Stabilizing Effect of Trapped Alpha Particles on Low-Frequency MHD Ballooning Modes in ITER Plasmas With Flat q(ψ) Profiles

The stabilizing compressibility effect of trapped alpha particles on low-frequency magnetohydrodynamic (MHD) ballooning modes (Re ω ≪ Im ω) in the International Thermonuclear Experimental Reactor (ITER) is investigated. It is found that this stabilization is the most effective one in the central region of the plasma column, where the unstable region of MHD ballooning modes is located for typical flat q(ψ) profiles in ITER. The alpha-particle distribution function is supposed to be isotropic and slowing down in energy. It has been found that the values of β_α/β_total ≅ 1.5 to 2.0% are sufficient to stabilize ballooning modes in the central low-shear region for the peaked pressure profiles [P(ψ) = P(0)(1 − ψ)^γ] proposed for ITER. The value of β_α/β_total remains almost unchanged to suppress the instability for all γ = 1.0 to 2.0.