Precise particle placement: Manipulating beam ion phase space to improve tokamak performance

David Pace

General Atomics

Friday, April 21, 2017

3:00pm

NW17-218

PSFC Seminars

Abstract: The presence of energetic ions, produced by fusion reactions and auxiliary heating, in tokamaks creates opportunities for improving plasma performance and stability. Steady state plasma scenarios, including those of ITER, are being developed with neutral beam injection as a heating and current drive source. The energetic ions resulting from beam injection are a non-Maxwellian population capable of driving plasma waves through a resonant energy exchange that can increase radial transport of the ions, thereby reducing beam heating and current drive. New experiments demonstrate that neutral beam injection can be tailored to populate ion phase (velocity) space such that the detrimental effects resulting from ion-wave interactions are reduced or eliminated. The DIII-D neutral beam system has been recently upgraded to allow for independent control of the neutral beam current (I) and voltage (V) during plasma shots. This first-ever capability greatly increases the parameter space of injected beam power and torque available for experiments. Shear Alfven waves that are typically excited by beam ions are shown to be largely avoided when the beam voltage, i.e., ion velocity, is reduced (at constant power) such that the ions are no longer resonant with the wave structure. The physics of energetic ion interactions with plasma instabilities in tokamak experiments will be presented, along with initial results using pre-programmed beam IV waveforms to manipulate energetic ion phase space to control these interactions. A further-in-the-future effort to develop phase space tailoring of the fusion-produced alpha particle population will also be described. That task involves the use of spin polarized fuel to increase the fusion reaction rate (through an effective increase in fusion cross-section) and control the birth velocity space distribution of fusion alphas. Modeling shows that a D+He3 test in DIII-D can demonstrate the survivability of polarized fuel in the tokamak environment. This work supported by the U.S. Department of Energy under award DE-FC02-04ER54698.

Bio: Dr. David Pace earned his degree in experimental plasma physics at the University of California-Los Angeles. He conducted postdoctoral research on energetic particle physics at the DIII-D and Alcator C-Mod tokamaks before joining the General Atomics Magnetic Fusion Energy group. His recent work involves energetic ion transport due to coherent plasma waves, including the measurement of energetic ion fluxes at the tokamak wall.