- Nazikian, R;
- Fu, GY;
- Austin, ME;
- Berk, HL;
- Budny, RV;
- Gorelenkov, NN;
- Heidbrink, WW;
- Holcomb, CT;
- Kramer, GJ;
- McKee, GR;
- Makowski, MA;
- Solomon, WM;
- Shafer, M;
- Strait, EJ;
- Van Zeeland, MA
Intense axisymmetric oscillations driven by suprathermal ions injected in the direction counter to the toroidal plasma current are observed in the DIII-D tokamak. The modes appear at nearly half the ideal geodesic acoustic mode frequency, in plasmas with comparable electron and ion temperatures and elevated magnetic safety factor (q_{min}>or=2). Strong bursting and frequency chirping are observed, concomitant with large (10%-15%) drops in the neutron emission. Large electron density fluctuations (n[over ]_{e}/n_{e} approximately 1.5%) are observed with no detectable electron temperature fluctuations, confirming a dominant compressional contribution to the pressure perturbation as predicted by kinetic theory. The observed mode frequency is consistent with a recent theoretical prediction for the energetic-particle-driven geodesic acoustic mode.