A measure of fast ion beta at marginal stability in the reversed field pinch

Abstract

Energetic ions are well confined in the core of the reversed field pinch and a substantial population develops rapidly during tangential neutral beam injection. A saturated fast ion density is observed within several milliseconds and coincides with the onset of energetic particle mode activity. First measurements of the fusion product profile determine the fast ion density and pressure profiles, the latter of which when combined with neutral particle analysis of the energy distribution. These are key measurements in explaining the limiting behavior of the fast ion profile in response to a combination of effects. Tearing mode activity affects the fast ion profile, and the fast ions noticeably influence the core-most tearing mode. The nonlinear situation settles into a marginally stable state where the EPM transports fast ions down a steep gradient to a region where their confinement is limited by stochastic orbit and charge exchange losses. In the state at marginal stability (achieved in one set of experimental conditions with full 1 MW neutral beam power), a critical fast ion beta gradient is measured that limits the core , or nearly four times the core thermal , but less than a strictly classical calculation predicts.

The reversed field pinch

“…The profile in figure 38 is the quasi-steady state profile at marginal stability and full beam power. The limit to the fast ion beta is believed to result from the onset of the EPM and MIAE activity, which redistributes the fast ions outward in radius where they are subject to stochastic transport [351,362]. Central β fi ∼ 8% is comparable to the alpha beta expected in a reactor plasma.…”

“…The fast ion profile has been measured using a columnated neutron detector [362]. The corresponding fast ion beta profile is shown in figure 38, with β fi (0) ∼ 8%.…”

Fast ion transport in the quasi-single helical reversed-field pinch

“…The redistribution of the fast ions associated with the bursting events is localized to the core, conserving the overall volume integration of the particle content. 26 Indeed, the bursting nature of the destabilized modes follows predator-prey relaxation oscillations characteristic of localized fast ion transport. 22 The absence of bursts at a high core-resonant mode amplitude shown in Fig.…”

Fast Ion Transport in the Three-Dimensional Reversed-Field Pinch

“…The change in the neutron flux is in response to the dynamically evolving tearing modes throughout the QSH transition. [22] Examining part (a. ), the fast ion driven neutron flux decreases with fixed, low, subdominant mode amplitude and increasing n = 5 amplitude.…”

“…Should the trend continue to reactor-level Lundquist number, the fast ion phase-space will be nearly free of subdominant mode-induced orbit losses and result in reduced transport similiar to that observed in the bottom-right of Figure 2 (a.). With well confined fast ions, the possibility of Alfvénic mode induced resonant transport may become an important factor (negligible in the NBI-heated MST case due to other dominant loss mechanisms [21,22]).…”