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Argon micro-plasmas a few centimetres long are generated at low gas pressure (6 mbar) in hollow-core photonic crystal fibres with 100 µm core diameter, using microwave excitation (2.45 GHz) based on a surfatron, with no damage to the host structure. Electromagnetic simulations assist the design of the surfatron cavity, finding optimal conditions for discharge ignition and maintenance. The plasma characterization reveals an electron density in the range 8 × 1014–1015 cm−3 (estimated from a semi-empirical analysis of the power coupled to the plasma, along the axial-direction) and a gas temperature of 1100–1300 K at the centre of the fibre (measured by optical emission spectroscopy diagnostics), thus showing an impressive ionization degree of ∼10−2 for power densities of ∼0.1 MW cm−3. The micro-plasma dynamics is investigated using a self-consistent one-dimensional (radial) fluid model, describing the charged particle and the electron energy transport, the electromagnetic excitation and the gas heating. Model results are used to confirm the diagnostics, revealing an extreme plasma confinement and a steep temperature gradient that give rise to the main operation features of this novel compact UV source. [ABSTRACT FROM AUTHOR]