Non-inductive current built-up by local electron cyclotron heating and current drive with a 28 GHz focused beam on QUEST*

The plasma current can be driven solely by injecting electron cyclotron waves (ECWs) in spherical tokamak (ST) configuration. A system of 28 GHz gyrotron (maximum power: 270 kW) is renewed and reinstalled on QUEST. A focused ECW beam, whose diameter is about 5 cm at the second harmonic resonance, is injected for local ECW heating and current drive. The local power density at resonance exceeds 75 MW/m$^2$ at an injection power of 150 kW. The incident ECW polarization can be adjusted employing the phase shifter consisting of two corrugated plates \footnote{ T.Ii $et$ $al.$, Rev. Sci. Instrum. 86, 023502 (2015)}. During 1.25 second pulse of ECH, plasma current is built up to $I_{\mathrm{p}}$ = 70 kA fully non-inductively with a core electron density of $n_{\mathrm{e}}$ $>$ 10$^{18}$ m$^{-3}$. The closed flux in such ST plasma is determined at the inboard limiter on the center stack. Energetic electrons are also responsible for the pressure and equilibrium.