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, DIII-D, and TFTR 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 ~ 30/tsec on PLT) but, in sone 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 a ~ 10% reduction in neutron emission in ~ 0.5 msec. The emission then decays more slowly during the final two phases. Ihermalization of reacting ions on a ~ 3T M time scale accounts for only ~ 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).

Energetic Ion Experiments in DIII-D

“…The redistribution is inferred from rapid reductions in the fusion reaction rates produced by 0.8-MeV 3 He ions, 12 80-keV beam ions, 12,15,25 and ;5-keV deuterons in a Maxwellian distribution. 26 The most recent beam-ion observations 15 are consistent with the standard theory of fast-ion redistribution at a sawtooth crash developed by Kolesnichenko. 27 Ordinarily, the fishbone instability barely perturbs the D-D fusion reaction rate, indicating that the instability has little effect on beam-ion confinement.…”

Fast-ion transport and neutral beam current drive in ASDEX upgrade

“…They appear periodically in the plasma center when the safety factor, q, is below one and cause a fast crash of the central pressure, followed by a recovery phase. The crashes strongly redistribute fast ions, as reported from D-T experiments at JET [4] and TFTR [5] and from deuterium plasmas at DIII-D [6], TEXTOR [7], MAST [8] and ASDEX Upgrade [9]. There are two mechanisms that are responsible for the strong redistribution.…”

The behaviour of fast ions in tokamak experiments

“…On PDX, inverted sawtooth oscillations were seen in the active charge exchange flux out to the edge of the plasma (Fig. 56), with a delay of less than 0.1 ms between the sawtooth crash and the rise in edge signal small (< 5%) drops in the total neutron emission in PLT [192] and TFTR [23] on the time-scale of the beam slowing-down time T~,. No redistribution of fast ions at the sawtooth crash was required to explain these results [192].…”

“…On PLT, a detector that was sensitive to central 3He tail ions measured normal (downward) reductions in signal at sawtooth crashes [55]. In later work on PLT, a different detector observed either normal or inverted sawteeth depending upon the relative locations of the resonance layer and the sawtooth inversion radius [23]. On JET, a detector that measured off-axis protons observed inverted sawteeth at sawtooth crashes [48, 601.…”