REPOSITORY > RESULTS

Abstract

In this thesis we present the development of the device for the removal of carbon deposits in
fusion reactors. Amorphous hydrogenated carbon deposits (a-C:H) represent a major problem
for smooth operation of future fusion reactors with divertors containing carbon tiles,
because they cause retention of radioactive tritium. There are techniques which can inhibit
the growth of a-C:H deposits, but these techniques, however, do not prevent deposition
completely. Therefore the occasional removal of a-C:H deposits in future fusion reactors
will be necessary. We investigated the possibility of cleaning a-C:H deposits in tokamaks
with neutral oxygen atoms. For this purpose we have developed a large plasma reactor,
where inductively coupled plasma is generated with radiofrequency generator. Removal of
a-C:H deposits with neutral oxygen atoms over large areas requires the development of a
huge source of oxygen atoms. Knowledge, required for the development of large plasma
system, was obtained on the smaller plasma system.
First a comprehensive characterization of small plasma reactor was made. In the small
reactor inductively coupled plasma was created with the use of 1.2 kW radiofrequency generator.
The behavior of some key discharge and plasma parameters of oxygen plasma as
a function of pressure and generator power was investigated. Density of neutral oxygen
atoms was measured by a standard catalytic probe. The systematic measurements were
performed in order to study the transitions between E- and H-mode. Approximately 35 cm
from the edge of the excitation coil the densities were around 1×10[SUP]21 m−3. The neutral
oxygen atom densities in H-mode were approximately two times higher than in E-mode.
The dissociation fraction in the vicinity of the catalytic probe was about ∼ 5 %. Hysteresis
behavior of oxygen plasma was investigated during continuous excitation with radiofrequency
generator. Hysteresis was observable at pressures higher than 15 Pa and the width
of hysteresis was increasing linearly with pressure.
Knowledge, obtained on the smaller plasma system, was used for the development of a
large plasma system. The discharge tube was 2 m long with the diameter of 20 cm, and
the excitation of inductively coupled plasma was performed with 8 kW radiofrequency generator.
First an optimization between high-frequency generator and low pressure plasma
was made with impedance matching network and dual-excitation coil. For optimization we
used the Smith chart and a reflection coefficient calculation of an electric circuit of inductively
coupled plasma. The suitability of dual-excitation coil was confirmed by comparing
oxygen emission line intensities of plasma generated in normal and in the dual-excitation
coil. The light intensity of plasma generated in dual-excitation coil at a certain coil voltage
was about 4-times higher than the light intensity of plasma generated in normal excitation
coil.
The after glow chamber was connected to the discharge tube with approximately 2 m
long glass connecting tube of a complex shape. Systematic measurements of the neutral
oxygen atom density, using standard catalytic probe, lead to the conclusion, that for certain
generator power there is an optimum pressure at which the maximum neutral oxygen atom
density is reached. In the after glow chamber the densities were up to 2,5×10[SUP]20 m−3 and
the dissociation fraction was up to 2 %. By comparing the neutral oxygen atom densities
on the edge of the discharge tube, measured with an optical catalytic probe, to the densities
measured in an after glow chamber, it was shown that the ratio between these two densities
is highly dependent on the effective pumping speed. The optimal effective pumping speed
was approximately equal to the conductance of the connecting tube. At the optimal effective
pumping speed the ratio between densities on the edge of the discharge tube and in an after
glow chamber was about ∼ 20. It was also shown, that the neutral atom density in the
after glow chamber can be increased, if a small amount (∼ 20 %) of noble gas is added to
oxygen.
The last set of measurements was dedicated to the study of a-C:H etching with neutral
oxygen atoms. Suitable etching rates were reached (up to 35 nm/s). It was found that the
etching rates of a-C:H deposits with neutral oxygen atoms are exponentially dependent on
sample temperature and linearly dependent on the density of neutral oxygen atoms. It was
shown, using AES profile analysis, that mixed deposits a-C:H:W can also be removed, but
at higher temperatures and higher neutral oxygen atom density is needed.
The original scientific achievements of this work are: neutral oxygen atom density measurements
in transitions from E- to H-mode, electrical and plasma parameters measurements
during hysteresis behavior of inductively coupled oxygen plasma, the study of the
connecting tube conductance and the effective pumping speed impact on the transfer of
neutral oxygen atoms, the detailed study of a-C:H deposits etching rates and the improved
measuring procedure for the systematical measurements of the atom densities with catalytic
probes.