Predicting High Harmonic Ion Cyclotron Heating Efficiency in Tokamak Plasmas

Observations of improved radio frequency (rf) heating efficiency in ITER relevant high-confinement ($H$-)mode plasmas on the National Spherical Tokamak Experiment are investigated by whole-device linear simulation. The steady-state rf electric field is calculated for various antenna spectra and the results examined for characteristics that correlate with observations of improved or reduced rf heating efficiency. We find that launching toroidal wave numbers that give fast-wave propagation in the scrape-off plasma excites large amplitude ($\sim \mathrm{kV}{\mathrm{m}}^{-1}$) coaxial standing modes between the confined plasma density pedestal and conducting vessel wall. Qualitative comparison with measurements of the stored plasma energy suggests that these modes are a probable cause of degraded heating efficiency.

Figure 1

Midplane profiles for NSTX shot 130608 as used in the simulations presented here. Electron density ($n_e$) and temperature ($T_e$) are from multipoint Thomson scattering data, ion temperature ($T_i$) is from charge exchange recombination spectroscopy data, and the NBI fast-ion density ($n_i$) is from TRANSP [16] simulation. $a=1.52\mathrm{m}$ is the minor radius.

Figure 2

Two-dimensional aorsa simulation results of the steady-state wave electric field amplitude for HHFW on NSTX. The dashed line shows an approximate fast-wave cutoff based on a zero-dimensional dispersion calculation using $k_{\parallel }\approx k_{\varphi }=n_{\varphi }/R$. The separatrix is shown in purple, closed magnetic flux surfaces of the core plasma are in red, and open flux surfaces of the scrape-off plasma are in blue.

Figure 3

Three-dimensional AORSA results for the steady-state wave electric field amplitude. The gold (solid) and gray (transparent) contours are $0.6\mathrm{kV}{\mathrm{m}}^{-1}$ and are inside and outside the LCFS, respectively. The antenna, center stack, and sample magnetic field line trajectory are shown. Figures were created using VisIt [14].