[en] Coupling of ICRF (ion cyclotron range of frequencies) power to the plasma is one of the standard methods to heat plasmas in toroidal devices with magnetic confinement. However voltage limits on the ICRF antenna used to launch the waves sometimes lead to a limitation of the power. These limits are related to a variety of high voltage breakdown phenomena in the presence of plasma that depend, in particular, on spatial charge effects and particle fluxes to the electrodes. An ICRF probe has been developed to study the high voltage phenomena. The open end of a coaxial line models the high voltage region of the antenna. The voltage limits were studied in well defined conditions in a test facility without magnetic field and in the real conditions of the peripheral plasma of the ASDEX Upgrade divertor tokamak. The ICRF probe was installed in the test facility and conditioned in vacuum by high power pulses to reliable operation with 60 kV, 200 ms or 80 kV, 20 ms pulses. During the conditioning, vacuum arcs occur mainly at the probe head. The arcs appear often when dark field emission currents are measured. The presence of a plasma density of 10¹⁵ m^-3 (delivered by a high aperture ion source) does not affect the voltage stand-off of the probe unless the pressure of working gas is increased beyond a critical level: a semi-self-sustained glow discharge is ignited at a pressure of 0.15 Pa for He and 0.03 Pa for air. These pressures are about one order of magnitude lower than the pressures required for ignition of a self-sustained glow discharge at 80 kV. Cathode spots on the surface of the inner conductor are formed in the semi-self-sustained discharge and often lead to the formation of the arc discharge. (orig.)