The note presents the first plasma density measurements collected by a novel microwave interferometer in a compact Electron Cyclotron Resonance Ion Sources (ECRIS). The developed K-band (18.5 ÷ 26.5 GHz) microwave interferometry, based on the Frequency-Modulated Continuous-Wave method, has been able to discriminate the plasma signal from the spurious components due to the reflections at the plasma chamber walls, when working in the extreme unfavorable condition λ ≃ L ≃ L (λ, L, and L being the probing signal wavelength, the plasma dimension and the plasma chamber length, respectively). The note describes the experimental procedure when probing a high density plasma (n > 1 ⋅ 10 cm) produced by an ECRIS prototype operating at 3.75 GHz.

Electron cyclotron resonance ion sources (ECRIS) are widely used plasma based machines for the production of intense ion beams in science and industry. The performance of modern devices is limited by the presence of the density cut-off, above which electromagnetic (EM) waves sustaining the plasma are reflected. We hereby discuss the systematic data analysis of electrostatic wave generation in an ECR prototype operating at 3.75 GHz-0.1 THz. In particular, electron Bernstein waves (EBW) have been excited. EBW have already been generated in large-scale plasma devices for thermonuclear fusion purposes. In ion sources where L c ∼λ RF (L c being the plasma chamber size and λ RF the pumping wave wavelength) the EM field assumes a modal behaviour; thus both plasma and EM field self-organize so that no optical-like wave launching is possible (i.e. the cavity effect dominates on the optical path). The collected data, however, supported by 3D full-wave simulations, actually demonstrate that a Budden-type X-B conversion scenario can be established above some critical RF power thresholds, operating in an off-ECR regime. The generation and absorption of the EBW has been demonstrated by the presence of three peculiar signatures: along with the establishment of an overdense plasma, generation of supra-thermal electrons and modification (non-linear broadening) of the EM spectrum measured within the plasma have been observed. At the threshold establishing such a heating regime, the collected data provide evidence for a fast rotation of the electron fluid.Keywords: plasma physics, overdense plasma, Bernstein waves, x ray diagnostics, langmuir probe diagnostics, magnetized plasma, plasma hole configuration interaction with low pressure (10 −6 -10 −3 mbar) gases or vapours. In presence of a magnetostatic field, the electromagnetic wave absorption is particularly efficient at the socalled 'electron cyclotron resonance' (ECR). Most of the parameters of the extracted beam, such as its intensity, emittance and shape in real space, depend in a decisive way on the characteristics of the plasma from which the beam is extracted [2]. Any further development of ECR-type ion sources is thus intrinsically dominated by physical properties of the plasmas. Additional limitations are given by the high-B mode conditions [3]. In plasmas sustained by microwaves, the

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