Sub-millisecond electron density profile measurement at the JET tokamak with the fast lithium beam emission spectroscopy system

Abstract

Diagnostic alkali atom (e.g., lithium) beams are routinely used to diagnose magnetically confined plasmas, namely, to measure the plasma electron density profile in the edge and the scrape off layer region. A light splitting optics system was installed into the observation system of the lithium beam emission spectroscopy diagnostic at the Joint European Torus (JET) tokamak, which allows simultaneous measurement of the beam light emission with a spectrometer and a fast avalanche photodiode (APD) camera. The spectrometer measurement allows density profile reconstruction with ∼10 ms time resolution, absolute position calculation from the Doppler shift, spectral background subtraction as well as relative intensity calibration of the channels for each discharge. The APD system is capable of measuring light intensities on the microsecond time scale. However ∼100 μs integration is needed to have an acceptable signal to noise ratio due to moderate light levels. Fast modulation of the beam up to 30 kHz is implemented which allows background subtraction on the 100 μs time scale. The measurement covers the 0.9 < ρ < 1.1 range with 6-10 mm optical resolution at the measurement location which translates to 3-5 mm radial resolution at the midplane due to flux expansion. An automated routine has been developed which performs the background subtraction, the relative calibration, and the comprehensive error calculation, runs a Bayesian density reconstruction code, and loads results to the JET database. The paper demonstrates the capability of the APD system by analyzing fast phenomena like pellet injection and edge localized modes.

Effect of the isotope mass on pedestal structure, transport and stability in D, D/T and T plasmas at similar β N and gas rate in JET-ILW type I ELMy H-modes

“…Due to its high spatial resolution, the HRTS is very valuable to determine the pre-ELM pedestal structure. For the electron density pre-ELM profiles, the HRTS information have been complemented with the Lithium beam [41] which provides high quality profiles in the scrape off layer (SOL) at 100 Hz. However, the Lithium beam does not reach the pedestal top in these pulses (for details, see [41]).…”

Identity of the JET M-mode and the ASDEX Upgrade I-phase phenomena

“…• The JET Li-BES system [23] consists of a beam injector aligned vertically at the top of the tokamak, shooting downwards and a beam emission detection system. The observation range of the detection system can be set by a turnable mirror in the limiter shadow displaced toroidally from the beam, looking quasi parallel with the field lines.…”

Characterization of edge and scrape-off layer fluctuations using the fast Li-BES system on COMPASS

“…Neutral alkali beams are routinely used in a number of fusion-related devices as non perturbative diagnostics for time evolving density profile measurements [10,11,12,13].…”