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 To allow direct comparison with the measurements, synthetic radiation temperatures are computed with the code ECRad.¹⁰ 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.

电子回旋共振加热（ECRH）可通过电子回旋电流驱动器（ECCD）驱动大电流密度。预计ECCD对于ITER和DEMO等未来聚变反应堆中的高性能等离子体至关重要，这使得ECCD的当前驱动效率成为未来反应堆的关键设计参数。在当今的设备中，已发现测量的电流驱动效率与预测的电流驱动效率之间存在良好的一致性。但是，为了确保未来机器的可靠性，需要直接验证电子动量分布函数。

作为朝着这个目标迈出的第一步，我们在本文中介绍了ASDEX升级托卡马克中低密度等离子体的斜电子回旋发射（ECE）测量。使用两个倾斜的ECE诊断程序可以同时测量与等离子体电流同向和反向流动的电子。分布函数的预测是使用反弹平均的Fokker-Planck码RELAX计算的。⁹为了直接与测量进行比较，使用代码ECRad计算合成辐射温度。¹⁰如果包括主要在低电子能量下发生的径向传输，则可以找到很好的一致性。

我们证明，倾斜的ECE诊断程序可以直接在相空间中的ECRH沉积位置测量电子分布功能。此外，它们对降低ECCD效率的大量倾斜角散射电子敏感。讨论了测量和建模的局限性和不确定性。