Identification of 1-MW infrared radiation in an electron-cyclotron maser as free-electron two-quantum Stark radiation

Abstract

We find that the transition from a virtual state to another virtual state is forbidden. Because the intermediate state in two-quantum Stark (TQS) radiation is virtual, TQS radiation is an effective one-photon radiation. We find that the ratio of the power of TQS radiation to the power of the corresponding one-photon radiation in the electron beam is many orders of magnitude larger than that in atomic radiation. We attribute this phenomenon to the fact that in free-electron TQS radiation, the free electron is forced to emit radiation by an electric wiggler over a length longer than the wavelength of the electric wiggler while in bound-electron TQS radiation, the bound electron only experiences the wiggler potential within a length on the order of the atomic size. We find that plasma waves with potential amplitudes on the order of 1 kV and wavelengths on the order of 1 cm can be generated due to the \(\mathcal{E}_r \hat r \times B\hat z\) rotational-drift and the helical (or rippled) magnetic field in an electron-cyclotron maser (ECM). We reason that these plasma waves decompose into shorterwavelength plasma waves such that the wavenumber (K) spectrum of the potential amplitude scales as 1/K2 in the region of K ≪ K i = 2π/l i , where l i is the mean inter-electron distance. We find that the wavelength and the power of free-electron TQS radiation driven by these short-wavelength plasma waves under the influence of the axial uniform magnetic field are in good agreement with those of the measured 1-MW infrared radiation.