Runaway discharges are studied in the small Brazilian Tokamak TBR-1 (R=30 cm, a=8 cm, Bt=0.5 T, It=12 kA). Low plasma density, up to 1012 cm−3, provides reproducible runaway discharges with the runaway current more than 95% of the total current, the Shafranov parameter up to 1.8, and the effective kinetic energy of the runaway electrons about 300–360 keV. The runaway-related relaxation phenomenon is triggered by the biased electrode technique and the plasma resistance is determined from the time decay of the loop voltage spikes. The experimental data are analyzed by comparison with the theory of runaway generation and equilibrium of the toroidal relativistic beam-plasma system.

Positive voltage spikes caused by a fast decrease of the plasma current are observed in runaway dominated discharges in the small Brazilian Tokamak TBR-1͓Kuznetsov et al., Phys. Plasmas 6, 4002 ͑1999͔͒. Comparison of the measured voltage spike with the value given by the solution of the one-dimensional diffusion equation for the toroidal electric field permits one to infer the plasma conductivity and initial electric field distribution. An additional runaway generation due to voltage spikes can be important. In this case, both the decay time of the voltage spike and the total toroidal current perturbation can decrease substantially.