Heating by an Electron Bernstein Wave in a Spherical Tokamak Plasma via Mode Conversion

The first successful high power heating of a high dielectric constant spherical tokamak plasma by an electron Bernstein wave (EBW) is reported. An EBW was excited by mode conversion (MC) of an $X$ mode cyclotron wave injected from the low magnetic field side of the TST-2 spherical tokamak. Evidence of electron heating was observed as increases in the stored energy and soft x-ray emission. The increased emission was concentrated in the plasma core region. A heating efficiency of over 50% was achieved, when the density gradient in the MC region was sufficiently steep.

Figure 1

Cross-sectional view of TST-2@K. Typical equilibrium flux surfaces and the locations of critical layers are shown. The rf leakage monitor is installed 120° away toroidally from the launcher. Fundamental ECR (electron cyclotron resonance) is located slightly at the high field side of the magnetic axis.

Figure 2

Calculated MC efficiency for $X\mathrm{\text{−}}\text{}B$ mode conversion as a function of density scale length ($L_{\mathrm{n}}\equiv n/|\frac{dn}{dx}|$). The solid line is $C_{\max }$ defined in [6].

Figure 3

Plasma discharge with 70 kW of net rf power (#301462). (a) Plasma current, (b) line-integrated density ${\overline{n}}_{\mathrm{e}}l$ at $R=0.38\mathrm{m}$ ($l\simeq 0.6\mathrm{m}$), (c) plasma stored energy, (d) densities measured at three locations inside the local limiter, and (e) density scale length.

Figure 4

Time evolutions of discharges with 90 kW net rf injection (#302787 in black) and without rf power (#302788 in gray). (a) Plasma current and rf leakage power, (b) forward, reflected, and net rf powers, (c) SX (1–10 keV) emission, (d) line-integrated density at $R=0.38\mathrm{m}$ ($l\simeq 0.55\mathrm{m}$), (e) $H_{\alpha }$, (f) $P_{\mathrm{rad}}$ (broadband radiation over the range 0.01–7 keV), and (g) plasma stored energy.

Figure 5

(a) Contour plot of PIN diode array signals, and (b) evolution of the peak intensity.

Figure 6

Comparison between calculated MC efficiency for a density profile consisting of steep (shallow) gradient regions [inside (outside) the local limiter] and the observed heating efficiency. The upper and lower bounds of the measured limiter edge density are based on the measured density at probe (1) and the density extrapolated to the limiter edge from probe (2).