Abstract
Electron cyclotron resonance heating (ECRH) can drive large current densities through electron cyclotron current drive (ECCD). ECCD is expected to be crucial for high-performance plasmas in future fusion reactors like ITER and DEMO, making the current drive efficiency of ECCD a critical design parameter for future reactors. In present-day devices, good agreement between measured and predicted current drive efficiency has been found. However, to ensure the reliability in future machines, a direct validation of the electron momentum distribution function is needed.As a first step towards this goal, we present in this paper oblique electron cyclotron emission (ECE) measurements of a low-density plasma in the ASDEX Upgrade tokamak. Two oblique ECE diagnostics are used to allow the simultaneous measurements of electrons streaming co- and counter-directionally with the plasma current. Predictions for the distribution function are computed with the bounce-averaged Fokker-Planck code RELAX. 9 9E Westerhof et al, Rijnhuizen report,1992. To allow direct comparison with the measurements, synthetic radiation temperatures are computed with the code ECRad. 10 10S Denk et al, Computer Physics Communications, p. 107175, 2020. Good agreement is found if radial transport occurring predominantly at low electron energies is included.We demonstrate that oblique ECE diagnostics measure the electron distribution function directly at the ECRH deposition site in phase space. Furthermore, they are sensitive to the abundance of pitch-angle scattered electrons that reduce the ECCD efficiency. Limitations and uncertainties of the measurements and the modeling are discussed.
Highlights
Electron cyclotron resonance heating (ECRH) is a technique that utilizes focused, MW-strong microwave beams to heat a magnetized plasma
This paper presents a first step to confirm the quasi-linear theory for electron cyclotron current drive (ECCD) using oblique electron cyclotron emission (ECE) measurements capable of resolving the electron distribution function at the ECRH deposition site in the phase space spanned by a radial coordinate and cylindrical momentum space
Since Te is a critical quantity for the electron distribution function during ECCD, this is another limiting factor for the results shown in this paper
Summary
See author list of H Meyer et al 2019 Nucl. Fusion 63 (2021) 015003 computed with the bounce-averaged Fokker-Planck code RELAX.. To allow direct comparison with the measurements, synthetic radiation temperatures are computed with the code ECRad.. We demonstrate that oblique ECE diagnostics measure the electron distribution function directly at the ECRH deposition site in phase space. They are sensitive to the abundance of pitch-angle scattered electrons that reduce the ECCD efficiency.
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