Electron cyclotron resonance (ECR) ion sources are largely used for production of intense beams in science and industry. Performances of modern devices are severely limited by the generation of suprathermal electrons, whose origin is still controversial. We hereby demonstrate that the electron energy distribution function is strongly influenced by the reciprocal displacement of magnetic field and plasma density profiles. The latter can be modified independently of the former, e.g. by changing the plasma anisotropic diffusion. Auxiliary electrons emitted by carbon nanotube based electron guns are used for Simon current compensation: this significantly reduces the ion losses, increases the output currents and successfully suppresses the hot electron generation. (Some figures may appear in colour only in the online journal)