Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Unexpected connection: Rotation reversal tied to energy confinement saturation

11.11.2011
News from the 53rd Annual Meeting of the APS Division of Plasma Physics

Research on the Alcator C-Mod experiment at MIT has made an unexpected connection between two seemingly unrelated but important phenomena observed in tokamak plasmas: spontaneous plasma rotation and the global energy confinement of the plasma.


Plasma fluctuations, that accompany the low density, low confinement regime and disappear promptly at the rotation reversal may provide the link between these two seemingly disparate phenomena. Credit: John Rice, MIT

Self-generated flows, the spontaneous plasma rotation which arises even when there is no external momentum input, can have a strong beneficial effect on plasma transport and stability. But in a fusion reactor, unlike most current experiments, there will be little or no external rotation drive—thus it is crucial to understand and be able to predict plasma rotation under these conditions. In the discharges studied at Alcator C-Mod, the flows can reverse direction at a very precise transition point, depending on plasma density and current.

This flow reversal turns out to be tightly connected to the global energy confinement of the plasma. Since the early days of tokamak research (1970s), it was known that the energy confinement time (the ratio of the energy content of the plasma and the total input power) increased as the density of the plasma increased. This generated a great deal of excitement since one of the goals of fusion research was to operate at high density with good confinement, so this improvement was a bonus. At high enough density, however, the energy confinement stopped increasing.

The results of these experiments suggest that energy confinement and rotation reversal are closely related. At low density, where the energy confinement time increases with the density, the plasma rotates in one direction at roughly +5 km/s. Then at the critical density, the rotation direction reverses direction to values around -20 km/s and the energy confinement saturates. The critical density depends on plasma conditions, increasing with plasma current and decreasing with machine size.

These observations reveal the fundamental connection between the two phenomena and how they both depend on the nature of the underlying plasma turbulence. One hypothesis is that at low density, the turbulence is driven by trapped electron modes, which strongly degrade the confinement and which propagate in a particular direction. As the density is raised, these modes are suppressed, and turbulence driven by ion temperature gradients dominates. These modes at higher density regulate the confinement and propagate in the opposite direction. Evidence for this explanation is emerging from careful measurements of plasma fluctuations.

Abstract:

NI2.00006 Rotation Reversal and Energy Confinement Saturation in Alcator C-Mod Ohmic L-mode Plasmas: A Novel Transport Bifurcation
Session NI2: Transport of Momentum and Particles,
Ballroom BD, Wednesday, November 16, 2011, 12:00PM:30PM

Saralyn Stewart | EurekAlert!
Further information:
http://www.aps.org

More articles from Physics and Astronomy:

nachricht Basque researchers turn light upside down
23.02.2018 | Elhuyar Fundazioa

nachricht Attoseconds break into atomic interior
23.02.2018 | Max-Planck-Institut für Quantenoptik

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Attoseconds break into atomic interior

A newly developed laser technology has enabled physicists in the Laboratory for Attosecond Physics (jointly run by LMU Munich and the Max Planck Institute of Quantum Optics) to generate attosecond bursts of high-energy photons of unprecedented intensity. This has made it possible to observe the interaction of multiple photons in a single such pulse with electrons in the inner orbital shell of an atom.

In order to observe the ultrafast electron motion in the inner shells of atoms with short light pulses, the pulses must not only be ultrashort, but very...

Im Focus: Good vibrations feel the force

A group of researchers led by Andrea Cavalleri at the Max Planck Institute for Structure and Dynamics of Matter (MPSD) in Hamburg has demonstrated a new method enabling precise measurements of the interatomic forces that hold crystalline solids together. The paper Probing the Interatomic Potential of Solids by Strong-Field Nonlinear Phononics, published online in Nature, explains how a terahertz-frequency laser pulse can drive very large deformations of the crystal.

By measuring the highly unusual atomic trajectories under extreme electromagnetic transients, the MPSD group could reconstruct how rigid the atomic bonds are...

Im Focus: Developing reliable quantum computers

International research team makes important step on the path to solving certification problems

Quantum computers may one day solve algorithmic problems which even the biggest supercomputers today can’t manage. But how do you test a quantum computer to...

Im Focus: In best circles: First integrated circuit from self-assembled polymer

For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.

In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...

Im Focus: Demonstration of a single molecule piezoelectric effect

Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale

Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

2nd International Conference on High Temperature Shape Memory Alloys (HTSMAs)

15.02.2018 | Event News

Aachen DC Grid Summit 2018

13.02.2018 | Event News

How Global Climate Policy Can Learn from the Energy Transition

12.02.2018 | Event News

 
Latest News

Basque researchers turn light upside down

23.02.2018 | Physics and Astronomy

Finnish research group discovers a new immune system regulator

23.02.2018 | Health and Medicine

Attoseconds break into atomic interior

23.02.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>