Accounting of the power balance for neutral-beam heated H-mode plasmas in NSTX

Abstract

A survey of the dependence of power balance on input power, shape, and plasma current was conducted for neutral beam heated plasmas in NSTX. Measurements of heat to the divertor strike plates and divertor and core radiation were taken over a wide range of plasma conditions.The different conditions were obtained by inducing an L-mode to H-mode transition, changing the divertor configuration [lower single null (LSN) vs. double-null (DND)] and conducting a NBI power scan in H-mode. Up to 70% of the net input power is accounted in the LSN discharges with 20% of power lost as fast ions, 30% incident on the divertor plates, 10% radiated in the core, and 10% radiated in the divertor. In contrast, the power accountability in DND is 85-90%. A comparison of DND and LSN data show that the remaining power in the LSN is likely to be directed to the upper divertor.

Divertor heat flux scaling with heating power and plasma current in H-mode discharges in the national spherical torus experiment

“…Indeed the peak heat flux was measured to be <1 MW/m 2 at the lower power levels, approaching the values observed in radiative and detached divertor studies in NSTX [9]. The fact that the data lie below the solid blue line, which represents detachment onset, is perhaps not surprising, given that the power accountability in these discharges was 70% at the higher power levels and down to 60% at the lowest power levels [7]. Including an anomalous power loss of 30-40%, the solid blue line would shift below the data, representing a lower bound as expected.…”

“…In this paper, we present the first results of outer divertor heat flux scaling with neutral beam heating power (P NBI ) and plasma current (I p ) in H-mode discharges in the National Spherical Torus Experiment (NSTX). This work builds upon previous NSTX reports of the peak heat flux scaling in Lmode discharges [6], and power accountability [7] and initial surveys of the heat flux characteristics [8] in H-mode discharges, as well as studies of heat flux management through radiation and detachment [9]. We focus on the outer divertor because the inner divertor remains detached through most of the operational window in NSTX [10].…”

“…Here loss power is defined as P loss = P OH + P NBI À dW MHD /dt ÀP core rad À P fast ion loss , where P OH is the ohmic heating power, P core rad is the core radiated power, and P fast ion loss is the fast ion loss. From our power accounting study [7], we use that P core rad $0.1(P OH + P NBI ) and P fast ion loss $ 0.2(P OH + P NBI ). A change in the slope is evident for P loss between 2.5 and 3 MW, i.e.…”

Progress towards high performance plasmas in the National Spherical Torus Experiment (NSTX)

“…Power accountability in both LSN and DND plasmas was found to be good [36], with up to 70% and 90% of the power being accounted for in the two configurations, respectively. The largest fraction of the power loss (35%) was deposited on the divertor plates, with an out-in ratio of up to 5 : 1.…”

Divertor heat flux reduction and detachment experiments in NSTX

“…The layout and description of NSTX edge and divertor diagnostics are given in Refs. [4,6,7]. The plasmas were fueled by deuterium from a high field side injector at a rate of C 6 3.8 · 10 21 s À1 .…”