Caneses Marin, Juan Francisco
Description
In this thesis, we investigate wave propagation and plasma
equilibrium in MAGPIE, a helicon based linear plasma device
constructed at the Australian National University, to study
plasma-material interactions under divertor-relevant plasma
conditions. We show that MAGPIE is capable of producing low
temperature (1–8 eV) high density hydrogen plasma (2–3×10^19
m-3) with 20 kW of RF power when the confining magnetic field is
converging.
The original research...[Show more] herein described comprises: (1)
Characterization of hydrogen plasma in MAGPIE, (2) Analysis of
the RF compensation of double Langmuir probes, (3) Excitation,
propagation and damping of helicon waves in uniform and
non-uniform magnetic fields and (4) Steady-state force balance
and equilibrium profiles in MAGPIE.
We develop an analytical model of the physics of floating probes
to describe and quantify the RF compensation of the DLP
technique. Experimental validation for the model is provided. We
show that (1) whenever finite sheath effects are important,
overestimation of the ion density is proportional to the level of
RF rectification and suggest that (2) electron temperature
measurements are weakly affected.
We develop a uniform plasma full wave code to describe wave
propagation in MAGPIE. We show that under typical MAGPIE
operating conditions, the helical antenna is not optimized to
couple waves in the plasma; instead, the antenna’s azimuthal
current rings excites helicon waves which propagate approximately
along the whistler wave ray direction, constructively interfere
on-axis and lead to the formation of an axial interference
pattern. We show that helicon wave attenuation can be explained
entirely through electron-ion and electron-neutral collisions.
Results from a two-dimensional full wave code reveal that RF
power deposition is axially non-uniform with both edge and
on-axis components associated with the TG and helicon wave
respectively.
Finally, force balance analysis in MAGPIE using a two-fluid
“Braginskii” type formalism shows that the electron fluid
exists in a state of dynamic (flowing) equilibrium between the
electric, pressure and thermal forces. The pressure gradient,
driven by the non-uniform RF heating, accelerates the plasma into
the target region to velocities close to the ion sound speed.
From the measured axial plasma flux we find that the plasma
column in MAGPIE can be divided into an ionizing and a
recombining region. For the conditions investigated, a large
fraction of the plasma created in the ionizing region is lost in
the recombining region and only a small fraction reaches the end
of the device. The equilibrium plasma density along the length
of
MAGPIE can be quantitatively explained using a 1D transport
calculation which includes volumetric particle sources and
magnetic compression. We show that the plasma is transported, by
the electron pressure gradient, from under the antenna
(0.5×10^19 m-3) into the target region where it reaches maximum
density (2-3×10^19 m-3).
Using the results herein presented, this thesis explores the
relationship between the RF power deposition in MAGPIE, parallel
plasma transport and the production of high density plasma in the
target region.
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