.
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2
Blistering stops at ∼10 atom/cm . This phenomenon may show up –
though at much higher critical doses – when metals are bombarded by H-ions.
Blistering may also have a non-radiation origin.
Another source of impurities are the numerous unipolar microarcs occur-
ring on the coating surface. As a matter of fact, they become a dominant source
of impurities at very high stresses. The microarcs occur because the electrons
travel at a much higher speed than the ions. As a result, any electrode contacting
with the plasma gets a negative ‘floating’ potential
kT m
U = e ln i ,
p
e 2 2 πm e Up=kTe2elnmi2πme,
where T is the plasma electron temperature and m is the ion mass. In hydro-
e
i
gen plasma, U ≈ 3kT , the result being that at T = 10 eV, the potential is high
e
e
p
enough to initiate an arc between the electrode and the plasma. The discharge
current in unipolar arcs is generally 10–50 A; in most cases, it is close to 20 A.
Arcing is particularly intense during a plasma column formation (20–30 ms)
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and at the end of an operating cycle. Arc burning time is 0.1–50 µs. From 10
18
to 10 impurity atoms are emitted from the cathode spot into the plasma during
each pulse. On average, there are 30 elementary charges travelled through an
electric circuit for each evaporated cathode atom.
Unipolar microarc suppression methods mostly consist in the use of ultra-
high-vacuum-aided removal of oil films, dielectric implants, adsorbing layers,
and surface microrelief levelling, as shown in Fig. 6.1.
A flux of ‘runaway’ electrons with energies of a few hundreds of keV may
also cause impurity influx splashes. These arise at the early stage of a plasma
column generation and exist for 0.01–10 µs. ‘Runaway’ electrons produce a
total current of around 1 kA. Intensive flows of impurity atoms can be driven
by discharge current disruptions, when during 0.3–1 ms a plasma column shifts
towards the wall and strikes it, causing a local overheating, thermal desorption
and evaporation of the wall material.