Relativistic effects on reconstruction of density profiles via reflectometry in ITER and potential for electron temperature measurements

Abstract

Relativistic effects on reflectometry density profile measurements have been evaluated for planned ITER operating scenarios. Standard X-mode, right-hand cutoff reflectometry reconstruction can produce large errors (up to 35% for T e (0) = 24 keV plasmas) in high-temperature ITER plasmas if relativistic effects are ignored. It will therefore be essential to account for these effects in ITER. The errors created in density profile reconstruction have been analysed using simulated temperature profiles, including uncertainties, as an input to a standard inversion routine. Modelling using random errors with maxima of ±10% across the electron temperature profile leads to errors in the inverted density profile ranging from zero at the edge to a maximum RMS error of ∼1% at the magnetic axis. In addition to this, a novel iterative inversion technique is demonstrated which eliminates the dependence on external T e information and allows T e itself, and hence electron pressure P e , to be extracted from the reflectometry measurement. The new technique utilizes phase data from both Oand X-mode propagation and has been successfully demonstrated numerically in peaked plasma profiles, as well as in the edge pedestal region. Integration of this novel approach with other planned measurement systems will serve to improve confidence in electron density and temperature measurements in ITER.

Investigation of the Thomson scattering-ECE discrepancy in ICRF heated plasmas at Alcator C-Mod

“…Future experiments to investigate the discrepancy should include, when possible, measurements of (or at least constraints on) the electron temperature using diagnostics other than TS and ECE. One example would be exploiting relativistic effects to extract T e from reflectometry measurements [34] or perhaps using a spatially resolving, spherically bent x-ray crystal spectrometer to assess T e from impurity lines [35]. Since high temperature plasmas T e (0) > 8 keV can be obtained relatively routinely now at C-Mod in the I-mode regime [21], it will be possible to continue to add to the existing database that was analyzed for this paper.…”

Survey of the TS-ECE Discrepancy and recent investigations in ICRF heated plasmas at Alcator C-Mod

“…Future experiments to investigate the discrepancy should include, when possible, measurements of, or constraints on, the electron temperature using other diagnostics (e.g. exploiting relativistic effects to extract Te from reflectometry [28] or perhaps using an x-ray crystal spectrometer to assess Te from impurity lines [29].…”

Active Diagnostics

“…The different temperature dependence of O-and X-mode cutoffs might be used with advantage in next-generation, high-temperature burning plasma experiments (BPXs) to reconstruct iteratively both the density and the temperature profile by applying reflectometry simultaneously in O-and in X-mode as proposed in [17]. Two independent reflectometers probing the same plasma column then need to be operated.…”