Beam optics of RF ion sources in view of ITER’s NBI systems

Abstract

A low beamlet divergence is crucial for the efficiency of the ITER-NBI systems, since it affects the transmission of the beam through the duct. There is a requirement of 7 mrad e-folding divergence for the ITER Heating Neutral Beam. Significantly higher divergences (10-15 mrad) have been observed in RF-source based experiments albeit at low beam energy. This could be the consequence of a broad perpendicular velocity distribution of the H-/D- particles before extraction. This paper explores this hypothesis and its implications for ITER. To estimate H-/D- perpendicular temperatures in the RF-driven BATMAN Upgrade test facility, spatially resolved measurements of the beam power density are compared with IBSimu calculations. The estimated perpendicular temperatures show a strong dependence on the source filling pressure, decreasing from approximately 4 eV at 0.3 Pa to 2 eV at 0.4 Pa. Ion-optics calculations of the ITER-HNB grid system are performed to evaluate whether the temperatures estimated in the BATMAN Upgrade test facility are tolerable in view of beam-grid interaction and beamline transmission. The beamline transmission is fairly insensitive to the perpendicular temperature, but the heat loads at the downstream grids increase with the perpendicular temperature.

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“…The analysis of the earliest time frame of the BUG beam target instead of the time frame at 0.5 s with the agreed assumptions of the task force, yields a lower beam divergence of 9 mrad for an RF source at 0.3 Pa [49]. These two values for the divergence are plotted in figure 10(b) together with the individual contributions to the divergence of the ITER beam [13]. The comparison of the measured values with the simulations indicate that the 7 mrad target can be achieved at the high acceleration voltages applied at ITER.…”

“…The prototype source represents a 1/8 scale of the full ion source and has been tested first at the BATMAN test facility with focus on the demonstration of the ion source parameters for short pulses (up to 4 s extraction) accompanied by extensive plasma diagnostics to get insights into the underlying ion source physics, whereas MANITU demonstrated the long pulse capability [9]. In 2018, BATMAN was upgraded to focus on beam properties (BUG) [12,13] and subsequently enhanced for steady state operation. For the latter, a steady state capable calorimeter has been developed in-house and a cryogenic pump was installed supported by EUROfusion, other changes concern the adequate temperature control of components.…”

“…These parameters are subject to change with the type of beam source and with pressure. The beam optics of the RF ion sources in view of ITER's NBI systems is discussed in [13].…”

“…The comparison of the measured values with the simulations indicate that the 7 mrad target can be achieved at the high acceleration voltages applied at ITER. Nonetheless, the comparison with the arc sources and the strong dependence of the beam divergence on the filling pressure triggered discussions on the reason [8,13], for which the perpendicular temperature of the negative ions is identified. The latter increases the divergence especially at low beam energies as can be seen in figure 10(b) for the assumed 1 eV.…”

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“…(iv) The ion acceleration by the sheath and their recombination on the metal walls induces energetic atoms D 0 and gas heating [25] in the plasma. Hot atoms D 0 bombarding the caesiated surface in the extraction region will produce D − with transverse energy resulting in D − beam optics with high divergence (>10 mrad) [27].…”