Neutral beam injection (NBI) plasmas have been initiated with the assistance of 5 kW, 2.45 GHz microwaves in a medium-sized heliotron, Heliotron J plasmas with a line-averaged electron density of over 1 × 1019 m−3 are generated with 1 MW NBI using a magnetic field between 0.63 and 1.25 T within 20 ms after turning on NBI. This technique does not require the electron cyclotron resonance layer for the plasma startup, which enables us to enlarge the operational space for the magnetic field strength. Electron cyclotron emission measured with a radiometer reveals that the condition of initial plasmas generated by 2.45 GHz microwaves is critical for reliable startup.

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A microwave reflectometer is developed for electron density profile measurement in Heliotron J. An amplitude modulation (AM) type system is adopted to reduce density fluctuation effects. The carrier frequency ranges from 33 to 56 GHz, and the modulation frequency is 200 MHz. The X-mode is selected as the propagation mode in order to measure a hollow density profile which is typically observed in ECH plasmas. A test-bench examination using an aluminum reflection plate shows that the measured phase shift agrees well with that expected from the change in the plate position. The initial measurement results show that the reconstructed density profile has hollow profile at low density and steep gradient at the edge in ECH plasma. The hollowness is weakened as the averaged density increases.