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 1015m-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. When the ICRF probe is installed in ASDEX Upgrade and is well conditioned (to the maximal voltages achieved in the test facility), high voltage breakdown on the probe often correlates with activity of edge localized modes (ELMs). The breakdown characteristics are similar to that of the cathode spots formation in the semi-self-sustained discharge glow discharge. The maximal RF voltage on the ICRF probe increases from shot to shot, i.e. an additional conditioning effect is observed during plasma operation. The voltage limit of the probe can be increased by application of a positive DC bias to the inner conductor while at the same time the rectified current associated with the collection of ions across magnetic field is suppressed. It was found that the appearance of ELMs and other intermittent events in the scrape-off-layer (SOL) plasma in the region of the probe head lead to a local dissipation of a high fraction of RF power. The role of ELMs as RF breakdown trigger is confirmed by observations during operation of the full-size AUG ICRF antenna. A reliable arc detection system is required for the ICRF antennas (not every breakdown triggered by ELMs is easy to detect), otherwise the overall performance of the antennas degrades due to appearance of quasi-stationary arc discharges. The antennas operates more reliably when the antenna conductors are conditioned with plasma. Measures to improve the antenna voltage stand-off in the presence of plasma are suggested: an optically closed Faraday screen; glow discharge conditioning; a form of antenna conductors to minimize ion collection across the magnetic field and minimize asymmetry of electrodes along the field; neutral density reduction inside the antenna. Further work should be focused on the choice of the antenna materials, parasitic absorption of the RF power and the antenna-plasma interaction for different DC boundary conditions of the antenna circuit.     «

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 dev...     »

Das Abstrahlen von Radiowellen nahe der Ionenzyloktronfrequenzen (ICRF) ist eine der üblichen Heizmethoden von magnetisch eingeschlossenen Plasmen. Jedoch begrenzt die Spannungsfestigkeit der ICRF Antenne die maximal abstrahlbare Leistung. Die Begrenzung kann die Folge unterschiedlicher Mechanismen sein, die von Raumladungseffekten und Teilchenflüssen zu den Elektroden abhängen. Eine ICRF Versuchsanordnung bestehend aus einem einseitig offenen koaxialen Resonator wurde für experimentelle Studien entwickelt. Dabei stellt das offene Ende ein Gebiet mit hoher Spannung in der Antenne dar. Die Spannungsfestigkeit dieses Bereichs wurde in einem Teststand unter definierten Bedingungen ohne Magnetfeld, und in dem Randplasmabereich des ASDEX Upgrade unter antennenähnlichen Bedingungen untersucht. Die Ergebnisse helfen die Überschlagsprozesse in einer ICRF Antenne besser zu verstehen und zeigen Methoden auf, mit denen die Spannungsfestigkeit von Antennen verbessert werden kann.     «

Das Abstrahlen von Radiowellen nahe der Ionenzyloktronfrequenzen (ICRF) ist eine der üblichen Heizmethoden von magnetisch eingeschlossenen Plasmen. Jedoch begrenzt die Spannungsfestigkeit der ICRF Antenne die maximal abstrahlbare Leistung. Die Begrenzung kann die Folge unterschiedlicher Mechanismen sein, die von Raumladungseffekten und Teilchenflüssen zu den Elektroden abhängen. Eine ICRF Versuchsanordnung bestehend aus einem einseitig offenen koaxialen Resonator wurde für experimentelle Studien...     »