Comparison of electron cyclotron heating results in the helically symmetric experiment with and without quasi-symmetry

The extraordinary wave at the second harmonic of the electron cyclotron frequency produces and heats the plasma in the helically symmetric experiment. Ray-tracing calculations predict 40% first pass absorption at a plasma density of 1.5 × 10¹⁸ m^-3 and an electron temperature of 400 eV. To measure the wave absorption, a set of absolutely calibrated microwave detectors is installed along the machine. It was found that the absorption efficiency is very high (about 0.9) in the quasi-helically symmetric (QHS) and mirror configurations, and it drops to 0.6 in the anti-mirror mode. The confinement of particles in the different configurations is studied in the neutral gas breakdown experiments. With the same gas pressure and heating power, the density for the QHS configuration has a larger growth rate (10⁴ s^-1) compared with the mirror (5 × 10³ s^-1) and anti-mirror modes (2 × 10³ s^-1). A study of the stored energy versus launched power and plasma density shows that it increases linearly (up to 50J) with power and has a maximum at a low plasma density (at about 0.4 × 10¹⁸ m^-3). The central electron temperature measured by Thomson scattering also rises linearly with heating power and reaches 600 eV at 100 kW of launched power.