Study of RF Stray Currents in ITER Neutral Beam Test Facilities

Abstract

The operation of SPIDER (Source for the Production of Ions of Deuterium Extracted from Radio-frequency plasma), full-scale prototype of ITER NBI (Neutral Beam Injector) radio-frequency ion source, pointed out deleterious effects caused by stray Radio-Frequency (RF) currents flowing in the electrical equipment not included in the RF power system. MITICA (Megavolt ITER Injector and Concept Advancement), the full-scale prototype of ITER NBI, is characterized by a similar design in terms of layout of the power supplies and connections to the beam source; thus, it is expected to be subject to the RF stray currents problem. SPIDER RF system is composed of four RF generators, four coaxial lines and four RF loads. Each RF generator is rated for operation at 200 kW in the frequency range 0.9 ÷ 1.1 MHz. The power is delivered to the four loads via as many RF coaxial lines, housed inside a multiconductor transmission line. Each load consists of a capacitive matching network and two plasma drivers in series. Due to the presence of stray connections at the generator and beam-source side (e.g., parasitic capacitances), unwanted RF currents can flow through alternative paths and negatively affect the components not intended for transmission of RF power, the output stages of power supplies and several diagnostics installed in the High-Voltage Deck (HVD) and at the beam source. This paper presents the development of a circuital model used to estimate the RF stray currents in SPIDER electrical system; the understanding of this phenomenon and the development of a model with predictive capabilities is fundamental for the assessment of the performance of both SPIDER and MITICA and, in general, of alternative RF system layouts with respect to the stray currents issue.

RF Stray Currents in SPIDER Power Circuits: Model Assessment and Experimental Results

“…The perunit length parameters were computed with Finite Elements Method Magnetics (FEMM) as in [5]. The model of one cell of the TL is depicted in Fig.…”

“…Digital Object Identifier 10.1109/TPS.2022.3186291 devices and to the overheating of passive components not specifically rated to sustain these RF currents [3]. Experimental activities were carried out to understand the origin of this phenomenon [3], [4], and to support the analysis, we developed a simplified circuital model of SPIDER [5], involving the electrical system components relevant for the study of RF stray currents, that provided a confirmation of the experimental observations from a qualitative point of view and helped in better understanding them. In SPIDER and Megavolt ITER Injector and Concept Advancement (MITICA), it is planned to replace the present tetrode-based RF generators with solid-state amplifiers [6], which implies a modification of the RF system layout.…”