Coupling of Neutral-Beam-Driven Compressional Alfvén Eigenmodes to Kinetic Alfvén Waves in NSTX Tokamak and Energy Channeling

An energy-channeling mechanism is proposed to explain flattening of the electron temperature profiles at a high beam power in the beam-heated National Spherical Torus Experiment (NSTX). Results of self-consistent simulations of neutral-beam-driven compressional Alfvén eigenmodes (CAEs) in NSTX are presented that demonstrate strong coupling of CAEs to kinetic Alfvén waves at the Alfvén resonance location. It is suggested that CAEs can channel energy from the beam ions to the location of the resonant mode conversion at the edge of the beam density profile, modifying the energy deposition profile.

Figure 1

Contour plots of magnetic-field perturbation for $n=8$ (a) and $n=4$ (b) CAE modes show resonant coupling to KAWs. The solid contour line on the $|\delta B_{\perp }|$ plots corresponds to the resonant condition ${\omega }_A(Z,R)=\omega$.

Figure 2

Radial profiles of magnetic-field perturbation for the $n=4$ CAE versus major radius. The CAE peaks near the magnetic axis, where $\delta B_{\parallel }\gg \delta B_{\perp }$.

Figure 3

Radial profile and contour plot of the effective potential $V_{\text{eff}}$ for the $n=8$ CAE with $\omega =0.48{\omega }_{ci0}$. The mode can exist for $V_{\text{eff}}<0$ with radial extent $0.86\mathrm{m}<R<1.45\mathrm{m}$.

Figure 4

Radial profile of Alfvén continuum, $\delta E_{\parallel }$ and beam-ion density profile, and contour plot of $\delta E_{\parallel }$ for the $n=8$ CAE mode.

Figure 5

Radial profiles of $\delta E_{\parallel }$ and radial component of Poynting vector $S=⟨E\times B⟩$ for the $n=4$ CAE mode.