Abstract
The large (size: 1 m × 2 m) radio frequency (RF) driven negative ion sources for the neutral beam heating (NBI) systems of the future fusion experiment ITER will be operated at a low filling pressure of 0.3 Pa, in hydrogen or in deuterium. The plasma will be generated by inductively coupling an RF power of up to 800 kW into the source volume. Under consideration for future neutral beam heating systems, like the one for the demonstration reactor DEMO, is an even lower filling pressure of 0.2 Pa. Together with the effect of neutral gas depletion, such low operational pressures can result in a neutral gas density below the limit required for sustaining the plasma. Systematic investigations on the low-pressure operational limit of the half-ITER-size negative ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility were performed, demonstrating that operation is possible below 0.2 Pa. A strong correlation of the lower pressure limit on the magnetic filter field topology is found. Depending on the field topology, operation close to the low-pressure limit is accompanied by strong plasma oscillations in the kHz range.
Highlights
The large radio frequency (RF) driven negative ion sources for the neutral beam heating (NBI) systems of the future fusion experiment ITER will be operated at a low filling pressure of 0.3 Pa, in hydrogen or in deuterium
Due to the high relevance of deuterium operation for both ITER and DEMO, pulses are done in hydrogen and deuterium, with the ELISE standard magnetic filter (IPG plus one row of permanent magnets, three IPG return conductors, see Figure 2a) and with the following modifications of the field topology intended to affect the field topology in different regions of the ion source:
The onset and strength of these oscillations strongly depend on the topology of the magnetic filter, as illustrated in Figure 4 for deuterium, which shows for the ELISE standard filter, the configuration without the central IPG return conductors and the configuration without the magnetic filter time traces of the filling pressure and the current impinging the top segment of the bias plate
Summary
The neutral beam heating (NBI) system of the international experimental reactor ITER will be based on large RF-driven sources for negative deuterium or hydrogen ions [1]. The following experimental and theoretical results indicate that the low-pressure operational limit of ion sources for NBI mainly depends on the strength and/or topology of the magnetic (filter) field, the source geometry and plasma parameters in the plasma generation volume. Comparing results obtained at different arc driven ion sources, a general reduction of the low-pressure limit with an increasing ratio of the ion source volume to the plasma loss area is found [23]. This may indicate that racetrack-shaped RF drivers, as foreseen for the European DEMO, allow operation with lower filling pressures compared to circular drivers. Investigations have been conducted for different topologies of the magnetic filter field in order to investigate its impact on the low-pressure operational limit
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