Page 205 - Fundamentals of Magnetic Thermonuclear Reactor Design

Vacuum and Tritium System Chapter | 6 187

The flow of impurities coming to the plasma from the wall is proportional to
the flow of electrons incident on the wall from the plasma:
W
W e
γ
N imp =⋅ ≈ γ ⋅ α
e
E e bnd E e bnd Nimp=γ⋅WeEe bnd≈γe⋅WαEe bnd,
where W and W are the electron flow power and α-particle flow power, respec-
α
e
tively, E is the electron energy in the boundary area, and γ is the amount of
e
e bnd
impurities knocked out of the wall by incident electron. The values of γ vary
e
widely depending on the type of material, impurity content, surface condition
and absorbed dose density, D. In the course of irradiation, the flow of desorbed
−7
atoms declines according to the empirical formula γ ≈ 4.8 × 10 D −0.63 . The
e
formula is valid for pure stainless steel. Stimulated gas emission also declines if
the surface is cleaned with a glow discharge. After exposure to open air, initial
γ is close to 0.1.
e
−7
At γ ≤ 10 , stimulated gas emission is insignificant compared with the
e
−4
wall sputtering. However, at γ ≥ 10 , radiation losses from oxygen atoms
e
prevail in plasma. This highlights the importance of an adequate selection of
the structural material, its production process and methods for its surface con-
ditioning as part of the MFR vacuum chamber manufacturing and operation
process.
6.4 PLASMA IMPURITY CONTROL
6.4.1 Sources of Impurities
The sources of impurities in hydrogen plasma include the following:
l He synthesis,
l gas emission from vacuum chamber structural and functional materials,
l impurities contained in a fuel mixture,
l impurities delivered to the chamber from reactor systems via the shared
vacuum pumping duct (e.g. neutral beam injectors, diagnostics equipment,
etc.),
l uncontrolled leaks in in-vessel components’ cooling channels), and
l plasma’s corpuscular and electromagnetic radiation impacting the FW.
Under stationary operating conditions, the balance of synthesised α-particles
and He atoms pumped out of the system is described by

( − R1 He ) [n He He ]V = cn n < σν >, 1−RHenHeVτHe=cnDnT<σν>,
DT
τ
where [n ] is the acceptable He concentration in the plasma, R is the pro-
He
He
portion of He atoms reflected by the wall and returned to the plasma and
τ is the α-particle hold time [11]. By setting the [n ] and τ values, one
He
He
He
can measure the return coefficient finite value. For instance, if n[ He ] n / pl = 0.1 [nHe]/npl=0.1