Global modes and high-energy particles in ignited plasmas

Abstract

Fusion-produced alpha particles may spontaneously enhance the stability of an ignited plasma against m=1 internal modes. Stable values of the poloidal beta of the thermal plasma component significantly in excess of the ideal magnetohydrodynamic threshold can be attained, provided the region where the magnetic helical parameter q is below unity is not too wide and the ignition temperature is not too high. A comprehensive analysis of the different instability regimes is presented, with special attention to so-called ‘‘fishbone oscillations’’ and their influence on the ignition energy balance. The theoretical predictions compare favorably with sawtooth suppression experiments in the JET tokamak [Phys. Rev. Lett. 60, 2148 (1988)] with high-power, ion-cyclotron resonant frequency heating, where energetic ions in the MeV range are produced.

The fishbone instability in the DIII-D tokamak

“…It should be noted that recent theoretical work [13][14][15] predicts that the trapped particle branch and the ion diamagnetic branch merge for w.j > 0.5, so identification of the dominant branch may not be possible for many of the DIII-D data. Perhaps this explains why the observed mode frequency does not agree particularly well with either f.j or f p .…”

“…Coppi et al suggested that it is this branch of the internal kink with w -oo*j that is the experimentally observed fishbone and emphasized the importance of the poloidal beta j3 p in determining stability. Recent work [13][14][15] has shown that these two distinct branches exist for o>*j s w*i/a> p < 0.5 but that the branches merge for d>*j > 0.5.…”

Edge transport barrier in JET hot ion H modes

“…Three different expressions for the radial correlation length have been proposed: ∆ cor ≈ ρ i ∝ ρ θi [7], ∆ cor ≈ √ ρ i a [10,11] and ∆ cor ≈ 3 ρ 2 i a [12]. The next concept is the idea of turbulence suppression by the finite ion Larmor radius [13][14][15]. We can expect this mechanism to be effective in suppressing the short wavelength turbulence with characteristic radial correlation length of the order of the ion Larmor radius.…”

“…Assuming that the pressure gradient within the edge transport barrier is constant, both the energy flux through the separatrix and the maximum plasma pressure at the top of the barrier are controlled by the width of the edge transport barrier. There has been a variety of theoretical proposals about the possible scaling for the width of the edge transport barrier since the discovery of H modes [8][9][10][11][12][13][14][15][16]. However, due to inadequate experimental information on the edge region, which exhibits steep gradients, it is still not clear which mechanisms control the width of the edge transport barrier.…”

Internal Kink Mode Dynamics in High-beta NSTX Plasmas

“…On the other hand, trapped energetic ions are also well known [15] to help stabilize the internal kink mode. In tokamaks, this stabilization can delay the occurrence of normal sawteeth, allow the current profile to penetrate on axis, and ultimately lead to monster sawteeth [16,17] with large inversion radius.…”