An experimental investigation of fast ion confinement on the ISX-B tokamak

An experimental investigation of fast ion confinement was conducted on the ISX-B tokamak at the Oak Ridge National Laboratory to ascertain that the beam ion behavior is properly described by classical processes. Data were collected during tangential injection of H⁰ beams (co-, counter-, and co- plus counter-) at power levels up to 1.9 MW in low plasma current (I_p = 80 to 215 kA) D⁺ discharges. Experimental energy spectra of energetic charge-exchange neutrals along several sightlines in the torus equatorial plane are compared with the predictions of Fokker-Planck and orbit-following Monte Carlo calculations to verify the validity of classical theory. A further tool used in this investigation is the comparison of predicted and experimental beam-plasma neutron emission during injection of beams doped with 3% D⁰. Both the fast neutral spectra and the beam-plasma neutron emission are in close agreement (within factors of ≤ 2) with the calculated values under a variety of plasma parameters, beam parameters, and injection geometries. Furthermore, measured decay rates of the beam-plasma neutron production following beam turn-off show that the beam slowing down—at energies close to the injection energy and in the plasma core—is classical within a 30% uncertainty. These results demon strate that classical theory describes well the behavior of the beam ions. More over, MHD activity is shown not to cause enhanced fast ion losses in the ISXB. Also, beam additivity experiments indicate that the fast ion density in the plasma volume is proportional to the injected beam power P_b. An unresolved issue is whether the central fast ion density is linear with P_b.

In addition, the analysis of charge-exchange spectra is critically evaluated. It is shown that the analysis need be integrated with a knowledge of the orbit topology to correctly interpret the spectra. Cases where the zero banana width, Fokker-Planck calculation is adequate/inadequate to predict fast neutral spec tra and the power deposited in the plasma are discussed.