20 research outputs found
Parallel transport modeling of linear divertor simulators with fundamental ion cyclotron heating
The Material Plasma Exposure eXperiment (MPEX) is a steady state linear
device with the goal to perform plasma material interaction (PMI) studies at
future fusion reactor relevant conditions. A prototype of MPEX referred as
Porto-MPEX is designed to carry out research and development related to source,
heating and transport concepts on the planned full MPEX device. The auxiliary
heating schemes in MPEX are based on cyclotron resonance heating with radio
frequency (RF) waves. Ion cyclotron heating (ICH) and electron cyclotron
heating (ECH) in MPEX are used to independently heat the ions and electrons and
provide fusion divertor conditions ranging from sheath-limited to fully
detached divertor regimes at a material target. A Hybrid Particle-In-Cell code-
PICOS++ is developed and applied to understand the plasma parallel transport
during ICH heating in MPEX Proto-MPEX to the target. With this tool, evolution
of the distribution function of MPEX/Proto-MPEX ions is modeled in the presence
of (1) Coulomb collisions, (2) volumetric particle sources and (3) quasi-linear
RF-based ICH. The code is benchmarked against experimental data from Proto-MPEX
and simulation data from B2.5 EIRENE. The experimental observation of
density-drop near the target in Proto-MPEX and MPEX during ICH heating is
demonstrated and explained via physics-based arguments using PICOS++ modeling.
In fact, the density drops at the target during ICH in Proto-MPEX/MPEX to
conserve the flux and to compensate for the increased flow during ICH.
Furthermore, sensitivity scans of various plasma parameters with respect to ICH
power are performed for MPEX to investigate its role on plasma transport and
particle and energy fluxes at the target. Finally, we discuss the pathway to
model ECH in MPEX using the Hybrid PIC formulation herein presented for kinetic
electrons and fluid ions
Helicon wave propagation and plasma equilibrium in high-density hydrogen plasma in converging magnetic fields
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 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
(0.5Ă—10^19 m-3) into the target region where it reaches maximum
density (2-3Ă—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
Wave modelling in a cylindrical non-uniform helicon discharge
A radio frequency (RF) field solver based on Maxwell's equations and a cold
plasma dielectric tensor is em- ployed to describe wave phenomena observed in a
cylindrical non-uniform helicon discharge. The experiment is carried out on a
recently built linear plasma-material interaction machine: the MAGnetized
Plasma In- teraction Experiment (MAGPIE) [B. D. Blackwell, J. F. Caneses, C.
Samuell, J. Wach, J. Howard, and C. S. Corr, submitted on 25 March 2012 to
Plasma Sources Science and Technology], in which both plasma density and static
magnetic field are functions of axial position. The field strength increases by
a factor of 15 from source to target plate, and plasma density and electron
temperature are radially non-uniform. With an enhancement factor of 9.5 to the
electron-ion Coulomb collision frequency, 12% reduction in the antenna radius,
and the same other conditions as employed in the experiment, the solver
produces axial and radial profiles of wave amplitude and phase that are
consistent with measurements. Ion-acoustic turbulence, which can happen if
electron drift velocity exceeds the speed of sound in magnetized plasmas, may
account for the factor of 9.5 used to match simulated results with experimental
data. To overcome the single m vacuum solu- tion limitations of the RF solver,
which can only compute the glass response to the same mode number of the
antenna, we have adjusted the antenna radius to match the wave field strength
in the plasma.(not finished because of the limited number of characters, please
see the full paper)Comment: Submitted to Physics of Plasmas on 30 April 201
RF compensation of double Langmuir probes: modelling and experiment
An analytical model describing the physics of driven floating probes has been developed to model the RF compensation of the double langmuir probe (DLP) technique. The model is based on the theory of the RF self-bias effect as described in Braithwaite's work [1], which we extend to include time-resolved behaviour. The main contribution of this work is to allow quantitative determination of the intrinsic RF compensation of a DLP in a given RF discharge. Using these ideas, we discuss the design of RF compensated DLPs. Experimental validation for these ideas is presented and the effects of RF rectification on DLP measurements are discussed. Experimental results using RF rectified DLPs indicate that (1) whenever sheath thickness effects are important overestimation of the ion density is proportional to the level of RF rectification and suggest that (2) the electron temperature measurement is only weakly affected
Initial characterization of Argon plasmas in the "MAGnetized Plasma Interaction Experiment" (MAGPIE)
RF magnetic fluctuation probes and symmetric double Langmuir probes have been utilized to characterize Argon helicon plasma in a converging magnetic field. We observe that when enough RF power is supplied an Ar II blue core mode is formed suggestive of t
Design and characterization of the Magnetized Plasma Interaction Experiment (MAGPIE): A new source for plasma-material interaction studies
The Magnetized Plasma Interaction Experiment (MAGPIE) is a versatile helicon source plasma device operating in a magnetic hill configuration designed to support a broad range of research activity and is the first stage of the Materials Diagnostic Facility at the Australian National University. Various material targets can be introduced to study a range of plasma-material interaction phenomena. Initially, with up to 2.1kW of RF at 13.56MHz, argon (1018-1019m3) and hydrogen (up to 1019m3 at 20kW) plasma with electron temperature 3-5eV was produced in magnetic fields up to 0.19T. For high mirror ratio we observe the formation of a bright blue core in argon above a threshold RF power of 0.8kW. Magnetic probe measurements show a clear m=+1 wave field, with wavelength smaller than or comparable to the antenna length above and below this threshold, respectively. Spectroscopic studies indicate ion temperatures <1eV, azimuthal flow speeds of 1kms1 and axial flow near the ion sound speed
Wave modelling in a cylindrical non-uniform helicon discharge
A radio frequency (RF) field solver based on Maxwell's equations and a cold plasma dielectric tensor is employed to describe wave phenomena observed in a cylindrical nonuniform helicon discharge
An experimentally constrained MHD model for a collisional, rotating plasma column
A steady-state single fluid MHD model which describes the equilibrium of plasma parameters in a collisional, rotating plasma column with temperature gradients and a non-uniform externally applied magnetic field is developed. Two novel methods of simplifying the governing equations are introduced. Specifically, a 'radial transport constraint' and an ordering argument are applied. The reduced system is subsequently solved to yield the equilibrium of macroscopic plasma parameters in the bulk region of the plasma. The model is benchmarked by comparing these solutions to experimental measurements of axial velocity and density for a hydrogen plasma in the converging-field experiment MAGPIE and overall a good agreement is observed. The plasma equilibrium is determined by the interaction of a density gradient, due to a temperature gradient, with an electric field. The magnetic field and temperature gradient are identified as key parameters in determining the flow profile, which may be important considerations in other applications.This work was supported by
Australian ARC project DP1093797 and FT0991899.
MAGPIE construction and operation was funded under the
NCRIS scheme of the Australian Government