Intense Geodesic Acousticlike Modes Driven by Suprathermal Ions in a Tokamak Plasma

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 ($q_{\min }\ge 2$). Strong bursting and frequency chirping are observed, concomitant with large (10%–15%) drops in the neutron emission. Large electron density fluctuations (${\tilde{n}}_e/n_e\simeq 1.5\%$) 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.

Figure 1

Evolution of (a) plasma current $I_p$ and 75 keV deuterium neutral beam power $P_{\mathrm{NBI}}$, (b) edge poloidal magnetic field fluctuations ${\tilde{B}}_{\theta }$ and neutron emission $n_s$, (c) time resolved power spectrum of ${\tilde{B}}_{\theta }$. Bursting and frequency chirping coincide with 10%–15% drops in the neutron emission. Plasma toroidal magnetic field $B_T=2\mathrm{T}$, major radius $R_0=1.7\mathrm{m}$, minor radius $a=0.6\mathrm{m}$.

Figure 2

Radial profiles of (a) the density fluctuation level ${\tilde{n}}_e/n_e$ and $q$ profile vs major radius $R$ measured using beam emission spectroscopy (BES), taken 6 cm above the plasma midplane; (b) ion temperature $T_i$, electron temperature $T_e$, electron density $n_e$ vs $\sqrt{\Psi }\mathrm{pol}$ where ${\Psi }_{\mathrm{pol}}$ is the poloidal flux; (c) $n=0$ GAM continuum evaluated using the NOVA code and the observed range of frequency (red band). $B_T=2\mathrm{T}$, $R_0=1.7\mathrm{m}$.

Figure 3

(a) 2D density perturbation (${\tilde{n}}_e/n_e$ normalized to unity) from linear hybrid simulation of the E-GAM showing $m=1$ standing wave. The dashed horizontal line is the elevation of the linear BES array at the time of interest. (b) Mode frequency (blue) and linear growth rate $\gamma /\omega (\%)$ (red) vs central fast ion beta ${\beta }_h(0)$.

Figure 4

Cosine of the phase ($\cos \varphi$) of the $n=0$ mode vs vertical height ($z$) measured along a vertical array of BES channels located at major radius $R=1.9\mathrm{m}$. The phase is shown at 4 times in a single discharge (131874), each corresponding to a different vertical height of the magnetic axis (dashed lines: $-3$, $-4$, $-5$, $-6\mathrm{cm}$, respectively). The point of phase inversion tracks the motion of the magnetic axis, indicative of an $m=1$ standing wave.