Having the ability to control the operation of advanced plasma heating methods is essential for the achievement of fusion-relevant plasmas in open traps. We describe a technique to deal with the increased transport brought about by the electron cyclotron heating in axisymmetric magnetic mirror device. The technique is based on shaping of the plasma potential by means of a set of specifically biased electrodes facing plasma at both open ends of a trap. In the experiments with combined microwave and neutral beam plasma heating performed in the gas-dynamic trap (GDT) facility at the Budker Institute, we show that a value of on-axis electron temperature up to 450 eV at plasma density 1.2 × 1019 m-3 can be supported steadily for 1.5 ms limited by the available heating and magnetic confinement systems. Stable hightemperature discharge, no longer degraded by low-frequency instabilities, offered a unique opportunity to confirm quantitatively the gas-dynamic plasma confinement model in a new range of parameters.