Abstract
Neutral beam injection (NBI) systems are required for heating and current drive in the next generation fusion experiment ITER, and strict requirements are placed on the beamlet core divergence (<7 mrad) for transmission into the tokamak. The measurement of single beamlet divergence is challenging due to the multi-beamlet nature of the negative ion sources that are required for such systems; diagnostic systems compatible with large high power ion beams can only provide spatially averaged measurements, leading to mixing of beamlet signals within diagnostic results. To improve the understanding of this effect, a direct comparison has been made between the single beamlet and multi-beamlet divergence by a combination of both Beam Emission Spectroscopy (BES) and 1D carbon fiber composite tile calorimetry, in a joint campaign by IPP and Consorzio RFX. The measurements performed so far in this campaign at the BATMAN Upgrade Test Facility have led to two major results. First, an excellent agreement is found for single beamlet divergences from the two diagnostics, showing that the results from these diagnostic systems can be compared for single beamlets. Second, the contribution of beamlet deflection, caused by an alternating magnetic field at the extract grid, to the divergence as measured using BES has been quantified with up to a factor of 3 increase when compared with the single beamlet value. This demonstrates that further investigation is needed into how mixing of information from a beamlet affects diagnostic results with a combination of both simulation and experiment, which will be performed in a next step.
Highlights
For plasma heating and current drive of the ITER fusion experiment, two neutral beam injection (NBI) systems operating in deuterium or hydrogen, are foreseen with a total power of 33 MW
The results presented in this work show a direct comparison between the measurements of single beamlet divergences from both Beam Emission Spectroscopy (BES) and 1D carbon fiber composite (CFC) tile imaging
It is the first time that BES divergence measurements have been performed simultaneously on both a group of beamlets and a single, isolated beamlet. These two comparison points have allowed for the cross-checking of divergence measurements by BES and CFC tile calorimetry, and excellent agreement has been shown between the two
Summary
For plasma heating and current drive of the ITER fusion experiment, two neutral beam injection (NBI) systems operating in deuterium or hydrogen, are foreseen with a total power of 33 MW. Test facilities for investigating the aspects of the beams required for ITER are all based on the same working principle: a hydrogen plasma is generated in the RF driver(s) with a power of typically up to 100 kW per driver and a frequency of 1 MHz. The plasma expands into one common expansion chamber and is cooled from Te ≈ 10 eV inside the driver to Te < 2 eV just in front of the first grid by a magnetic filter field in order to minimize the destruction of negative ions via collisions with electrons. The Doppler shifted Hα emission provides a measure of the velocity distribution of particles crossing the line of sight; the e-folding half-width of a Gaussian fitted to the shifted peak gives a value for the divergence.6,9 This method intrinsically combines data from multiple apertures as the line of sight inevitably passes through many beamlets. The first comparison between divergence measured by CFC tile imaging and by BES for a single beamlet and the first comparison of BES divergence measurements from a single beamlet and a group of beamlets extracted simultaneously from the same system are shown
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