Study of breakdown and plasma formation in the KTM tokamak with the massive conductive vacuum chamber

•The experiments were accomplished to verify and optimize the massive KTM vacuum chamber calculation model;•The analysis of the plasma initiation stage in the KTM tokamak was performed using TRANSMAK code;•The avalanche ohmic breakdown was achieved in toroidal electric field of 1.4-1.6 V/m with a toroidal field of 1.14 T.•The plasma discharge with the discharge duration of 65 ms and the maximum plasma current of about 100 kA was obtained. The article presents and discusses works on achieving ohmic breakdown at the KTM tokamak. The vacuum chamber of the KTM tokamak has a number of design features that significantly distinguish it from other installations - massive low-resistance vacuum chamber with unevenly distributed elements. The first experiments showed that the presence of massive asymmetrically distributed conductive elements of the KTM vacuum chamber leads to the complexity and inaccuracy of modeling the induced eddy currents and, accordingly, to the inaccuracy of calculating the field null configuration. Due to the limited value of the toroidal electric field of about 1.6 V / m and the toroidal magnetic field of 1 T, there are high requirements for the value of the stray poloidal magnetic field in the breakdown region for its implementation. So, it was required to carry out comprehensive work to achieve a breakdown at the KTM tokamak.