Expected accuracy of fuel ion ratio measurements by collective Thomson scattering at TEXTOR

摘要

Introduction In magnetically confined fusion experiments and for future reactors, measurements of the fuel ion ratio - the ratio between fuel ion densities - are of intrinsic scientific interest and will further be important for plasma control and machine protection. The techniques currently used for fuel ion ratio measurements - most notably neutral particle analysis - may not be applicable in the core of a fusion reactor [1]. It is therefore important to develop alternatives. Microwave based collective Thomson scattering (CTS) diagnostics are well suited for reactor environments and provide access to the dynamics of confined ions by measuring the spectrum of probe radiation scattered by plasma fluctuations excited by ion motion. CTS measurements thus yield information on the ion velocity distribution and on certain plasma waves. For specific scattering geometries, CTS spectra contain signatures of ion Bernstein waves (IBWs) which are highly sensitive to the ion species mix. Measurements of lBW signatures in CTS spectra have therefore been suggested as a new diagnostic principle for measurements of the fuel ion ratio. Previous feasibility studies have found that such a diagnostic could fulfill the measurement requirements for ITER [2], and that it could be integrated in the CTS system foreseen to measure fast ion velocity distributions on ITER [3]. The next natural step is to perform proof-of-principle experiments in current machines demonstrating the ability to measure IBW signatures in CTS spectra, their sensitivity to plasma composition, and the ability to infer the fuel ion ratio - or in current machines the equivalent hydrogen to deuterium density ratio R_H =nH/(nD +nH). The CTS receiver at TEXTOR was recently modified for such experiments, which require higher frequency resolution than was possible with the existing system [4]. Here we discuss the theoretical sensitivity to R_H of spectra measured with the modified receiver, and the expected accuracy with which R_H could be inferred from such spectra. Results of experiments to measure IBW signatures will be reported elsewhere.