Characterization of Lyman-Alpha Absorption for Calibrating DIII-D's LLAMA Diagnostic

We present experimental studies of the effects of environmental parameters on the calibration of DIII-D’s Lyman-α neutral hydrogen density diagnostic, LLAMA [1]. Lyman-α (121.6nm) light is readily absorbed by air, therefore the Krypton calibration source (123.6nm) that mimics Lyman-α plasma edge emissions is housed in a dedicated vacuum chamber. During calibration, unexpected Lyman-α absorption in the chamber results in ~25% variation in the intensity measurement of the calibration source. Vacuum causes a coating of unknown composition to form on the magnesium fluoride entrance window of the Krypton source, which is avoided by taking measurements in atmospheric argon pressure [2]. However, impurities (namely oxygen) accompany venting, and the chamber walls outgas trapped moisture. Minimizing the concentration of these impurities, and thus their associated absorption, is key to maintaining a consistent calibration environment. We find that this environment requires a vacuum pressure of at least 10^-4 torr in both the chamber and vent lines prior to argon venting. Environmental consistency yields higher precision calibration, and thus improved neutral density measurements with LLAMA.

[1] Rosenthal RSI 92, 033523 (2021)

[2] Towle APS DPP, CP12.00038 (2025)

Work supported by US DOE under DE-FC02-04ER54698, DE-AC02-09CH11466, DE-SC0014264, and the SULI program.