Neutron sawtooth behavior in the PLT, DIII-D, and TFTR tokamaks

Abstract

The effect of the sawtooth instability on the 2.5 MeV neutron emission in the PLT [Plasma Physics Controlled Nuclear Fusion Research, 1976 (IAEA, Vienna, 1977), Vol. I, p. 21], DIII-D [Plasma Physics Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159], and TFTR [Plasma Physics Controlled Nuclear Fusion Research, 1984 (IAEA, Vienna, 1985), Vol. I, p. 29] tokamaks is studied. In thermonuclear plasmas, the instability typically results in a 20% reduction in emission. The time evolution of the thermonuclear neutron signal suggests that the sawtooth crash consists of four phases. First, the electron density profile flattens rapidly (in approximately 30 μsec on PLT) but, in some cases, there is little associated change in neutron emission, suggesting that most reacting ions remain confined in the sawtooth region but do not completely mix. After the electron sawtooth, the ions continue to mix, resulting in an approximate 10% reduction in neutron emission in about 0.5 msec. The emission then decays more slowly during the final two phases. Thermalization of reacting ions on an approximate 3τii time scale accounts for only about 20% of the slow drop. Most of the slow drop seems to be caused by loss of ion energy from the mixing region (an ion heat pulse).