Effects of sawtooth crashes on beam ions and fusion product tritons in JET

Abstract

The JET neutron emission profile monitor is used to measure the 2.5 MeV and 14 MeV neutron emission line integrals before and after sawtooth crashes in high d-d neutron yield, hot ion H mode plasmas in the Joint European Torus (JET). Deuterium-deuterium (d-d) fusion produces 2.5 MeV neutrons and 1 MeV tritons (t) at nearly equal rates from its two reaction channels. A plasma current of 3 MA is sufficiently high to contain most of the fusion product tritons, which have birth orbit gyroradii and velocity space distributions similar to those of the 3.5 MeV or particles from d-t fusion. By examining neutron emission line integrals and tomographically deduced local emissivity profiles, an upper limit of 10% can be placed on the net fraction of fusion product tritons which are displaced from the plasma axis by those sawtooth crashes studied. This is a much smaller net fraction than that typically observed, 35-55%, for displaced injected neutral beam deuterium ions. A study of the response of beam injected deuterium ions to a sawtooth crash shows that the change in their axial density depends on the precrash spatial width of the neutron emissivity profile. The fusion product tritons, which have a large precrash spatial width, respond weakly to a crash. This weak response is consistent with the behaviour of the analogous d-d beam thermal neutrons when extrapolated to the corresponding emissivity spatial width. The implication of these observations is that beam ions and 3.5 MeV alpha particles in JET may be relatively resilient to sawtooth clashes, when the spatial width of their density is sufficiently large

Neutron Emission Profile Measurement and Fast Charge Exchange Neutral Particle Flux Measurement for Transport Analysis of Energetic Ions in JT-60U

“…The effect of sawtooth on the redistribution of energetic ions in tokamaks has been studied both experimentally [31] and theoretically [32]. Figures 14 (a) and (b) show the time traces of the electron temperature measured from the ECE measurement [33].…”

Global Confinement, Sawtooth Mixing, and Stochastic Diffusion Ripple Loss of Fast ICRF-driven H+ Minority Ions in TFTR

“…The influence of sawtooth oscillations on fast ions in tokamaks is of considerable interest because theoretical studies showed they can lead to a significant redistribution of the ions [12] . The influence of sawtooth effects on injected neutral beam ions and fusion products in DD plasmas has been discussed elsewhere [13,14] . During DT experiment on TFTR, the PCX diagnostic revealed strong transport of trapped MeV alpha particles radially outward well beyond q=1 surface after sawtooth crashes [15 -17] .…”

Alpha-Physics and Measurement Requirements for ITER

“…Evidence for a weak radial redistribution of alpha-like 1 MeV tritons during a sawtooth crash in JET is illustrated in Fig. 2 [13]. Sawtooth-induced radial redistribution of H-minority tail ions has also been measured in JET using impurity charge exchange[l4], and with alphas in " R using the pellet charge exchange (PCX)[lS] and alpha-CHERS [16] diagnostics.…”