February 16, 2012 | 1 Comment
Paul Bellan, professor of applied physics in the Division of Engineering and Applied Science at the California Institute of Technology (Caltech) and graduate student Auna Moser have discovered a surprising phenomenon about basic plasma behavior.
Plasma is a gas so hot that atoms are stripped of their electrons. The plasma jets act like electrical wires providing a throughway for speeding electrons.
At the cosmic scale hunks of plasma, called coronal mass ejections can escape the sun because of a process of magnetic field lines breaking and merging with other lines, which is called magnetic reconnection. When those field lines are broken and reconnecting considerable energy is released. At the same time huge chunks of plasma from the sun’s surface can escape and some find a path toward Earth.
The coronal mass ejections headed Earth’s way can also snap the Earth’s magnetic field lines, causing charged particles to speed toward Earth’s magnetic poles. That in turn sets off the shimmering light shows we know as the northern and southern lights.
There is a lot of energy involved. Magnetic reconnection is a key issue in developing thermonuclear fusion as a future energy source using plasmas in the laboratory. It would seem that Eric Lerner’s work with Focus Fusion at Lawrenceville Plasma Physics is exploiting the behavior of micro plasma to get make microfusion a practical power source. That may make the Caltech work highly significant.
Bellan and Moser use high-speed cameras to look at jets of plasma in the lab and have discovered a surprising phenomenon that provides clues to just how magnetic reconnection occurs. They describe their results in a paper published in the February 16 issue of the journal Nature.
For their experiment Moser fired jets of hydrogen, nitrogen, and argon plasmas at speeds of about 10 to 50 kilometers per second across a distance of more than 20 centimeters in a vacuum. A run of the experiment requires 200 million watts of power to produce jets that are a scorching 20,000 degrees Kelvin and carry a current of 100,000 amps. To study the jets, Moser used cameras that can take a snapshot in less than a microsecond, or one millionth of a second.
The motion of the flowing electrons in the plasma jet generates a magnetic field, which then exerts a force on the plasma. These electromagnetic interactions between the magnetic field and the plasma can cause the plasma jet to twist up forming a rapidly expanding corkscrew. The behavior is called “kink instability”.
Moser looked closely at this behavior in her experimental plasma jets and saw something entirely unexpected, more often than not the corkscrew shape that developed in her jets grew exponentially and extremely fast, forming 20 centimeter long coils in just 20 to 25 microseconds. She also noticed tiny ripples began appearing on the inner edge of the coil just before the jet broke right at the moment when there was a magnetic reconnection.
Moser and Bellan say that at the start they did not know what they were seeing, but they knew it was strange. “I thought it was a measurement error,” Bellan says. “But it was way too reproducible. We were seeing it day in and day out. At first, I thought we would never figure it out.” Months of additional experiments determined that the kink instability actually spawns a completely different kind of phenomenon, called a Rayleigh-Taylor instability.
The Rayleigh-Taylor instability happens when a heavy fluid sitting on top of a light fluid tries to trade places with the light fluid. Ripples form and grow at the interface between the two, allowing the fluids to swap places. What Moser and Bellan realized is that the kink instability creates conditions that give rise to a Rayleigh-Taylor instability. As the coiled plasma expands due to the kink instability it accelerates outward. In these experiments it isn’t gravity drawing the heavier atoms, rather the plasma tries to swap places with the trailing vacuum forming ripples that then expand. Moser and Bellan learned the instability shown by the by the ripples on the trailing side of the accelerating plasma forms up in about a microsecond.
Connecting the two events is quite an achievement. Kink instability has been researched nearly 60 years and Rayleigh-Taylor instability for more than 100. Until now no one’s considered the possibility that Rayleigh-Taylor instability would be caused by kink instability. Bellan observes the two types of instabilities are so different that to see them so closely coupled was a shock. “Nobody ever thought there was a connection.”
The team’s observations note that the two instabilities occur at very different scales, while the twisted coil created by the kink instability spans about 20 centimeters, the Rayleigh-Taylor instability is much smaller, making ripples just two centimeters long and those smaller ripples rapidly erode the jet, forcing the electrons to flow faster and faster through a narrowing channel. Bellan explains, “You’re basically choking it off. Soon, the jet breaks, causing a magnetic reconnection.”
The observations may be worthwhile information as Lerner’s Focus Fusion develops fusion at the microscale.
At the macroscale on the sun’s surface the magnetic reconnection involves phenomena that span scales from a million meters to just a few meters. At the larger scales the physics seem relatively simple and straightforward. Moser and Bellan show at the microscale the physics becomes more subtle and complex and it is in this regime that magnetic reconnection takes place.
The key advance is being able to relate phenomena at large scales, such as the kink instability, to those at small scales, such as the Rayleigh-Taylor instability. Cautiously they note that although kink and Rayleigh-Taylor instabilities may not drive magnetic reconnection in all cases, this mechanism is a plausible explanation for at least some scenarios in nature and the lab.
Hopefully the Caltech team has developed information that will be useful for Lerner and other microscale plasma research. The Rayleigh-Taylor effects might just be a source of answers to problems or solutions for improving power results at Lawrenceville Plasma Physics. The observations could also apply to more mundane work such as plasma cutting and welding. Time will tell.