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Thermal energy confinement scaling in PDX limiter discharges

Experiments were performed on the PDX tokamak to study plasma heating and beta scaling with high-power, near-perpendicular neutral-beam injection. The data taken during these experiments were analysed, using a time-dependent data interpretation code (TRANSP), to study the transport and thermal confinement scaling over a wide range of plasma parameters. This study focuses on results from experiments with D0 injection into H+ plasmas using graphite rail limiters, a = 40–44 cm, R = 143 cm, Ip= 200–480 kA, BT = 0.7–2.2 T, and, typically, n̄e = (2.5–4.2) X 1013cm−3. The results of this study indicate that for both Ohmic and neutral-beam-heated discharges the energy flow out of the plasma is dominated by anomalous electron losses which are attributed to electron thermal conduction. The ion conduction losses are not inconsistent with neoclassical theory; however, the total ion loss influences the power balance significantly only at high toroidal fields and high plasma currents. Therefore, except for these cases, the total thermal energy confinement time for the neutral-beam-heated discharges follows the scaling of electron energy confinement. While the confinement is found to have little dependence on toroidal field, it is a strong function of plasma current, increasing with increasing Ip. In contrast, Ohmic confinement times are found to scale as n̄eq½. Because of the dominant effect of the electrons, the total confinement is found to depend heavily on the variation of the electron thermal diffusivity, χe, in the outer region of the plasma (a/2 ≤ r ≤ 7a/8). The values of χe in this region of the plasma show an approximately inverse dependence on the local value of the poloidal magnetic field.