Physics of RF heating systems on proto-MPEX

Piotrowicz, Pawel A.

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https://hdl.handle.net/2142/105619 Copy

Description

Title

Physics of RF heating systems on proto-MPEX

Author(s)

Piotrowicz, Pawel A.

Issue Date

2019-06-28

Director of Research (if dissertation) or Advisor (if thesis)

Ruzic, David N

Doctoral Committee Chair(s)

Ruzic, David N

Committee Member(s)

• Andruczyk, Daniel
• Caughman, John
• Curreli, Davide
• Jin, Jianming

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Proto-MPEX, plasma waves, plasma physics, plasma material interactions, helicon, plasma source, plasma heating, RF heating, ion cyclotron heating, linear plasma source, ion heating

Abstract

Realizing controlled fusion as a commercial energy source is faced with many challenges. One of the main challenges being the development of Plasma Facing Components (PFC) that can survive the extreme environment encountered in a fusion reactor. To expedite the testing and development of PFCs Oak Ridge National Laboratory (ORNL) is building the Materials Plasma Exposure eXperiment (MPEX), which is a linear device purposed specifically for studying Plasma Material Interactions (PMI). Current linear devices cannot produce plasmas with fusion divertor relevant electron and ion temperatures and instead rely on electrostatic biasing of the target to simulate the relevant ion energies. This methodology inhibits studying the interaction of the eroded material and recycled neutral gas with a fusion relevant divertor plasma and does not properly simulate the angular energy distribution of the ion fluxes, therefore, PMI studies on these linear devices omit a vast amount of rich physics important to PFC development. MPEX will enable the study of fusion relevant PMI by producing fusion divertor relevant plasma conditions in front of a target station using RF technology. Proto-MPEX is the device that is currently operating at ORNL, where the viability of this RF technology is being demonstrated. The electron density, electron temperature, and ion temperature of the target plasma will be controlled independently with separate RF heating systems. This thesis focuses on the electron density production system and the ion heating systems on Proto-MPEX and their viability for MPEX. The electron density production on Proto-MPEX is accomplished by a helicon plasma source. Helicon plasma sources have been shown to efficiently produce high-density plasmas for a relatively low amount of RF power. Efficient electron density production of helicon plasma sources in light ions is hypothesized to be enabled by strong core power deposition when the plasma conditions allow for the formation of helicon normal modes. Experimental evidence supporting this hypothesis is presented in the form of B-dot probe and IR camera measurements, showing the increase of on-axis RF magnetic field strength and the formation of eigenmode structure concurrently with an increase in core power deposition at the expense of power deposition in the periphery of the plasma column. An RF full-wave model of the helicon region is made, which predicts the formation of cavity-like structures of the RF magnetic field when core power deposition is increased. Next, the problem that Proto-MPEX’s helicon source has been shown to not operate efficiently at higher magnetic field strengths is addressed. The hypothesis that the power balance does not allow enough density production for the helicon antenna to sustain a mode of operation that enhances core power deposition is tested by coupling a 2D axisymmetric full-wave simulation of the helicon antenna to a volume integrated 0D power/particle balance. This model is compared to experimental measurements of electron density and shows that there is a decrease in electron density production due to a decrease in core power coupling in the region where electron density decreases on Proto-MPEX. The model shows that if the Proto-MPEX helicon plasma source is operated at even higher magnetic fields strengths than efficient electron density production is recovered. Finally, the performance of Proto-MPEX is compared with other experimental devices by calculating the ionization cost of the plasma source, which shows that improvements to efficiency can practically be achieved to match the ionization cost of other plasma sources. Experimental improvements to the helicon source region are suggested and quantified with the couple RF and particle/power balance model. The ion heating on Proto-MPEX is accomplished by ion cyclotron heating via the beach heating technique. This technique is expected to increase ion temperatures on Proto-MPEX to values of Ti = 20 eV or more. The beach heating technique has been successfully demonstrated on previous devices, however, these devices were operating at much lower electron densities than Proto-MPEX. A theoretical route to core ion heating is first explored. At magnetic field strengths near the ion cyclotron resonance, the higher electron density in Proto-MPEX brings the existence of the Alfv ́en resonance into the Proto-MPEX plasma. This layer acts to cut-off the cold plasma slow wave, called the inertial Alfv ́en wave for the high-density core plasma. On the high-density side of the Alfv ́en resonance, the kinetic Alfv ́en wave can propagate when the electron temperature allows. In Proto-MPEX this wave is thought to be responsible for the core heat- ing of ions in the device. A simplified kinetic plasma tensor is implemented in COMSOL to simulate the propagation of this wave and to show that at Proto-MPEX relevant conditions this wave is responsible for core heating of the ions. Next, experimental evidence for core ion heating is presented in the form of ion temperature and target heat flux measurements. However, the core heating is shown to transiently cool, which is proposed to be due to the charge exchange with neutral gas born from plasma recombination at the target. When the target material is changed from carbon to stainless steel, the heat flux at the target reaches an increased steady state and higher ion temperatures are measured throughout the plasma column. The axial peak of the ion temperature is also located closer to the target for the case of the stainless steel target. These phenomena are hypothesized to be due to the increased reflection coefficient of the target material, and a model for quantifying this hypothesis shows that the flux of energetic neutral particles born from the reflection of sheath accelerated ions could explain these observations. Finally, experimental optimization of the magnetic field in the ICH region is shown with COMSOL simulations showing good agreement with these experimental results. The numerical simulations are then used to explore the parameter space of driving frequency, antenna length, and distance of the antenna to the ion cyclotron resonance on the predictions of heating in the ion cyclotron region.

Graduation Semester

2019-08

Type of Resource

text

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Copyright 2019 Pawel Piotrowicz

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