ALPHA-PARTICLE DIAGNOSTICS USING IMPURITY PELLET INJECTION (INVITED)

We have proposed using impurity pellet injection to measure the energy distribution of the confined alpha particles in a reacting plasma [R. K. Fisher et al., Fusion Technol. 13, 536 (1988)]. The ablation cloud surrounding the injected pellet is thick enough that an equilibrium fraction F0infinity (E) of the incident alphas should be neutralized as they pass through the cloud. By observing neutrals created in the large spatial region of the cloud which is expected to be dominated by the heliumlike ionization state, e.g., Li+ ions, we can determine the incident alpha distribution dn(He-2+)/dE from the measured energy distribution of neutral helium atoms dn(He-0)/dE using dn(He-0)/dE = dn(He-2+)/dE.F0infinity (E,Li+). Initial experiments were performed on the Texas Experimental Tokamak (TEXT) in which we compared pellet penetration with our impurity pellet ablation model [P. B. Parks et al., Nucl. Fusion 28, 477 (1988)], and measured the spatial distribution of various ionization states in carbon pellet clouds [R. K. Fisher et al., Rev. Sci. Instrum. 61, 3196 (1990)]. Experiments have recently begun on the Tokamak Fusion Test Reactor (TFTR) with the goal of measuring the alpha particle energy distribution during D-T operation in 1993-94. A series of preliminary experiments are planned to test the diagnostic concept. The first experiments will observe neutrals from beam-injected deuterium ions and the high energy He-3 tail produced during ion cyclotron (ICH) minority heating on TFTR interacting with the cloud. We will also monitor by line radiation the charge state distributions in lithium, boron, and carbon clouds. Later experiments are planned to measure the energy distribution of the 3.7 MeV alphas created by He-3-D reactions during ICH minority heating. Observations of 3.7 MeV alphas should allow single-particle alpha physics to be studied now and result in a fully tested diagnostic prior to D-T operation of TFTR.