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Title	18: High Collisionless NBI Torque Drive for GAMs, aka the VH-mode path?
Name:	John deGrassie degrassie@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Turbulence & Transport	Presentation time:	Not requested
Co-Author(s):	George McKee, Terry Rhodes	ITPA Joint Experiment :	No
Description:	*Use high power NBI co-torque to transiently drive Geodesic Acoustic Modes and measure the plasma response and mode properties with BES. The model requires that "enough" prompt NBI radial current be injected to raise the E field "fast enough" so that the plasma rings in this fashion (see Background below). Actually, the proposed target plasma and suddenly switched-on NBI level are reminiscent of the VH-mode recipe. *Counter-Ip operation with the off axis beam will be evaluated as a possible enhanced prompt E-field driver. However, BES will hopefully be a critical diagnostic.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	*Select a target plasma with low collisionality, with q95 ~ 6. We probably want a DND biased up with normal BT to stave off the H-mode transition as long as possible. 3 NBI co-sources are turned on simultaneously and BES is deployed to look for a GAM response. Other turbulence diagnostics will be useful. If struck, perhaps the GAM response can be followed with only the one (150) beam for some time. Perhaps we will be able to do a number of measurements with various beam mixtures after the thump and ideally see if there is any correlation between the GAM response and any subsequent H-mode transition, or transport barrier formation.
Background:	*NBI torque injected by ions into promptly trapped orbits results in a radial fast ion current that delivers this torque via Jfast X B. The low collisionality plasma responds as a dielectric for times much shorter than the momentum transport timescale, that is, a return polarization current is generated in the bulk ions. This polarization is calculable for collisionless orbits, and depends upon the details of the orbit topology for an ion. For timescales much shorter than the thermal ion bounce time the gyro-orbits shift, giving the so-called classical polarizability. For timescales longer than a bounce time the banana orbits shift giving the neoclassical polarizability, about 100 times larger than the classical value. Passing-trapped ion collisions bring the plasma response to a common neoclassical value. *So, the plasma dielectric in this regime is a function of frequency (timescale). Striking the plasma fast enough with a radial current source results in GAM generation as described in Hinton and Rosenbluth, PPCF vol 41, A653 (1999). These GAM oscillations are then collisionally damped. *We need to get the E-field to rise fast enough in a thermal ion bounce time in order to modify the orbit. An estimate shows that the prompt radial fast ion current scales with the local plasma beta, and Ip^2. So we want a low beta target (and low collisionality is important for longer GAM damping time), and low Ip, i.e. higher q95, say 5-6. The estimate indicates 3 co-sources would be enough. Hopefully, less will work to give a range to study.
Resource Requirements:	1 day. NBI. Gyrotrons.
Diagnostic Requirements:	Standard. BES. Other fast diagnostics (turbulence, mhd, AE, ...)
Analysis Requirements:
Other Requirements: