Observation of quasi-periodic frequency sweeping in electron cyclotron emission of nonequilibrium mirror-confined plasmaM. E. Viktorov, A. G. Shalashov, D. A. Mansfeld and S. V. GolubevEPL, 116 5 (2016) 55001DOI: https://doi.org/10.1209/0295-5075/116/55001