The Fusion and Astrophysical Plasma Physics Group within the Physics Department and the Center for Astrophysics and Space Sciences at the University of California, San Diego is led by Professor Patrick H. Diamond, Distinguished Professor of Physics, and consists of two research scientists-- Mikhail Malkov, Fred Hinton, and two graduate student researchers-- and is managed by an administrative assistant.

**Identification of Predator-Prey Dynamics in Gyrokinetic Simulations** - *Sumire Kobayashi, Ozgur D. Gurcan and Patrick H. Diamond*

The interaction between spontaneously formed zonal flows and small-scale turbulence in nonlinear gyrokinetic simulations is explored in a shearless closed field line geometry. It is found that when clear limit cycle oscillations prevail, the observed turbulent dynamics can be quantitatively captured by a simple Lotka-Volterra type predator-prey model. Fitting the time traces of full gyrokinetic simulations by such a reduced model allows extraction of the model coefficients. Scanning physical plasma parameters, such as collisionality and density gradient, it was observed that the effective growth rates of turbulence (i.e., the prey) remain roughly constant, in spite of the higher and varying level of primary mode linear growth rates. The effective growth rate that was extracted corresponds roughly to the zonal-flow-modified primary mode growth rate. It was also observed that the effective damping of zonal flows (i.e., the predator) in the parameter range, where clear predator-prey dynamics is observed, (i.e., near marginal stability) agrees with the collisional damping expected in these simulations. This implies that the Kelvin-Helmholtz-like instability may be negligible in this range. The results imply that when the tertiary instability plays a role, the dynamics becomes more complex than a simple Lotka-Volterra predator prey.

To the memory of Professor Marshall N. Rosenbluth - an American plasma physicist and member of the National Academy of Sciences - who passed away on September 23, 2003.

We are working primarily on the theory of fusion plasma including:

- Nonlinear dynamics of plasmas and fluids
- Anomalous transport
- Self-organized criticality in confined plasma
- L-H transition, transport barrier physics
- Dynamo theory
- Nonlinear waves in space plasma

Our work is primarily funded by the Office of Fusion Energy Sciences of the U.S. Department of Energy.

Grant No. DE-FG02-04ER54738.