Studies of energetic ion confinement during fishbone events in PDX

Abstract

ABSTRACT. The 2.5 MeV neutron emission from the beam-target d(d, n) 3 He fusion reaction has been examined for all PDX deuterium plasmas which were heated by deuterium neutral beams. The magnitude of the emission was found to scale classically and to increase with T* n as expected when electron drag is the primary energy degradation mechanism. The time evolution of the neutron emission through fishbone events was measured and used to determine the confinement properties of the energetic beam ions. Many of the experimental results are predicted by the Mode Particle Pumping Theory.

The fishbone instability in the DIII-D tokamak

“…This theory was consistent with all experimental measurements of fastion transport at fishbones in PDX [3,[6][7][8][9][10] and could even be reconciled with the fast-ion losses observed during tangential injection in PBX [23]. White's theory does not agree well with the DIII-D data, however.…”

“…The magnetic perturbation measured at the outer midplane wall was B e /B e = 2.5% at this burst. In PDX, magnetic perturbations of 1% at the outer wall (the wall conforms to the plasma more closely in DIII-D than it typically did in PDX) produced comparable neutron behaviour [3]. In DIII-D, magnetic perturbations of B 9 /B fl = 4 X 10" 4 correlate with 1 % drops in neutron emission; a similar threshold was observed in PDX [3].…”

“…When large fishbones occur during D° -D + injection, neutron scintillators [3] measure coincident reductions in neutron emission (Fig. 5).…”

“…Like precursors to the sawtooth instability [2], the fishbone was an n = 1, m = 1 mode, but the m = 0 reduction in soft X-ray emission was smaller and the concurrent m > 2 magnetic oscillations were measured with Mirnov coils [1]. The mode frequency was higher than at a sawtooth [1] and 'whistled down' during a burst from an initial value of f = 22 kHz to a final value of f « 11 kHz [3]. Fishbones were observed in PDX whenever the product of toroidal beta and safety factor exceeded a critical value (/3 T q ^ 2.5%) [4].…”

“…Fishbones were observed in PDX whenever the product of toroidal beta and safety factor exceeded a critical value (/3 T q ^ 2.5%) [4]. Since the instability ejected large numbers of fast ions [1,[3][4][5][6][7][8][9][10] it limited the maximum /3 achievable in PDX [4], Two main theoretical interpretations of the fishbone instability have been advanced. Chen et al [11] discovered that beam ions trapped within the q = 1 radius can kinetically destabilize a branch of the internal kink mode with frequency w = w p , where w p is the precession frequency of the resonant beam ions; they suggested that fishbones occur when the pressure of these hot, trapped beam ions (j3 M ) is sufficiently high.…”