One of the most natural things to see is the flickering of a candle flame. The flicker is caused by a chemical reaction in the weak plasma flame. It is a simple example of a plasma instability. Other types of instabilities are driven by electrical charges alone. These reaction are much faster, too fast for the human eye to see. Sometimes instabilities do not impact the process, but in many industrial applications plasma instabilities are a serious limit to the potential for plasma processes to work effectively.
A great example of a basic plasma control system is the remote plasma in PPPL Laboratory in Princetown. This plasma can be run remotely online and controlled over the internet. This shows that scientist are actively working on plasma control to remove the flicker and increase the useful applications for plasma technology.
Plasma technology in rapidly replacing many processes that have been used for many centuries, even millenia. For example, cutting has been carried out by blades, ranging from stone edges to stainless steel blades. Several decades ago, plasma began to be used to treat the surface of steel blades to produces hardened surfaces. In recent years the blade is being replaced directly by a plasma as plasma cutters, and etchers are used to cut metals and even silicon and glass.
As plasma technology advances into new applications, plasma control has become a major issue for many of these application. Ideally, engineers would like to measure the plasma density and increase or decrease the power applied to maintain a constant density. In reality, plasmas react to many parameters in a complex and often poorly understood way, so that such direct control is extremely difficult . For example, in fusion reactors the plasmas reacts to magnetic fields, but the currents in a plasma also create a magnetic field. The current flow is a maximum where the plasma density is highest, often the plasma centre, so the current will travel through the high density region. This current then creates a magnetic field around this high density area which tends to confine electrons in that region leading to a further increase in ionization and plasma density. This type of positive feedback is very common in plasma and can lead to arcs forming, it is this exact same mechanism that creates a lighting bolt. Another example would be fusion plasma where the plasma is confined in a magnetic bottle, such as the Tokamak and the high central current leads to a sawtooth instability., which is a relaxation-oscillation of the core plasma and is ubiquitous in most large plasma devices.
Run the media clip below to see how a magnet field effects a density plasma.
The above media clip from PPPL shows how unstable a plasma can be and how hard it is to maintain a constant process.
In a Tokamak, the sawtooth cycle starts with an increasing current in the center of the plasma, this produces a high magnetic shear and pressure and lower q. To avoid instabilities a safety factor has to be maintained above one. The safety factor q, is the ratio of the times a particular magnetic field line travels around a toroidal confinement area’s “long way” (toroidally) to the “short way” (poloidally). The prevent sawtooth instabilities a mechanism using additional heating creates what is called a dynamo effect. This dynamo effect establishes a voltage in the central part of the tokamak that opposes the applied drive voltage. This reduces the current or flattens the current profile. The q factor is maintained close to 1, and a saw tooth free plasma can be maintained. The mechanism, called flux pumping solves one of the biggest problems with controlled fusion plasma. Stable fusion plasmas will allow the generation of power in a process that mimics the reaction that drives the sun. This is the combination of two hydrogen atoms to form helium. Helium is a stable nucleus and is not radioactive. So, the fusion process has less environmental impact than normal nuclear reactors which use fission to break up heavy atoms like uranium to form radioactive by-products. The fusion reaction is more efficient, with each reaction producing much more energy than a fission reaction: The difference is the same as between an atomic bomb which uses fission and a hydrogen bomb which uses fusion. But, the reactor needs to control the reaction, fission reactions happen easier and tend to run away and melt down the reactor, whereas a fusion reaction is harder to get going and the reaction stops if there is a breakdown. It cannot runaway.
A recent paper outlines details of the most recent work on controlling Fusion plasma. This work is regarded as a significant breakthrough in that the modelling has led to a sufficient understanding of the mechanism that the researchers are now confident that they can engineer a stable sun in a magnetic bottle, this earth bound plasma will be more stable and engineered to remain under control.
Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations
Krebs, S. C. Jardin, S. Gunter, K. Lackner, M. Hoelzl, E. Strumberger,
and N. Ferraro.
from scientist at Max-Planck/Princeton Research Center for Plasma Physics and
Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543, USA and the Max Planck Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany.
which was Received 16 June 2017 and accepted for publication after peer review on the 5 September 2017; published online 27 September 2017.
the paper shows the results modelling a self-regulating magnetic flux pumping mechanism in tokamaks that maintains the core safety factor at q = 1, thus preventing sawteeth, is analyzed in nonlinear 3D magnetohydrodynamic simulations using the M3D-C1 code.
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Mike B Hopkins