Absolute calibration of the Lyman- α measurement apparatus at DIII-D

The LLAMA (Lyman-Alpha Measurement Apparatus) diagnostic was recently installed on the DIII-D tokamak [Rosenthal et al., Rev. Sci. Instrum. (submitted) (2020)]. LLAMA is a pinhole camera system with a narrow band Bragg mirror, a bandpass interference filter, and an absolute extreme ultraviolet photodiode detector array, which measures the Ly-α brightness in the toroidal direction on the inboard, high field side (HFS) and outboard, low field side (LFS). This contribution presents a setup and a procedure for an absolute calibration near the Ly-α line at 121.6 nm. The LLAMA in-vacuum components are designed as a compact, transferable setup that can be mounted in an ex situ vacuum enclosure that is equipped with an absolutely calibrated Ly-α source. The spectral purity and stability of the Ly-α source are characterized using a vacuum ultraviolet spectrometer, while the Ly-α source brightness is measured by a NIST-calibrated photodiode. The non-uniform nature of the Ly-α source emission was overcome by performing a calibration procedure that scans the Ly-α source position and employs a numerical optimization to determine the emission pattern. Nominal and measured calibration factors are determined and compared, showing agreement within their uncertainties. A first conversion of the measured signal obtained from DIII-D indicates that the Ly-α brightness on the HFS and LFS is on the order of 1020 Ph sr^{−1} m^{−2} s^{−1}. The established calibration setup and procedure will be regularly used to re-calibrate the LLAMA during DIII-D vents to monitor possible degradation of optical components and detectors.

莱曼α测量装置（Lyman-Alpha Measurement Apparatus，简称LLAMA）诊断系统近期已安装于DIII-D托卡马克装置上[Rosenthal等人，《Review of Scientific Instruments》，已投稿，2020年]。该系统为针孔相机构型，配备窄带布拉格（Bragg）镜、带通干涉滤光片以及绝对式极紫外光电二极管探测器阵列，可在环向方向上测量内侧高场侧（HFS）与外侧低场侧（LFS）的莱曼α辐射亮度。本研究针对121.6 nm处的莱曼α谱线，介绍了一套绝对定标装置与定标流程。LLAMA的真空内部组件被设计为紧凑且可迁移的构型，可安装于配备经美国国家标准与技术研究院（National Institute of Standards and Technology，简称NIST）校准的莱曼α辐射源的异地真空舱内。研究采用真空紫外光谱仪表征莱曼α辐射源的光谱纯度与稳定性，并通过经NIST校准的光电二极管测量该辐射源的辐射亮度。针对莱曼α辐射源发射的非均匀性问题，本研究通过扫描辐射源位置并采用数值优化方法确定其发射分布的定标流程予以解决。研究确定了标称定标因子与实测定标因子并进行对比，结果显示二者在不确定度范围内吻合良好。对DIII-D装置采集的首批实测信号进行转换后结果表明，高场侧与低场侧的莱曼α辐射亮度量级约为10²⁰ 光子·sr⁻¹·m⁻²·s⁻¹。本次搭建的定标装置与流程将在DIII-D装置每次真空排气期间定期用于LLAMA的重新定标，以监测光学元件与探测器可能出现的性能退化。