We demonstrate plasma discharges with extremely high temperature of bulk electrons at the large axially symmetric magnetic mirror device GDT (Budker Institute, Novosibirsk). According to Thomson scattering measurements, the on-axis electron temperature averaged over several sequential shots is 660 ± 50 eV with peak values exceeding 900 eV in few shots. This corresponds to at least threefold increase as compared to previous experiments both at the GDT and at other comparable machines, thus demonstrating the maximum quasi-stationary (∼ 1 ms) electron temperature achieved in open traps. The breakthrough is made possible with application of sophisticated electron cyclotron resonance heating in addition to standard heating by neutral beams. The reported increase of the electron temperature along with previous experiments, which demonstrated highdensity plasma confinement with β ≈ 60%, provide a firm basis for extrapolating to fusion relevant applications of open magnetic systems.

This paper summarizes the results of experiments on electron cyclotron resonance heating (ECRH) of plasma obtained at the axially symmetric magnetic mirror device gas dynamic trap (GDT) (Budker Institute, Novosibirsk). The main achievement is the demonstration of plasma discharges with extremely high temperatures of bulk electrons. According to the Thomson scattering measurements, the on-axis electron temperature averaged over several sequential shots is 660 ± 50 eV with peak values exceeding 900 eV in a few shots. This corresponds to an at least threefold increase as compared to previous experiments both at the GDT and at other comparable machines, thus demonstrating the maximum quasi-stationary (∼0.6 ms) electron temperature achieved in open traps. The breakthrough is made possible with the successful implementation of a sophisticated ECRH scheme in addition to standard heating by neutral beams (NBs). Another important result is the demonstration of the significantly increased lifetime of NB-driven fast particles with the application of ECRH, leading to a 30% higher plasma energy content at the end of the discharge. All available data including the previously demonstrated possibility of plasma confinement with β as high as 60%, allows us to consider fusion applications of axially symmetric magnetic mirror machines on a realistic basis.

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