Principles of fuel ion ratio measurements in fusion plasmas by collective Thomson scattering

Abstract

For certain scattering geometries collective Thomson scattering (CTS) measurements are sensitive to the composition of magnetically confined fusion plasmas. CTS therefore holds the potential to become a new diagnostic for measurements of the fuel ion ratio-i.e. the tritium to deuterium density ratio. Measurements of the fuel ion ratio will be important for plasma control and machine protection in future experiments with burning fusion plasmas. Here we examine the theoretical basis for fuel ion ratio measurements by CTS. We show that the sensitivity to plasma composition is enhanced by the signatures of ion cyclotron motion and ion Bernstein waves which appear for scattering geometries with resolved wave vectors near perpendicular to the magnetic field. We investigate the origin and properties of these features in CTS spectra and give estimates of their relative importance for fuel ion ratio measurements.

Resolving the bulk ion region of millimeter-wave collective Thomson scattering spectra at ASDEX Upgrade

“…• , are signatures of the ion cyclotron motion and of weakly damped ion Bernstein waves exited by the ion motion 36 . They appear at frequency intervals corresponding to the cyclotron frequencies of the main ion species present in the plasma -i.e.…”

“…Signatures of the ion cyclotron motion and ion Bernstein waves are of particular interest for their sensitivity to ion density ratios. This has been used for measurements of the isotopic composition [30][31][32][33][34] , and it could potentially be exploited for measurements of the fuel ion ratio in burning DT-plasmas 35,36 . However, accessing the detailed information contained in the bulk ion region of CTS spectra requires a frequency resolution beyond what is generally achieved with the cavity filters commonly used for fast-ion measurements.…”

Recent development of collective Thomson scattering for magnetically confined fusion plasmas

“…An additional application of the CTS diagnostic is the characterization of the bulk-ion parameters such as main ion temperature, main ion drift velocity, and bulk ion composition [13]. These measurements are particularly relevant for stellarator devices due to the general difficulty in diagnosing ion temperatures at high electron densities.…”

Thomson scattering diagnostic for the measurement of ion species fraction

“…This technique provides a non-invasive way to measure bulk plasma parameters such as electron and ion temperature, electron density, plasma flow, and ionization state [2][3][4]. Thomson scattering has also been used to determine ion species fraction in the presence of a background magnetic field [5]. We show that it is possible to measure the ion species fraction to better than ±0.06 for a range of non-magnetized plasmas using Thomson scattering.…”