Subdivertor fuel isotopic content detection limit for JET-DTE operation

In preparation for JET Deuterium-Tritium Experiment 2 (DTE-2, ca. late 2019) the detection limit capability for fuel isotope concentrations in JET the subdivertor region has been studied based on a re-examination of the DTE-1 Penning optical spectroscopy data [1, 2]. The analysis indicated a 1% T/(H+D+T) lower limit, which is considered sufficient for DT operation, while detection down to <~1% concentrations is desired for neutron budget control during TT operation and during DD (following DT). In support of this study, an advanced spectral fitting program at IRFM CEA, with full error statistics capability, has been operated in simulation mode, successfully simulating the error that was exhibited in the DTE1 data. This detection limit degrades over time due to attenuation of Penning source-emitted light by source-produced thin film deposition. An upgraded diagnostic for DTE-2 is aiming to reduce light losses, partly by mitigating window coatings is being completed in time for this critical for JET-DTE2 measurement and likely the principal way to diagnose low-level fuel isotopic concentration in ITER plasmas [2]. [1] D.L. Hillis et al., RSI 70,1999 [2] C.C. Klepper et al., 2017 JINST 12 C10012