In the ITER Neutral Beam Test Facility (NBTF) in Padova (Italy), the Ion Source and Extraction Power Supplies (ISEPS) are currently in operation for the SPIDER experiment and in power supply integrated commissioning phase for MITICA. The procurement of ISEPS by OCEM Energy Technology foresees four stages, corresponding to four units: one ISEPS for SPIDER, one ISEPS for MITICA and two ISEPS systems for the Neutral Beam Injectors of ITER. The first two stages have been completed, while the others are currently in progress. The ISEPS systems include 4 RF generators based on self-excited free-running tetrode oscillators. The reasons of the selection of this technology laid on the experience on RF ion sources for NBI at IPP Garching, the belief that self-excited oscillators could assure stability in power and frequency in a wide range of load parameters and on the positive feedbacks from the experimental results until the time of the definition of the procurement scope.. Moreover, at that time, no sufficient evidence was available that alternative technologies could have given better performance. The choice of the technology was anyway left open in the technical specifications, but all potential suppliers proposed solutions based on tetrode oscillators for the above mentioned reasons. However, subsequent feedbacks from ion sources operation at IPP reported on the observation of sudden frequency variations of the oscillator, so-called "frequency-flip", which limited the controllability of the RF generators. Afterwards, RF amplifiers based on solid state components were tested with positive results in BATMAN and ELISE facilities at IPP. Observations of frequency flips came also from the first experimental campaigns of SPIDER; the accompanying analyses explained the intrinsic and unavoidable limits of the application of this technology to the resonant loads of Neutral Beam Injectors based on RF ion sources. The comprehension of the phenomenon allowed properly controlling the oscillators so as to avoid the frequency flip occurrence and to achieve long pulse operation, but the impossibility to operate at the desired load resonance condition was also fully clear, preventing the achievement of the nominal power. All these evidences led to the decision to replace the RF generators in SPIDER and MITICA and change the current ITER baseline from self-excited freerunning tetrode oscillator technology to solid state amplifier technology. The paper will present and discuss the issues found in SPIDER with the operation of the RF generators based on self-excited tetrode oscillators and the advantages expected from their substitution with RF solid state amplifiers. The assessment of the procurement for SPIDER and MITICA will be also described, including the necessary adaptation of the site interfaces and the strategy to test and install the amplifiers.
Radio Frequency Generators Based on Solid State Amplifiers for the NBTF and ITER projects
Gaio E;Recchia M;Toigo V;
2021
Abstract
In the ITER Neutral Beam Test Facility (NBTF) in Padova (Italy), the Ion Source and Extraction Power Supplies (ISEPS) are currently in operation for the SPIDER experiment and in power supply integrated commissioning phase for MITICA. The procurement of ISEPS by OCEM Energy Technology foresees four stages, corresponding to four units: one ISEPS for SPIDER, one ISEPS for MITICA and two ISEPS systems for the Neutral Beam Injectors of ITER. The first two stages have been completed, while the others are currently in progress. The ISEPS systems include 4 RF generators based on self-excited free-running tetrode oscillators. The reasons of the selection of this technology laid on the experience on RF ion sources for NBI at IPP Garching, the belief that self-excited oscillators could assure stability in power and frequency in a wide range of load parameters and on the positive feedbacks from the experimental results until the time of the definition of the procurement scope.. Moreover, at that time, no sufficient evidence was available that alternative technologies could have given better performance. The choice of the technology was anyway left open in the technical specifications, but all potential suppliers proposed solutions based on tetrode oscillators for the above mentioned reasons. However, subsequent feedbacks from ion sources operation at IPP reported on the observation of sudden frequency variations of the oscillator, so-called "frequency-flip", which limited the controllability of the RF generators. Afterwards, RF amplifiers based on solid state components were tested with positive results in BATMAN and ELISE facilities at IPP. Observations of frequency flips came also from the first experimental campaigns of SPIDER; the accompanying analyses explained the intrinsic and unavoidable limits of the application of this technology to the resonant loads of Neutral Beam Injectors based on RF ion sources. The comprehension of the phenomenon allowed properly controlling the oscillators so as to avoid the frequency flip occurrence and to achieve long pulse operation, but the impossibility to operate at the desired load resonance condition was also fully clear, preventing the achievement of the nominal power. All these evidences led to the decision to replace the RF generators in SPIDER and MITICA and change the current ITER baseline from self-excited freerunning tetrode oscillator technology to solid state amplifier technology. The paper will present and discuss the issues found in SPIDER with the operation of the RF generators based on self-excited tetrode oscillators and the advantages expected from their substitution with RF solid state amplifiers. The assessment of the procurement for SPIDER and MITICA will be also described, including the necessary adaptation of the site interfaces and the strategy to test and install the amplifiers.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
