Collisional Damping of Electron Bernstein Waves and its Mitigation by Evaporated Lithium Conditioning in Spherical-Tokamak Plasmas

The first experimental verification of electron Bernstein wave (EBW) collisional damping, and its mitigation by evaporated Li conditioning, in an overdense spherical-tokamak plasma has been observed in the National Spherical Torus Experiment (NSTX). Initial measurements of EBW emission, coupled from NSTX plasmas via double-mode conversion to $O$-mode waves, exhibited $<10\%$ transmission efficiencies. Simulations show 80% of the EBW energy is dissipated by collisions in the edge plasma. Li conditioning reduced the edge collision frequency by a factor of 3 and increased the fundamental EBW transmission to 60%.

Figure 1

(a) The central $T_e$ (dashed line), $T_e$ at the MC layer (dashed line with error bars indicating error in laser Thomson scattering measurement) and $B\mathrm{\text{−}}X\mathrm{\text{−}}O$ transmission efficiency (solid line) are plotted as a function of time. The vertical dashed line indicates the time of the $L\mathrm{\text{−}}H$ transition. (b) The solid line indicates the measured $T_{\mathrm{rad}}$ (second harmonic EBE at 24 GHz) for an $H$-mode plasma. The EBE simulated $T_{\mathrm{rad}}$ without (bottom solid line) and with (long dashed line for $Z_{\mathrm{eff}}=2$ and short dashed line for $Z_{\mathrm{eff}}=4$) collisional damping effects are shown as a function of time. The shaded grey region indicates the uncertainty in the measured $T_{\mathrm{rad}}$. The error bars for the EBE simulations include the error in the $n_e$ and $T_e$ measurements as well as the magnetic equilibria.

Figure 2

Comparison of (a) ${\upsilon }_{\mathrm{ei}}/\omega$ and (b) ray intensity as a function of major radius for two time points: $t=0.148\mathrm{s}$, during high emission (solid line), and $t=0.348\mathrm{s}$, after the decay of EBE (dashed line). The grey region indicates the location of the MC layer.

Figure 3

The (a) edge density and (b) electron temperature at the UHR layer are shown as a function of time for a plasma without Li conditioning (solid line) and a case with Li conditioning (dashed line). Error bars are indicated by the vertical lines.

Figure 4

(a) The plasma current, $I_p$, and central $T_e$ are plotted as a function of time for an $H$-mode plasma. The measured (b) $T_{\mathrm{rad}}$ and (c) $B\mathrm{\text{−}}X\mathrm{\text{−}}O$ transmission efficiency as a function of time are plotted for $0\mathrm{mg}/\min$ (solid line) and $19\mathrm{mg}/\min =286$ total mg of evaporated lithium (dashed line).

Figure 5

Comparison of (a) ${\upsilon }_{\mathrm{ei}}/\omega$ and (b) ray intensity as a function of major radius for two cases: without lithium conditioning (dashed line) and with lithium conditioning (solid line). The grey region indicates the location of the MC layer.