Alfvén Cavity Modes, Fast Ions, Alpha Particles and Diagnostic Neutral Beams

Abstract

ITER will have a high-energy population of the fusion product alpha particles with a velocity distribution function f α (v, r) called the slowing-down distribution. At each radius r, or more precisely, on each magnetic flux surface Ψ, the distribution in energy E = 1 2 m α v 2 of the particles is derived from their birthrate of the 3.5 MeV alpha particles produced by nuclear reaction rate σv DT times the product of the local deuterium and tritium densities [Bosch and Hale (1992)]. After their birth, the alpha particles slow down and scatter in their direction from the Coulomb collisions with the thermal plasma. In contrast, the 14.1 MeV neutrons born with the alpha particles travel in straight lines through the chamber wall into the outer boron-lithium blanket designed for breeding new tritium atoms for maintaining the tritium fuel supply. The blanket also converts the neutron kinetic energy into thermal power to drive the electric generators for fusion electric power. ITER will test tritium breeding blanket designs, but the conversion of the thermal energy into electric power awaits the next machine called DEMO.The phase-space distribution of the magnetically-trapped alpha particles is of primary interest since this distribution function is the source of a high-frequency ioncyclotron wave and the low-frequency Alfvén eigenmodes. The tritium experiments in JET [Keilhacker, et al. (1999)] showed the presence of both these instabilities. The high-frequency, ion-cyclotron waves were easily observed in the first DT experiments on TFTR (1996) and then more clearly on JET [Jet Team (1998)]. The power at the fundamental cyclotron frequency of the alpha particle, which is close to the deuteron cyclotron frequency, showed peaks in the wave frequency power spectrum with several cyclotron harmonics excited by the alpha particle distribution function. The amplitude of the fundamental wave at approximately 18 MHz increased linearly with the alpha-particle power as reported in the IAEA publication for TFTR discharges with the progression of 1.8, 4.7 and 8.9 MW of neutral beam heating power.The alpha particle density was inferred from measurements of the spectrum of the knock-on deuterons produced in the elastic nuclear collisions of the Z = 2 alpha particles with the plasma ions D + and T + . The measurements of the spectrum used the neutral particle analyzer that collected the D and T atoms leaving the 47 ITER Physics Downloaded from www.worldscientific.com by NATIONAL UNIVERSITY OF SINGAPORE on 10/03/15. For personal use only. ITER Physics Downloaded from www.worldscientific.com by NATIONAL UNIVERSITY OF SINGAPORE on 10/03/15. For personal use only.