For nuclear fusion to power the world, the energy produced and the plasma used to create it must be contained within a generating device. Research from Khalifa University has unlocked new understanding into how plasma interacts with container solid surfaces such as those that may be used in nuclear reactors of the future.
Dr. Ioannis Kourakis, Associate Professor, has investigated the characteristics of a phenomenon that occurs when a superheated plasma interacts with a solid surface. With Dr. Mohammed Mohsen Hatami, Khaje Nasir Toosi University of Technology, Iran, Dr. Kourakis calculated how the boundary between the plasma and solid may be affected by different concentrations of positively charged ions in the plasma. The findings represent the first time researchers have been able to characterize the properties of the plasma sheath in a plasma containing three positive ion species. The results were published in Nature Scientific Reports.
Plasma is an interesting research challenge to scientists across disciplines. So much of the universe is made of plasma — it comprises over 99 percent of the visible universe as stars, nebulas, auroras, lightning and neon signs.
Plasma is often called the “fourth state of matter” after solid, liquid, and gas. When gas is sufficiently heated, the molecules get more energetic and excitable, moving around more and more freely. At a high enough temperature, the atoms themselves will break apart, with electrons separating from their nuclei, leaving behind charged particles known as ions amid a swirl of electrons. This is plasma.
“Plasmas are large ensembles of charged particles or ionized gases which either occur in various forms in nature or may be produced artificially in the laboratory,” Dr. Kourakis said. “In the laboratory, plasmas are fabricated in large chambers hosting electric discharge experiments, in which the plasma is separated from the wall surface by a thin positively charged region called the sheath. This arises from the difference in mobility between the ions and the electrons.”
The plasma sheath is formed when plasmas come in contact with solid surfaces. It balances the fluxes of fast electrons and slow ions to keep the plasma neutral. A plasma is a superheated gas with electron temperature usually equal to or higher than that of ions. Since electrons are much lighter than ions, they can escape from plasma at much faster speed if there is no confining potential barrier. Once the electrons are mostly depleted from the boundary interface between the plasma and the solid surface, a region with only positive ions and neutrons will be formed. This is the plasma sheath. At the same time, the solid surface becomes negatively charged relative to the plasma. As the potential increases, more and more electrons are reflected by the sheath.
Positive charges in the sheath can push more ions to diffuse out of plasma. Eventually, the loss rate of electrons and ions will reach an equilibrium.
“Although plasma-sheath formation is one of the oldest problems in plasma physics, it is still far from being thoroughly understood and attracts attention among researchers,” Dr. Kourakis said. “This is due to its importance in the modification of the surface properties of the materials, in excitation of electrostatic waves, and its relevance in magnetic confinement fusion plasma.”
In plasmas composed of different gases, the resulting ions may have different velocities. In a two-ion-species plasma derived from argon and xenon gases, the argon and xenon ions move at the same speed. When a third species is added, the first two species move at different speeds and this change is related to the concentration of the third ion species. How this impacts the plasma sheath remains unclear.
Dr. Kourakis and his collaborator investigated this phenomenon in plasma comprising argon, xenon and krypton ions. By combining mathematical investigations with kinetic modeling and experimental results, the researchers were able to accurately compute the velocity of the ion species at the sheath edge. This then allowed them to investigate their effect on the structure and properties of the plasma sheath.
“We found that increasing the krypton concentration causes a decrease in the space-charge density, which leads to a decrease in the sheath thickness and an increase in the sheath potential,” Dr. Kourakis said.
Understanding how the plasma sheath can be manipulated by adding different ion species to the plasma mixture could be crucial to our efforts to achieve nuclear fusion on Earth. Any attempt to harness the power of nuclear fusion requires containing plasma superheated to over 100 million degrees Celsius within the walls of a device. Plasma, meet solid surface. Plasma this hot and energetic is impossible to contain in any normal vessel, with current solutions involving magnetic fields to draw the plasma away from the walls and keep everything contained.
Much of this relies on controlling the direction of the ions in the plasma and reducing the volume of positive ions that collide with walls of the fusion containment device. The researchers’ results show different concentrations of ion species impact the collision force, which in turn impacts the width of the plasma sheath. These findings could help design new nuclear fusion experiments and reactors.
12 October 2022