High β experiment and confinement regimes in a compact helical system

A volume-averaged equilibrium β value 〈β_eq〉 of 2.14% is achieved in a compact helical system using two neutral beam lines with balanced injection and intense wall conditioning with Ti gettering. This value is the highest β value realized so far in helical systems. Reheat mode, where the stored energy increases after turn-off of a strong gas puff, is employed in the experiment. Discharge conditions are as follows: B_t = 0.61 T; beam power through the port, 1.1 MW (coinjection) and 0.8 MW (counterinjection); line-averaged electron density n_e = 6.5 × 10¹³cm^-3. Amplitudes of magnetic fluctuations integrated over the frequency range from 3 kHz to 100 kHz become saturated at 〈β_eq〉 higher than 1%. Dominant coherent modes are m n = 2 1 and 1 1 when 〈β_eq〉 is lower and higher respectively than 1%. Dependence of the energy confinement time τ_E on n_e (up to 8 × 10¹³ cm^-3) and B_t (from 0.6 to 1.8T) is also studied in this high β experiment. When the density increases τ_E degrades compared with the LHD scaling; the density dependence exhibits Bohm-like behaviour. On the contrary, τ_E scales as B_t^≈0.75, which is rather close to the LHD scaling (gyro-Bohm-like behaviour).