Abstract
In the previous divertor campaign, the Wendelstein 7-X (W7-X) device injected 3.6 MW of neutral beam heating power allowing for the achievement of densities approaching 2 × 1020 m−3, and providing the first initial assessment of fast ion confinement in a drift optimized stellarator. The neutral beam injection (NBI) system on W7-X is comprised of two beam boxes with space for four radio frequency sources each. The 3.6 MW of heating reported in this work was achieved with two sources in the NI21 beam box. The effect of combined electron-cyclotron resonance heating (ECRH) and NBI was explored through a series of discharges varying both NBI and ECRH power. Discharges without ECRH saw a linear increase in the line-integrated plasma density, and strong peaking of the core density, over the discharge duration. The presence of 1 MW of ECRH power was found to be sufficient to control a continuous density rise during NBI operation. Simulations of fast ion wall loads were found to be consistent with experimental infrared camera images during operation. In general, NBI discharges were free from the presence of fast ion induced Alfvénic activity, consistent with low beam betas. These experiments provide data for future scenario development and initial assessment of fast-ion confinement in W7-X, a key topic of the project.
Highlights
In the previous divertor campaign, the Wendelstein 7-X (W7-X) device injected 3.6 MW of neutral beam heating power allowing for the achievement of densities approaching 2 × 1020 m−3, and providing the first initial assessment of fast ion confinement in a drift optimized stellarator
The neutral beam injection (NBI) system on W7-X is comprised of two beam boxes with space for four radio frequency sources each
While Alfvenic activity was present in many discharges on W7-X, such modes could not be correlated with the presence of fast ions [5]
Summary
The recent inertially cooled divertor campaign on Wendelstein 7-X (W7-X) included the commissioning and operation of the neutral beam heating system on the device (among other milestones). Experiments began with injection into plasmas sustained by electron cyclotron resonance heating (ECRH), allowing for control of target plasmas and densities These experiments provided a wealth of information to help validated both neutral beam deposition [3] and wall loss estimates [4]. A post-mortem inspection of the vessel showed no signs of damage or excessive loading of the steel panels Thomson data for this discharge (figure 2) indicates a slight decrease in core temperature and increase in density at all radii when the NBI is injected. Spectroscopic measurements show a strong rise in iron and carbon light which quickly returns to levels before TESPEL injection within 500 ms Such behavior is consistent with ECRH discharges showing a prompt flushing of impurities from the core [17]. Alfven modes did appear in ECRH discharges the exact nature of such modes is still an active topic of research
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