To investigate the hot electrons population in highly charged electron cyclotron resonance ion source (ECRIS), the axial emitted bremsstrahlung spectra, along with microwave signal emitted from the ECRIS plasma are measured on SECRAL-II (Superconducting ECR ion source with Advanced design in Lanzhou No. II) ion source. The evolution of the hot electrons spectral temperature, Ts, is studied through the variation of magnetic configuration. The experimental results have shown that when the ratio of the minimum field to the resonance field (i.e. Bmin /Becr) is less than 0.75~0.80, there is a linear dependence of the spectral temperature on the ratio of Bmin /Becr, above this threshold Ts saturates and electron cyclotron instability appears simultaneously. This phenomenon may be due to the fact that the spectral temperature Ts provides an indication of the temperature of the hot electrons, significant losses of the hot and warm electrons caused by electron cyclotron instability will lead to a saturation of Ts. In addition, this investigation has also shown that Ts decreases with the increase of the gradient at the resonance zone at low mirror ratio and is insensitive to the gradient at high mirror ratio when Bmin is constant.
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